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Editorial Board 

R. E. BURK, Cleveland, Ohio; H. MARK, New York, N. Y. 
G. S. WHITBY, Teddington, Middx. 

Volume I 

Collected Papers of W. H. Carothers on High Polymeric Substances 
Edited by H. MARK and G. S. WHITBY 


New York 






Edited by 


Adjunct Professor of Organic Chemistry, Director of Chemical Research, Chemical Research 

Brooklyn Polytechnic Institute Laboratory, Tedddington, Middx. 

With 35 illustrations 



New York 

Copyright, 1940, by 

215 Fourth Avenue, New York, N. Y. 

Printed in U. S. A. 


While physics is directing its main effort to exploring the structure of 
matter and the nature of energy in the sub-atomic realm, organic chemistry 
is becoming increasingly concerned with molecular structures of (a) greater 
and greater complexity, and (b) greater and greater size. In both these 
latter respects, the study of organic chemistry tends, in its most fruitful and 
significant latter-day developments, to become less the wide-ranging study 
of "the compounds of carbon" and more the study of the "organic" aspects 
of carbon compounds, i. e. t the study of those aspects bearing on the struc- 
ture and behavior of living organisms. In both respects, the behavior of 
the living cell represents the final problem to the understanding of which 
the whole development trends. 

Two conditions are necessary for the development of life, viz.: 

(a) A structural system, acting as a skeleton, which protects the func- 
tionally-active living matter from disturbances and at the same time per- 
mits the continuation of diffusion and of chemical reactions in the medium 
contained in the system. 

(b) Another system of highly reactive and diffusible compounds which 
serve as reactants and catalysts iti the processes necessary to maintain life 
and direct to a common final result a great number of chemical reactions 
and colloidal factors. 

In the present generation ever since what was once described as 
"Chemical Physiology" and later "Physiological Chemistry" acquired full 
status as "Biochemistry" much attention has been paid to the latter 
condition, and much illuminating work has been carried out on the chemis- 
try of metabolism and on catalysis and promoters of vital processes. At 
the same time, arid independently of this, a keen interest has developed iri 
the study of the natural skeletal and storage substances of organisms -in 
cellulose, keratin, starch, rubber, etc. and as a result a new field of research 
has defined itself, namely, the chemistry of high polymers. 

During the last twenty years the study of this field has been steadily ex- 
tended and intensified. Important forward steps can already be signalized 
in the preparation of synthetic high-polymers; in the elucidation of the 
mechanism and kinetics of polymerization processes; in the determination 
of the chemical structure of natural and synthetic high-polymers; in the 
determination and understanding of their colloidal behavior, and in the 
development of formulas capable of describing quantitatively the proper- 
ties of high-polymers. Generalized structural principles applicable to all 
the large molecules in question have become apparent, and, although still 



far from satisfactory in all details, obviously represent a useful frame of 
reference within which high-polymers may be classified and described. To 
refine and improve the structural picture for every significant high-molecu- 
lar material, in the hope of arriving at a fuller and more satisfying explana- 
tion of its chemical, colloidal and physical properties, is one of the tasks to 
which the further study of high -polymers must be addressed. 

New representatives of high-polymers are continually being synthesized, 
and thus offering for comparative quantitative investigation an increasingly 
large body of experimental material. And, too, new experimental technics, 
especially physical technics, are continually being applied to the study of 
high-polymers with illuminating results. In short, the whole field is ex- 
panding rapidly. 

In these circumstances it is felt that a useful purpose should be served 
by an attempt, such as this series of volumes represents, to bring together 
our present knowledge of the subject in a manner both comprehensive and 
critical. Such a survey of the subject should serve to throw into clear relief 
the generalizations which it is already possible to make; it should anchor 
the new branch of high-polymeric chemistry firmly to the fundamentals of 
chemical science, and, generally, it should contribute to fostering and facili- 
tating the further study of the subject. 

In addition to being of purely scientific interest, high-molecular ma- 
terials are of great general and economic importance. Much of our daily 
life depends on their use, and a large proportion of the wealth of all civilized 
countries is invested in industries which either use natural high-polymers as 
raw materials or manufacture and fabricate synthetic polymers. It is cer- 
tainly a conservative estimate if one assumes that more than 30% of all 
graduate chemists employed in industries are engaged in work connected 
with high -polymeric materials. In making such an estimate, one must bear 
in mind the fact that high polymeric substances are closely connected with 
the various branches of the textile industry dealing with natural and arti- 
ficial fibers, the production of natural rubber and its synthetic congeners 
and the fabrication of goods therefrom, many branches of the food industry, 
the production of molded plastics, of lacquers, of synthetic resins and of 
varnishes based on synthetic resins, the industries concerned with starch 
and its conversion products, and so on. All who have had experience in 
such industries cannot but be aware of the sore need for advancing our 
fundamental knowledge of high-polymeric materials, and cannot but have 
noted how, speedily, every forward step in such knowledge is reflected in 
improvement in the technical processes and in the products of the relevant 


It may, further, be noted that a very substantial proportion of the stu- 
dents obtaining university degrees in chemistry in various parts of the world 
are likely to find themselves called upon to apply their knowledge to indus- 
tries concerned in some degree or other with high-polymeric materials, and 
hence should benefit directly by at least some acquaintance with the funda- 
mentals of high-polymeric chemistry. 

It should be added that, as notified in the title of the series, it is proposed 
to include in the series, not only volumes on High Polymers in the strict 
sense, but also certain volumes on "Allied Subjects." This is done because 
it is thought undesirable to exclude from the survey certain solid systems, 
such as, e. g., those represented by bitumen and clay, which, although not 
strictly high-polymeric, nevertheless behave in many respects, especially 
Theologically, in a manner closely related to, and in some cases partly de- 
pendent on, high-polymers. 

It should perhaps be mentioned that, as this series of volumes, although 
employing the English language, is international in character, it has been 
considered appropriately catholic that contributors to the volumes which 
it will comprise should be free to follow in their various contributions either 
English or American spellings, where these differ. 

R. E. BURK, 
May, 1940 Editors. 



It is perhaps fair to say that the progress of organic chemistry in our 
day is most distinctively characterized by two particularly attractive and 
promising lines of research, namely by the investigation of biochemical 
catalysts ferments, vitamins, hormones and auxins and by the sys- 
tematic development of the chemistry of high polymeric substances. 
Both these lines of research are founded solidly on the experimental 
methods and theoretical viewpoints of classical organic chemistry. The 
former reaches far into biology, physiology and medicine, and touches 
some of the central problems of the functioning of living organisms. The 
latter cuts deeply into the fundamentals of the structure of matter 
especially of those parts of the structure of organisms which are not 
obviously involved in their metabolism and, further, is the basis of a 
field of chemical industry, already wide, and with great possibilities of 

In this series on high polymers, of which the present monograph is the 
first volume, an attempt will be made to offer a collection of treatises 
covering the most significant studies made in the field, in order to show 
where the subject now stands and to facilitate the further development 
which its study is surely destined to have. 

Our present knowledge of high polymers is due to a considerable and 
now rapidly growing number of workers, who have contributed essential 
material from various points of view. Some have, with success, employed 
chiefly the tried and well known methods of organic chemistry. Others 
have, with illuminating results, applied the methods of modern physics 
and physical chemistry to the study of high polymers. Again, the work 
of numerous investigators, whose concern was not primarily or wholly 
with the study of high polymers and polymerization, has contributed ex- 
perimental facts and theoretical conceptions which form a not unimpor- 
tant part of our present body of knowledge of the subject. 

No investigator has excelled Wallace Hume Carothers in advancing 
our knowledge of high polymeric chemistry and at the same time providing 
a basis for the development of technically useful synthetic polymeric 
materials. He may rightly be called the outstanding personality in this 
new branch of organic chemistry. His brilliant experimental technique, 
the abundance of his ideas, and the constructive manner in which he 
applied his critical faculty, together with the excellent facilities at his 
disposal and the competent collaborators who assisted him, enabled 



Carothers to carry out in the short space of twelve years more than most 
of us accomplish in a lifetime, and to produce an output which is at once 
very considerable in volume and compact in character. His publications 
are deservedly to be considered as "classical" : they will always remain an 
essential part of the foundation on which the high polymeric chemistry 
of the future will be erected. Their study should be undertaken by all 
who wish to be familiar with the chemistry of high polymers. They are 
highly inspiring, and at the same time they point out many of the diffi- 
culties which work in this field is bound to meet. 

With such thoughts in mind, it was felt that there could be no better 
start for this series than to publish, as the first volume in it, a collection 
of the papers embodying Carothers' studies of high polymers and closely 
related matters. The two editors regard it as a privilege to prepare this 
edition, not only because the issue of Carothers' papers in a convenient, 
collected form will, it is felt, be an addition to chemical literature which 
will be generally appreciated, but also because it will serve as an expres- 
sion of the admiration and esteem which they and other workers in the 
field of high polymers feel for a great chemist, whose premature death was 
a severe loss to chemistry and humanity. 

Carothers' work on polymerization and polymers divides itself into 
two distinct series of studies devoted to (1) "Studies on Polymerization 
and Ring Formation," (2) "Acetylene Polymers and their Derivatives." 
The papers in the first of these series are reprinted in Part One of this 
volume; those in the second, in Part Two. A few other papers, mostly 
concerned with general discussions of the subject of polymerization, are 
reprinted in Part Three. 

Part One is concerned essentially with condensation polymers produced 
by the recurring condensation of long, so-called polyfunctional molecules, 
at both ends of which are reactive groups capable of reacting, with the 
elimination of water, with the reactive end groups of other molecules of 
the same of another type to yield long condensed-polymeric chains, the 
products being, typically, polyamides, polyesters and polyanhydrides. 
The work, in addition to contributing in the most important way to our 
knowledge of condensation polymers, forms the basis of an interesting 
industrial development, namely, the manufacture of a synthetic poly- 
amide product ("Nylon"), which may be considered, chemically, as repre- 
senting a simplified protein-like material, and which can be drawn into 


fibers ranging in size from finest textile fibers suitable for making hosiery 
and having a tensile strength at least as great as that of natural silk to 
coarser fibers suited to serve as "bristles." Not only is the work the basis 
of the particular industrial development just mentioned; it also carries 
possibilities of other practical developments in the future not yet realized 

Part Two is concerned with polymers, in the narrower sense of the 
term, produced by the self-addition of conjugated systems, especially 
conjugated butadienoid systems containing chlorine and other substituents. 
In it, also, a good deal of attention is given to the chemical reactions of 
vinylacetylene and divinylacetylene, two new polymers of acetylene, 
which are the source of the butadienoid systems in question and are the 
first readily-available compounds containing the interesting conjugated 
enine system, H 2 C=CH C=C . This work is the basis on which there 
has been developed the commercial manufacture of a synthetic rubber, 
"Neoprene." A survey of the work is given in the Introduction to Part 

A few early papers of Carothers, dealing with subjects other than 
polymerization, are omitted from this volume. A bibliography of Caro- 
thers' publications relative to polymerization is given (p. 423 et seq.)\ 
it includes United States patents. 


April, 1940 


The editors and publishers wish to thank the numerous individuals and 
organizations for their assistance in the preparation and publication of this 
book and without whose valuable aid, this volume might not have been 

We desire especially to mention the cooperation and encouragement 
given the editors by du Pont de Nemours and Company, in which labora- 
tories the greater part of Carothers' work was carried out. 

We are deeply indebted to : 

Dr. E. K. Bolton, Chemical Director of du Pont de Nemours and Com- 
pany, in Wilmington, Del. 

Dr. A. P. Tanberg, Director of the Experimental Station, and 
Dr. A. W. Kenney, of the Experimental Station, Wilmington, Del. 

We have also had the opportunity to discuss our plans with several 
former collaborators, friends and pupils of W. H. Carothers and wish to 
express our gratitude to them. We are particularly grateful to Dr. E. O. 
Kraemer and Dr. H. Kienle for their valuable assistance. 

It is through the cooperation of those societies and publishers who pub- 
lished the original papers, that we were enabled to reprint them. 

For this reason, we are very grateful to : 

The American Chemical Society, and its Secretary, Mr. Charles W. 
Parsons, who permitted the reprinting of all the papers from the 
"Journal of the American Chemical Society," and the use of the 

The Faraday Society, and its Secretary, Mr. G. S. W. Marlow, who 
permitted the reprinting of a paper from the "Transactions of the 
Faraday Society." 

Mr. H. E. Howe, for permission for the reprinting of a paper from 
"Industrial & Engineering Chemistry." 

The Williams and Wilkins Company, Baltimore, Mel., for permission 
to reprint a paper from the "Chemical Reviews." 

Professor Roger Adams and the National Academy of Sciences, Wash- 
ington, D. C., for permission to reprint the Biography of W. H, 
Carothers by Roger Adams, and for the block for the frontispiece. 

The index has been carefully prepared by Dr. Dora Stern, New York. 



The editors wish finally to express their gratitude for the cooperation 
and assistance given them by Interscience Publishers, Inc. and especially, 
by Dr. E. S. Proskauer of this firm. 

July, 1940 



Introduction to the Series, "High Polymers" V 

Introduction to Volume I VIII 

Table of Contents VIII 

Biography by Roger Adams XV 

Part One 

Studies on Polymerization and Ring Formation 1 

Introduction by H. Mark 3 

I. An Introduction to the General Theory of Condensation Polymers. . . 4 

II. Polyesters 17 

III. Glycol Esters of Carbonic Acid 29 

IV. Ethylene Succinates 42 

V. Glycol Esters of Oxalic Acid 54 

VI. Adipic Anhydride 63 

VII. Normal Paraffin Hydrocarbons of High Molecular Weight Prepared by 

the Action of Sodium on Decamethylene Bromide 68 

VIII. Amides from e-Aminocaproic Acid 78 

IX. Polymerization 81 

X. The Reversible Polymerization of Six-Membered Cyclic Esters 141 

XI. The Use of Molecular Evaporation as a Means for Propagating Chem- 
ical Reactions 154 

XII. Linear Superpoly esters 156 

XIII. Polyamides and Mixed Polyester-Polyamides 165 

XIV. A Linear Superpolyanhydride and a Cyclic Dimeric Anhydride from 

Sebacic Acid 168 

XV. Artificial Fibers from Synthetic Linear Condensation Super polymers. . 179 

XVI. A Poly alcohol from Decamethylene Dimagnesium Bromide 190 

XVII Friedel-Craf ts Syntheses with the Polyanhydrides of the Dibasic Acids . 192 

XVIII. Polyesters from co-Hydroxydecanoic Acid 195 

XIX. Many-Membered Cyclic Anhydrides (1) 202 

XX. Many-Membered Cyclic Esters 212 

XXI. Physical Properties of Macrocyclic Esters and Anhydrides. New 

Types of Synthetic Musks 221 

XXII. Stereochemistry and Mechanism in the Formation and Stability of 

Large Rings 225 

XXIII. e-Caprolactone and Its Polymers 235 

XXIV. Cyclic and Polymeric Formals 240 

XXV. Macrocyclic Esters 248 

XXVI. Meta and Para Rings 259 

XXVII. Polydecamethylene Oxide 263 

XXVIII. Preparation of Macrocyclic Lactones by Depolymerization 265 




Part Two 

Acetylene Polymers and Their Derivatives 271 

I. Introduction by G. Stafford Whitby 273 

II. A New Synthetic Rubber: Chloroprene and Its Polymers 281 

III, The Addition of Hydrogen Chloride to Vinylacetylcne 306 

IV. The Addition of Hydrogen Bromide to Vinylacetylene, Bromoprene and 

Dibromobutene 311 

V. The Polymerization of Bromoprene (Third Paper on New Synthetic 

Rubbers) 314 

VI. Vinylethinylmagnesium Bromide and Some of Its Reactions 321 

VII. Sodium Vinylacetylide and Vinylethinylcarbinols 323 

VIII. a-Alkyl-/3-Vinylacetylenes 329 

IX. l-Alkyl-2-chloro-l,3-butadienes and their Polymers (Fourth Paper on 

New Synthetic Rubbers) 331 

X. The Chlorination of the Hydrochlorides of Vinylacetylene 335 

XI. Dichloro-2,3-butadiene-l,3 and Trichloro-l,2,3-butadiene-l,3 340 

XII. The Addition of Thio--cresol to Divinylacetylene 344 

XIII. The Action of Chlorine on Dinvinylacetylene 348 

XIV. The Dihydrochloride of Divinylacetylene 357 

XV. Halogen-4-butadienes-l,2. The Mechanism of 1,4-Addition and of a,y- 

Rearrangcment 301 

XVI. The Preparation of Orthoprenes by the Action of Grignard Reagents 

on Chloro-4-butadiene-l,2 308 

XVII. Mercury Derivatives of Vinylacetylene 372 

XVIII. l-Halogen-2-vinylacetylenes 375 

XIX. The Structure of Divinylacetylene Polymers 378 

XX. The Addition of Alcohols to Vinylacetylene, etc 381 

XXI. Homologs of Chloroprene and Their Polymers (Second Paper on New 

Synthetic Rubbers) 384 

XXII. The Synthetic Rubber Problem 391 

Part Three 

Miscellaneous Papers 399 

I. Association Polymerization and the Properties of Adipic Anhydride. . 401 

II. Ueber die angeblichen Isomerien bei cyclischen Oxalsaeure-estern 402 

III. 6-Caprolactone 407 

IV. Polymers and Polyf unctionality 409 


Bibliography and Patents 423 

Authors Index 433 

Subject Index 439 



Wallace Hume Car-others, who died on April 29, 1937, was born in Bur- 
lington, Iowa, on April 27, 1896. His contributions to organic chemistry 
were recognized as outstanding and, in spite of the relatively short span of 
time for his productive accomplishments, he became a leader in his field 
with an enviable international reputation. 

His paternal forbears were of Scotch origin and settled in Pennsylvania 
in prerevolutionary days. His father, Ira Hume Carothers, taught coun- 
try school at the age of 19. Later he entered the field of commercial educa- 
tion and for forty-five years has been engaged in that type of work as 
teacher and vice-president. 

His maternal ancestors were of Scotch-Irish stock and were also, for the 
most part, farmers and artisans. They were great lovers of music, and 
this may account for the intense interest in and appreciation of music which 
Carothers possessed. His mother, who was Mary Evalina McMullin of 
Burlington, Iowa, exerted a powerful influence and guidance in the earlier 
years of his life. 

On February 21, 1930, he married Helen Everett Sweetman of Wilming- 
ton, Delaware. A daughter, Jane, was born November 27, 1937. 

Wallace was the oldest of four children. His education began in the 
public schools of Des Moines, Iowa. In 1914 he was graduated from the 
North High School. As a growing boy he had zest for work as well as play. 
He enjoyed tools and mechanical things and spent much time in experi- 
menting. His school work was characterized by thoroughness and his high 
school classmates testify that when he was called upon to recite, his answers 
revealed careful preparation. It was his habit to leave no task unfinished 
or done in a careless manner. To begin a task was to complete it. 

He entered the Capital City Commercial College in the fall of 1914 and 
was graduated in the accountancy and secretarial curriculum in July, 1915, 
taking considerably less time than the average. He entered Tarkio College, 
Tarkio, Missouri, in September, 1915, to pursue a scientific course, and 
simultaneously accepted a position as assistant in the Commercial Depart- 
ment. He continued in this capacity for two years and then was made an 
assistant in English, although he had specialized in chemistry from the 
time he entered college. During the World War the head of the depart- 
ment in chemistry, Dr. Arthur M. Pardee, was called to another institution, 
and Tarkio College found it impossible to secure a fully equipped teacher of 



chemistry. Carothers, who previously had taken all of the chemistry 
courses offered, was appointed to take over the instruction. Since he was 
rejected as a soldier on account of a slight physical defect, he was free to 
serve in this capacity during his junior and senior years. It is interesting 
that during his senior year there were four senior chemistry-major students 
in his class and every one of them later completed work for the doctorate, 
studying in the universities of this country and abroad. Today they bear 
testimony to the fact that as undergraduates they owed much to the in- 
spiration and leadership of Carothers. 

Upon entering college his interest in chemistry and physical sciences was 
immediate and lasting, and he rapidly outdistanced his classmates in ac- 
complishment. As a student he showed mature judgment and was always 
regarded by his fellow students as an exceptional person. Invariably he 
was the brightest student in the class regardless of the subject. Financial 
necessity required that he earn a large portion of his educational expenses. 
He always found time, however, to associate with the other students, 
though he showed little interest for the boisterous enthusiasms of the aver- 
age underclassman. During his last two years in college he was entrusted 
with a number of student offices to which he gave freely of his time and 

Leaving Tarkio College in 1920 with his bachelor of science degree, he 
enrolled in the chemistry department of the University of Illinois where he 
completed the requirements for the master of arts degree in the summer of 
1921. His former instructor at Tarkio College, then head of the chemistry 
department at the University of South Dakota, desired a young instructor 
to handle courses in analytical and physical chemistry and was fortunate 
in securing Carothers for this position during the school year, 1921-1922. 
He went to South Dakota only with the intention of securing sufficient 
funds to enable him to complete his graduate work, but the careful and 
adequate preparation of his courses, as well as his care of the students under 
his direction, showed that he could be a very successful teacher of chemistry. 
He was still the same quiet, methodical worker and scholar, not forceful as 
a lecturer, but careful and systematic in his contact with the students. 
He always required adequate preparation of assigned work and was able 
to get a large volume in student accomplishment. 

Simultaneously with his teaching work he started to develop some inde- 
pendent research problems. He was especially interested in the 1916 paper 
of Irving Langmuir on valence electrons and desired to investigate some of 
the implications it held in organic chemistry. Pursuing this idea he car- 
ried out laboratory studies which were reported in his first independent 


contribution to the Journal of the American Chemical Society, "The 
Isosterism of Phenyl Isocyanate and Diazobenzene-Imide." His second 
independent paper, published while still a student, was that on 'The Double 
Bond." In this he presented the first clear, definite application of the 
electronic theory to organic chemistry on a workable basis. He described 
the electronic characteristics of the double bond and in essence included in 
his discussion everything that has since been written on this particular 

He returned to the University of Illinois in 1922 to complete his studies 
for the degree of doctor of philosophy, which he received in 1924. His 
major work was in organic chemistry with a thesis under the direction of 
Dr. Roger Adams, on the catalytic reduction of aldehydes with platinum- 
oxide platinum-black and on the effect of promoters and poisons on this 
catalyst in the reduction of various organic compounds. His minors were 
physical chemistry and mathematics. He exhibited the same brilliance in 
all of his courses and in research which characterized his earlier accom- 
plishments. Although specializing in organic chemistry, he was consid- 
ered by the physical chemists to have a more comprehensive knowledge of 
physical chemistry than any of the students majoring in that field. In 
1920-1921 he held an assistantship for one semester in inorganic chemistry 
and for one semester in organic chemistry. He was a research assistant 
during 1922-1923, and during 1923-1924 held the Carr Fellowship, the 
highest award offered at that time by the department of chemistry at 
Illinois. During these two years his seminar reports demonstrated his wide 
grasp of chemical subjects. The frequency with which his student col- 
leagues sought his advice and help was indicative of his outstanding ability. 
At graduation he was considered by the staff as one of the most brilliant 
students who had ever been awarded the doctor's degree. A vacancy on 
the staff of the chemistry department of the University of Illinois made it 
possible to appoint him as an instructor in organic chemistry in the fall of 
1924. In this capacity he continued with unusual success for two years, 
teaching qualitative organic analysis and two organic laboratory courses, 
one for premedical students and the other for chemists. 

Harvard University, in 1926, was in need of an instructor in organic 
chemistry. After carefully surveying the available candidates from the 
various universities of the country, Carothers was selected. In this new 
position he taught during the first year a course in experimental organic 
chemistry and an advanced course in structural chemistry, and during the 
second year he gave the lectures and laboratory instruction in elementary 
organic chemistry. 


President James B. Conant, of Harvard University, was professor of 
organic chemistry at the time that Carothers was instructor. He says of 

"Dr. Carothers' stay at Harvard was all too short. In the brief space of time during 
which he was a member of the chemistry department, he greatly impressed both his col- 
leagues and the students. He presented elementary organic chemistry to a large class 
with distinction. Although he was always loath to speak in public even at scientific 
meetings, his diffidence seemed to disappear in the classroom. His lectures were well 
ordered, interesting, and enthusiastically received by a body of students only few of 
whom planned to make chemistry a career. In his research, Dr. Carothers showed even 
at this time that high degree of originality which marked his later work. He was never 
content to follow the beaten track or to accept the usual interpretations of organic reac- 
tions. His first thinking about polymerization and the structure of substances of high 
molecular weight began while he was at Harvard. His resignation from the faculty to 
accept an important position in the research laboratory of the du Pont Company, was 
Harvard's loss but chemistry's gain. Under the new conditions at Wilmington, he had 
facilities for carrying on his research on a scale that would be difficult or impossible to 
duplicate in most university laboratories. Those of us in academic life, however, always 
cherished the hope that some day he would return to university work. In his death, 
academic chemistry, quite as much as industrial chemistry, has suffered a severe loss." 

In 1928 the du Pont Company had completed plans to embark on a new 
program of fundamental research at their central laboratory, the Experi- 
mental Station at Wilmington, Delaware. Carothers was selected to head 
the research in organic chemistry. The decision to leave his academic po- 
sition was a difficult one. The new place demanded only research and 
offered the opportunity of trained research men as assistants. This over- 
balanced the freedom of university life and he accepted. From then on 
until his death his accomplishments were numerous and significant. He 
had the rare quality of recognizing the significant points in each problem 
he undertook and unusual ability for presenting his results in a most ex- 
plicit and precise way, which led to clarity and understanding. In these 
nine years he made several major contributions to the theory of organic 
chemistry and discoveries which led to materials of significant commercial 
importance. Dr. Elmer K. Bolton, Chemical Director of the du Pont 
Company, writes concerning Carothers 

"At the time the du Pont Company embarked upon its program of fundamental re- 
search in organic chemistry in the Chemical Department, Dr. Carothers was selected to 
direct this activity, because he had received the highest recommendations from Harvard 
University and the University of Illinois, and was considered to have unusual poten- 
tiality for future development. There was placed under his direction a small group of 
excellently trained chemists to work on problems of his own selection. The results of 
his work, extending over a period of nine years, have been of outstanding scientific in- 


terest and have been considered of great value to the Company as they have laid the 
foundation for several basically new developments of commercial importance. 

"In our association with Dr. Carothers, we were always impressed by the breadth and 
depth of his knowledge. He not only provided inspiration and guidance to men under 
his immediate direction, but gave freely of his knowledge to the chemists of the depart- 
ment engaged in applied research. In addition, he was a brilliant experimentalist. 
Regarding his personal characteristics, he was modest, unassuming to a fault, most un- 
complaining, a tireless worker deeply absorbed in his work, and was greatly respected 
by his associates. He suffered, however, from a nervous condition which in his later 
years was reflected in poor health and which became progressively worse in spite of the 
best medical advice and care, and the untiring efforts of his friends and associates. His 
death has been a great loss to chemistry and particularly to the Chemical Department. 
In my judgment, he was one of the most brilliant organic chemists ever employed by the 
du Pont Pompany." 

His reputation spread rapidly; his advice was sought continually, not 
only by his colleagues but also by chemists throughout the world. In 1929 
he was elected Associate Editor of the Journal of the American Chemical 
Society; in 1930 he became an editor of Organic Syntheses. He took an 
active part in the meetings of the organic division of the American Chemical 
Society. He was invited frequently to speak before various chemical 
groups. He addressed the Johns Hopkins summer colloquium in 1935 on 
"Polymers and the Theory of Polymerization." That year he also spoke 
on the same subject before the Faraday Society in London, when his paper 
was considered one of the outstanding presentations on the program. His 
achievements were recognized by his election to the National Academy 
of Sciences in 1936 the first organic chemist associated with industry to 
be elected to that organization. During these years from 1928-1937 
several attractive academic positions were offered him but he chose to re- 
main to the end with the Company which had given him his opportunity for 
accomplishment . 

Carothers was deeply emotional, generous and modest. Pie had a lov- 
able personality. Although generally silent in a group of people, he was a 
brilliant conversationalist when with a single individual, and quickly dis- 
played his broad education, his wide fund of information on all problems of 
current life, and his critical analysis of politics, labor problems and business, 
as well as of music, art, and philosophy. With all his fine physique he had 
an extremely sensitive nature and suffered from periods of depression which 
grew more pronounced as he grew older, despite the best efforts of his 
friends and medical advisers. * 

* This biography is an abbreviated reprint of Roger Adams, Wallace Hume Carothers, 
1896-1937. National Academy of Sciences of the U, S., Biographical Memoirs. 20. 
12th Memoirs. 1939. 






This series of articles extends from 1929 to 193(5 and comprises twenty- 
eight papers. 

Reactions which produce giant molecules can be divided into poly- 
condensation and polymerization processes; Carothers has called them 
C-polymers and A-polymers, respectively. The articles of Part 1 are 
especially devoted to the study of C-polymerization. The following are 
the outstanding problems which are open for investigation. 

(a) If two bifunctional molecules, e. g., one dibasic acid and one glycol, 
or diamide, react, two possibilities occur. The reaction can result (1) in 
a chain polymer of lower or higher molecular weight, which still bears 
either hydroxyl or carbonyl terminal groups or (2) in a smaller or larger 
ring, which does not contain the reactive group. The following questions 
arise: Under what conditions does either of these two possibilities take 
place and what is the molecular weight of the resulting compound. Papers 
I to XIV are chiefly concerned with this problem. A large number of 
dibasic acids, glycols, diamides, co-hydroxy and ammo-carbonic acids were 
included in the investigation and many formerly unknown compounds were 
synthesized and described. 

If 5- or 6-membered rings can be formed during a polycondensation of 
the type described, the reaction almost invariably leads to ring formation ; 
this may be put forward as a general result. There is still a certain possi- 
bility also for the production of 7- and 8-membered rings, while larger 
rings are not formed during such polycondensation reactions. This rule 
is associated with the stereochemical points of view of Sachse and Mohr. 

(b) The second problem is concerned with the behavior of the high 
polymeric compounds, which can be produced by polycondensation, and 
their possible technical utilization. Polyesters and poly amides may be 
very readily cast into films or drawn into filaments which display very 
interesting technical properties. Papers XV, XVIII and XIX deal in part 
with this question. In general, as soon as the degree of polymerization 
exceeds a number between 30 and 40, the material begins to exhibit strength 
and extensibility. Such products can be cast or spun from solution and 
also from the molten state; they can later undergo a stretching process 



which increases the strength and pliability of the resulting samples and also 
causes the material to assume a lustrous appearance. This investigation 
can certainly be regarded as an outstanding example of successful scientific 
work, which bears at the same time considerable technical importance. 
One of the polyamides, which has been prepared and investigated, was 
selected for technical production, and is now coming into great importance 
under the trade name of "Nylon." 

(c) Another attractive line of investigation was the more thorough 
study of the second type of compounds which result from polycondensation 
reactions, namely poly-membered rings. Ruzicka has shown that such 
compounds exhibit peculiar properties and are of considerable interest from 
the point of view of their odor. In papers XX, XXI, XXII, XXIII and 
XXVI, a new method for the preparation of such rings is described; their 
formation and properties are studied. It turns out that the specific odor 
is closely connected with the number of bonds and varies with it in a 
characteristic way. 

It is of importance to point out that an experimental method has been 
developed to deal with such high polymeric chains or multi-membered 
rings or mixtures of the two. Pure samples suitable for quantitative 
characterization could hardly be obtained by the normal experimental 
methods of organic chemistry. Therefore, a new type of molecular still 
has been developed, which is briefly described in paper XI and which is 
extensively applied in all these investigations. 

In the following, each single paper is reprinted without any change; 
a short abstract of its contents is given at the beginning in italics; and a 
few additional references concerning the recent development and literature 
are added. The purpose of this brief introduction is to emphasize the main 
general results and to refer briefly to their scientific and technical im- 

I. An Introduction to the General Theory of Condensation 


This first paper contains a general introduction to the theory of polymers. 
With the aid of a comprehensive table Carothers distinguishes between 
two classes of polymers: 

A-polymers, produced by recurring addition of monomers. The molecular 
formula of the monomer is identical with that of the structural unit. The 

*W. H. Carothers; Journ. Am. Chem. Soc., 51, 2548-59 (1929); Contribution No. 10 
from the Experimental Station of E. I. du Pont de Nemours and Co. 
Received April 13, 1929. Published August 7, 1929. 


monomer can be obtained from the polymer by thermal or photochemical de- 
composition; the polymer is formed by self -addition, 

C-polymers, produced by recurring condensation of monomers. The 
molecular formula of the monomer differs from that of the structural unit. 
The monomer can be obtained from the polymer by a hydrolytic process; 
the polymer is formed by a polyintermolecular condensation.* 

Then the conditions for the formation of C-polymers are explored. The 
term "functionality is introduced. A functional group in the monomer 
is such an arrangement of atoms as might lead to a reaction step. 
Such groups are, e. g., OH, NH 2 , COOH, SO 3 H, etc. According as 
the monomer contains one, two, three, etc., of them it is called mono 
functional, bifunctional, etc. 

Multifunctional monomers can react intra- or intermolecularly . In the 
first case they lead to rings, in the second to C-polymers. 
If during a bifunctional reaction a 5- or 6-ring can be formed intra- 
molecuarly, then it almost invariably happens that the reaction leads to 
ring formation. 

Bifunctional reactions which if intramolecular would lead to larger than 
6 -rings generally proceed intermolecularly and give C-polymers. The 
formation of 7 -rings, however, may possibly occur, while 8 -rings are much 
less probable. 

This empirical rule, which has of course also several exceptions is dis- 
cussed in the paper from the point of view of the stereochemical considera- 
tions of Sachse and Mohr. 

Polymerization frequently leads to substances of very high molecular 
weights, and the problem of the structure of high polymers is attracting 
a great deal of attention, especially because such important materials as 
rubber, cellulose, proteins and resins either are high polymers or have 
certain properties which are common to high polymers. 

The conditions which Berzelius (1) was concerned to recognize by the 
term polymer, were the presence of the same atoms in the same proportions 
in compounds having different molecular weights. These conditions are 
satisfied by the members of a great many thousand pairs of compounds 
which are not now regarded as polymers. Thus, of the compounds paracet- 
aldehyde, butyric acids and hydroxycaproic acids, only the first would 
now be considered a polymer of acetaldehyde, although there is nothing in 
the conditions of the Berzelius definition to exclude the others. Hence, 

* In modern literature A-polymers are frequently called polymerisation products, 
while C-polymers are termed polycondensation products. The shorter terminology of 
Carothers would certainly be practical. 


whatever the term polymer may mean now, it does not mean precisely 
what Berzelius intended, and the conditions which he set up are not 
sufficient to define it. In current attempts to define this term (2) it is still 
stated that a polymer and its monomer must have the same atoms in the 
same proportions. But this condition is not satisfied by the polyoxy- 
methylenes (see Table I) which are universally considered to be polymers 
of formaldehyde. It seems desirable, therefore, to attempt to formulate 
a definition which will be in so far as possible in accordance with both 
the current usage and the essential facts. 

The structures of a good many polymers, including some of very high 
molecular weights, are known either completely or in part and an examina- 
tion of their formulas shows some interesting relationships (see Table I). 
They are characterized by a recurring structural unit, so that if this is 
represented by R , the structure of these polymers may be represented 
in part by the general formula R R R R R R R R , etc., or 
( R -) n . In this formula n may be small as in paracetaldehyde (n = 3), 
or it may be very large as in the polyoxymethylenes. The end valences 
may be united as they are in paracetaldehyde to form a ring, or they 
may be saturated by univalent groups such as H and OH to form an 
open chain of the type H( R ) n OH as they are in a-polyoxymethylene. 
It seems probable that cellulose and silk fibroin are of this type, and in 
any event it may be observed that no high polymer is certainly known to 
be cyclic (3). There are polymers which do not conform to the type 
( R ) n but those which do will be called linear whether the chain is open 
or closed; and the subsequent discussion is concerned only with these. 

The structural units R are bivalent radicals which, in general, are 
not capable of independent existence. The presence of a recurring struc- 
tural unit is, of course, characteristic of most organic compounds (e. g., 
CH 2 in aliphatic compounds), but in the case of polymers there exists 
a molecule, the monomer, corresponding to the structural unit, and from 
which the polymer may be formed or to which it may be degraded. 

Examination of the formulas of Table I will show that two types of 
polymers may be distinguished. In the first type, which includes paracet- 
aldehyde, rubber, polystyrene and polyoxymethylenes, the molecular 
formula of the structural unit is identical with that of the monomer, i. e. t 
the formula of the structural unit, R , is isomeric with that of the 
monomer. In the second type, which includes the polyethylene glycols, 
cellulose and silk fibroin, the molecular formula of the structural unit 
differs from that of the monomer by H 2 O, t . e., the monomer is H R OH. 
The transformation of polymers of the first type into their monomers is 


brought about simply by heating, and the reverse transformation (poly- 
merization) occurs spontaneously or by the action of catalysts. In the 
second type, degradation to the monomer occurs by hydrolysis, and if 
the reverse process were to take place it would require the elimination of 
water among many molecules. This would be polyintermolecular con- 
densation (4). 

These two classes will be distinguished as (1) addition or A polymers. 
The molecular formula of the monomer is identical with that of the struc- 
tural unit. The monomer can be obtained from the polymer by ther- 
molysis or the polymer can be synthesized from the monomer by self- 
addition. (2) Condensation or C polymers: the molecular formula of 
the monomer differs from that of the structural unit. The monomer can 
be obtained from the polymer by hydrolysis or its equivalent or the polymer 
can be synthesized from the monomer by polyintermolecular condensation. 
Polymerization then is the chemical union of many similar molecules 
either (A) without or (C) with the elimination of simpler molecules (HaO, 
HC1, NaCl, NH 3 , etc.) (5). 

Assuming that polyintermolecular condensation exists, the above ex- 
amples and definitions and their implications provide ample reason for 
referring to this process as a type of polymerization. These examples, 
of course, do not provide any proof that this process as distinct from 
and independent of A polymerization does exist. This proof will appear 
incidentally in the following discussion, which is concerned with the 
general principles involved in the formation of condensation polymers. 

Polyfunctional Compounds. Polyintermolecular condensation re- 
quires as starting materials compounds in which at least two functional 
groups are present in the same molecule (e. #., hydroxy acids, HORCOOH, 
might lead to poly-esters, HORCOORCOORCOORCOORCOORCO , 
etc.; amino acids, to poly-amides, NH 2 RCONHCONHRCONHR- 

Among compounds having more than one functional group, those of the 
type x R y may be called bifunctional, R"x 3 , trifunctional, etc. In 
these formulas R stands for a bivalent radical (R* for a trivalent radical) 
and x and y for functional groups capable of reacting with each other in 
a known fashion to form the new functional group z. Thus x R y > 
R z , where R z evidently represents a structural unit which 
will appear in the product and which may be present 1, 2, 3, ... n times. 
Reactions of this type will be called bifunctional regardless of the num- 
ber of molecules involved. Reactions of the type x R x + y R'- y 
> z R z R' may be called bi-bifunctional. All such reac- 


tions may, at least by hypothesis, pass through the stage x R x R' y 
which is equivalent to x R y, and for purposes of discussion they 
may therefore be classed as simple bifunctional reactions. Reactions 
of the type R'x 3 + Ry 2 > product will be called tri-bifunctional. Simi- 
larly there may be tetra-bifunctional, tri-trifunctional reactions, etc., 
and all these may be classed together as polyfunctional reactions. The 
present discussion is concerned only with bifunctional reactions. 

Bifunctional Reactions. These always present two possibilities: they 
may be intramolecular or intermolecular. If intramolecular they can 
lead only to the simple monomeric ring I. If intermolecular they may 
lead either to a polymeric ring II or to a polymeric chain III. 

x R y - > R z I 

- > ( R z ) II 

- ^ X __(__R_ Z __)_R__y HI 

n- 1 

These obviously represent three possibly competing reactions. 

The question now arises, what factors will determine which of these 
possible courses a bifunctional reaction will take? It is obvious, for 
example, that in general dilution would favor intra- over intermolecular 
reactions (6). Temperature and catalysts might favor either one or the 
other. It appears, however, that structural and stereochemical factors will 
usually be more important than any others. That is, though it may be 
possible in some bifunctional reactions to control the choice between intra- 
and intermolecular reaction by suitable adjustment of experimental condi- 
tions, this choice will, in general, be almost completely determined by the 
nature of the reacting molecules. 

The effects of these factors may be stated as follows. (1) If the product 
of intramolecular reaction would be a ring which, on stereochemical 
grounds, is incapable of existence, reaction will be intermolecular. This 
is apparently the case with p-N^CcEUCI^CHkCl, which reacts with itself 
intermolecularly (7), and not as had previously been supposed (8) with the 
formation of the so-called dihydro-^-indole (9). The utility of this very 
obvious principle is somewhat diminished by the fact that, in the present 
state of stereochemical knowledge, it is sometimes impossible to predict 
whether a given ring system will be capable of existence or not. 

(2) Bifunctional reactions which can lead to the formation of 5- or 6- 
rings almost invariably proceed intramolecularly. This well-established 






















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fact is responsible for the existence of the majority of the very large 
number of 5- and 6-rings which is known. 

(3) Bifunctional reactions which, if intramolecular, could lead only to 
larger-than-6-rings, generally proceed intermolecularly and lead to poly- 
meric products. If this rule were free from any exceptions, it would neces- 
sarily follow that the polymeric products would always be of the open- 
chain type, III. 

Although a great many bifunctional reactions have been studied in the 
hope of forming large rings, our information as to the precise nature of the 
products of these reactions is very meager. That they do not usually 
proceed intrarnolecularly follows from the fact that very few large rings 
are known. It is true that such reactions frequently do not proceed at 
all under conditions which, in analogous cases, lead to the formation of 
5- and 6-rings; but the formation of the latter often proceeds under con- 
ditions which do not permit intermolecular reaction even among uni- 
functional compounds (compare the formation of 7-lactones in the pres- 
ence of a large excess of water, with the esterification of acetic acid). 
There are, however, a good many types of reaction which are intermolecular 
among simple compounds, which are practically free from side reactions, 
and which by suitable adjustment of experimental conditions may be 
forced to completion. It should be possible to conduct reactions of these 
types even when bifunctional, so as to obtain analytically homogeneous 
products whether they proceed intrarnolecularly or not. These simple 
principles have been repeatedly applied to bifunctional reactions in this 
Laboratory, and they have seldom failed to lead to analytically homogene- 
ous products. Moreover, where the formation of 5- or 6-rings was ex- 
cluded by the nature of the reacting materials, these reactions have, with- 
out exception, led to high polymers. This fact provides a possible explana- 
tion for the meagerness of the information which is available concerning 
the precise nature of the products of such bifunctional reactions. High 
polymers frequently have properties which make their investigation very 
difficult. Moreover, if a chemist is expecting a reaction to lead to ma- 
terials of simple properties, he is usually inclined to regard the appearance 
of a resinous or sirupy product which neither can be crystallized nor dis- 
tilled as signifying only that the experiment has failed. When the prod- 
ucts of such reactions have been capable of purification and have shown 
the expected analytical composition, they have frequently been assumed 
to be dimeric (and cyclic) for no other reason than that they were ob- 
viously not monomeric. In this connection it should be emphasized that 
substances of very high molecular weights may neverthless be micro- 
crystalline and very soluble (see the following paper). 


Some of the points discussed above are illustrated by the following 

Anhydrides of the acids of the series HOOC(CH 2 ) a: COOH are known 
in which x has all values from 1 to 8 inclusive. Of these only succinic and 
glutaric anhydrides are monomeric. These are, respectively, 5- and 6- 
rings. The other anhydrides which, if monomeric, would be 4-, 7-, 8-, 
9-, 10- and 11 -rings, are all highly polymeric (10). 

Hydroxy-acids of the series HO(CH 2 ) Z COOH may condense with them- 
selves. Only those in which x is 3 or 4 yield monomeric lactones (11). 
These lactones are 5- and 6-rings, respectively. This series of acids now 
includes those in which x has all values from 8 to 16 inclusive (12). The 
lactones corresponding to the acids in which x has the values 12 to 16 are 
also known (12b), as well as the 17-membered lactone of the unsaturated 
acid, hexadecene-(7)-ol-(16)-acid-(l) (13). All of these lactones are per- 
fectly stable substances and can be distilled without decomposition (14); 
and yet none of these lactones has been synthesized by a bifunctional re- 
action (15). They were synthesized by the oxidation of the corresponding 
cyclic ketones with persulfuric acid. By heating the acids, products are 
indeed obtained which have the same analytical composition as the lac- 
tones, but (12a) they are "polymeric-like." The properties of the known 
lactones of this series indicate that lactones cannot be intermediates in the 
formation of these polymers. The formation of these must, therefore, 
involve C polymerization. 

Amino acids of the series NH 2 (CH 2 ), r COOH are known in which x has 
the values 3, 4, 5 and 6. When x is 3 and 4, intramolecular anhydride 
formation occurs spontaneously at the melting point of the acids, the 
products being the 5- and 6-membered rings, pyrrolidone and piperidone 
(16). When x is 5 the acid is, on heating, converted to the extent of 20 to 
30% of the theoretical into the 7-membered lactam. "The rest of the acid 
is converted into a viscous gelatinous mass which could be obtained in a 
state of only approximate purity and which, according to the analysis, 
is isomeric with the 7-membered lactam; that is, it is a polymeric prod- 
uct" (17). Where x is 6 no trace of the corresponding lactam could be ob- 
tained by heating the acid, although this lactam (suberone-isoxime) is 
a known and stable substance (18). By heating the acid a product was ob- 
tained having the same composition as this lactam, but it was an un- 
distillable solid, insoluble in most solvents (17). 

The compounds Br(CH 2 ) I NH 2 , where x is 4, 5 and 6, react with them- 
selves with the formation of secondary amines containing the structural 
unit ( CH 2 ) 7 NH . Where x is 4 and 5 the products are the mono- 


meric 5- and 6-rings, pyrrolidine and piperidine (19). Where x is 6 the 
amount of the monomeric base (7-ring) formed was too small to permit 
purification (20). The product was for the most part an oil which solidified 
to a waxy solid. It could not be distilled in vacuum without decomposi- 
tion, and though no molecular weight determination is recorded, v. Braun 
seems inclined to regard it as dimeric. Its properties are obviously more 
consistent with a more highly polymeric structure. 

An interesting example of a bi-bifunctional reaction is found in the 
reaction between glycols and acetaldehyde (or acetylene). This presents 


the possibility of forming cyclic acetals, CH 3 CH R, or polyacetals, 

HO R O CH O R O CH O R O CH O , etc. This re- 

action has been studied by Hill and Hibbert (21). Ethylene and tri- 
methylene glycols gave in excellent yields the cyclic acetals which are 5- 
and (5-rings. Tetramethylene glycol gave in poor yield a volatile com- 
pound which was apparently the monomeric cyclic acetal containing a 
7-ring. A considerable part of the product was an undistillable sirup. The 
products from octamethylene and decamethylene glycols were also un- 
distillable sirups. No molecular weight determinations are recorded. Hill 
and Hibbert recognized of course that these sirups might have the polymeric 
linear structure indicated above, but seemed inclined to regard them as 
simple rings of a peculiar kind. 

The only reaction which has led to the formation of large carbon rings 
from open-chain compounds is the action of heat on certain salts of di- 
basic acids (22). In no case are these large rings the chief products of this 
reaction, nor can it be established that they are primary products, since 
the reaction takes place at high temperature. If polymeric products 
were formed they would be decomposed thermally. High polymers 
could not appear in the distillate from which the products are isolated, 
because high polymers cannot be distilled. These features sharply dis- 
tinguish this reaction from other truly bif unctional reactions which might, 
but which do not, so far as our information goes, lead to rings of the same 
type (23). 

Any theory which attempts to explain why 5- and 6-rings are formed 
very readily and larger rings with very great difficulty must take into 
account the fact that these larger rings are not less stable than the smaller. 
A satisfactory theory is already available. The Baeyer strain theory in 


its original form is made untenable by the existence of the higher cyclo- 
paraffins, and has been replaced by the Sachse-Mohr theory which per- 
mits the existence of non-planar and strainless rings and which has be- 
sides a great deal of other evidence in its support (24). The features of this 
theory essential to the present argument are exceedingly simple. They 
involve the assumption of the tretrahedral angle in the carbon valences 
and of free rotation about each carbon-carbon single bond in a chain. 
No description of stereochemical relations can be as convincing as a 
demonstration with suitable models (25). Such models show that there is 
a certain inevitability in the formation of 5- and 6-rings and a large degree 
of fortuity in the formation of larger rings. This question has been con- 
sidered in some detail by Mohr (26). Regarding the possibility of the 
formation of a cyclic ketonic ester by the internal condensation of sebacic 
ester he says : 

The molecule of sebacic ester is a very long chain. The multiplicity of forms which 
this molecule can assume is extraordinarily great. It is clear that by the random col- 
lisions of the molecules, fewer molecules of sebacic ester will in unit time assume the 
form necessary for ring closure than is the case with adipic ester under the same condi- 
tions, since this contains in its molecule only four methylene groups, and four less points 
of rotation than does sebacic ester. To bring the two ends of the sebacic ester molecule 
together in the least forced fashion apparently requires a very small amount of energy 
or none at all. Unfortunately, however, we have not the means arbitrarily and with the 
least possible expenditure of energy to bring about the desired change in form of the 
molecules and to hinder the undesired. In this connection we are left entirely to chance, 
that is, to random collision, which will bring about a given form the more rarely the 
more forms are possible, i. e., the longer the chain between the two carboxyl groups. 

In perhaps two dozen cases it is well established that a bifunctional 
reaction proceeds fairly smoothly with the formation of a larger-than-6- 
ring. These cases include more 7-membered than larger rings and more 
aromatic than aliphatic compounds. The Sachse-Mohr theory would 
lead one to expect that the formation of 7-rings would not be very much 
more difficult than the formation of 6-rings. The bifunctional reactions 
discussed in the examples offered above all involve purely aliphatic com- 
pounds, and for the most part the chains are unsubstituted. In most of 
these examples the formation of a 7-ring occurs to a certain extent, and 
the formation of the corresponding 8-ring not at all. It appears that in 
such 7-atom chains the probabilities of intra- and intermolecular reaction 
are about equal. The addition of one more atom to the chain diminishes 
the probability of intra- with respect to intermolecular reaction to such an 
extent that only the latter appears. These relations may be somewhat 


modified by the presence of substituents on the chains and even by the 
nature of the reacting groups in ways which it is, at present, impossible to 
predict. It is evident, however, that if two atoms of such a chain are 
adjacent atoms of a benzene ring their position with respect to each other 
is fixed, and the chances of intramolecular reaction are greater than in an 
analogous simple chain of the same length. (The latter will have one 
more axis of rotation than the former.) It is therefore not surprising 
that in NH 2 (CH 2 )6C1 intramolecular reaction occurs only to a slight ex- 
tent, and in o-NI^CeH^CH^Cl almost quantitatively (27). Other similar 
examples might be cited. 

There is also some evidence to indicate that even simple substituents 
such as methyl groups on a chain may increase the tendency toward the 
formation of larger rings (28). Researches in the diphenyl series have es- 
tablished that substituent groups suitably placed may completely inhibit 
rotation about a nearby single bond, and a similar effect in aliphatic chains 
is at least conceivable. Any restriction of the freedom of rotation of the 
atoms of a chain would, on the basis of the Sachse-Mohr theory, increase 
the chances of ring formation. 

The process of polyintermolecular condensation finds no mention in 
treatises on polymerization. This may be due to the fact that such a 
process is not admitted to exist, or that it is not admitted to be polymeriza- 
tion. The examples cited above, and others to be described later, prove 
that such a process does exist, and that it may result in the formation 
of very large molecules. Whether it is to be regarded as polymerization 
or not will depend upon the definition which is adopted for that term. 
The definitions offered above include it as a special type of polymerization. 
This classification finds more to justify it than the analogies which are 
recognized in these definitions. 

The process of A polymerization (the only type which appears to have 
been generally recognized) results in the formation of large molecules 
from small; and it has come about that any process which has this result 
is called polymerization. Since, however, A polymerization is by defini- 
tion a process of self -addition, chemists have often been misled to the 
assumption that condensation leads directly only to small molecules, and 
that if large molecules are formed from small as the apparent result of 
condensation, this is due to the intervention of some unsaturated molecules, 
capable of undergoing A polymerization (29). It is quite certain, however, 
that in many such cases (i. e.,m all cases of true C polymerization) no such 
intermediate products occur. This is a matter of important practical im- 
plications. The reactions involved in the formation of A polymers must, 


in the nature of the case, be for the most part reactions which are peculiar 
to the process of polymerization. For this reason the mechanism of A 
polymerization still remains somewhat obscure. Hence, the mere assump- 
tion that unsaturated intermediates intervene in a reaction which leads to 
high polymers contributes little to one's understanding of the mechanism 
of the process, or the structure of the product. In those cases in which 
this assumption is wrong its use leads one to regard as complicated and 
mysterious a process which may be simple and obvious. C polymerization 
merely involves the use in a multiple fashion of the typical reactions of 
common functional groups. Among bifunctional compounds these reac- 
tions may proceed in such a way as to guarantee the structure of the struc- 
tural unit, R , in the polymer, ( R ) n , formed. It is one of the im- 
mediate objects of the researches to be described in subsequent papers to 
discover how the physical and chemical properties of high polymers of this 
type are related to the nature of the structural unit. 


Linear polymers confrom to the type R R R R R , etc., which 
is characterized by a recurring structural unit. The structural unit R 
is a bivalent radical. Two types of polymers are recognized. (1) Addition 
of A polymers : the polymeric molecule is converted by heat into a monomer 
having the same composition as the structural unit, or the polymer is 
formed by the mutual addition of a number of such monomers. (2) Con- 
densation or C polymers : the polymeric molecule is converted by hydrolysis 
or its equivalent to a monomer which differs in composition from the struc- 
tural unit by one H 2 O (or HC1, NH 3 , etc.), or the polymeric molecule is 
formed from numbers of the monomers by a process of polyintermolecular 
condensation. Rubber, polystyrene, polyoxymethylene and paracetalde- 
hyde are A polymers. Cellulose, silk fibroin and hexa-ethylene glycol are 
probably C polymers. 

Substances of the type x R x and x R y are called bifunctional. In 
these formulas R represents a bivalent radical and x and y functional 
groups capable of reacting with each other in a known fashion to form the 
new functional group, z. Reactions of the type x R y > product are 
called bifunctional and those of the type x R x + y R y > product 
are called bi-bifunctional. Such reactions will lead to compounds contain- 
ing the structural units R z and R z R z . Bifunctional reac- 

tions will be intramolecular and will lead to the monomeric product, R z 



when this can be a 5- or 6-ring. If the monomeric product can only be a 
larger-than-6-ring, reaction will usually be intermolecular and the product 
a polymer of the type R z R z R z R z R z , etc. 

Bibliography and Remarks 

(1) Berzeiius, Jahresbericht, 12, 63 (1833). 

(2) Hess,"Chetnie der Zellulose," Leipzig, 1928, p. 577; Meerwein,Houben-Weyl,"DieMethoden 
der organischen Chemie," Leipzig, 1923, zweite Auflage, dritter Band, p. 1013; Staudinger, Ber., 58, 
1074 (1920). 

(3) However, Staudinger, in his latest papers, favors the view that polystyrene and rubber are very 
large rings, Ber., 62, 241 (1929). 

(4) The term condensation is used here to name any reaction which occurs with the formation of a 
new bond between atoms not already joined and which proceeds with the elimination of elements (H 2 , 
Na, etc.) or of simple compounds (HsO, CaHtOH, NH 3 , NaBr, etc.) Examples are the Wurtz reaction, 
Friedel-Crafts reaction, esterification, etc. 

(5) It would not be difficult to suggest examples in which a single polymer might belong to either or 
to both of these classes depending upon the method by which it was synthesized. So far as cellulose 
is concerned there is little to justify its classification as a C rather than as an A polymer other than the 
criteria which are set forth in the above definitions, and these may at present appear somewhat arbi- 
trary. Thus even if cellulose were synthesized by the dehydration of glucose, this might occur by the 
formation of a glucosan which subsequently polymerized by self-addition. It may be observed, how- 
ever, that of the two trimethylglucosans which might conceivably polymerize in this way to form tri- 
methylcellulose, one is known and does not polymerize, and the other is probably incapable of existence 
on stereochemical grounds. See Freudenberg and Braun, Ann., 460, 288 (1928). 

(6) Cf. Ruggli, Ann., 392,92 (1912), where the recognition of this principle made possible the syn- 
thesis of large rings containing an acetylenic linkage. 

(7) Ferber, Ber., 62, 183 (1929). 

(8) v. Braun and Gawrilow, ibid., 45, 1274 (1912). 

(9) For some other examples, see Titley, J. Chem. Soc., 2571 (1928). 

(10) For malonic anhydride see Staudinger and Ott, Ber., 41, 2214 (1908). For the other 
anhydrides see Voerman, Rec. trav. chim., 23, 265 (1904). The cryoscopic data which Voerman 
presents on phenol solutions have no bearing on the molecular weights of the anhydrides, since even 
succinic anhydride reacts rapidly with phenol to form phenyl succinate; Bischoff and von 
Hedenstrtim, Ber., 35, 4076 (1902). Farmer and Kracovski have recently obtained adipic anhydride 
in a monomeric form, J. Chem. Soc., 680 (1927). 

(11) On this point no data are available for those in which x is 5 and 6 [/. Am. Chem. Soc., 46, 2838 
(1924); Ber., 60, 605 (1927); ibid., 33, 864 (1900)]. The analogous acids, CH 3 CHOH(CH 2 ) 2 CH- 
(CH 3 )CH 2 COOH [Baeyer and Seuffert, ibid., 32, 3619 (1899)] and C 2 H*CHOH(CH 2 )4COOH [Blaise 
and Kohler, Compt. rend., 148, 1772 (1909) ], have been prepared, and by heating are converted at least 
partially into the 7-membered lactones. The acid C 2 HjCHOH(CH 2 )5COOH has also been prepared. 
On heating, it also loses water, but the 8-membered lactone is not formed. The product is a lactide 
like material which cannot be distilled without decomposition (Blaise and Kohler). 

(12) (a) Lycan and Adams, J. Am. Chem. Soc., 51,625 (1929) ; (b) Ruzicka and Stoll, #<?/*>. Chim. 
Ada., 11, 1159 (1928). Since this paper was written a great deal of further information concerning 
these acids has become available in the paper of Chuit and Hausser, ibid., 12, 463 (1929). 

(13) Kerschbaum, Ber., 60, 902 (1927). 

(14) This indicates that large heterocyclic rings are not less stable than their lower homologs. 
Their physical properties are such as would be expected from the known properties of their lower 
homologs The mere presence of a large ring does not result in the development of any of those 
unusual secondary, residual or supermolecular forces which are sometimes supposed to confer 
"polymeric" properties on relatively simple molecules. 

(15) Kerschbaum (ref. 13) records various attempts to prepare ambretollid by the methods com- 
monly used for the preparation of lactones. By heating the acid he obtained traces of an oil which had 
the odor of the lactone. (It is powerfully odorous.) The product was, however, for the most part, a 
gelatinous material soluble in alkali. For the usefulness of the method proposed in Gorman Patent 
Application 150,677, see Ruzicka and Stoll, ref. 12b. 

(16) Gabriel, Ber., 22, 3338 (1889); Schotten, ibid., 21, 2240 (1888). 


(17) v. Braun, Ber., 40, 1840 (1907). 

(18) B. p. 156 at 8 mm. It is, however, not prepared by a bi functional reaction, but from cy- 
cloheptanone oxime by the Beckmann rearrangement. Wallach, Ann., 312, 305; 309, 18 (1899). 

(19) v. Braun and Beschke, Ber., 39, 4121 (1906); Blank, ibid., 25, 3044 (1892); v. Braun and 
Steindorf.iTm*., 38, 172 (1905). 

(20) v. Braun and Steindorf, ibid., 38, 1083 (1905); v. Braun, ibid. 43, 2853 (1910). 

(21) Hill and Hibbert, /. Am. Chem. Soc., 45, 3108, 3124 (1923). See also Franke and Gigerl, 
Monatsh.,49 t 8 (1928). 

(22) Ruzickaand co-workers, Helv. Chim. Acta, 9, 230, 249, 339, 389, 399, 499, 715, 1008 (1926); 
10, 695 (1927); 11, 496, 670, 686, 1159 1174 (1928). 

(23) For example, the action of metals on a dihalide, Br(CH2)zBr. Thus decamethylene bromide 
reacts rapidly and very smoothly with metallic sodium, but no cyclodecane is formed. See Franke and 
Kienberger, Monatsh., 33, 1189 (1912). This reaction will be discussed in a later paper 

(24) See Hiickel, "Der gegenwartige Stand der Spannungstheorie," Fortschritte Chem. Physik und 
physik. Chem., Band 19, Heft 4, 1927. 

(25) Ordinary (preferably quite small) wire tetrahedra may be joined by short pieces of rubber tub- 
ing in such a way that the arms which are being connected overlap. This provides free rotation about 
the union, but prevents bending. This method of union insures that any model which is built up is 
practically strainless (. e., the tetrahedral angle is always retained) and at the same time is sufficiently 
mobile to illustrate the multiplicity of forms which long chains and large rings can assume. 

(26) Mohr, /. prakt. Chem., 98, 348 (1918). 

(27) v. Braun and Bartsch, Ber., 32, 1270 (1899). 

(28) See "Annual Reports of the Progress of Chemistry," 1927, p. 100; Moyer and Adams, J. Am. 
Chem Soc , 51, 630 (1929). 

(29) Thus, Drummond, Inst. Rubber Ind., 4, 43 (1928), .states, "Where resins are formed by a pre- 
liminary condensation as are the phenol-formaldehyde products, it is logical to assume that in the 
preliminary reaction, unsaturated atomic linkings are introduced which provide the necessary arrange- 
ment for polymerization subsequently to occur." Scheiber, Chem. Umschau, Fette, Oele, Wachse 
Harze, 15, 181 (1928), is even more explicit: "Fur den Aufbau organischer Stoffe kommen bekanntlich 
nur zwei Prozesse in Betracht, und zwar Kondensationen und Polymerisationen. Vorgange der ersteren 
Art fvihren im allgemeinen zu Molekulverbanden beschr&nkter Grosse . .Bei Polymerisationen hinge- 
gen kommt es in manchen Fallen zur Ausbildung extrem grosser Molekiile. . ." 

II. Polyesters* 

In this paper a bi-bifunctional reaction is studied by condensing di- 
basic carboxylic acids with dihydric alcohols. The number of carbon 
atoms in the structural units are 7, 8, 9, 10, 11, 12, 14, 15, 16, 18 and 22. 
Consequently] only C-polymers and no other products are obtained. 
Their molecular (or better average molecular) weights were measured by 
the freezing and boiling point methods. They range between 2500 and 
5000. With the exception of the liquid ethylene malonate all polyesters 
are microcrystalline materials, with a melting point, which is not quite 

* W. H. Carothers and G. A. Arvin; Journ. Am. Chem. Soc., 51, 2660-70 (1929); 
Contribution No. 1 1 from the Experimental Station of E. I. du Pont de Nemours and 

Received April 13, 1929. Published August 7, 1929. 

t See the thesis developed on pages 10 and 11. 


The following combinations were prepared* 

ethylene malonate hexamethylene succinate 

ethylene succinate hexamethylene adipate 

ethylene adipate hexamethylene sebacate 

ethylene sebacate hexamethylene phthalate 

ethylene maleale decamethylene succinate 

ethylene fumarate decamethylene adipate 

ethylene phthalate decamethylene sebacate 

trimethylene succinate decamethylene phthalate 
trimethykne adipate 
trimethylene sebacate 
trimethykne phthalate 

An example of a bi-bifunctional reaction is found in the reaction between 
a dibasic acid and a dihydric alcohol, HOOC R' COOH + HO R" 
OH, which, if it is conducted so as to involve both functional groups of 
each reactant, must lead to an ester having the structural unit, OC R' 
CO O R" O = R . In accordance with the thesis developed in 
the previous paper, esters formed in this way will be polymeric unless the 
number of atoms in the chain of the structural unit is less than seven. 
In this paper esters are described in which the number of atoms in the 
chain of the structural unit is 7, 8, 9, 10, 11, 12, 14, 15, 16, 18 and 22 
atoms. All these esters are highly polymeric, and, although some of 
them have been prepared by various methods, no monomeric form of any 
of them has as yet been isolated. 

Preparation of the Esters 

The following method was used for the preparation of the solid esters whose prop- 
erties are listed in the table. The acid together with a 5% excess of the glycol was placed 
in a Claisen flask provided with a receiver and condenser, and the flask was heated in a 
metal-bath. At about 160 (bath temperature) reaction set in. Water distilled off 
freely during the first hour (temp., 175-185) and very slowly if at all during the succeed- 
ing two hours at the same temperature. The receiver was now changed, the flask pro- 
vided with a very fine capillary and heating continued under a good vacuum (usually 
less than 0.2 mm.) for about three hours, the temperature of the bath being raised to 
200-250. During this period little or no distillate collected (provided only a 5% excess 
of glycol was used). The residue, which was a slightly dark and, at 150, more or less 
viscous liquid, was poured from the flask. The amount of this residue corresponded 
with the theoretical (based on the acid used), and the amount of water actually collected 
approached the calculated more closely the larger the sample used (60-90%). The 
esters were purified by crystallization. 

* Mostly by direct condensation of the acid with the glycol in different propor- 
tions, in one case (ethylene succinate) by combining silver succinate with ethylene 
bromide (comp. p. 25). 


Ethylene malonate was prepared by heating ethyl malonate and glycol in the same 
fashion. There was some decomposition (evolution of gas) when the residue was heated 
to 240 in vacuo. This residue was a thick sirup which could not be induced to crys- 
tallize. It was dissolved in acetone, filtered and heated for several hours at 175-190 
in high vacuum. Nothing corresponding to a monomeric ethylene malonate was found 
on redistilling the distillates from this preparation. 

The preparation and properties of some esters not included in the table are described 
in the Experimental Part of this paper. 

Structure of the Esters 

The conclusion that these compounds are esters and that they contain 
the structural unit R indicated above follows directly from the 
method of preparation and is supported by the analytical data and chemi- 
cal behavior. Comparison of Cols. 4 and 5 in Table I shows that the 
carbon and hydrogen percentages in general agree with those calculated 
for R . So also did saponification numbers where these were de- 
termined. The products of saponification of the esters are the acids and 
glycols from which they were prepared. 

That the esters are not monomeric 7-, 8-, 9-, . . ., 22-membered rings is 
indicated by their physical properties and by their molecular weights. 
In Col. 7 are given the molecular weights calculated from the observed 
boiling point elevations or freezing point lowerings, and in Col. 8 the 
solvents and methods used. The solvents used in the ebullioscopic de- 
terminations include chloroform, ethylene chloride, benzene and acetone. 
Those used in the cryoscopic determinations include benzene, glacial 
acetic acid and diphenyl ether. 

In these determinations it was necessary to use rather large samples 
to obtain boiling point or freezing point changes of 0.02 to 0.05. Judged 
by their self-consistency, the values obtained with the use of freezing 
benzene have about the same relative accuracy as is ordinarily attained 
with simple compounds. With other solvents, and especially in the 
boiling point determinations, observations were much less consistent. 
This was due in part at least to changes of surface tension, associated no 
doubt with molecular size, since there was much foaming. The values 
obtained by this method are recorded, since for some of the esters they are 
the only ones available, and for others they clearly indicate that the order 
of magnitude of the apparent molecular weights is quite independent of the 
character of the solvent used. 

The lowest molecular weight observed for any of the solid esters was 
2300 and the highest 5000. The mean of all the determinations is 3200, 
and for purposes of calculation the molecular weight of each ester has 




53 v* 

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5 I 8 a> 

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^JJJSjg JiJ.JzJ ft 

<<OOO < < -^'<UO'S 
d, a, ex d d a d d d d 

ffi O 
3 1 




1 1 

CO Ol to O O 

o e* o> oo CN 

t-.' oo oo oo o> 

CO O C C 00 

h- O O CO C3l 



^ CO CO cSl 1-1 O 



co c3 
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H rH O CN 00 CO 

b. CD ^ CO O O 

M t H 1 i it 


been assumed to be approximately 3000. Such a molecular weight cor- 
responds with a value of 8 to 20 (depending on the length of the chain 
of the structural unit) for n in the general formula ( CO R' CO 
O R" O ) tt . It can scarcely be supposed that the molecules present 
in a given sample are all identical so far as the values of n are concerned, 
but their crystallinity and a certain homogeneity in such physical behavior 
as solubility indicate that the varieties of molecular species present in 
a given sample probably do not include a very wide range. 

The disposition of the valences at the ends of the chains in the above 
formula cannot as yet be definitely decided. A cyclic structure is ren- 
dered improbable by the same considerations which led us to expect that 
these esters would be polymeric and which have been discussed in the 
previous paper. Esters prepared by the method indicated above are 
definitely not acidic, and since the detection of a car boxy 1 group in a 
molecule having a weight of 3000 to 5000 should present no difficulties, 
no carboxyl groups can be present at the ends of the chains. Attempts 
to detect the presence of hydroxyl groups have not as yet succeeded. 
Kthylene succinate crystallizes unchanged from acetic anhydride, and is 
not affected by phenyl or naphthyl isocyanate. Nevertheless we are 
inclined to assume that hydroxyl groups are present at each end of the 
chain due to the presence of one more molecule of glycol than acid : HO 
R" O ( CO R' CO O R" O ) M H. The failure of these 
groups to react with reagents may be ascribed to the operation of the same 
factors which set a limit to the size of the molecules formed. 

In general the analytical values agree more closely for such a formula 
than for a cyclic formula. In Col. 6 of Table I are given the carbon and 
hydrogen percentages calculated for this formula assuming a molecular 
weight as close as possible to 3000. The means of the deviations between 
the carbon and hydrogen percentages found and those calculated for the 
( R ) n formula are 0.48 and 0.01% ; while the deviations calcu- 
lated for the HO ( R ) w R' OH formula are -0.31 and -0.11%. 

Assuming a molecular weight of 3000 the chains, H [ O (CH 2 ) y 
O CO (CH 2 ) CO 1 n O (CH 2 ) y OH, would contain 170-200 
atoms and their lengths would lie between 240 and 280 A. (1). Their 
lengths are therefore of the same order of magnitude as that assumed by 
Meyer (1) for the cellulose chains (100 glucose units per chain = 510 A.). 
The molecular cohesions calculated from the data presented by Meyer (2) 
lie between 250,000 and 300,000 cai. or 3.3 to 4 times the heat of separa- 
tion of a carbon-carbon bond (75,000 cal.) . These molecular cohesions are, 
however, only about 10% of those calculated for a cellulose chain of 500 A. 


The structure of these esters will be discussed in more detail later in 
the light of experimental work which is not yet completed. 

Properties of the Esters 

All the esters of the type (CO (CHa)* CO O (CH 2 ) y O ) 
with the exception of ethylene malonate are crystalline solids. The 
property of crystallinity is not very highly developed, but it is quite defi- 
nitely present (3). This fact is of interest because examples of crystalline 
highly polymeric substances are not very numerous. As with all crystal- 
line substances of this class, it is not possible to develop large crystals. 
Ethylene succinate when it separates from a melt or from concentrated 
solutions (in chloroform) crystallizes in doubly refracting spherolites 
(microscopic) which grow to what appear to be star-like groups of needles. 
From dilute alcohol solutions it separates in discrete microscopic needles 


Crystallized ethylene succinate and decamethylene sebacate are dusty 
powders which have a great tendency to become electrified. Some of the 
intervening members are quite soft and even somewhat sticky. Ethylene 
succinate solidifies from a melt as a hard, brittle, opaque, white mass. 
Decamethylene sebacate solidifies to a white, brittle, waxy solid. 

The non-crystalline members include ethylene malonate, ethylene 
fumarate and ethylene, trimethylene, hexamethylene and decamethylene 
phthalates. The phthalates and the fumarate evidently have less sym- 
metry than the saturated straight-chain purely aliphatic type. The lack 
of crystallinity of the malonate may be associated with the low melting 
point of the malonic acid; or it may be due to the fact that considerable 
decomposition occurred during the process of esterification. 

The melting points of the solid esters are not very sharp and they depend 
somewhat upon the rate of heating. This latter effect is noticed especially 
with ethylene succinate. 

When the crude reaction mixture is dissolved in chloroform and precipitated with 
benzene, the resulting dusty white powder melts in a capillary tube at about 102. 
If, however, the tube is placed in a bath which has already been heated to 96 the 
sample melts at once. The two melting points are called the slow and instantaneous 
melting points, respectively, and they are best observed on a bloc Maquenne. Samples 
which are dusted on the bloc when it is below 96 do not melt until it reaches 102, 
but as soon as the bloc reaches 96 samples melt the instant they touch it. Both these 
temperatures may be changed somewhat by repeated crystallization, but a sample whose 
melting point has been raised by repeated crystallization may suddenly show the origi- 
nal melting point on further crystallization. By long extraction with boiling absolute 
alcohol in which it is practically insoluble, ethylene succinate is modified so that the in- 


stantaneous and slow melting points coincide at about 107-108, and from the extracts 
may be isolated a very small amount of lower-melting material. When such a high- 
melting sample is again crystallized from a mixture of chloroform and benzene or ether, 
the melting point usually drops, and one observes again the instantaneous melting 
point of 96 and the slow melting point 102. The melting point of such samples may 
also be raised by long heating just below 100, and both melting points then gradually 
approach 107. When a 96, 102 sample is quickly melted, then allowed to solidify in 
an agate mortar and powdered, the instantaneous melting point is lowered somewhat, 
and the slow melting point may be either raised or lowered, depending upon the time 
during which the molten material is heated. 

Attempts to define the melting point more clearly by heating or cooling curves on 
fairly large samples were unsuccessful (5). Apparently the viscosity of the melt is so 
great as to make the process of crystallization slow even in the presence of previously 
formed crystals and under a considerable temperature gradient . 

The erratic behavior of the melting points may be associated with the fact that all 
these esters are hygroscopic, but the absorption of water cannot be the sole cause for 
this behavior, for samples of ethylene succinate having various melting points were 
stored over 20% sulfuric acid for ten days and all of them gained considerably in weight 
without changing in physical appearance or melting point. 

Anomalies in melting points are not unknown among compounds of definite con- 
stitutions and relatively low molecular weights. One may cite, for example, the fact 
that sucrose separates from methyl alcohol in crystals melting at 169-170 and from 
other solvents in crystals melting at 179-180, and these two forms do not show any 
detectable chemical differences (fi). But the case of ethylene succinate appears to be 
more complicated and confused than this. Its melting point depends upon its history in 
a fashion which can as yet be defined only to the following extent it always rises and 
approaches 107 as it is, while still in the solid state, heated or extracted with boiling 
absolute alcohol. 

Among the other esters these irregularities were less pronounced. 
Nevertheless, the melting points are not very characteristic and no attempt 
was made to crystallize them to constant melting points. Instead they 
were crystallized two to six times depending upon the color and softness 
of the sample. The melting points recorded are the highest observed for 
complete melting and are usually within 5 of the lowest observed for any 
sample of the same material. The melting range was usually less than 
2. The melting point of the decamethylene ester of a given acid was 
higher than that of the hexamethylene ester. The melting point of the 
hexamethylene ester was higher than that of the trimethylene ester, 
which was in turn lower than that of the ethylene ester. With one ex- 
ception, the adipic ester of each glycol melted lower than either its suc- 
cinic or sebacic ester. 

All of these esters are, when molten, quite viscous. Those which do not 
crystallize on cooling become more or less hard, tough and glassy. The 
use as resins of esters formed by the action of polybasic acids on poly- 


hydric alcohols has been covered by numerous patents (7). The impres- 
sion seems to prevail that all such esters are resins. Of the esters described 
in this paper the majority are crystalline. The phthalates which we have 
prepared are, however, all resinous. Ethylene phthalate is quite tough 
and moderately hard. Neither this ester nor the trimethylene, hexa- 
methylene nor decamethylene esters of the same acid have been described 
in the scientific literature. As the length of the alcohol chain increases, 
these phthalates become progressively softer. Decamethylene phthalate 
has the consistency of a moderately thick sirup. 

None of these esters is volatile. Each of them has been heated to 
200 in vacua without showing any tendency to distil. Ethylene sue- 
cinate heated for three hours at 250 under a pressure of 2 mm. is not 
changed in properties, and its analytical composition (carbon, hydrogen 
and saponification number) is not changed. It shows no tendency to 
distil or to evolve any volatile products when heated to 280 at a pressure 
of 1 micron. At 350 ethylene succinate undergoes complete thermal 
decomposition, the products being ethylene, acetaldehyde, succinic anhy- 
dride, carbon, etc. (8). 

All of these esters are insoluble, or nearly so in water, alcohol, petroleum 
ether and ether (9). The least soluble of the saturated aliphatic esters is 
ethylene succinate. It is more or less soluble in hot ethyl acetate, acetic 
acid, acetic anhydride, ethyl succinate and acetone, and it crystallizes 
from these solvents on cooling. It is insoluble in benzene. The other 
saturated aliphatic esters with the exception of ethylene malonate are 
readily soluble in cold benzene and at least moderately soluble in acetone, 
ethyl acetate and glacial acetic acid. The solutions are noticeably vis- 
cous only when fairly concentrated (e. g. t 10% or stronger). This fact 
and the rapidity with which solution occurs lead us to believe that the 
solutions are true molecular dispersions. 

Although none of these esters is soluble in water they are all somewhat 
hygroscopic. This property is especially pronounced in those polyesters 
which are not crystalline. The drying of these resins was very difficult 
and, as the analytical results indicate, was not always successful. Drying 
to constant weight was complicated by the tendency to foam when heated 
in vacua (10). 

Other Methods of Preparation 

The only detailed study of polyesters of the type here described which 
we have found is reported in a paper by Vorlander (11). 


Ethylene succinate was first prepared by Lourenco (12) by heating sue- 
cinic acid and ethylene glycol to 300 (m. p. about 90). Subsequently 
Davidoff (13) prepared Lourengo's ester and identified it with one which he 
obtained by heating silver succinate with ethylene bromide. Vorlander 
studied this latter compound in detail and showed that its chemical be- 
havior was in all respects what would be expected of an ethylene ester 
of succinic acid. He also identified it with Lourengo's ester and with one 
obtained by the action of succinyl chloride on the disodium derivative of 
ethylene glycol. 

Vorlander found that his ethylene succinates prepared from silver suc- 
cinate and from succinyl chloride had apparent molecular weights ranging 
from 265 to 321 in freezing phenol and in freezing acetic acid. He sup- 
posed them to be dimeric, and in accordance with this view was able to 
prepare the same compound from silver succinate anddi-(/3-chloro-ethyl)- 
succinate. This method of synthesis evidently merely establishes that 
the compound was probably not monomeric. 

The properties which Vorlander records for ethylene succinate agree in 
general with those which we have observed. We have found that it is 
possible to prepare ethylene succinates having, within certain limits, vari- 
ous melting points (and molecular weights) by heating succinic acid and 
glycol in various proportions. The highest melting (102) is that de- 
scribed above. It was prepared from acid and excess glycol. Vorlander 
and other writers on this subject ascribe to ethylene succinate a melting 
point of 88-90. We have obtained samples showing this melting point 
by using the acid and glycol in equivalent amounts; and, by using excess 
acid, have obtained lower-melting samples. These ethylene succinates 
differ somewhat in solubility, but they are alike in physical appearance 
in their lack of volatility and in their viscous character when molten. All 
of these materials are highly polymeric. We have also prepared ethylene 
succinate from silver succinate and ethylene bromide according to Vor- 
lander's directions. This reaction does not proceed smoothly, and our 
product was slightly colored and melted at 75 instead of at 90. Never- 
theless, molecular weight determinations in boiling ethylene chloride gave 
values ranging from 1400 to 2000. We think, therefore, that Vorlander's 
molecular weight determinations must be in error; and this conclusion 
is supported by the fact that the properties which he records and which 
we have observed are not consistent with so low a molecular weight as 288. 

We have also prepared ethylene succinate from ethyl succinate and 
glycol. It is also highly polymeric. Thus the methods which have been 
used for the preparation of this compound include four separate and dis- 


tinct methods commonly used in the preparation of esters. They all lead 
to products of the expected composition but of high molecular weights. 
There is not the slightest reason for supposing that the monomeric ethylene 
succinate should be incapable of existence, or even that it should be un- 
stable, but it still remains unknown. These facts find their explanation 
in the thesis developed in the previous paper. 

Vorlander also prepared ethylene maleate and ethylene fumarate by the 
silver salt method. He could not obtain any consistent values for the 
molecular weights of these materials. We have prepared the fumarate 
from ethyl fumarate and the maleate from maleic anhydride. Our prepa- 
rations differ from those of Vorlander in several respects. Thus, our male- 
ate was crystalline and our fumarate resinous. The reverse was true for 
Vorlander's compounds. Neither our analytical values nor his agree well 
with the calculated. These esters appear to be much more complicated 
in their behavior than ethylene succinate. They both become completely 
insoluble on heating. 

Experimental Part 

Ethylene Phthalate No. 1. Ethylene glycol 62 g. (I mole) and phthahc anhydride 
74 g. (0.5 mole) were heated together for eight hours at 190 under ordinary pressure 
and for three hours at 300 under 3 mm. The viscous residue was heated with boiling 
water for twenty minutes, dissolved in chloroform, filtered and precipitated with ether 
and then dissolved in acetone, filtered and precipitated with water. It was then dried 
by heating to 100-1 70. The resulting glassy resin was fairly hard when cold, and be- 
came softer on heating. All attempts to induce it to crystallize failed. It was neutral: 
2 g. required 0.03 cc. of 0.23 N NaOH for alkalinity toward phenolphthalein. It was 
soluble in chloroform, acetone, ethyl acetate and acetic acid; insoluble in petroleum 
ether, ether, benzene, alcohol and water. 

Anal. Substance dried to constant weight in high vacuum at 70. Calcd. for 
CioH s O 4 : C, 62.48; H, 4.19; molecular weight, 192; saponification number, 96. Calcd. 
forH[O(CH 2 ) 2 OOCC 6 H 4 CO] 26 0(CH 2 ) 2 OH = (W^Oio*: C, 62.15; H, 4.28; molecular 
weight, 4804; saponification number, 97.3. Found: C, 01.86, 61.95; H, 4.29, 4.30; 
molecular weight by micro boiling point method in ethylene chloride, 4830, 5070, 4680, 
4690; saponification number, 95.7. 

Ethylene Phthalate No. 2. This was prepared in the same way as No. 1 , but with 
a 20% excess of phthalic anhydride and was purified in a similar fashion. Appearance 
and solubility were the same as for No. 1 . 

Anal. After drying to constant weight in high vacuum at 70. Found: C, 62.11, 
62.07; H, 4.29, 4.33; molecular weight, by method of Menzies and Wright in ethylene 
chloride, 2940, 2700, 3020; by micro boiling point method in ethylene chloride, 3030, 
2930; saponification number, 96.2. 

Ethylene Phthalate No. 3. Prepared by heating diethyl phthalate (0.5 mole) with 
ethylene glycol (1 mole) in the same fashion and purified as before. Appearance and 
solubility were the same as for No. 1. 


Anal, after drying to constant weight in high vacuum at 70. Found: C, 62.35, 
62.14; H t 4.40, 4.37; molecular weight, by freezing point lowering in diphenyl ether, 
2070, 2030; by method of Menzies and Wright in ethylene chloride, 1990, 2100, 2050, 
1770; by micro boiling point method in ethylene chloride, 2100, 1870. 

Ethylene Phthalate No. 4. This was prepared by stirring vigorously 19 g. of glycol 
(added slowly) with 60 g. of phthalyl chloride and 51 g. of dry pyridine in 125 cc. of 
chloroform at 0-5. The reaction mixture was washed thoroughly with dilute acid and 
dilute sodium carbonate and water, and decolorized with Darco. After drying the 
chloroform solution the ester was precipitated with ether. The yield was 53 g. It 
resembled the other ethylene phthalates, but its solutions in chloroform were less viscous 
0.016 poise for a 20% solution at 27 as compared with 0.027-0.031 poise for the other 
ethylene phthalates. Its apparent molecular weight was also lower. 

Anal, after drying to constant weight in high vacuum at 70. Found: C, 61.64, 
61.64; H, 4.28, 4.30; molecular weight, by method of Menzies and Wright in ethylene 
chloride, 1550, 1610; saponification number, 99.5, 99.6. 

Hydrolysis of Ethylene Phthalate. Twenty grams of ethylene phthalate was re- 
fluxed for sixty-four hours with 60 g. of 48% hydrobromic acid. After neutralization 
with sodium carbonate, the reaction mixture was steam distilled. From the distillate 
was isolated 16.7 g. or 86.6% of the calculated amount of ethylene bromide, b. p. 131- 
134. The residue on acidification gave 16.3 g. or 93.6% of the calculated amount of 
phthalic acid. This on conversion to the anhydride melted at 130-131 . 

Trimethylene Phthalate. Seventy-four grams (0.5 mole) of phthalic anhydride 
and 38 g. (0.5 mole) of trimethylene glycol were heated at 250 for two hours under 
ordinary pressure and under diminished pressure for two hours. The residue was dis- 
solved in benzene, treated with Darco, filtered and precipitated with ether. When cold 
it was a clear, glassy solid, somewhat softer than ethylene phthalate. It was soluble 
in chloroform, benzene, acetone, ethyl acetate and acetic acid; slightly soluble in alco- 
hol; insoluble in ether, petroleum ether and water. 

Anal. Calcd. for C n HioO 4 : C, 64.08; H, 4.89; molecular weight, 206. Calcd. for 
H[OOCC6H 4 COO(CH 2 ) 3 ] 1 60(CH 2 ) 3 OH = C m H m O^: C, 63.63; H, 5.05; molecular 
weight, 3168. Found: C, 63.68, 63.51; H, 5.17, 5.12; molecular weight by micro boil- 
ing point method in ethylene chloride, 3180, 3030. 

Hexamethylene Phthalate. Twenty-nine and six-tenths g. of phthalic anhydride 
and 23.6 g. of hexarnethylene glycol were heated to 180-190 under atmospheric pressure 
for two and one-half hours and then for one and one-half hours at 250 under 5 mm. 
In addition to the water, some phthalic anhydride collected in the receiver. The resid- 
ual dark gum was purified by dissolving in benzene, decolorizing with Darco, and pre- 
cipitating with ether. It formed a clear, light brown, sticky gum. Solubility: solvents 
are the same as for trimethylene phthalate, but hexamethylene phthalate is more soluble. 

Anal, after drying to constant weight in high vacuum at 70. Calcd. for Ci4HioO 4 : 
C, 67.75; H, 6.50; molecular weight, 248. Calcd. for H[O(CH 2 )6OOCC6H 4 CO]7O- 
(CH 2 ) 6 OH = Ci 94 H 126 O 3 o: C, 67.35; H, 6.85; molecular weight, 1855. Found: C, 
66.74, 66.84; H, 6.75, 6.85; molecular weight by freezing point lowering in benzene, 
1700, 1830. 

Decamethylene Phthalate. Seven and four-tenths g. of phthalic anhydride and 
9 g. of decamethylene glycol were heated at 190-200 for two hours under atmospheric 


pressure, and at 210-220 for one and one-half hours at 5 mm. The residue was dis- 
solved in benzene, decolorized with Darco and precipitated by petroleum ether. It 
was a clear, light brown, thick, sticky sirup. Solubility: solvents for decamethylene 
phthalate are the same as for hexamethylene phthalate, but the former is more soluble. 

Anal, after drying to constant weight in high vacuum at 70. Calcd. for CiaHa^: 
C, 71.01; H, 7.95; molecular weight, 304. Calcd. for H[O(CH2)ioOOCC 6 H 4 CO]7O- 
(CH 2 )i OH = Ci 3 H 19 oO 3 o: C, 70.80; H, 8.39; mol. wt. 2303. Found: C, 70.66, 70.44; 
H, 8.21, 8.26; molecular weight by freezing point lowering in benzene, 2250, 2060. 

Ethylene Fumarate. Fifty-seven and three-tenths g. of diethyl fumarate 
(0.33 mole) and 25 g. of ethylene glycol (0.4 mole) were heated for ten hours in a stream 
of nitrogen, the temperature being gradually raised from 190 to 230 and the pressure 
being reduced at the end to 4 mm. The residue weighed 35 g. or 75% of the calculated 
amount. It was accompanied by some insoluble material from which it was freed by 
solution in chloroform, filtration and precipitation with ether. It was washed several 
times with dry ether. It formed a transparent, slightly yellow, moderately tough mass. 
After drying it became insoluble in the common solvents. 

Anal, after drying to constant weight in high vacuum at 70. Calcd. for C HeO4: 
C, 50.70; H, 4.22. Found: C, 51.95, 51.89. H, 6.06, 6.12. Molecular weight de- 
terminations could not be made because of the lack of solubility after drying 

Ethylene Maleate. Thirty-two and five-tenths g. of maleic anhydride (0.33 
mole) and 18.6 g. (0.30 mole) of glycol were heated at 195-200 for four hours, and 
then for some time at 210-215 under reduced pressure. The residue (40 g.) was 
separated from some insoluble material by solution in warm ethylene chloride and fil- 
tration. It was precipitated by cold ether. The product separated as an oil but 
solidified on standing at 5-10 for two hours. It was a white powder. Most of it 
melted between 88 and 95. After drying in vacua it had become insoluble in the com- 
mon solvents including ethylene chloride and it did not melt below 250 . 

Anal, after drying to constant weight in high vacuum at 70. Calcd. for CeHeO^ 
C, 50.70; H, 4.22. Found: C, 49.87, 49.70; H, 4.36, 4.28. Molecular weight de- 
terminations could not be made because of the lack of solubility after drying. 

For his kind assistance in the analytical work we here express our thanks 
to Mr. Wendell H. Taylor. 


The following esters have been prepared: ethylene malonate, ethylene 
succinate, trimethylene succinate, ethylene adipate, trimethylene adipate, 
hexamethylene succinate, hexamethylene adipate, ethylene sebacate, tri- 
methylene sebacate, decamethylene succinate, hexamethylene sebacate, 
decamethylene adipate, decamethylene sebacate, ethylene maleate, ethyl- 
ene fumarate, ethylene phthalate, trimethylene phthalate, hexamethylene 
phthalate and decamethylene phthalate. Their molecular weights have 
been determined. They are all highly polymeric. Their properties are 
described and their structures are discussed. 


Bibliography and Remarks 

(1) The values C = 1.5 A. and O 1.1 A. are taken from Meyer, Naturwtsscnschaftcn, 42, 782 

(2) Ref. 1, p. 21. 

(3) So far as they have been examined in this respect, all these esters give sharp x-ray diffraction 

(4) Incidentally these are the forms in which Hess* trimethylcellulose crystallizes, and the photo- 
micrographs (Figs. 78, 80 and 81) which he presents ("Die Chemie der Zellulose," Leipzig, 1928, p 
432) for this substance would serve almost perfectly as pictures of ethylene succinate. The crystal- 
linity of this trimethylcellulose therefore provides no guarantee either that it contains only a single 
molecular species, or that the molecules present are not very large. 

(5) Some experiments were kindly made for us by Dr. E. L. vSkau with the special apparatus 
which he has devised for the precise determination of the melting points of pure organic compounds. 

(6) Pictet and Vogel, Helv. Chim. Ada, 11, 901 (1928). 

(7) See for example U. S. Patents Nos. 1,108,332; 1,091,627; 1,091,628; 1,091,732; 1,108,329; 
1,108,330; 1,108,331; 1,642,079; 1,678,105; 1,098,776; 1,098,777, 1,424,137, 1,413,144; 1,413,145, 
1,667,199; 1,667,200; 1,119,592; 1,141,944; 1,663,183. 

(8) Tilitschejew, /. Russ. Phys. Chem. Soc., 57, 143 (1925). 

(9) It is interesting to note that Freudenberg and Braun's trimethylcellulose, like ethylene sue 
cinate, is insoluble in ether and carbon tetrachloride, but quite soluble in chloroform [Ann., 460, 288 

(10) It is interesting to observe that inulin, a polymeric substance of a different type, but probably 
of the same degree of molecular complexity as the esters here described is also quite hygroscopic. Cf 
Drew and Haworth, J. Chem. Soc., 2690 (1928). 

(11) Vorlander, Ann.. 280, 167 (1894). See also Bischoff, Ber.. 27, 2940 (1894); 40, 2779, 2803 
(1907). Glycol esters of adipic acid are referred to in German patent 318,222, Chem. Zentr., II, 536 

(12) Lourenco, Ann. chim. phys., 293 (1863). 

(13) Davidoff, Ber., 19, 406 (1886). 

III. Glycol Esters of Carbonic Acid* 

This paper describes different glycol esters of carbonic acid. The aim is 
to check whether condensations which can lead to 5- and 6 -rings remain 
intramolecular, while such as can give only larger rings lead to C- 
polymers. In fact it was shown that as long as 5- or 6-rings can be formed 
(ethylene carbonate and trimethylene carbonate) well crystallized, low 
molecular compounds, with molecular weights of about 90 and 110, are 
produced; as soon as only larger rings can be formed C-potymers, with 
molecular weights up to about 3000, result). ^ 
Trimethylene carbonate 


< >CH 2 

X) CH 2 ' 

* W. H. Carothers and F. J. van Natta; Journ. Am. Chem. Soc. 52, 314-26 (1930) ; 
Contribution No. 19 from the Experimental Station of E. I. du Pont de Nemours and 

Received July 24, 1929. Published January 8, 1930. 

t Compare the thesis developed on pages 10 and 12. 


which contains a 6 -ring can be obtained in both forms: as a monomeric di- 
ester with a molecular weight of 104 and as a polymer with a polymerization 
degree between 38 and 45. This latter is apparently formed by an A - 
polymerization of the monomeric trimethylene carbonate. 
The preparation and the properties of the following substances are described 
and discussed: 

trimethylene carbonate decamethylene carbonate 

tetramethylene carbonate diethylene carbonate 

pentamethylene carbonate p-xylylene carbonate 
hexamethylene carbonate 

The glycol esters of dibasic acids described in a previous paper (1) are all 
highly polymeric. This was expected from the generalization (2) that bi- 
functional reactions which, if intramolecular, could lead only to larger-than- 
(i rings proceed intermolecularly. It seemed desirable to examine an ho- 
mologous series of similar compounds in which the length of the structural 
unit might be as short as 5 or 6 atoms, since in these cases reaction should 
be intramolecular and the products monomeric, and in the longer chains 
of the same series reaction should be intermolecular and the products 

Such a series is found in the glycol esters of carbonic acid. The first 

yO CH 2 
member of this series, ethylene carbonate, O=C<f \ , has long been 

X) CH 2 

known (3). It is a crystalline solid which boils at 238 arid it is definitely 
established to be the monomeric 5-ring by both cryoscopic and vapor 
density data (4). 

We have now prepared the trimethylene, tetramethylene, hexamethyl- 
ene, decamethylene, diethylene and p-xylylene esters of carbonic acid. 
The properties of these esters, together with the apparent mean molecular 
weights determined by cryoscopic and ebullioscopic methods, are indicated 
in Table I. The analytical compositions of all these esters correspond with 
the formulas of their structural units O -(CH 2 ) M CO, but where 
the length of the chain of this unit is 5 or 6, only one such unit is present 
in a molecule of the ester. On the other hand, where the length of the 
chain of the structural unit is 7, 8, 9 or 13, eight to twenty-two structural 
units are present in each molecule. The method of preparation and the 
analytical compositions of the polymeric esters indicate a structure which 
may be represented by the general formula O (CH 2 ) M -O CO O 
(CH 2 ) n -O-~- CO O(CH 2 ) M -O CO O (CH 2 ) W O CO-, etc. To 





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complete this formula, it is necessary to discover whether the valences 
at the ends of the chains are saturated mutually with the formation of a 
ring, or are saturated by univalent groups to give open chains. The 7-, 
8-, 9- and 13-membered rings which might be formed in this reaction are 
not found, and there is no reason to suppose that much larger rings would 
be formed more readily. For some polyesters similar to those described 
here we have direct experimental proof of the open-chain structure. It 
seems quite certain, therefore, that these polycarbonates are also open 
chains. From the method of preparation it follows that the ends of the 
chains must bear hydroxyl groups (from an extra molecule of glycol) or 
carbethoxy groups (from an extra molecule of ethyl carbonate). In this 
connection it is interesting to observe that when an excess of ethyl car- 
bonate was used in the preparation of hexamethylene carbonate, pure 
dicarbethoxyhexane, C 2 H 6 O CO O(CH 2 ) 6 O CO OC 2 H 6 , was isolated, 
and an oil which, judging by its composition, was composed chiefly of C 2 H&- 
O CO 0(CH 2 ) 6 CO 0(CH 2 ) 6 O CO O(CH 2 ) 6 O CO OC 2 H 6 . 

Properties of the Polycarbonates. The physical properties of these 
polycarbonates again illustrate the fact that neither melting points nor 
solubilities offer any general criteria of molecular size. 

In this connection trimethylene carbonate is of especial interest since 
it has been obtained in both the monomeric form and a form in which 
the degree of polymerization is 38-45. The monomeric form is hygro- 
scopic and is soluble in water, benzene and alcohol, but only slightly soluble 
in ether and ligroin. The polymeric form is not soluble in water and alco- 
hol, but is still soluble in benzene. Solubility here diminishes with con- 
siderable increase in molecular size, but it does not disappear. Prac- 
tically all the polymeric esters described in this and the previous paper 
are insoluble in water and alcohol, but soluble in chloroform and somewhat 
soluble in acetone. Those polyesters in which the structural units con- 
tain polymethylene chains (CH 2 )^, in which x is greater than 3-5, show 
great solubility in benzene. 

Polyesters of high molecular weight may be either liquid (diethylene car- 
bonate, ethylene malonate (1), decamethylene phthalate (1)) or solids. 
Most of the solid polyesters are crystalline. These crystalline polyesters 
differ from analogously constituted crystalline esters of low molecular 
weight in that the crystals are always microscopic. In some cases it is not 
possible to decide even by microscopic examination whether these solid 
polymers are crystalline, but x-ray examination always shows definite crys- 
tallinity. Thus hexamethylene carbonate was an opaque, tough, horny 
material with considerable elasticity. Nevertheless, it had a fairly definite 


melting point and it gave a quite sharp x-ray diffraction pattern 
(Fig. 3). 

The following generalizations concerning the influence of the structure 
of the structural units of polyesters on their physical properties can now 
be offered. As the lengths of the polymethylene chains, (CH2)*, which 
separate the ester groups increase, solubility in organic solvents increases 
and viscosity of the molten ester diminishes. This would be expected 
from the fact that carbonyl groups contribute much more heavily to in- 
termolecular association forces than do methylene groups. With one 
exception (ethylene malonate (1)) all of the linear polyesters in which the 
ester groups are separated only by polymethylene chains, (CH^)*, are 
solids. Less symmetrical linear polyesters such as the alkylene phthalates 
are resins (transparent glasses or sirups). The solid polyesters when 
melted and then cooled yield opaque masses. These may be brittle and 
porcelain-like or soft and waxy. The waxy quality increases with the 
lengths of the polymethylene chains just as it does in the glycols and the 
acids from which the polyesters are derived. 

The problem of the more precise expression of the relationship between 
the structure and the properties of high polymers is complicated by the 
fact that some of the properties of this class of substances which are of 
the greatest practical importance and which distinguish them most sharply 
from simple compounds cannot be accurately measured and indeed are 
not precisely defined. Examples of such properties are toughness and 
elasticity. We have found these two properties in only one of the poly- 
esters which we have prepared. This polyester is hexamethylene car- 
bonate. Like pentamethylene and decamethylene carbonates, this ma- 
terial is crystalline; it has a fairly definite melting point (55-60) and it 
gives a sharp x-ray diffraction pattern (see Fig. 3). But the higher 
and lower members of this series separate from solution as powders, while 
hexamethylene carbonate separates in the form of rubbery flakes. It 
is not yet certain that this peculiar combination of properties is inherently 
associated with the molecular structure of this particular ester (i. e., with 
the linear union of a large number of the structural units CO O 
(CH 2 )e O ) rather than with some accidental features of its formation 
which may have been absent in the preparation of other members of the 
series, but the important problem of the relationship between chemical 
or physical structure and rubber-like properties is so poorly provided with 
material suitable for inductive argument that the discovery of a new syn- 
thetic and analytically homogeneous material exhibiting these properties 
is of some significance. So far as we are aware there is nothing in current 


theories which would lead one to expect that polymeric hexamethylene 
carbonate wonld be elastic. 

As indicated above, high polymers, like materials of low molecular 
weight, may show great variety in certain physical properties they may 
be either liquids or high-melting solids; they may show slight or very 
great solubility. The following properties, however, are inherently asso- 
ciated with the highly polymeric state. 

(1) Lack of Volatility. None of the polyesters which we have 
prepared can be volatilized as such even at very low pressures and high 
temperatures. Volatility diminishes continuously with increase in mo- 
lecular size due to the increase in intermolecular forces. It is probably 
true in general that compounds whose molecular cohesions lie above 75,000 
cal. (the heat of separation of a carbon-carbon bond) cannot be distilled. 
The calculated molecular cohesion (about 250,000 cal.) (5) of our poly- 
esters lies far above this value. 

(2) Viscosity in Liquid State. This would be expected to increase 
with the intermolecular forces and so with molecular size. All of the poly- 
esters which we have studied are extremely viscous in the molten state. 

(3) Micro-crystallinity in the Solid State. This has been referred 
to above. In this connection it is interesting to observe that considerable 
difficulty was experienced in preparing and keeping crystals of rnonomeric 
trimethylene carbonate sufficiently small to yield a powder diagram in 
an x-ray diffraction experiment. On the other hand, it has not been pos- 
sible to prepare macroscopic crystals of any polymeric ester. 

Trimethylene Carbonate. Among all the esters formed from bi- 
f unction al reactions which we have studied, trimethylene carbonate and 
ethylene oxalate (0) present the peculiar property of exhibiting reversible 
transformation between a rnonomeric and a polymeric form. These rnono- 
meric esters contain 6-membered rings. The polymerization of trimethyl- 
ene carbonate is brought about by heating and is catalyzed by a trace of 
potassium carbonate. The polymer is a perfectly colorless, transparent 
glass, which on long standing becomes opaque (crystallization). X-ray 
diffraction patterns of the rnonomeric and polymeric forms are shown in 
Figs. 1 and 2. The polymer has the same apparent analytical composition 
as the monomer. Its degree of polymerization as measured by molecular 
weight determinations in boiling benzene is 38-45. On heating in vacua, 
it distils and the distillate is found to be the rnonomeric form. Examina- 
tion of the literature shows that the property of undergoing reversible 
polymerization is common to many 6-rings containing ester linkages. 
Thus, the lactones of 6-hydroxyvaleric acid and of the hydroxyethyl ether 



of glycolic acid polymerize on heating (7) and so does glycolide (8). The 
action of heat on lactic acid leads to polylactyl lactic acids, and these on 

further heating are converted to 
the 6-ring lactide (9). 

The unique position occupied 
in this respect by 6-rings contain- 
ing an ester linkage is indicated 
by the following facts. No record 
of the polymerization of a 7-lac- 
tone (5-ring) is found. Our own 
attempts to polymerize ethylene 
carbonate (5-ring) were unsuc- 
cessful. The polyesters which we 
have prepared and in which the 
length of the chain of the] struc- 
tural unit is greater than 7 are not 
depolymerized on heating. The 
monomeric form of none of these 

Fig. 1. Monomeric trimethylene car- 
bonate (Mo radiation). The intensity of 
the numerous outer rings is weak and these 
do not appear in the reproductions. 

esters is known. 

In an attempt to force the de- 
polymerization of tetramethylene 
carbonate, it was heated above 300 at 0.9 mm. This caused it to de- 
compose with the evolution of consider- 
able gas. From the distillate was iso- 
lated a very small amount (about 1% of 
the calculated) of a crystalline solid 
having the analytical composition of 
tetramethylene carbonate. Molecular 
weight determinations indicated that it 

is a dimer: COO(CH 2 ) 4 OCOO(CH 2 ) 4 O. 
The absence of a monomer was not defi- 
nitely established, but it could only have 
been present in very small amount. 
Thus, the behavior of polymeric tetra- 
methylene carbonate is very different 
from that of its next lower homolog. 
In the self-esters of some of the 

higher cu-hydroxy fatty acids, HO(CH 2 )*COOH, both the monomeric lac- 
tones and the polyesters are known (10), The polyesters, however, are 


2. Polymeric trimethylene 
carbonate (Cu radiation). 



not depolymerized on heating, and the polyesters, not the lactones, are 
formed on heating the acids. 

These facts, together with others which will be discussed later, indicate 
clearly that polyesters formed by bifunctional reactions in which the 
length of the structural unit is greater than 6 atoms are the direct result of 
intermolecular condensation (C polymerization) (2). The formation of 
polymeric trimethylene carbonate from its monomer is obviously A poly- 
merization, and the same is true of the polymerization of lactides and of 

A fairly satisfactory explanation of these peculiarities may be found 
in the Mohr theory (11), which has proved adequate to explain all of the 
previously observed influences of structural features on the course of bi- 
functional reactions (2). In accordance 
with the terms of this theory, 5-rings 
will be readily formed and very stable 
since they are free from strain. The 
formation of seven-membered and larger 
rings by bifunctional reactions is very 
improbable, but such rings once they 
are formed will be quite stable, since 
they too are practically free from strain. 
Six-rings will be readily formed, but 
they present the possibility of strains 
which are absent or present only to a 
diminished degree in larger rings. Thus, 
the Mohr theory predicts the possible ex- 
istence of two cyclohexanes (cis and trans) 

but only one is known. However, where structural features would pre- 
clude the possibility of the intercon version of these two forms, as in dec- 
alin, both forms are known. It is concluded, therefore, that in simple 
6-rings, such as cyclohexane, equilibrium with reversible interconversion 
of the two forms exists. At every such interconversion the molecule must 
pass through a position of considerable strain. This picture presents the 
features necessary to rationalize the peculiar position of 6-membered ester 
rings. A polyester in which the recurring structural unit is a chain of 
six atoms would be readily converted into the corresponding monomeric 
6-ring by a process of ester interchange, as indicated below : 

Fig. 3. Hexamethylene carbonate 
(Cu radiation). 



If the chain of the structural unit were longer than 6 atoms, this reaction 
would be less probable for the same reason that the higher co-hydroxy 
fatty acids do not yield lactones, i. e., the number of points of rotation 
in the chain between carbonyl and alcoholic oxygen are so numerous that 
the probability of close intramolecular approach" of these groups is very low. 
The polymerization of the 6-ring esters seems also to involve ester inter- 
change. This process may involve the coalescence of two such rings, 
as indicated below 

C -- 0-Csr-C 


r/0 \ 

c o- 

V ^-R ^ x 

The resulting 12-ring might then coalesce with another six-ring, and so 
011, with the formation of very large rings. It is possible also that traces 
of water may intervene at some stage in the process and that the large 
molecules produced are open rather than closed chains. In any event 
the peculiar mobility of the 6-ring esters may be ascribed to the fact that 
these rings are subject to strains, and that the strains may be relieved by 
an ester interchange resulting in an enlargement of the molecule. 

The precise mechanism of the polymerization of the 6-menibered ester 
rings shares to a certain extent the obscurity which is common to most 
cases of A polymerization, but further attention is being devoted to this 
problem and the study is being extended to the corresponding amides. 
In this connection the analogy which exists between lactides and diketo- 
piperazines on the one hand, and polyesters and polypeptides on the 
other hand, is of especial interest because of its possible relationship to the 
structure of proteins. 

Dimeric Tetramethylene Carbonate. The dimeric tetramethylene 
carbonate referred to above deserves special mention. There can be little 
doubt that it is a 14-membered cyclic ester, and as such it represents a 
class of compounds of which few members are known. Other dimeric cy- 
clic esters have been obtained and will be described in later papers. 

Previous Work on the Carbonates of Dihydric Alcohols. Reference 
to ethylene carbonate has already been made. Carbonates of the three 
dihydroxybenzenes were prepared by Bischoff and v. Hedenstrom (12). 


The carbonate of catechol is apparently monomeric since it can be distilled. 
The w-phenylene carbonate is a non-volatile, brown, glassy material of 
uncertain melting point (197-202) and low solubility, and the corre- 
sponding para compound shows similar properties. These two compounds 
are evidently polymeric. 

Experimental Part 

Preparation of Carbonates. The carbonates here described were all 
prepared by ester interchange (alcoholysis) between the glycol and ethyl 
carbonate. This reaction proceeds very smoothly in the presence of an 
alkaline catalyst and may be forced to completion; the process need not 
be modified to take into account the solubility of the resulting ester, as 
is necessary when phosgene and the glycol are used as starting materials. 

Trimethylene Carbonate. A mixture of 60.8 g. (0.8 mole) of trimethylene glycol, 
114 g. (16% excess) of ethyl carbonate and a small cube of sodium was warmed in a 
Claisen flask until the sodium had completely dissolved. The flask was then immersed 
in an oil-bath at 130. During three hours the temperature was allowed to rise to 170. 
The evolved alcohol condensed and collected weighed 69.1 g. or 94% of the calculated. 

The oily residue was dissolved in benzene, washed twice with 20 cc. portions of 
water and dried over calcium chloride. After removal of the solvent, the residue was 
subjected to vacuum distillation. A small initial fraction, b. p. 120-135 at 20 mm., 
was unchanged glycol. The pure trimethylene carbonate then distilled fairly constantly ; 
b. p. 160-165 at 6 mm.; 135 at 4 mm., 105 at 0.2 mm.; weight 53 g., or 65% of the 
calculated. It completely crystallized in the receiver; recrystallization from absolute 
ether produced colorless needles, m. p. 47-48. This hygroscopic ester is very soluble in 
benzene, alcohol and water and slightly soluble in ether and ligroin. It decomposes on 
distillation at ordinary pressure. This ester gave a sharp x-ray powder diagram (Fig. 1) 
slightly scattered due to the difficulty of obtaining sufficiently small crystals. For 
analysis, see Table I. 

Polymerization of Trimethylene Carbonate. In the distillation of trimethylene 
carbonate it was frequently observed that the distilling residue was very thick and 
viscous while the warm distillate was comparatively thin and mobile. The distillation 
of 53 g. of trimethylene carbonate was interrupted while still incomplete. The distillate 
was monomeric trimethylene carbonate, m. p. 48. The undistilled residue (15.6 g.) 
was very viscous even when hot and it cooled to a colorless sticky resin. Molecular 
weight determinations showed it to be polymeric ; molecular weight in boiling benzene, 
found: 2390, 2320. 

This residue was distilled at 10 mm. It yielded 13.6 g. of monomeric trimethylene 
carbonate (observed b. p. 150-190) which completely crystallized on cooling; mol. wt., 
calcd. for monomeric trimethylene carbonate: 102. Found: (in boiling benzene) 
115, 118. 

The tarry residue (1.7 g.) remaining in the flask was insoluble in the common organic 

A trace (0.1% of finely powdered anhydrous potassium carbonate was added to a 
sample of the pure crystalline monomeric ester and the mixture was heated at 130. 


After shaking and heating for ten minutes the mixture suddenly became very viscous 
and a slight amount of gas was evolved. After five hours of heating the colorless product 
formed a clear glassy mass on cooling; mol. wt., found: (in boiling benzene) 4670, 
3880. For analysis see Table I. After standing for a week, this glassy polymer became 
opaque and x-ray examination showed it to be crystalline (Fig. 2). 

Under similar conditions (temp. 130; time of reaction, four hours) ethylene car- 
bonate did not polymerize. 

When polymeric trimethylene carbonate was heated to 210 at ordinary pressure, 
it was decomposed. Allyl alcohol was identified as one of the products of this reaction 
by its boiling point and by the melting point of its phenylurethan. This fact indicated 
decomposition according to the scheme 


O CH 2 CH 2 CH 2 O C > CH 2 =-CH CH 2 OH + CO 2 

The allyl alcohol isolated accounted for only about 25% of the ester decomposed. 
Higher boiling unsaturated materials present in the products of this reaction were not 

Tetramethylene Carbonate. This was prepared from 9.0 g. of tetramethylcne 
glycol, to which a small piece (2 mm. 3 ) of sodium had been added and 11.8 g. of ethyl 
carbonate. The mixture was heated to 120 and then the temperature was allowed 
gradually to rise to 160 during seven hours. The distillate was poured back several 
times to insure complete reaction of the ethyl carbonate. From the distillate was iso- 
lated 5.7 g. or 62% of the calculated amount of ethyl alcohol. 

The non- volatile reaction product was dissolved in benzene and the solution washed 
with water, dilute hydrochloric acid and water. It was dried with calcium chloride and 
the solvent was removed by distillation. On heating the residue in vacua there was ob- 
tained a very small amount of distillate boiling at 100-110 at 1 mm. This was chiefly 
unchanged tetramethylene glycol since it gave a di-p-nitrobeiizoate, m. p. 175. No 
further distillation could be effected when the bath was heated to 250 and the pressure 
reduced to 0.2 mm. 

The residue was purified by dissolving it in chloroform and precipitating it with 
absolute alcohol while cold. The oily precipitate soon crystallized to a cream-colored 
powder; weight 6.3 g. or 54% of the calculated; m. p. 55-59; very soluble in cold 
benzene, chloroform, acetone and acetic acid; insoluble in ether, alcohol and petroleum 
ether. It gave a sharp x-ray diffraction pattern. Before analysis it was dried for four 
days at 80 in high vacuum. For analytical data see Table I. 

Di--nitrobenzoate of Tetramethylene Glycol. Prepared from the glycol and the 
acid chloride in pyridine ; crystallized from boiling acetic acid ; m p. 175. 

Anal. Calcd. for Ci S H 16 O,N 2 : C, 55.67; H, 4.12. Found: C, 56.19, 56.00; H, 
4.31, 4.14. 

Thermal Decomposition of Tetramethylene Carbonate. An attempt was made to 
bring about the depolymerization of tetramethylene carbonate by heating it to a high 
temperature in vacua. The tetramethylene carbonate was prepared from 9.0 g. of glycol 
and 11.8 g. of ethyl carbonate in the same manner as described above. It was heated at 
0.9 mm. in a small Claisen flask by means of a m.tal bath. Very slight distillation 
occurred as the temperature of the bath was raised from 270 to 300. Between 300 and 
325 evolution of gas occurred and the pressure increased. From the distillate were is j- 


la ted 0.1 g. of crystalline solid and 2.8 g. of oil. The residue remaining in the distilling 
flask weighed 3 g. and was apparently identical with the polymeric tetramethylene 
carbonate described above; it melted at 55 and had the same physical appearance 
and solubility behavior. 

The crystalline distillate separated from alcohol in minute prisms melting at 175- 
176. Analysis and molecular weight determinations showed it to be dimeric tetra- 

r i 

methylene carbonate, CO O(CH 2 )4O CO O(CH 2 )4O. 

Anal. Calcd. for (C B H 8 8 )2: C, 51.71; H, 6.94; mol. wt., 232. Found: C, 51.77; 
51.80; H, 6.95; 7.02; mol. wt., in boiling ethylene chloride, 194, 231; mol. wt., in freez- 
ing benzene, 204. 

The liquid distillate had a pungent mint-like odor and readily absorbed bromine 
in acetic acid solution. It is a mixture and no chemical individuals have as yet been 
isolated from it. 

Pentamethylene Carbonate. This was prepared from 5.9 g. of ethyl carbonate and 
5.2 g. of pentamethylene glycol to which a small amount of sodium had been added. 
The mixture was heated for twelve hours at 120-160; 76% of the calculated amount 
of alcohol was found in the distillate. The residue after it had been freed from sodium 
was heated at 250 under 0.15 mm. The distillate (1.1 g.) boiled at 110-130 and con- 
sisted chiefly of unchanged glycol; di-/>-nitrobenzoate, m. p., 104-105. The non- 
volatile sirupy residue crystallized on standing to a hard waxy material; weight, 4.1 
g. or 63% of the theoretical. It was purified by precipitation with absolute alcohol 
from a cold chloroform solution as a granular and slightly colored powder; m. p., 44- 
46; very soluble in benzene, chloroform, acetone and acetic acid; insoluble in ether, 
alcohol and petroleum ether. Before analysis it was dried to constant weight in high 
vacuo. For analysis see Table I. 

Di--nitrobenzoate of Pentamethylene Glycol. From the glycol and the acid 
chloride in pyridine; crystallized from a mixture of benzene and alcohol; m. p., 104-105. 

Anal, Calcd. for C 19 H 18 O 8 N 2 : C, 56.71; H, 4.51. Found: C, 56.95, 57.02; H, 
4.59, 4.69. 

Hexamethylene Carbonate. This was prepared from 12 g. of ethyl carbonate and 
12 g. of hexamethylene glycol to which a small piece of sodium had been added. The 
mixture was heated from 130 to 170 during two hours; 86% of the calculated amount 
of alcohol was found in the distillate. The residue after the removal of the sodium 
was subjected to distillation in high vacuum. A very small amount of distillate was 
found to consist of unchanged glycol. The residue, which amounted to 10 g. or 67% of 
the calculated, solidified on cooling to a light colored, horny, tough mass. It was soluble 
in benzene, acetone and chloroform; insoluble in ether and alcohol; m. p. 55-60. A 
sharply defined x-ray diffraction pattern (Fig. 3) showed it to be crystalline. For 
analysis see Table I. 

Dicarbethoxyhexane. In an experiment similar to the above, ethyl carbonate and 
glycol were used in the ratio of 2 moles to 1 (24 g. of ester and 12 g. of glycol). The 
calculated amount (9.2 g.) of alcohol was collected. The residue after removal of sodium 
was subjected to vacuum distillation. It yielded 5.5 g. of distillate, b. p. 130140 at 
0.8 mm. The residue could not be distilled, and neither the distillate nor the residue 
could be induced to crystallize. No ethyl carbonate was found in the distillate. The 
distillate was dicarbethoxyhexane; rc 2 ,? 1.4310; dig 1-065; d\l 1.056. 


Anal. Calcd. for Ci2H 2 2O 6 : C, 54.94; H, 8.41; mol. wt., 262. Found: C, 55.00; 
H, 8.41; mol. wt. in boiling benzene, 236, 240, 245. 

The oily residue was very soluble in alcohol and ether. Its analytical composition 
corresponded with the formula C 2 H 5 O CO O(CH 2 ) 6 O CO O(CH 2 ) 6 O CO O- 
(CH 2 ) 6 CO OC 2 H 6 . 

Anal. Calcd. for C 2 6H 4 6Oi 2 : C, 56.73; H, 8.36; CO 2 on hydrolysis, 33.1%; mol. 
wt., 532. Found: C, 56.77, 57.00; H, 8.41, 8.53; CO 2 , 34.15; mol. wt. in boiling ben- 
zene, 573, 549, 528. The CO 2 was determined by saponifying with alcoholic sodium 
hydroxide and weighing the sodium carbonate formed. 

Carbonate of Diethylene Glycol. This was prepared in 43% yield by the method 
used in the preparation of the other carbonates. It was a light colored sirup which 
could not be induced to crystallize; insoluble in alcohol and ether, quite soluble in 
acetone and benzene, very soluble in chloroform and hot ethyl acetate. On standing it 
appeared to decompose to some extent into acetaldehyde and carbon dioxide and for 
this reason it could not be dried to constant weight. For analysis see Table I. 

Decamethylene Carbonate. This was prepared in 75% yield by the method used 
for the preparation of the other carbonates. It was soluble in benzene and chloroform 
but insoluble in alcohol. It was purified by precipitation from chloroform with alcohol 
in the cold. The oily product gradually solidified to a cream-colored powder; m. p. 
55; very soluble in chloroform, slightly soluble in ether, benzene, acetone and acetic 
acid; insoluble in alcohol and petroleum ether. This powder gave a sharply defined 
x-ray diffraction pattern. Before analysis it was dried to constant weight in high vacuum 
at 90. For analysis see Table I. 

/>-Xylylene Carbonate. This was prepared from />-xylylene glycol and ethyl car- 
bonate as in the preparation of the other carbonates. The residue after completion of 
the ester interchange was insoluble in hot benzene, absolute alcohol, chloroform and 
carbon tetrachloride ; slightly soluble in acetic acid. It dissolved more or less completely 
in ethylene chloride, anisole and dioxane. It was thoroughly triturated with water and 
with cold alcohol to remove sodium and excess glycol and ethyl carbonate. The amount 
of the white solid remaining corresponded to 82% of the calculated. It was separated 
into two fractions by extraction with ethylene chloride. 

(a) Soluble Fraction. This was precipitated with ether from ethylene chloride 
in the form of white flocks; m. p. 137-138. For analysis see Table I. 

(b) Fraction Insoluble in Ethylene Chloride. This was insoluble in the common 
organic solvents; m. p. 177-185 (rather indefinite). For analysis see Table I. 

Attempted Preparation of Methylene Carbonate. Silver carbonate (81 g.) was 
heated with methylene bromide (51 g.) in dry toluene solution. Formaldehyde and 
carbon dioxide were formed and free silver was produced. From the reaction mixture 
there was isolated only a small amount of oil with a sweet odor probably methylene car- 
bonate. This liberated formaldehyde on heating: CH 2 CO 3 -* H 2 CO + CO 2 . 

We are indebted to Dr. A. W. Kenney and Mr. Henry Aughey for the 
x-ray diffraction pictures, to Mr. W. H. Taylor for many molecular- 
weight determinations and to Mr. G. A. Jones for determinations of carbon 
and hydrogen. 



The following compounds have been prepared by the action of the ap- 
propriate glycols on ethyl carbonate : trimethylene carbonate, tetramethyl- 
ene carbonate, pentamethylene carbonate, hexamethylene carbonate, deca- 
methylene carbonate, diethylene carbonate and p-xylylene carbonate. 
Their properties are described. In accordance with a generalization al- 
ready set forth, ethylene carbonate and trimethylene carbonate are mono- 
meric and the other carbonates are polymeric. Trimethylene carbonate 
undergoes reversible A polymerization. A stable dimeric form of tetra- 
methylene carbonate has been prepared by thermal decomposition of its 
usual polymeric form. Hexamethylene carbonate is tough and elastic. 

Bibliography and Remarks 

(1) Carothers and Arvin, Journ. Ant. Chem. Soc., 51, 2560 (1920). 

(2) Carothers, ibid., 51, 2548 (1929). 

(3) Nemirowski, J. prakt. Chem., (2} 28, 439 (1883); Chem. Zenlr., 23 (1884). 

(4) Vorlander, Ann , 280, 186 (1894). 

(5) Meyer and Mark, Z. angew. Chem., 41, 943 (1928). 

(6) See Bischoff, Ber., 40, 2803 (1907). Our own studies of alkylene oxalates will be described 

(7) Hollo, Ber. t 61, 895 (1928). 

(8) Bischoff and Walden, Ann., 279, 45 (1894). 

(9) Dietzel and Krug, Ber., 58, 1307 (1925). 

(10) Ruzicka and Stoll, Helv. Chim. Acta, 11, 1159 (1928); Lycan and Adams, Journ. Am. Chem. 
Soc., 51, 625 (1929); Chuit and Hausser, Helv. Chim. Ada, 12, 463 (1929); Lycan and Adams, Journ 
Am. Chem. Soc., 51, 3450 (1929). 

(11) Mohr, J. prakt. Chem., 98, 348 (1918). 

(12) Bischoff and v. HedenstrSm, Ber., 35, 3431 (1902). 

IV. Ethylene Succinates* 

This contribution contains a more thorough investigation of ethylene suc- 
cinate, its formation, structure and bearing on the association theory of 
high polymers. 

Neutral ethylene succinates are prepared by heating succinic acid with 
excess glycol at 180 until the distillation of water stops: they are chains 
of about 3000 m. w. 

Acidic ethylene succinates are obtained if an excess of succinic acid is ap- 
plied. The reactants are heated to 180 until no more water is evolved and 
then the temperature is raised to 200-240 for one to five hours to make 

* W. H. Carothers and G. L. Borough; Journ. Am. Chem. Soc., 52, 711-21 (1930); 
Contribution No, 20 from the Experimental Station of E. I. du Pont de Nemours and 

Received August 7, 1929. Published February 6, 1930. 


reaction as complete as possible. The products are always acidic. 
The polymerization degree varied from 6 to 23. Molecular weights were 
determined by the boiling point method and by titration of the end-standing 
acid group. Excellent agreement was obtained between the two methods* 
and the theoretical values for the above mentioned polymerization degrees. 
The mecJianism of the formation of chain polymers is investigated and the 
possibility of an association of a number of 8-rings through exceptionally 
strong van der Waals forces discussed. 
Carothers expresses himself in favor of the main valence chain theory. 

Ethylene succinate as a typical example of a condensation polymer (1) 
has been submitted to further study with the view of gaining more in- 
formation in regard to its structure. The presence of the structural unit, 
(1) I, in this ester has been established by the methods used in its synthe- 
sis (2, 3), by its chemical behavior and by its analytical composition (2, 3). 
O CH 2 CH 2 O OC CH 2 CH 2 CO (I) 

Of the various polymeric forms described in this paper, the apparent 
molecular weights indicate average values of 6 and 23, respectively, for 
the number of structural units contained in each molecule of the lowest 
and highest polymers. It is assumed that these units are joined together 
in a linear fashion by real primary valences, as in II. Since it is very im- 
probable that there are free valences at the ends of the resulting chains, 
the problem of the structure of these polymers resolves itself into finding 
whether the end valences are mutually saturated with the formation of 
very large rings or are saturated by univalent groups of some kind. 

~O~-(CH 2 ) 2 O OC(CH 2 ) 2 CO O (CH 2 ) 2 O OC (CH 2 ) 2 CO O 

(CH 2 ) 2 O OC (CH 2 ) 2 CO , etc. (II) 

Any of the reactions by which this ester is prepared offers the formal 

possibility of establishing either rings (closed chains) of 8, 16, 24, 

(S)n members or of open chains corresponding to each of these rings. The 
observed molecular weights indicate the presence of 50 to 180 atoms in 
each chain. This fact and the physical properties of the ethylene succi- 
nates indicate the absence of more than traces of rings of less than 32 mem- 
bers. Since the same theoretical considerations (1, 4) which predict the 
improbability of the formation in bifunctional reactions of rings of 8 to 32 
members apply a fortiori to still larger rings, an open chain seemed much 
more probable than a cyclic structure, and our efforts were directed to- 
ward the detection of the univalent groups which would constitute the ends 

* Titration values were 1016, 1344, 1795 and 3417; boiling point values 1070, 
1380, 1582 and 3110; theoretical values 982, 1414, 1847 and 3432, respectively. 


of the open chains. We have proved the presence of such groups, and the 
open-chain structure for the polymeric ethylene succinates is clearly es- 

Neutral Ethylene Succinate. This ester is prepared by heating suc- 
cinic acid with excess glycol at 180 until the distillation of water ceases 
and then removing the excess glycol by heating in high vacuum at 200- 
250. The ester is purified by crystallization and melts at 102 (5). 

Its apparent molecular weight is about 3000. 

If this ester is an open chain, its method of formation and its observed 
molecular weight would lead one to assign to it the structure III, and it 

H [O (CH 2 ) 2 O OC -(CHj)r- CO ] O (CH 2 ) 2 OH (III) 
should therefore show the reactions of a dihydric alcohol. The possibility 
of applying to it many of the typical reactions for the detection of pri- 
mary alcohol groups was excluded for one reason or another. Thus the 
presence of ester linkages excluded any reactions which might result to 
hydrolysis. The fact that the compound is hygroscopic and insoluble in 
ether made it useless to attempt to apply any such methods as the Zere- 
witinoff. The compound crystallizes from acetic anhydride and is appar- 
ently unchanged even on long boiling in this solvent in the presence of 
catalysts, but in so large a molecule acetylation might occur without 
any apparent change in physical properties, and the amount of acetic acid 
which would be liberated on hydrolysis of even a completely acetylated 
product would be so small as to make its estimation difficult. The ester 
reacts very slowly and incompletely with phosphorus tribromide. 

Six grams of the ester heated for five hours in 30 cc. of boiling chloroform with 
6 cc. of phosphorus tribromide after thorough washing and repeated crystallization was 
found to contain 0.58% Br. 

The ester was also recovered apparently unchanged after treatment 
with phenyl and naphthyl isocyanates under various conditions. These 
failures appear to be due to a reluctance of the hydroxyl groups to react, 
and they are not altogether surprising in view of the diminished reactivity 
which is frequently associated with increased molecular size. 

Reaction occurred completely in the expected sense with succinic and 
with ^-bromobenzoic anhydrides at elevated temperatures. These reac- 
tions led to the dibasic acid which agreed in properties and composition 
with Formula IV, and to a ^-bromobenzoyl ester which agreed in com- 
position with Formula V. 

HO OC (CH 2 ) 2 CO [0 (CH 2 ) 2 OC (CH 2 ) 2 CO ] 28 OH (IV) 
/>-BrC 6 H 4 CO [ O (CH 2 ) 2 O OC (CH 2 ) 2 CO ] 22 O (CH 2 ) 2 O OC 

C 6 H 4 Br-/> (V) 


Preparation of IV. Twelve grams of neutral ethylene succinate was heated with 
3 g. of succinic anhydride at 175-180 for three hours. The reaction mixture was 
washed three times with hot water, dried, precipitated from chloroform with ether and 
crystallized 6 times from acetone. Its properties identified it as the dibasic acid IV, 
which would be formed by the reaction of each hydroxyl group of III with one molecule 
of succinic anhydride. 

The ester III melted at 102, had an apparent molecular weight of 
about 3000, was neutral and did not form a sodium salt. IV prepared 
from III melted at 98 and had an apparent molecular weight of 3110 
(method of Menzies and Wright in ethylene chloride, observed values 
3170, 3040) and a neutral equivalent of 1708. The action of sodium bi- 
carbonate on IV led to a sodium salt which melted at 104 and contained 
1.23% of sodium. The analytical data are given in Table I. 



M. p., 

C. equiv. 

Mol. wt. calcd. 
from neut. 

Mol. wt. X = 
found struc- 
in boiling tural 
ethylene units per 
chloride molecule 

Mol. wt. 























Anal calcd , 




1708 3417 

Anal found, % 
C H 




Sodium sj 
. Calcd. 



., sodium 

p., C. 

48.86 5 


48 42 48. 


5.57 5. 





49.22 5 


48.84 48 


5.74 5, 





49.38 5 


49.34 49 


5.86 5, 





49.66 5 


49.41 49. 


5.70 5. 





Preparation of Di--bromobenzoyl Derivative of III. Two and one-half grains of 
III was heated for five hours with 0.75 g. of -bromobenzoic anhydride at 175-185. 
The reaction mixture was dissolved in chloroform and precipitated by ether, washed 
with hot water, dried and precipitated from acetone by ether. It melted at 93 . Analy- 
sis showed it to be the expected di--bromobenzoyl derivative, V, of III. 

Anal. Calcd. for V as CmHmO^Bra : C, 49.30; H, 5.20; Br, 4.84. Found: C, 
49.47, 49.39; H, 5.59, 5.60; Br, 4.84, 5.01, 4.88, 4.91. 

Acidic Ethylene Succinates. If glycol and succinic acid would react 
completely in the proportions in which they are brought together, it 
would be possible to prepare chains of various lengths by using the glycol 
and acid in various ratios. In a number of experiments various excess 
amounts of succinic acid were allowed to react with glycol. The reactants 
were heated together at 180 until no more water was evolved and the 


temperature of the bath was then raised to 200-240 for one to five hours 
to make the reaction as complete as possible. Only traces of acid and 
glycol appeared in the distillates. The residues were always acidic and, 
when the excess of acid was large, always contained some unchanged suc- 
cinic acid. Moreover, products of high molecular weight could be iso- 
lated from these residues even when the ratio of acid was as high as 2:1. 
This showed that under the conditions of these experiments complete con- 
trol of the length of the chain produced was not .possible by adjustment of 
the ratios of the reactants. On the other hand, this factor had some in- 
fluence in controlling the lengths of these chains, and by making use of 
such control as it offered, together with fractional crystallization, it was 
possible to isolate samples of material representing chains of various 
lengths, each of which was quite homogeneous in its physical behavior. 
A comparison of the analytical data and molecular weights for these acidic 
ethylene succinates clearly indicates that their structure may be repre- 
sented by VI, in which x has various average values for different fractions. 

HO OC (CH 2 ) 2 CO [ O (CH 2 ) 2 O OC (CH 2 ) 2 CO ]* OH (VI) 
Via, x = 6 VIb, x - 9 Vic, x = 12 

Preparation of VI, a, b and c. Ethylene glycol, 41 g., and succinic acid, 93.5 g. 
(20% excess), were heated at 200-210 for four hours, and the residual water was then 
removed as completely as possible by heating in vacua. The residue was dissolved in 
chloroform and precipitated with ether. The powder which resulted from the drying 
of this precipitate weighed 92.2 g. and melted at 87.5-90. It was extracted several 
times with boiling water. The residue (59 g.) was dissolved in chloroform and pre- 
cipitated by ether. It melted at 90. This constituted fraction Vic. The hot aqueous 
extracts on cooling deposited 11 g. of solid which after drying melted at 73-74 . 

In another similar experiment the acid and glycol were heated for two hours at 
190-210 under ordinary pressure, and then immediately extracted several times with 
boiling water (3 X 300 cc.). The residue amounted to 10.5 g. After solution in 
chloroform and precipitation by ether, it melted at 82-83. This constituted fraction 
VIb. On cooling, the aqueous extract deposited considerable white solid. This after 
precipitation from chloroform by ether melted at 73. This material together with 
the fraction from the previous experiment melting at the same temperature constituted 
fraction Via. By evaporation of the aqueous extracts from this experiment there was 
obtained another fraction melting at 86 . The analytical data for this fraction indicated 
that it lay between the 73 and the 82 fractions. 

Fraction IV. was prepared from III in the manner already indicated. 

The analytical data for these fractions (see Table I) indicate that we 
are here dealing with a polymeric series in which the average values for the 
degree of polymerization vary from 6 up to 23. These fractions closely 
resemble each other in physical properties: they have the same appear- 
ance under the microscope, and they show similar solubility relations. All 


the fractions are soluble in cold chloroform and in hot 50% alcohol. The 
lowest fraction is quite soluble in hot water and the higher fractions only 
very slightly soluble. The melting points rise with increasing degree of 

All these fractions form sodium salts on treatment with sodium bicar- 
bonate, and these resemble very closely in properties the acids from which 
they are derived. Thus they are readily soluble in cold chloroform and in 
warm acetone. They are only slightly soluble in cold water. All of them, 
however, are readily soluble in warm water, and in this respect the higher 
members are sharply differentiated from the corresponding acids. The 
magnitude of the change in properties which is produced by the transfor- 
mation of the slightly polar carboxyl group into the completely polar 
sodium salt will be expected to vary with the size of the chains to which 
these groups are attached, but we found it somewhat surprising, never- 
theless, that these sodium salts should melt only a few degrees above the 
melting points of the corresponding acids. This difference diminishes con- 
tinuously with the increase in length of the chain and in the highest poly- 
mer, IV, amounts to only 6. 

The general plan of the structure of these acid esters is clearly estab- 
lished by the formation of the sodium salts. Attempts to prepare other 
derivatives from acids met with little success. Treatment with thionyl 
chloride and with phosphorus pentachloride under various conditions 
furnished products which contained chlorine, but it could not be estab- 
lished that these materials were really acid chlorides. Attempts to pre- 
pare amides by heating the sodium salts of the acids with ^-toluidine hy- 
drochloride led to the formation of N-/>-tolyl succinimide. Heating the 
sodium salts with />-bromophenacyl bromide led to the formation of de- 
rivatives, but these contained less than the calculated amount of bromine. 
Thus the sodium salt of Via led to a derivative, m. p. 73, containing 
8.66% of bromine, while the calculated value for a di-(^-bromophenacyl) 
ester of Via is 11.61% Br. Similarly, the sodium salt of Vic led to a p- 
bromophenacyl derivative containing 4.12 instead of the calculated 7.13% 
of bromine. 

Mechanism of the Formation of Ethylene Succinate. The formation 
of an open-chain polyester of the type exemplified by II might occur in 
either one of two ways. (1) The cyclic monomeric ester VII might first 
be formed, and this might then undergo A polymerization (self-addition) 
with the formation of II. (2) II might be formed directly from the acid 
and the glycol (C polymerization) by a series of successive reactions. 
That polyesters of this general type may be formed by the first mecha- 


nism is clearly indicated by the fact (6) that the monomeric form of tri- 

methylene carbonate can be isolated by the distillation of its polymer, 

and, by heating the monomer can be changed to the poly- 

vCO Ox meric form. This is also true of other polyesters, but, so 

CH 2 CHi far as our information extends now, only of those in which 

CH CH *ke monomer is a 6-membered ring. The 5-ring esters 

\CO O/ ^ no ^ Ply mer i ze ; the polyesters whose monomers would 
Vii be larger-than-6-rings are not depolymerized; and, al- 

though monomers of this type are known (7) and are 
stable, none of them has ever been prepared by a bifunctional reaction. 
The general theory underlying these facts has already been discussed (1, 6). 

The evidence that ethylerie succinate is formed by Mechanism 2 (C 
polymerization) rather than 1 is fairly conclusive. Experimental evidence 
in support of Mechanism 2 may be found in the fact that the molecules 
of ethylene succinate are open chains with functional groups at the ends. 
The supposition that the monomeric ring VII is first formed involves the 
tacit assumption that 8-rings are readily formed, but are unstable, whereas 
fact and theory alike indicate that such rings are formed only with great 
difficulty, but are stable when they have been formed. We have made a 
good many attempts to isolate the monomeric 8-ring, VII. All of these 
attempts have failed, and all of the observations which we have made are 
best interpreted on the assumption that the long chains are built up by a 
series of successive reactions and that rings are not formed at any stage of 
the process. Some light is thrown at one or two points on the details of the 
mechanism of the reaction by the following observations. 

When glycol was allowed to react with succinic acid in the proportions 
of 1 to 2 moles, the product was found to be a mixture composed chiefly 
of unchanged succinic acid and polyester of fairly high molecular weight. 
The compound, HO OC (CH 2 ) 2 CO O (CH 2 ) 2 O OC (CH 2 ) 2 
CO OH, which might be expected to form under these stoichiometrical 
conditions was not found. 

Neutral ethylene succinate was prepared by heating the acid with excess 
glycol and, after some time, distilling off the excess glycol as completely 
as possible at high temperature and under greatly reduced pressure. Under 
these conditions the formation of some di-/3-hydroxy ethyl succinate, VIII, 
might be expected. This ester was in fact formed, and it distilled 

HO (CH 2 ) 2 O OC~(CH 2 ) 2 CO O (CH 2 )2~OH (VIII) 

out of the reaction mixture when this was heated in vacua. It was obtained 


in a state of approximate purity by redistillation in a carefully cleaned all- 
glass apparatus under high vacuum. 

Di-(/3-hydroxyethyl)-succinate. vSixty-two grams of ethylene glycol (1 mole) and 
39.3 g. of succinic acid ( l / 9 mole) in a pyrex Claisen flask fitted with ground-glass stop- 
pers and provided with a receiver were heated for six hours by means of an oil-bath at 
174-180; 8.8 g. of water, b. p. 100-102, w 2 D 8 1.3323, collected in the receiver. The 
residue was heated in vacuo finally to a temperature of 250 at 0.015 mm., until dis- 
tillation ceased. The residue was a pale yellow viscous liquid which readily solidified 
on cooling. It was dissolved in 100 cc. of warm chloroform and precipitated as a 
powder by the addition of 250 cc. of benzene. After drying, this powder weighed 36 g. 
(0.25 g. equivalent). Its instantaneous melting point was 98 and its slow melting 
point 105.5 . The distillates were redistilled and yielded 9.97 g. or 0.648 mole of water, 
33.9 g. or 0.548 rnole of glycol and 14.5 g. or 0.07 mole of di-(/3-hydroxyethyl) succinate. 
The total esters (0.7 mole + 0.25 g. equivalent) accounted for 0.32 mole of the 0.333 
mole of succinic acid used, and these esters plus the glycol recovered accounted for 
94.6% of the glycol used, while the water isolated was 87.5% of the calculated amount. 

The di-/3-hydroxyethyl succinate was never isolated in a state of purity. When 
distilled in a carefully cleaned all-glass (pyrex) Claisen flask, it boiled at 176-180 at 
0.001 mm. as a colorless viscous liquid, leaving only a trace of residue. 

Anal. Calcd. for CuHi 4 O 6 : C, 46.58; H, 6.88. Found: C, 45.76, 45.70; H, 
6.86, 6.95. 

Treatment with ^-nitrobenzoyl chloride in pyridine yielded a di-(/>-nitrobenzoate) 
as white needles from alcohol; m. p. 90-91 . 

Anal. Calcd. for C 2 2H2oOi2N 2 : C, 52.35; H, 3.97. Found: C, 52.36, 52.36; 
H, 4.19, 4.14. 

The di-phenylurethan formed by the action of phenylisocyanateondi-(/3-hydroxy- 
ethyl) succinate and crystallized from a mixture of benzene and petroleum ether melted 
at 113. 

Anal. Calcd. for C 2 2H 2 4O 8 N 2 : C, 59.45; H, 5.41. Found: C, 59.40, 59.56; H, 
5.53, 5.52. 

When heated slowly in vacuo di-/3-hydroxyethyl succinate was com- 
pletely converted into neutral (polymeric) ethylene succinate and ethylene 
glycol, the former appearing in the distillate and the latter remaining in 
the distilling flask. This process evidently involves an ester interchange: 
glycol is eliminated between two molecules of the dihydroxy ester with 
the formation of a new dihydroxy ester containing two of the structural 
units of ethylene succinate. Repetition of this process finally results in 
such a molecule as III. It is evident that so long as one of the products 
of a possible ester interchange can be eliminated in this way, no merely 
stoichiometrical factors can set up a limit to the length of the molecules 
which might be produced. Nevertheless a fairly definite limit exists: 
polyintermolecular esterification has never led us to molecules of greater 
average length than about 200 atoms. No doubt various factors are in- 


volved in this point. There can be no question that the reactivity of func- 
tional groups diminishes with the size of the molecules which contain them. 
It is apparently this factor which accounts for the failure of the ester III to 
react with many of the typical reagents for hydroxyl groups. Moreover, 
as Staudinger has frequently emphasized (8), the thermal stability of mole- 
cules must diminish with increase in their size. These two factors act in 
opposition. To force the completion of the reaction between succinic acid 
and glycol we have used high temperature (250) to increase reaction 
velocity and high vacuum to remove the water as completely as possible. 
The ester produced under these conditions has a molecular weight of about 
3000. It is interesting to observe that in the thermal polymerization of 
styrene the product formed at 250 also has an apparent molecular weight 
of about 3000, while the polystyrenes formed at lower temperatures have 
much higher molecular weights. And so also the A polymerization of tri- 
methylene carbonate described in the previous paper (6), although it also 
involves an ester interchange, leads to a polyester with a molecular weight 
considerably above 3000, since the reaction consists merely in self-addition 
and proceeds rapidly at 100. 

Dimeric Ethylene Succinate. Tilitschejew (9) has reported the isola- 
tion of a new ethylene succinate from the volatile materials formed by 
heating the product of the action of succinic acid on glycol (m. p. 88-89) 
to 340-390 under 3-4 mm. pressure. The new product was distinguished 
from the usual form not only by its different melting point (129-130), but 
by its definite macrocrystallinity. Cryoscopic data on acetic acid solu- 
tions of the new ethylene succinate indicated a double formula and, since 
Vorlander had already reported (10) that the old form was dimeric, Tilit- 
schejew regarded his new ester as an isomer of the old. It has now been 
established that the usual form of ethylene succinate is not dimeric, but 
much more highly polymeric; the new compound cannot, therefore, be 
simply an isomer of the old. Aside from this, however, repetition of Tilit- 
schejew's experiments has completely verified the correctness of his claims 
as to the nature of the new compound. It crystallizes in thin plates melt- 
ing sharply at 130. The analytical data clearly indicate that it is dimeric 
ethylene succinate. 

Anal. Calcd. for (CeH 8 O 4 ) 2 : C, 50.00; H, 5.50; mol. wt., 288; saponification 
equivalent, 72. Found: C, 49.85, 50.00; H, 5.60, 5.57; mol. wt., in boiling ethylene 
chloride, 302, 299; in freezing benzene 279; saponification equivalent, 71.97, 71.95, 

Attempts to partially saponify this ester were unsuccessful a part of 
the ester was recovered unchanged and the remainder was degraded to 


sodium succinate and glycol. Nevertheless, it seems fairly certain that 
this compound is the 16-membered ring, IX. No alternative formula seems 

(CH 2 ) 2 ~0OC (CH 2 ) 2 CO O (CH 2 ) 2 O OC (CH 2 ) 2 COv 

> (IX) 
(CH 2 ) 2 CO O (CH 2 ) 2 O OC (CH 2 ) 2 CO O (CH 2 ) 2 (X 

plausible. It is very improbable that this compound is present as such 
in the polymeric ester from which it is prepared. The dimer is quite 
soluble in hot absolute alcohol; the neutral polymer is quite insoluble. 
Continuous extraction of the polymer with hot absolute alcohol resulted 
in the solution of only a very small amount of material and this had the 
properties of the polymer not of the dimer. The dimer must therefore 
be formed during the process of thermal decomposition. The trans- 
formation of polymeric trimethylene carbonate into mononieric ethylene 
carbonate described in the previous paper (6) proceeds smoothly and prac- 
tically quantitatively at about 200. The formation of the dimeric 
tetramethylene carbonate, described in the same paper, and of dimeric 
ethylene succinate occurs only at a much higher temperature. Large 
amounts of gaseous and liquid products are formed as well as considerable 
carbonaceous residue. The yields of the dimers are quite small, e. ., 
3-4% of the theoretical under the best conditions (Tilitschejew, however, 
reports 5.5%). In these respects the reaction resembles that used by 
Ruzicka for the preparation of the large cyclic ketones (11). As yet we 
have not succeeded in isolating any monomeric ethylene succinate from 
the products of this reaction. 

Ethylene Succinate and the Association Theory of High Polymers. 
The discussion in this and the previous papers will, it is hoped, have 
amply demonstrated the adequacy of the ordinary structural theory of 
organic chemistry to deal with the polyesters, a fairly complicated class 
of high polymers. 

A possible explanation of the structure of the polyesters which has 
not yet been considered is the following. The chemical unit or molecule 
of ethylene succinate is the monomeric 8-ring, VII. Because of the great 
strains in this structure, or for some other reason, it exhibits exceptionally 
strong residual or lattice forces, so that the osmotic unit becomes an 
aggregate of a great many of these ultimate chemical units. There is 
such a complete lack of any general theoretical justification for this view 
that there would be no need to consider it in connection with the poly- 
esters were it not that in the field of natural polymers such as cellulose, 
rubber, etc., in various slightly differing forms it has been defended at 
great length by so many investigators (12). 


Among the many facts which are quite incompatible with any association 
theory of the structure of polyesters, the following deserve special mention. 

(1) All of the known cyclic esters containing larger-than-6-rings are 
stable substances with definite properties corresponding with their simple 
formulas. They do not show any tendency to associate in solution. 

(2) The polymeric esters corresponding to these monomers show no 
tendency to dissociate. 

(3) In the polymeric ethylene succinates the chemical and osmotic 
units have been shown to be identical within the limits of experimental 
error, i. e., ebullioscopic data give the same values for the molecular weights 
that are given by the determination of hydroxyl or carboxyl. 

The association theory is not clearly enough defined to permit any 
more crucial tests than these, but the following experiment at any rate 
agrees with these in the conclusions to which it leads. 

If two different association polymers (A)n and (B)m are mixed in 
solution or in the liquid state so that they constitute a single phase, then 
since (A)n < > nA and (B)m < > mB, the resulting mixture should be com- 
posed at least in part of (AB) P . 

Ethylene sebacate (m. p. 78) and ethylene succinate (m. p. 103) 
were melted together and thoroughly mixed. After cooling, the mass 
was extracted with benzene, which is a solvent for the sebacate but not 
for the succinate. The residue melted at 103 (unchanged succinate). 
To the benzene solution petroleum ether was added; the precipitated 
solid melted at 78-79 (unchanged sebacate). An ester was prepared 
by the action of glycol on equivalent amounts of succinic and sebacic 
acids. This was quite different in its properties from either the succinate 
or sebacate and it was homogeneous in its solubility behavior (m. p. 
38-40; mol. wt. 1540). This demonstrates the existence of a mixed 
polymer and the absence of any reversible relationship of association be- 
tween it and the two corresponding simple polymers. 

In the 6-ring esters a reversible relationship exists between the mono- 
meric and polymeric forms, and some of the statements made above do 
not apply to these polymers. But it is possible, nevertheless, to account 
satisfactorily for the behavior of these esters without the assumption of 
any special or peculiar kinds of valence (6) . 

The view that the ordinary structural theory of organic chemistry 
is adequate to deal with high polymers has been now for several years 
ably defended by Staudinger and his collaborators (13), and recently 
the same view has been applied in a brilliant fashion to such natural poly- 
mers as cellulose, rubber, silk fibroin, etc., by Meyer and Mark (14). So 


far as the minor differences (15) in the views of these two groups of investi- 
gators are concerned, our own experiments on polyesters incline us to favor 
those of Staudinger. That is, we can find no real objection to referring to 
primary valence chains as molecules, and among the polyesters these mole- 
cules are experimentally identical with the osmotic units of their solutions. 

The writers are indebted to Mr. W. H. Taylor for the molecular weight 
determinations and to Mr. G. A. Jones for the determinations of carbon 
and hydrogen. 


The polymeric ethylene succinate previously described is shown to be 
a long chain made up of the recurring unit O (CH 2 )2 O CO 
(CH 2 ) 2 CO and bearing hydroxyl groups at its ends. Acidic ethylene 
succinates made up of similar chains of various lengths and bearing car- 
boxyl groups at their ends have been prepared. Molecular weight deter- 
minations of these esters based on ebullioscopic measurements agree with 
those based on chemical evidence (estimation of hydroxyl or carboxyl). 

A study of the ethylene succinate prepared by Tilitschejew verifies 
his claim that it is dimeric, and this ester is undoubtedly a 16-membered 

Bibliography and Remarks 

(1) Carothers, Journ. Am. Chem. Soc., 51, 2548 (1929). 

(2) Vorlander, Ann., 280, 167 (1894). 

(3) Carothers and Arvin, Journ. Am. Chem. Soc., 51, 2560 (1929). 

(4) Mohr, J. prakt. Chem., 98, 348 (1918). 

(5) Capillary tube melting point. See Carothers and Arvin, Journ. Am. Chem. Soc., 51, 2560 
(1929), for a description of the peculiar melting point behavior of this compound. 

(6) Carothers and van Natta, J. Am. Chem. Soc., 52, 314 (1930). 

(7) Ruzicka and Stoll, Helv. Chim. Acta, 11, 1159 (1928). 

(8) See Staudinger, Ber., 59, 3019 (1926). 

(9) Tilitschejew, J. Russ. Phys.-Chem. Soc., 57, 143-150 (1925); Chem. Zentr., I, 2667 (1926). 

(10) Vorlander, Ann., 280, 167 (1894). 

(11) Ruzicka and co-workers, Helv. Chim. Acta, 9, 230, 249, 339, 389, 399, 499, 715, 1008 (1926); 
10, 695 (1927); 11, 496, 670, 686, 1159, 1174 (1928). 

(12) See for example Bergman, Knehe and Lippmann, Ann., 458, 93 (1927); Hess, Trogus and 
Friese, ibid., 466, 80 (1928); Schlubach and Eisner, Ber., 61, 2358 (1928); Pummerer, Nielsen and 
Gundel, ibid., 60, 2167 (1927). 

(13) Staudinger and co-workers, Helv. Chim. Acta, 5, 785 (1922); 7, 23, 842 (1924); 8, 41, 65, 67 
(1925); 9,529(1926); 11,1047,1052(1928); Ber., 53, 1073 (1920); 57,1203(1924); 60,1782(1927); 
59,3019(1926); 61,2427(1928); 62,241,263,442(1929); Ann., 447, 97, 110 (1926); 467,73(1928); 
Z. physik. Chem., 126, 425 (1927); Kautschuk, 237 (1927); Z. angew. Chem., 42, 37, 67 (1929); Z. 
Krist., 70, 193 (1929). 

(14) Meyer and Mark, Ber., 61, 593, 1932, 1939 (1928); Meyer, Naturwissenschaften, 16, 790 
(1928); Z. angew. Chem., 41, 935 (1928). 

(15) Meyer, Naturwissenschaften, 17, 255 (1929). 


V. Glycol Esters of Oxalic Acid* 

This is a study of the glycol esters of oxalic acid. 

Monomeric ethylene oxalate is a macrocrystalline substance with a melting 
point of 144; it undergoes polymerization under various conditions and 
yields microcrystalline, white, dusty polymers with melting points up to 
172. This material is insoluble and no molecular weight determination 
was possible. An intermediate polymer showed a melting point at 158- 
159; it was called B -polymer, is soluble and gave in boiling acetonitrile a 
molecular weight around 2400. The polymers can be depolymerized and 
fractionated by extraction with acetonitrile. 

Propyleneoxalate was obtained as monomeric crystalline material with a 
melting point of 142 and as polymeric resinous substance with an ap- 
parent molecular weight of 700.\ 

Trimethylene oxalate is obtained as dimeric and polymeric material with 
molecular weights of 270 and about 2000, respectively. 
Ilexamethylene oxalate is a white powder with am. p. of 79 and a molecu- 
lar weight around 1150. 

Melting points ranging from 110 to 172 have been ascribed to ethylene 
oxalate (1). Bischoff (2), by distillation, prepared a form melting 
at 142 to 143, which he showed to be monomeric. He observed that the 
melting points of this and higher-melting forms change spontaneously 011 
standing, and he ascribed this change to reversible polymerization but 
without the support of any comparative molecular weight data. In 
connection with a study of glycol esters of dibasic acids (3) we have made 
some further observations on ethylene oxalate and have also prepared some 
other alkylene oxalates. 

Preparation of Ethylene Oxalate. The ester was prepared by heating 
ethylene glycol and ethyl oxalate in a Claisen flask provided with a re- 
ceiver. Alcohol distilled off fairly rapidly when the heating bath was kept 
at 180-190. The residue was heated in a vacuum for a time to remove 
unchanged reactants. The distillate was found to contain some ethyl 
(/3-hydroxyethyl) oxalate. 

Ethyl (0-Hydroxyethyl) Oxalate. Color less liquid; b. p. (0.2 mm.), 108-110; 
<%> 1.2241; n 2 D 1.4405. 

Anal. Calcd. for CeHnA,: C, 44.42; H, 6.22. Found: C, 44.01; H, 6.02. 

* W. H. Carothers, J. A. Arvin and G. L. Borough; Journ. Am. Chem. Soc., 52, 
3292-3300 (1930); Contribution No. 35 from the Experimental Station of E. I. du Pont 
de Nemours and Co. 

Received April 23, 1930. Published August 5, 1930. 

t Corresponding to a polymerization degree of about 7 to 8. 



The residue after crystallization from glacial acetic acid or from ethyl 
oxalate was a dusty white powder that usually melted at about 153. 
This material was polymeric. 

Anal. Calcd.forC 4 H 4 O 4 : C, 41.38; H,3.45; mol. wt., 116; saponification equiva- 
lent, 58. Calcd. for CwH^Ow = C 2 H 5 O (COCO O(CH 2 ) 2 O)i 3 H: C, 41.70; 
H, 3.73; mol. wt., 1554; saponification equivalent, 59.4. Found: C, 41.66, 4197; 
H, 3.76, 4.05; mol. wt. (in boiling 
acetonitrile), 1510, 1580, 1610; 
saponification equivalent, 59.0, 60.8, 

When this polymer was 
heated in a vacuum, distilla- 
tion occurred and from the 
distillate Bischoff's 143 ester 
was obtained (observed m. p., 
143-144) . Molecular weight 
determinations in boiling aceto- 
nitrile agreed with those in 
freezing acetic acid reported 
by Bischoff, indicating that 
this material is monomeric. 

Anal Calcd. for C 4 H 4 O 4 : C, 
41.38; H, 3.45; mol. wt., 116. 
Found: C, 41.42, 41.44; H, 3.56, 
3.45; mol. wt. (in boiling acetoni- 
trile), 118, 120, 123, 126. 

Small samples of polymer, 
if distilled rapidly, gave better 
than 50% yields of the mono- 
mer. Under other conditions 
the yield was smaller owing 
to thermolysis (Id) to ethyl- 
ene, carbon dioxide, ethylene 

Fig. 1. Crystals of monomeric ethylene 
oxalate. X 94. (These crystals were placed 
for observation in a microscopic culture plate 
and sealed against the access of air and moisture 
by means of a cover glass. Mechanical dis- 
turbances were avoided, and subsequent ob- 
servations, except the last, were made from 
exactly the same position as the first.) 

carbonate, carbon monoxide 

and other gaseous, liquid and tarry products. 

Properties of Ethylene Oxalate and its Polymers. Monomeric 
ethylene oxalate is definitely macrocrystalline and shows a relatively high 
solubility (Table II). It is so readily hydrolyzed that it may be titrated 
directly with warm tenth normal alkali. On standing at room tempera- 
ture, the sharply defined crystals of the monomer rapidly disintegrate 



(Figs. 1-3) with the appearance of having been violently disrupted, and 
finally are transformed into very minute crystals, which, if undisturbed, 
may become spontaneously oriented on a glass surface to thread-like aggre- 
gates (Fig. 4). 

This transformation is due to polymerization. It is accelerated by 
moderate heat and catalyzed by acids or alkalies. The polymerization is 
accompanied by changes in melting point and solubility. In the first 
stages of the polymerization the melting point drops (e. g., to 106-110); 
higher-melting polymers are formed by heating the monomer in the pres- 
ence or absence of solvents or 
catalysts, and the final prod- 
uct is an insoluble material 
melting at 172-173. 

A sample of monomeric 
ethylene oxalate was heated 
at 135-140 for seven hours, 
and then extracted repeatedly 
with cold acetone. The ma- 
terial that separated on evapo- 
ration of the acetone was iden- 
tified as unchanged monomer. 
The acetone-insoluble mate- 
rial was a dusty white powder 
which after drying melted at 
171-172. It was insoluble in 
all common organic solvents, 
so no molecular weight deter- 
minations could be made. 

Fig. 2. The crystals of Fig. 1 after five days 
at room temperature. X 94. 

Anal. Calcd. for (C 4 H 4 O 4 )x: 
C, 41.38; H, 3.45; saponification 
equivalent, 58.0. Found: C, 41.48, 
41.59; H, 3.71, 3.96; saponification equivalent, 58.45. 

By fractional crystallization of polymerized monomer and of the 153 
polymer formed directly from glycol and oxalic ester, a great variety of 
samples was obtained showing such melting points as 106-108, 122-125, 
140-142, 148-150, 153, 155-157, 157-159, 160-163, 163-164, 172- 
173, as well as others having much wider melting ranges. Attempts to 
segregate homogeneous samples from these fractions met with difficulties 
due to the rapidity with which spontaneous transformations occurred even 
in the absence of solvents and at room temperature. Portions of three 



sharply melting samples were stored in glass-stoppered bottles, and melting 
point determinations were made at intervals with the following results. 


Nature and m. p. 
of sample 

144 (monomer) 
153 (polymer) 
172 (polymer) 

* M. p. after standing at room temperature 
2 months 4 months 5 l /2 months 

105-140 105-135 105-122 (A) 
149-155 149-155 149-155 (B) 
160-170 160-170 160-170 (C) 

To avoid these changes during the process of fractionation, it was 
necessary to work rapidly and 
to use cold solvents as far as 
possible. Solubility data estab- 
lished the homogeneity of the 
monomer (144) and the ap- 
parent homogeneity of the 
highest (172) polymer. Inter- 
mediate fractions, unlike the 
172 polymer, were completely 
soluble in warm acetonitrile, 
and since the solubility of the 
latter was not affected by the 
presence of monomer, inter- 
mediate polymers must have 
been present in these fractions. 
This led to the hope that it 
might be possible to isolate 
polymers sufficiently low for a 
study of structure. 

The fractions A, B and C of Fig. 3. The crystals of Fig. 1 after seven days 
Table I were recrystallized with at room temperature. X 113. 

the following results: extrac- 
tion of A with cold acetone removed a small amount of monomer. The 
residue was treated with warm acetonitrile, which dissolved all but a 
small amount of material that melted at 170-172 (high polymer). The 
material that separated from the acetonitrile solution, after recrystalli- 
zation, melted at 157-159. Similarly from B, fractions were isolated 
corresponding with monomer, high (172) polymer, and 157-159 polymer. 
C yielded a small amount of material melting at 155-157. The rest 
was unchanged 172 polymer. Extractions of various other samples 
were made, and in one case a small fraction melting at 106-108 and hav- 



ing an apparent molecular weight of about 900 was isolated. There is still 
considerable doubt, however, concerning the homogeneity of this fraction. 
The 153 material, which was the usual (recrystallized) product of the 
action of ethylene glycol on ethyl oxalate, was not homogeneous in spite of 
its fairly sharp melting point, for its apparent solubility changed with 
changing ratio of solute to solvent as shown below. 

Sample, g. 

Acetonitrile, cc. 

Apparent soly. in 
g. per 100 g. at 25 



A four-gram sample of the 153 material was then extracted repeatedly 
with 25 cc. portions of acetonitrile. With each extraction the apparent 
solubility decreased until a constant value was reached. 




Apparent soly. in 
g. per 100 g. at 25 









M. p. of residue, ' 


The apparent solubility of the residue did not change on increasing the 
ratio of solute to solvent five-fold, so this material must be regarded as 
essentially homogeneous. Its apparent molecular weight in boiling aceto- 
nitrile was about 2380 (observed values, 2070, 2480, 2670, 2370, 2275, 2520). 

One may conclude that at least two polymers of ethylene oxalate exist: 
a soluble form melting at 158-159 of molecular weight about 2400, and an 


Nature of oxalate sample 

Monomeric ethylene 
Polymeric ethylene 
Polymeric ethylene 
Monomeric propylene 
Monomeric ethylene 

heated at 90 for 

two weeks 

These values have no quantitative significance since the sample was not homo- 

M. p., C. 


mol. wt. 






<0 01 

<0 01 









insoluble form melting at 172 of unknown but probably much higher mo- 
lecular weight. Either of these forms may arise spontaneously from the 
other, and both of them may be formed from the monomer. Definite 
evidence for the existence of other polymers of ethylene oxalate is lacking. 
Chemical Properties of the Polymeric Ethylene Oxalates. Mono- 
meric ethylene oxalate is hydrolyzed with extraordinary rapidity. To a 
certain extent this property is shared by its polymers. Hence, although 
the polymers show acid reactions toward litmus in contact with water, 
this fact cannot be used to 
argue for the presence of long 
primary valence chains bearing 
carboxyl groups at the end. 
Attempts to prepare sodium 
salts from the polymers by 
the action of cold sodium bi- 
carbonate solution led to the 
isolation of sodium oxalate and 
unchanged polymer. This ease 
of hydrolysis is associated with 
great sensitivity toward other 
reagents. The attempt to de- 
tect hydroxyl groups or car- 
boxyl groups by heating poly- 
meric ethylene oxalate with 
m-bromobenzoic anhydride 
and with phenylhydrazine led 
to the isolation of ethylene- 
fo's-w-bromobenzoate and to 
the phenylhydrazide of oxalic 
acid. The latter reaction is 
what would be expected from 
an ester having the structure 
O (CH 2 )2 O , etc., whether the chain is open or closed, but the first 
reaction requires the elimination of CO CO O residues. These 
fragments of the molecules appeared as carbon monoxide and carbon diox- 
ide, which were observed to be evolved from the reaction mixture. 

Ethylene &-w-Broniobenzoate. Prepared by the action of w-bromobenzoic 
anhydride on ethylene glycol or on ethylene oxalate; crystallized from a mixture of 
chloroform and alcohol; m. p. 78-79. 

Anal. Calcd. for QeH^B^: C, 44.86; H, 2.80; Br, 37.38; mol. wt., 428. Found. 

Fig. 4. The crystals of Fig. 1 after two weeks 
at room temperature. X 185. 


C, 44.85, 45.06; H, 2.92, 2.98; Br, 37.26, 37.03; mol. wt. (in boiling benzene), 

Propylene Oxalate. By heating propylene glycol and ethyl oxalate 
together and removing unchanged reactants in a vacuum, a colored viscous 
resin was obtained. In boiling acetonitrile this showed an apparent mo- 
lecular weight of about 700 (observed, 670, 660). No crystalline material 
could be isolated from it. When this resin was strongly heated at a pres- 
sure of 5 mm., distillation occurred. A crystalline solid that separated 
from the liquid distillate melted at 142 after recrystallization from hot 
alcohol. This was identified as monomeric propylene oxalate. 

Anal. Calcd. for CsHeOi: oxalic acid, 69.24; saponification equivalent, 65; 
mol. wt., 130. Found: oxalic acid, 69.82; saponification equivalent, 64.5, 65.1; mol. 
wt. (in boiling acetonitrile), 131, 147. 

Carbon and hydrogen values (Pregl method) were consistently low, perhaps owing 
to the loss of methane. 

The monomeric methyl ethylene oxalate showed no tendency to poly- 
merize spontaneously at room temperature, but on being heated to 140- 
150 for eight hours it was converted to a white powder that melted at 176- 
178 and was insoluble in all common organic solvents. 

Trirnethylene Oxalate. This ester has been prepared by Tilitcheev (4) 
as a solid melting at 82-84 by heating methyl oxalate with trimethylene 
glycol first at atmospheric and then under diminished pressure, dissolving 
the residue in chloroform, and precipitating it with methyl alcohol. On 
distillation at 3-4 mm. pressure it was converted into an "isomeric" form 
melting at 186-187. 

Since the length of the chain of the structural unit of trimethylene oxalate 
is seven atoms, the reaction between oxalic ester and trimethylene glycol 
should be intermolecular and the product polymeric, in accordance with 
the generalization based on the study of other similar reactions (3, 5). 

We prepared trimethylene oxalate by heating ethyl oxalate with tri- 
methylene glycol. After three crystallizations from a mixture of chloro- 
form and ethyl alcohol it melted at 87-88. Its apparent molecular weight 
in boiling acetonitrile was about 2000 (observed value, 2040, 1980). 

Anal. Calcd. for C 6 H 6 O4: C, 46.15; H, 4.65. Found: C, 46.03, 46.34; H, 4.90, 

When this polymeric material was heated at 250 at 3-4 mm., thermoly- 
sis and distillation occurred. Gaseous, liquid and carbonaceous products 
were formed. The liquid distillate from 52 g. of polymer weighed 23.6 g., 
and on being cooled and treated with alcohol it yielded a small amount of 


crystalline solid, which after repeated crystallization melted at 186-187. 
This was Tilitcheev's "isomeric" trimethylene oxalate. 

Anal. Calcd. for (CsHsO^: C, 4.165; H, 4.65; raol. wt., 260. Found: C. 
45.75,46.12; H, 4.61,4.69; mol. wt. in boiling acetonitrile, 282, 278; in freezing phenol, 
272, 265. 

The molecular weight determinations prove this material to be dimeric. 

The liquid distillate from which the dimeric trimethylene oxalate was 
isolated was redistilled. It boiled from 70 at 20 mm. to 185 at 0.25 mm. 
From the higher-boiling fractions a considerable amount of trimethylene 
carbonate was isolated. This is a product that Tilitcheev assumed to be 
intermediate in the thermolysis of trimethylene oxalate, but he was unable 
to isolate it. Saponification of the remaining liquid led to the isolation 
(as calcium oxalate) of about 1.5 g. of oxalic acid. Hence it is possible 
that some monomeric trimethylene oxalate may have been present in the 

These results lead to the following conclusions. The action of trimethyl- 
ene glycol on ethyl oxalate proceeds intermolecularly and leads to an ester 
of the type CO CO O (CH 2 ) 3 O CO CO O (CH 2 ) 3 O 
CO CO O (CH 2 ) 3 O , etc. This on being heated to a high tempera- 
ture undergoes thermal decomposition and yields a complicated mixture of 
products containing a small amount of the 14-membered cyclic ester 

CO CO O (CH 2 ) 3 O CO CO O (CH 2 ) 3 O -, 

These reactions are analogous to those observed in the formation and 
decomposition of ethylene succinate (6) and tetramethylene carbonate (7). 
No truly reversible relationship between a monomeric and polymeric form 
of trimethylene oxalate exists, as it does in ethylene oxalate and in tri- 
methylene carbonate (7). The dimeric trimethylene oxalate shows no 
tendency to polymerize spontaneously. 

Hexamethylene Oxalate and Decamethylene Oxalate. From con- 
siderations which have already been set forth it is to be expected that 
these esters, by whatever method they are prepared, will be linear con- 
densation polymers. They are readily prepared by heating ethyl oxalate 
with the corresponding glycols at first under atmospheric pressure and 
finally in high vacuum. 

Hexamethylene Oxalate. White powder purified by precipitation from chloro- 
form by methyl alcohol, m. p. 66. 

Anal. Calcd. for C 8 Hi 2 O 4 : C, 55.79; H, 7.03; mol. wt., 172. Found: C, 55.77, 
55.58) H, 7.17, 7.08; mol. wt. (in boiling benzene), 1050, 1160, 1120. 


Became thylene Oxalate. White powder purified by precipitation from chloro- 
form by methyl alcohol, m. p. 79. 

Anal. Calcd. for Ci 2 H 2 4O 4 : C, 62.57; H, 9.63; mol. wt. f 232. Found: C, 62.89, 
62.73; H, 8.97, 8.90; mol. wt. (in boiling benzene), 1160, 1190. 


Ethylene oxalate exists in three mutually interconvertible forms: a 
monomer (m. p. 144), a soluble polymer (m. p. 159) and an insoluble 
polymer (m. p. 172). Ethylene oxalates showing other melting points 
are probably mixtures of these three forms, since evidence for the existence 
of any other individual forms is lacking. None of these forms are stable at 
ordinary temperature. The monomer polymerizes spontaneously, and the 
purified polymers are partially depolymerized. 

Propylene oxalate exists in at least two mutually interconvertible forms : 
a monomer and a polymer. Monomeric propylene oxalate polymerizes 
much less rapidly than ethylene oxalate. 

Trimethylene oxalate (m. p. 86) prepared from ethyl oxalate and tri- 
methylene glycol is a linear condensation polymer. It shows no tendency 
to depolymerize spontaneously. At high temperature it undergoes ther- 
mal decomposition, and one of the products of this reaction is the dimeric 
14-membered heterocycle, m. p. 187. This is stable and shows no tend- 
ency to polymerize further. 

Hexamethylene oxalate and decamethylene oxalate prepared by the 
action of the glycols on ethyl oxalate are linear condensation polymers. 

Ethyl (^-hydroxy ethyl) -oxalate and ethylene-fos-ra-bromobenzoate are 

Bibliography and Remarks 

(1) (a) Bischoff, Ber., 27, 2939 (1894); (b) 40, 2803 (1907); (c) Adams and Weeks, Journ. Am. 
Chem. Soc., 38, 2518 (1916); (d) Tilitcheev, Ber., 56, 2218 (1923). 

(2) Bischoff, ibid., 40, 2803 (1907). 

(3) Carothers and Arvin, Journ. Am. Chem. Soc., 51, 2560 (1929); Carothers and van Natta, 
ibid., 62, 314 (1930); Carothers and Borough, ibid., 62, 711 (1930). 

(4) Tilitcheev, J. Russ. Phys.-Chem. Soc., 68, 447 (1926); C. A. 21, 3358 (1927); Chem. Zentr., 
II, 440 (1927). 

(5) Carothers, Journ. Am. Chem. Soc., 51, 254g (1929). 

(6) Carothers and Dorough, Journ. Am. Chem. Soc., 52, 718 (1930). 

(7) Carothers and van Natta, ibid., 62, 314 (1930). 


VI. Adipic Anhydride* 

J. W. Hill describes the preparation of adipic anhydride. In accordance 
with the general rule put forward by Carothers, the anhydride of adipic 
acid should be capable of existing as a 7 -ring and as a long chain polymer. 
The author has succeeded in preparing both forms and also in converting 
the monomer (m. w. 128) into the polymer of a molecular weight around 
800 (polymerization degree 6-7). The reaction of the monomer with 
aniline and of the polymer with aniline and phenol was studied. 

The anhydrides of succinic and glutaric acids are known and are mono- 
meric rings of five and six atoms, respectively. The monomeric anhydride 
of the next member of the series, adipic acid, is a seven-atom ring, and 
therefore the direct preparation of this anhydride by the removal of water 
from the acid should give a polymolecular product, in accordance with the 
generalizations formulated in the previous papers in this series (1). This 
has been found to be the case. We have also been successful in preparing 
monomeric adipic anhydride for the first time. 

Adipic anhydride has been prepared by Voerman (2) and by Farmer and 
Kracovski (3), both of whom describe it as a solid. Voerman prepared the 
compound by treating adipic acid with acetyl chloride, removing volatile 
compounds in vacuo y and finally crystallizing from benzene He gives a 
melting point of 98. On the basis of a molecular weight determination in 
phenol, with which, however, we have found it to react, he seemed to 
regard the compound as a monomeric ring. The abnormally high molecu- 
lar weight obtained in boiling acetone, the lack of definite crystallinity 
and the relatively low solubility of the compound, however, led him to 
consider that it might be polymeric. He does not make his viewpoint 
altogether clear, but apparently he regarded the anomalous properties of 
the compound as due to some sort of association of simple rings and did not 
consider the probability of the existence of a long chain or large ring. 

Farmer and Kracovski state that adipic anhydride is definitely uni- 
molecular and ascribe to it a melting point of 97 This is certainly not 
correct since we have found the monomer to melt at a very much lower 

* J. W. Hill; Journ. Am. Chem. Soc., 52, 4110-14 (1930); Communication No. 40 
from the Experimental Station of E. I. du Pont de Nemours and Co. 

Received July 26, 1930. Published October 6, 1930. 

This paper does not bear the name of W. H. Carothers, but it is appropriate to 
include it in this volume and give a short abstract. This will maintain the full con- 
tinuity of this interesting and important series of contributions to high polymeric chem- 
istry. Compare also paper No. XVII on page 192. 


We have prepared adipic anhydride by both of these methods and have 
found that the product before subjection to distillation is polymeric, as 
we had anticipated and Voerman suspected. Polymeric adipic anhydride 
separates from hot benzene as a microcrystalline powder. Samples pre- 
pared in various experiments showed melting points ranging from 70 to 85. 
This variation and the difference between the melting points of our prod- 
ucts and those of previous investigators may be due to differences in molecu- 
lar size. A molecular weight determination in boiling benzene gave a value 
about six times the normal value. This value is probably low as the 
substance is very sensitive to moisture, and only the ordinary precautions 
were taken to exclude it. The compound hydrolyzes in boiling water to 
yield the acid and reacts rapidly with aniline and phenol. It is hygro- 
scopic and on standing unprotected gradually reverts to the acid. It is not 
distillable as such, but when heated in vacua to somewhat above 200, it 
breaks down and yields the true monomeric adipic anhydride, which distils 

Monomeric adipic anhydride is a liquid freezing at about 20 and boiling 
at about 100 at 0.1 mm. It reacts with cold water quickly with the evolu- 
tion of much heat to yield the acid. It reacts almost instantly with 
aniline to give a very high yield of pure monoanilide. On being heated at 
100 for a few hours, it changes to a polymer melting at 80-85. On 
standing at room temperature for ten days or more, it gradually solidifies. 
It is probable that traces of water bring about the polymerization. When 
a sample of liquid anhydride is transferred to a vessel which has not been 
very carefully dried, an amorphous skin soon coats the glass. 

The reactions of the monomeric and polymeric adipic anhydrides with 
aniline establish the structures of these compounds, the former as a seven- 
membered ring and the latter as a long chain, which may be a large ring or 
an open chain with terminal carboxyl groups. Monomeric anhydride (I), 
which has a symmetrical structure, can react with aniline to give but one 
product, adipic acid monoanilide (II). This has been verified experi- 

(CH,) 4 


mentally. Polymeric adipic anhydride, to which we may assign formula 
III, on the other hand, is no longer symmetrical as soon as one anhydride 
group reacts with aniline. The product formed, therefore, depends on 


which side of the oxygen atom the next anhydride group along the chain 
breaks. It will be seen from the scheme above that the reaction of poly- 
meric adipic anhydride with aniline may lead to three products: adipic 

oj o o jo oiro "1 

4 C-l-0-C(^ 

H H H | NHC 6 H fi \ H 

1 L J* 

o o o o o 

(CH 2 ) 4 C 
C 6 H 6 NH 

monoanilide adipic acid dianilide, etc. 


acid, adipic acid monoanilide and adipic acid dianilide. All of these com- 
pounds have been isolated from the reaction of the polymer with aniline. 
On the assumption that a random reaction takes place and that the car- 
boxyl groups which are probably at the ends of the chain may be neglected, 
these products should be formed in the proportion of one mole of acid to 
one mole of dianilide to two moles of monoanilide. In a quantitative ex- 
periment a 25% yield of dianilide was obtained. The adipic acid and 
monoanilide formed were not estimated quantitatively. Similarly the 
solid anhydride yields diphenyl adipate on warming with phenol. 

The experiments with aniline show definitely that the solid adipic an- 
hydride is a true condensation polymer in which the structural units are 
held together by primary valence forces. It is presumed that the polymers 
are open chains with carboxyl groups at the ends as has been demonstrated 
in the case of some of the polymeric ethylene succinates (4) . This view is 
further supported by the presence of adipic acid in the distillate when the 
polymeric anhydride is cracked to form the monomer. 

The writer wishes to thank Dr. W. H. Carothers for hrs interest and 
advice, and Mr. S. B. Kuykendall for determinations of carbon and 

Experimental Part 

Preparation of Polymeric Adipic Anhydride. One hundred grams of recrystallized 
adipic acid and 300 cc. of redistilled acetic anhydride were refluxed together for four 
to six hours, and the volatile constituents were removed in a vacuum at 100. The 
residue melted at about 70. It was repeatedly recrystallized from dry benzene, from 
which it separated as a micro-crystalline powder. Various samples prepared in this 
same way melted at temperatures varying from 70 to 85. At temperatures above the 
melting point, all these samples were viscous liquids, which solidified on cooling to waxy 
masses. In all experiments moisture was carefully excluded. In the most careful 
preparation carried out, moisture was excluded by phosphorus pentoxide tubes, the 
residue was heated overnight in an exhausted system which contained caustic potash 


in an adjacent vessel and then recrystallized four times with one nitration in a closed 
system. After each recrystallization the mother liquor was removed by careful de- 
cantation. The product melted at 73-75. The polymeric anhydride melted under 
boiling water and gradually went into solution. On cooling adipic acid crystallized out. 

Anal. Calcd. for (CeHgOa)^ saponification equivalent, 64.0. Found: saponi- 
fication equivalent, 63.2, 63.4. 

Reaction of Polymeric Adipic Anhydride with Aniline. A sample of polymer pre- 
pared directly from adipic acid and acetic anhydride (m. p. 75) was added to a slight 
excess of aniline and triturated in a mortar. Reaction took place spontaneously with 
the evolution of heat. The mixture was treated with dilute hydrochloric acid and the 
solid precipitate was filtered off. The filtrate was evaporated to dryness and the residue 
was then taken up in a volume of water not quite sufficient for complete solution, filtered 
and washed. The residue after crystallization from water was identified as adipic acid 
by a mixed melting point determination. The residue from the hydrochloric acid treat- 
ment was boiled with water and filtered. The filtrate deposited needles on cooling, which 
after recrystallization from water, were identified as adipic acid monoanilide by a mixed 
melting point with an authentic sample. The residue insoluble in boiling water, con- 
sisting of adipic acid dianilide, was recrystallized from alcohol; needles, m. p. 240-241 . 
(5). This experiment was carried out with various samples of polymeric anhydride: 
crude polymer prepared directly from adipic acid and acetic anhydride, recrystallized 
polymer, polymer formed by heating the monomer and still-residue from distillation of 
the monomer. In every case dianilide was isolated and identified. The experiment with 
the heat polymer was carried out quantitatively and gave a 25% yield of dianilide. 

Anal. Calcd. for C, 8 H,oO 2 N2: C, 72.96; H, 6.81. Found: C, 73.31, 73.24; H, 
6.95, 7.12. 

Reaction of Polymeric Adipic Anhydride with Phenol. Polymeric adipic anhydride 
was warmed with phenol to fusion and poured into cold water. On agitation a white 
solid separated out. This, after two recrystallizations from a mixture of alcohol and 
water (1 : 1), separated as lustrous plates of m. p. 105.5-106. The method of prepara- 
tion and the ultimate analysis showed this compound to be diphenyl adipate. 

Anal. Calcd. for Ci 8 H 18 O 4 : C, 72.45; H, 6.08. Found: C, 72.63, 72.19; H, 6.07, 

Preparation of Monomeric Adipic Anhydride. One hundred grams of adipic acid 
and 300 cc. of acetic anhydride were refluxed together for four hours. The acetic acid 
formed in the reaction and the excess acetic anhydride were removed by distillation in a 
vacuum. The residue was transferred to a Claisen flask and heated under vacuum. 
After the removal of the residual acetic anhydride at 0.1 mm. no further distillation took 
place up to 210 bath temperature. At this temperature the pressure rose, and in differ- 
ent experiments from 50 to 60 g. distilled between 105 and 125 under pressure from 
3 to 8 mm., the bath temperature ranging from 210 to 235. The residue was viscous 
when hot and solidified to a dark colored mass on cooling. The distillate consisted of a 
light pink liquid containing a small amount of crystalline material. The liquid and solid 
phases were separated by centrifuging. The crystalline material was washed with ben- 
zene, recrystallized from ethyl acetate, and identified as adipic acid by a mixed melting 
point determination. The liquid was redistilled in an all-glass apparatus. The boiling 
point was 98 to 100 at 0.1 mm. at a bath temperature of 117 to 120. Only about half 
of the material could be recovered by distillation since the residue gradually polymerized. 


Samples of the colorless distillate were sealed in small, carefully dried glass tubes for 
freezing point determinations. The compound froze at 19 to large translucent plates 

Anal. Calcd. for CeH 8 O 8 : C, 56.25; H, 6.30; mol. wt., 128; saponification equiv., 
64.0. Found: C, 55.89, 55.98; H, 6.51, 6.52; mol. wt. (in boiling benzene), 134, 131; 
saponification equiv., 63.7, 63.6. 

Polymerization of Monomeric Adipic Anhydride. When monomeric adipic an- 
hydride was transferred to a vessel which had not been carefully dried, a translucent coat- 
ing appeared on the surface of the glass. The coating was amorphous and in places 
skin-like. It melted at 60-65. When the liquid was heated in a sealed tube at 100, 
it polymerized completely and a maximum melting point of 80-84 was reached in about 
seven hours. On being heated at 138, a maximum melting point of 81-85 was reached 
in about two hours. In neither experiment did further heating affect the melting point. 

Anal. Calcd. for (CeHsOa)^ saponification equiv., 64.0. Found: saponification 
equiv., 63.5, 63.3; mol. wt. (in boiling benzene), 860, 710. 

Reaction of Monomeric Adipic Anhydride with Aniline. One cubic centimeter 
of anhydride was added to about eight cubic centimeters of aniline with stirring. Re- 
action was immediate and much heat was evolved. The excess aniline was dissolved in 
dilute hydrochloric acid. The solution was diluted to 200 cc. and filtered. The crude 
product was dried and weighed; m. p. 152-153; yield 80%. The monoanilide of adipic 
acid has previously been prepared by heating C 6 H 6 NHCOCH(COOH)(CH2) 8 COOH (6). 

Recrystallization from water did not raise the melting point. The crude com- 
pound was completely soluble in hot water (absence of dianilide). A similar experiment 
was carried out and worked up in smaller volume; m. p. (crude), 151-1.53; yield, 87%. 

Anal. Calcd. for C^HuOsN: C, 65.12; H, 6.80; neutralization equiv., 221. 
Found: C, 64.76, 64.86; H, 6.81, 7.01; neutralization equiv., 222, 223. 


The monomeric and polymeric forms of adipic anhydride have been 
prepared, the former for the first time, and they have been shown to be 
mutually interconvertible. The reactions of the compounds with aniline 
have shown the monomer to be a seven-atom ring and the polymer a long 
chain or large ring. Diphenyl adipate is described. 

Bibliography and Remarks 

(1) Carothers, Journ. Am. Chem. Soc., 51, 2548 (1929); Carothers and Arvin, ibid., 51, 2560 
(1929); Carothers and van Natta, ibid. t 52, 314 (1930); Carothers and Dorough, ibid. t 52, 711 (1930). 

(2) Voerman, Rec. trav. chim. t 23, 265 (1904). 

(3) Farmer and Kracovski, J. Chcm. Soc., 680 (1927). 

(4) Carothers and Dorough, Journ. Am. Chem. Soc., 52, 711 (1930). 

(5) BSdtker \Ber., 39, 2765, 4003 (1906)] gives 240; Balbiano [Gazz. chim. ital., 32, I, 446 (1902)] 
gives 240-241; Bouveault and Titry [Bull. soc. chim., [3] 25, 444 (1901)] gives 235. 

(6) Dieckmann, Ann., 317, 62 (1901). The melting point given is 152-153. 


VII. Normal Paraffin Hydrocarbons of High Molecular 
Weight Prepared by the Action of Sodium on Decamethylene- 


Long chain paraffins can be obtained by treating decamethylenebromide 
with metallic sodium in ethereal solution. 

After an enumeration and discussion of the different reactions, which are 
feasible in this system^ the preparation of the hydrocarbon mixture and its 
separation are described. The latter is carried out in a molecular still at a 
pressure of about 10~ 5 mm. Hg. Between the melting points of 35 and 
106 eighteen fractions were separated. The distillates were grouped 
together into eight fractions and redistilled; they crystallized with con- 
stant melting points and proved to be hydrocarbons up to C 7 oHi42. A 
curve showing the melting points as a function of chain length is given 
and thoroughly discussed. 

Finally a detailed description of the preparation of decamethylene- 
bromide is given. 

No rationally synthetic and practical methods for the preparation of 
giant individuals of the simpler homologous series are available, and one 
of the objects of the experiments here reported was to explore the possi- 
bilities of bifunctional Wurtz reactions as a means of access to this realm. 

The first step in simple Wurtz reactions is the formation of the corre- 
sponding sodium alkyl, RNa (1). This normally couples with the halide, 
forming the hydrocarbon R-R, but it may react in other ways and yield 
by-products. Sodium ethide reacts with diethyl ether 

C 2 H 6 Na + (C 2 H 5 ) 2 O = C 2 H 6 + C 2 H 6 ONa + C 2 H 4 (2) 

and one may expect the formation of considerable amounts of the hydro- 
carbon RH when the Wurtz reaction is carried out in that solvent. 

The experiments presently described are concerned with the action of 
sodium on decamethylene bromide in ethereal solution, and the normal 
course of this reaction including that due to the participation of the solvent 
(but ignoring for the moment the possibility of intramolecular reaction) 
may be formulated as follows 

Br(CH 2 )ioBr + 2Na - Br(CH 2 )ioNa + NaBr 
Br(CH 2 ) 10 Na + (CiH,)jO - H(CH 2 ) 10 Br + CaHiONa 
Br(CH : )ioNa + Br(CH 2 )ioBr - Br(CH 2 ) 20 Br + NaBr 

* W. H, Carothers, J. W. Hill, G. E. Kirby and R. A. Jacobson; Journ. Am. Chem. 
Soc., 52, 5279-88(1930); Contribution No. 46 from the Experimental Station of E. I. 
du Pont de Nemours and Co. 

Received October 9, 1930. Published December 18, 1930. 


H(CH 2 )i Br + 2Na = H(CH 2 ) 10 Na + NaBr 
H(CH 2 )ioNa + (C 2 H 6 ) 2 O - H(CH 2 ) 10 H + C 2 H 6 ONa + Q^ 
H(CH 2 ) 10 Na -f H(CH 2 )i Br - H(CH 2 ) 20 H + NaBr 
H(CH 2 )ioNa + Br(CH 2 )i Br = H(CH 2 ) 20 Br -f NaBr 
H(CH 2 )ioNa + H(CH 2 ) 20 Br - H(CH 2 ) 30 H + NaBr 
H(CH 2 )ioNa + Br(CH 2 ) 20 Br = H(CH 2 ) 80 Br + NaBr 
H(CH 2 ) 3 oBr + 2Na = H(CH 2 )soNa -f NaBr 
H(CH 2 ) 80 Na + Br(CH 2 ) 20 Br = H(CH 2 ) 60 Br + NaBr, etc. 

The product will be composed of individuals of the general formula 
H[(CH 2 )ioLH and possibly of similar chains terminated at one or both ends 
by bromine atoms. The length of these chains will be determined by the 
relative rates of reaction of the sodium compounds with the solvent and 
with the halides. Solubility effects may also come into play. 

The suitability of this reaction as a source of straight chain hydrocarbons 
will depend upon the extent to which it is possible to avoid undesirable 
side reactions, and some indications on this point may be had from the 
known behavior of simple halides. Sodium and w-heptyl bromide at the 
boiling temperature give tetradecane, 67%; heptane, 9%; heptene, 3%; 
heneicosane, 3%; hydrocarbons of higher molecular weight, some (3). 
Sodium and butyl bromide in ether under specified conditions give n- 
octane, 68%; butane ?, ?%; butene, ca. 1.5%; dodecarie, ca. 0.15%; 
hexadecane, ca. 0.05% (4). In both these typical cases the principal by- 
product is probably the hydrocarbon RH. In the reaction of the dihalide, 
this type of by-product would be identical with one of the normal reaction 
products. The higher hydrocarbons from butyl bromide and from heptyl 
bromide are probably not straight chains, and the formation of analogous 
products from the dihalide would be definitely pernicious because of the 
difficulty of separating isomeric individuals of high molecular weight. 
The quoted data on butyl bromide indicate however that in simple Wurtz 
reactions in ether not more than traces of these higher by-products need be 

The action of sodium wire on decamethylene bromide in absolute ether 
has been studied by Franke and Kienberger (5). The reaction proceeded 
smoothly to completion, and the products isolated were: w-decane, 34%; 
C 20 hydrocarbon, 30%; a small amount of C 40 hydrocarbon; and a con- 
siderable amount of solid hydrocarbon of higher molecular weight. All of 
these products were free of halogen. The melting point of the C 2 o hydro- 
carbon identifies it as n-eicosane. The formation of large amounts of 
w-decane apparently puzzled Franke and Kienberger, and they repeated 
their experiments with elaborate precautions to exclude water and alcohol 
from the solvent and the halide. The results were the same. n-Decane 
and w-eicosane are, however, strictly normal products of the reaction as it 


has been formulated above. The fact that Franke and Kienberger could 
find neither decene nor cyclodecane in the products is worthy of note, for 
any side reaction would almost certainly be accompanied by the formation 
of some decene, and any intramolecular reaction by the formation of some 
cyclodecane. These data indicate that the course of this reaction is, at 
least for the most part, strictly normal and exclusively intermodular. The 
major part of the product is composed of relatively short chains, but this 
is a matter which may be expected to be susceptible of some control by 
changes in the experimental conditions, and, as it appears to be in fact, 
especially by increase in the surface of the sodium. 

We had already treated decamethylene bromide in ether with finely di- 
vided sodium with stirring before Franke and Kienberger's paper had come 
to our attention. The product, obtained in good yield, was an ether-in- 
soluble solid, and no material boiling as low as w-decane was present. This 
product, by extraction and crystallization, was separated into C 2 oH42, 
C 3 oH 62 and C^Haz, each fairly pure, representing together about 25% of 
the total, and a higher fraction, m. p. 108-112, representing about 75% 
of the total. This contained 1.39% organic halogen, from which it was 
freed by heating and stirring it with a small amount of molten sodium in 
boiling butyl ether. The apparent molecular weight of the resulting hydro- 
carbon was about 1000. No individuals could be isolated from it by re- 
peated fractional crystallization, and it could not be distilled at 0.1 mm. 
pressure. It was finally separated into a series of individuals by distilla- 
tion in the molecular still and by crystallization of the distillates. 

Preparation of the Hydrocarbon Mixture. In a 500-cc. flask provided with a 
reflux condenser and a mercury-sealed mechanical stirrer, 95 g. (1.52 atoms) of sodium 
was pulverized under hot xylene. The xylene was removed and replaced by absolute 
ether, and then 75 g. (0.25 mole) of decamethylene bromide was added. The reaction 
mixture was stirred continuously. It soon developed a deep blue color. It was gently 
heated for one hour and then allowed to boil without external heating for two and one- 
half hours. Finally it was heated for two and one-half hours more and then allowed to 
stand overnight. The excess sodium in the thick mass was decomposed by alcohol, a 
large volume of water was added and the mixture was filtered with suction. The only 
material found in the ethereal layer of the filtrate was 0.5 cc. of an oil boiling at 240- 
330. The soft white residue on the funnel liquefied almost completely on being 
stirred with boiling water and solidified to a crystalline mass on cooling. It weighed 
30.5 g. (87.1% calculated as CH 2 ). It melted from 85 to 100 and contained bromine 
(found, 2.17, 2.27%). This solid was heated and vigorously stirred for five hours 
with 7 g. of molten sodium in boiling butyl ether. The excess sodium was decomposed 
by alcohol, a large volume of water was added and the mixture was filtered. The solid 
residue on the funnel was washed with boiling water and dried. It weighed 21 g. 
It was free of halogen and melted from 87 to 105. This is the material which was sub- 



mitted to fractionation in the molecular still. After the butyl ether had been removed 
from the non-aqueous layer of the distillate, there remained 6 g. of a soft waxy solid 
melting at 4&-54 . 

Separation of the Hydrocarbon Mixture. Eight grams of the solid 
hydrocarbon, m. p. 87-105, described above was heated in a small mo- 
lecular still (6) provided with a water-cooled condenser and a trap cooled 
with liquid air. The pressure in the system was continually maintained 
below 10~ 6 mm. Fractions were collected as follows. 

Temp, of bath, C. 















Wt. of distillate, g. 
















Total 4.02 
Residue 3.16 
Loss 0.82 

M. p. of distillate, C. 



- 55 

- 75 

- 78 

- 79 

- 81 

- 84 

- 89 

- 91 

- 90.5 
5- 92 
5- 97 

- 98 

- 99.5 


Solvent used 



M. p. 
found, C. 

for crys- 

te o mp., 







- 35 


Abs. EtOH 










- 66 

Abs. EtOH 4- Et 2 O 








C4 H 82 


,5- 81 

Ethylene chloride 










,9- 92 


Ligroin -f petroleum 









C(M)H 122 


5- 99, 


Butyl acetate 








C 7 oH 14 2 




Butyl acetate 








residue 110 -114 Butyl acetate 

Not distillable 










The loss is due to the impossibility of quantitatively removing the distil- 
late from the condenser. In this distillation the average area of the evapo- 
rating surface was about 18 sq. cm. About twenty hours was required 

for the collection of each of the 
above listed fractions. When 
the temperature of the heating 
bath was raised above 300 with 
the view of distilling the residue, 
the pressure rose and no distilla- 
tion could be effected. 

The distillates were grouped 
together into eight fractions and 
redistilled. The rate of distilla- 
tion from these partially puri- 
fied materials was very much 
higher than from the initial mix- 
ture. The distillates were crys- 
tallized to constant melting 
points. The properties of all 
the fractions thus obtained are 
indicated in Table I. 

Identity and Properties of 
the Hydrocarbons. The melt- 
ing points 36.7 (7) and 65.6- 
66 (8), respectively, have been 
assigned to w-eicosane and n-tri- 
acontane, and these values are 
in good agreement with our 
observations. The other four 
hydrocarbons listed in Table I 
are all new, and the last one 



20 40 60 80 100 
Melting points of hydrocarbons, C. 
Fig. 1. Normal paraffins: , Gascard, 
Ann.Chim., [9] 15,332 (1921); + , Hildebrand 
and Wachter, Journ. Am. Chem. Soc., 51, 2487 
(1929); O, new data. Cyclopolymethylenes : 
A, Ruzicka and co-workers, Helv. Chim. Acta, 
9, 499 (1926); 11, 496 (1928). 

stands six atoms above any 
paraffin previously described. 
Several [intermediate members 
of this same series have been 
described, however, and from 

the available data it is possible to construct a smooth curve of melt- 
ing points covering the range Ci 8 -C 7 o. The usefulness of such a curve 
in estimating the most probable values for individual hydrocarbons 
has already been emphasized (9), and in the curve presented in Fig. 1 the 


crosses are Hildebrand and Wachter's estimates of the best values for hy- 
drocarbons in the range Ci 9 -C 3 6. We have used a method of plotting simi- 
lar to that suggested by Austin (10) and have included the values for the 
three previously known hydrocarbons above C 3 6H 7 4. These are C 6 4Hno, 
C 6 2Hi26 and C64Hi 30 , all described by Gascard (8). Our data and those of 
Gascard fall very close to the same smooth curve which fits the best data 
for the lower hydrocarbons, and this fact incidentally affords confirmation 
of Hildebrand and Wachter's contention that the melting points accepted 
by the "International Critical Tables'* for some of the hydrocarbons in the 
range C 2 4 to C 32 are much too high. Hell and Haegele described dimyricyl 
in 1889 (11) and ascribed to it the formula CeoHm. This ascription ac- 
quired the sanction of recognition by Beilstein (third and fourth editions) 
and remained unquestioned until evidence that the myricyl radical contains 
31 carbon atoms appeared. In 1921 Gascard (8) concluded that Hell and 
Haegele's hydrocarbon was C 6 2Hi 2 4 and that hexacontane had never been 
prepared. Our data confirm this conclusion. Hexacontane melts at 
98.5 to 99.3, while the observed melting points ascribed to dimyricyl have 
ranged from 100.5-102. 

Independent proof of the identity of the six hydrocarbons is furnished 
by x-ray diffraction patterns which have been obtained by Dr. A. W. Ken- 
ney and will be described in a separate publication. 

Melting point data are available for only five cyclic polymethylenes 
above Cio. These fragmentary data do not fall on any smooth curve (see 
Fig. 1), but they are all widely enough separated from the data for the 
normal paraffins to provide additional assurance that none of our paraffin 
compounds are cyclic. 

Under the heading distillation temperature in Table I are listed the mini- 
mum bath temperatures required to effect evaporation at a moderate rate 
in the molecular still. These temperatures are quite characteristic since 
they are, in each case, only a few degrees above the temperatures at which 
no distillation occurs. Using the Langmuir formula (6) it is possible to 
make a rough calculation of the order of magnitude of the vapor pressures 
of these hydrocarbons from the minimum distillation temperatures. Such 
calculations indicate a value less than 0.01 mm. for heptacontane at 300 
and a value less than 1 mm. for triacontane at 100. Heptacontane has a 
molecular weight of 983, and it is probable that no organic compound of 
much higher molecular weight than this can ever be distilled under any ex- 
perimental conditions however favorable. The residue remaining from the 
removal of the heptacontane undoubtedly contained some octacontane, 
but none of this distilled out at a bath temperature of 300 and when the 


temperature was raised higher than this, decomposition set in. This ex- 
perimental result agrees remarkably well with the inferences to be drawn 
from the data presented by Meyer and Dunkel (12). From these data one 
can calculate that the molecular cohesion of heptacontane will be about 
71,000 calories and that of octacontane about 81,000 calories. Since the 
heat of separation of the carbon-carbon bond is about 75,000 calories, it 
should be possible to distil the first of these compounds, but not the second. 
Saturated paraffins have, in general, a lower molecular cohesion for a given 
molecular weight than any other types of compounds, and hence the limit 
of distillability will be found at a lower molecular weight for other com- 
pounds than hydrocarbons. These results are also in agreement with data 
of a different kind presented by Burch (13) who, by distilling a Pennsyl- 
vania petroleum in a molecular still, obtained as the highest distillable frac- 
tion a material having an apparent average molecular weight of 801 and a 
residue of apparent molecular weight of 1550. 

The physical properties of w-heptacontane are similar to those ascribed 
to dohexacontane and tetrahexacontane. It is very slightly soluble in 
boiling alcohol, ether or petroleum ether, but crystallizes well in the form 
of minute needles from hot butyl acetate or benzene. It dries to a starch- 
like powder which has a great tendency to become electrified. 

It was hoped that it might be possible to isolate hydrocarbons much 
higher than heptacontane, since information concerning simple individuals 
of very high molecular weight would be of great importance to the study 
of macromolecular materials generally, but there seems little probability 
of achieving this result without some change in method or advance in 
technique. The residue from which the n-heptacontane was distilled is a 
mixture which undoubtedly contains n-octacontane, w-nonacontane, n- 
decacontane and still higher hydrocarbons. It is readily soluble in hot 
butyl acetate (except for a trace of insoluble and infusible material), and 
it separates as a powder melting at 110-114. Its apparent average 
molecular weight in boiling benzene is about 1300. (Calcd. for CgoHm, 
1263.) By extraction with hot ethylene chloride, it was separated into 
fractions melting at 100-107, 106-111 and 110-114. In a sense this 
mixture is a polymeric homologous series, but its solubility is much lower 
than that of such supposedly analogous series of higher molecular weight as 
polystyrene and hydro-rubber. It resembles a probably similar but more 
complex mixture of hydrocarbons obtained by Fischer and Tropsch by 
the catalytic hydrogenation of carbon monoxide (14). 

There is one point bearing on the behavior of highly polymeric materials 
which is capable of a rough preliminary test with the series of hydrocar- 


bons described above. It has been suggested (15) that the apparent de- 
crease of molecular weight (decrease of viscosity, etc.), which is observed 
when rubber and other very high polymers are gently heated or treated 
with certain mild chemical agents or subjected to mechanical stresses, is 
real and is due to the fact that the thermal stability of molecules decreases 
continuously with increasing size, and in these materials has reached so low 
a value that cracking occurs at slightly above room temperature. Samples 
of each of the hydrocarbons from C 3 o to CTO were sealed off in small bulbs 
under nitrogen and heated side by side in a metal-bath. After five minutes 
at 400 all the melting points were unchanged. After five minutes at 410 
the melting point of C 70 was lowered from 106-107 to 104-105. The 
melt, moreover, was not clear as before but turbid. The turbidity dis- 
appeared rather sharply at 1 10 like a liquid crystal. The melting points 
of the other hydrocarbons were substantially unchanged, but, except for 
C 30 , they all gave hazy melts which cleared up only at temperatures some- 
what above their true melting points. After five minutes more at 420 all 
the melting points had become lower (that of 70 most) and there was some 
coloration. The C 30 was white, the C 4 o was cream-colored and the higher 
hydrocarbons in regular order were increasingly darker. When the tubes 
were finally opened after five minutes more at each of the temperatures 
430, 450 and 470, they were found to contain considerable gas most 
above the Cyo and least above the C 30 . Thus, the decrease in thermal 
stability with increasing molecular weight which is so marked in passing 
from methane (ca. 700) (16) to ethane (ca. 550) (17), and from ethane to 
hexadecane (ca. 470) (18) is still detectable in going from triacontane to 
heptacontane, but it has already fallen to such a small value that it would 
be somewhat unsafe to infer that a paraffin hydrocarbon of molecular 
weight 200,000 or even greater might not persist at room temperature. 

Mechanism of the Action of Sodium on Decamethylene Bromide. It 
has already been pointed out that all the observed products of this reaction 
are accounted for by assuming replacement of the bromine atoms of the 
halide by sodium, and the subsequent coupling of this with other molecules 
of the halide, or its reaction with the solvent. The absence of cyclic hy- 
drocarbons is not surprising since, although the higher cycloparaffins are 
no less stable than cyclohexane (19), the formation of large rings in bifunc- 
tional reactions occurs only under very exceptional conditions (20). In 
the formation of esters from co-hydroxy acids, amides from amino acids and 
anhydrides from dibasic acids, intramolecular reaction occurs exclusively 
only if there is a possibility of forming a 5- or a 6-membered ring. The 
behavior of alkylene halides toward metals however is peculiar in that there 


seems to be little tendency toward intramolecular reaction even when a 5- 
or a 6-membered ring might be formed. Thus the action of magnesium on 
alkylene halides leads to considerable coupling, but even from pentamethyl- 
ene halides no cyclic hydrocarbon is formed (21). This suggests that the 
coupling perhaps occurs largely at the metal surface where the atoms at 
the ends of the chains may be fixed by forces of adsorption and their free- 
dom of intramolecular approach hindered. 

Our main objective in the experiments described above was to prepare 
normal paraffins of very high molecular weight, and we made a number of 
attempts to adjust the experimental conditions so as to increase the length 
of the reaction chains. We treated decamethylene bromide with molten 
sodium in boiling butyl ether, with liquid sodium-potassium alloy in ethyl 
ether, with molten sodium in boiling octane and with finely divided sodium 
in the absence of a solvent. There was no evidence of the formation of 
considerable amounts of paraffins higher than Cioo in any of these experi- 
ments. We also prepared decamethylene dimagnesium bromide and 
treated it with cupric chloride in the expectation that products of the 
formula BrMg[(CH 2 )io]*MgBr would be formed. The chief product 
(71%), however, after the addition of water, was a volatile liquid appar- 
ently chiefly composed of decane and decene. The amount of higher boil- 
ing material (20%) did not exceed that which would arise from the coupling 
products usually produced in the formation of such reagents as decamethyl- 
ene dimagnesium bromide (21). 

Preparation of Decamethylene Bromide. The preparation, in good 
yields, of a whole series of polymethylene bromides from the glycols via 
the esters of the corresponding dibasic acids has been described by Chuit 
(22) but without great experimental detail. We first attempted to reduce 
ethyl sebacate by Levene and Allen's modification (23) of the original 
Bouveault procedure (24). In this modification, which has given good re- 
sults in the reduction of esters of monobasic acids, the sodium is first granu- 
lated by stirring in hot toluene, and the reaction mixture is vigorously 
stirred during the addition of the alcohol-fester mixture. This procedure 
gave very poor yields of decamethylene glycol; the yields were increased 
by diminishing the speed of stirring and became quite good when the stir- 
ring was omitted altogether. The procedure finally adopted was almost 
identical with that of Bouveault, and the details are given below. 

A condenser having a length of 2 meters and an inside diameter of about 2.5 cm. is 
connected with the side neck of a 5-liter two-necked round-bottomed flask, and the 
central neck of the flask is provided with a 1 -liter dropping funnel. One hundred and 
fifteen grams (5 atoms) of sodium in a single piece or in two or three large pieces is 


placed in the flask. One hundred and twenty-nine grams (0.5 mole) of ethyl sebacate 
is dissolved in one liter of thoroughly dried absolute ethyl alcohol and poured into the 
dropping funnel. One hundred and fifty to two hundred cubic centimeters of the al- 
cohol-ester mixture is allowed to fall onto the sodium at once. After two or three 
minutes the reaction becomes vigorous and the sodium melts. As soon as the sodium 
has melted another portion (about 100 cc.) of the alcohol-ester mixture is suddenly 
added. This causes the molten sodium to break up into fine particles, and the reaction 
becomes much more vigorous. The rest of the alcohol-ester mixture is then added as 
rapidly as possible (five to six minutes). As soon as the vigor of the reaction has some- 
what subsided, the flask is heated by an oil-bath to 110-115 until most of the sodium 
has dissolved. The ethyl alcohol is then removed from the reaction mixture by steam 
distillation, and the glycol is removed from the residue by extended (about seventy-two 
hours) continuous extraction with ether. The glycol is purified by distillation. The 
combined yield from nine such runs was 593 g. or 75% of the theoretical. 

Continuous ether extraction as a means of isolating glycols produced by the reduc- 
tion of esters has been successfully used by C. S. Marvel (25) ; otherwise the above pro- 
cedure is similar to that which has also been used by Miiller for preparing a series of 
glycols (26). 

Using the method described by Chuit (22), we obtained from decamethylene glycol 
only a 70% yield of bromide and this was contaminated with bromohydrin. The 
following modification of this method gave a purer product and a better yield. 

Dry hydrogen bromide was led into a well-stirred melt of 255 g. (1.47 moles) of 
decamethylene glycol in an open flask until the mass was saturated. The temperature 
of a metal-bath surrounding the flask was kept at 90-95. After saturation was com- 
plete the bath temperature was raised to 135-140 and a slow stream of hydrogen bro- 
mide was led in for six hours. The reaction mass was then cooled, diluted with benzene 
and decanted from a small amount of water remaining in the flask. The benzene was 
then removed by distillation, and the product distilled under diminished pressure; 
yield, 376 g. (85.5%); b. p., 168-172 (10 mm.). 

Anal. Calcd. for Ci H 20 Br 2 : Br, 53.33. Found: Br, 53.16, 53.16. 

We are indebted to Mr. W. L. McEwen for the preparation of deca- 
methylene glycol. 


By the action of sodium on decamethylene bromide in ether a mixture 
of paraffins of the general formula H [(CH 2 )io]* H has been prepared. 
By fractional distillation in the molecular still and by crystallization, the 
following individuals have been isolated from this mixture: w-eicosane, 
w-triacontane, w-tetracontane, n-pentacontane, n-hexacontane, n-hep- 
tacontane. The last four of these are new, and the last one has a higher 
molecular weight than any pure paraffin hydrocarbon hitherto described. 
The residue remaining from the separation of the hydrocarbons named 
amounts to about 25% of the total and is composed of still higher members 


of the same series which it has not yet been possible to separate. The 
mechanism of the reaction and the properties of the products are discussed. 

Bibliography and Remarks 

(1) Schorigin, Ber., 40, 3111 (1907); 43, 1931 (1910); Schlubach and Goes, ibid., 55, 2889 
(1922); Ziegler and Colonius, Ann., 479, 135 (1930). 

(2) Schorigin, Ref. 1; Ber., 41, 2723 (1908). 

(3) Bachmann and Clarke, Journ. Am. Chem. Soc., 49, 2089 (1927). 

(4) Lewis, Hendricks and Yohe, ibid., 50, 1993 (1928). 

(5) Franke and Kienberger, Monatsh., 33, 1189 (1912). 

(6) Washbura, Bur. Standards J. Research, 2, 476 (1929). 

(7) Krafft, Ber. t 19, 2220 (1886). The Cao hydrocarbon of Franke and Kienberger (Ref. 5) 
which melted at 36 was unquestionably n-eicosane; and their Cw hydrocarbon, which melted at 72, 
was probably a mixture of n-triacontane and n-tetracontane. 

(8) Gascard, Ann. chirn., [9] 15, 332 (1921); Peterson* Ber., 12, 741 (1879). 

(9) Hildebrand and Wachter, Journ. Am. Chem. Soc., [9] 51, 2487 (1929). 

(10) Austin, ibid., 52, 1049 (1930). 

(11) Hell and Haegele, Ber., 22, 502 (1889). 

(12) Meyer, Naturw., 16, 781 (1928); Dunkel, Z. physik. Chem., 138 42 (1928). 

(13) Burch, Proc. Roy. Soc. (London), 123, 271 (1929). 

(14) Fischer and Tropsch, Brennstoff-Chcmie, 8, 165 (1927). 

(15) Staudinger, Ber., 59, 3037 (1926). 

(16) Bone and Wheeler, /. Chem. Soc., 81, 542 (1902). 

(17) Williams-Gardner, Fuel Science Practice, 4, 430 (1925). 

(18) Gault and Hessel, Ann. chim., [10] 2, 319 (1924). 

(19) Ruzicka and co-workers, Helv. Chim. Ada, 9, 499 (1926); 11, 496 (1928). 

(20) Carothers, Journ. Am. Chem. Soc., 61, 2548 (1929); and subsequent papers of this series. 

(21) Zappi, Bull. soc. chim., [4J 19, 249 (1916); v. Braun and Sobecki, Ber., 44, 1918 (1911). 

(22) Chuit, Helv. Chim. Acta, 9, 264 (1926); Chuit and Hauser, ibid., 12, 850 (1929). 

(23) Levene and Allen, J. Biol. Chem., 27, 443 (1916); cf. Adams and Marvel, Univ. Illinois 
Bull., 20, 50 (1922). 

(24) Bouveault and Blanc, Bull. soc. chim , [3] 31, 666 (1904). 

(25) Private communication. 

(26) Mailer, Monatsh., 49, 27 (1928). 

VIII. Amides from e-Aminocaproic Acid* 

e-Aminocaproic acid when heated reacts intra- and intermolecularly , and 
there is formed 20-30% of the seven-membered lactam ring and 80-70% of 
a C-polymer, containing probably at least ten molecules of the amino acid. 
No information can be given as to the end group of the chain molecule, 
but it is improbable that the molecule is a multi-membered ring. 

-Aminocaproic acid on being heated reacts with itself and yields two 
products the corresponding 7-membered lactam, 20-30%, and an un- 
distillable material of approximately the same composition, 80-70% (1). 

* W. H. Carothers and G. J. Berchet; Journ. Am. Chem. Soc., 52, 5289-91 (1930) ; 
Contribution No. 42 from the Experimental Station of E. I. du Pont de Nemours and 

Received October 9, 1930. Published December 18, 1930. 


The following related facts are of interest. 5-Aminobutyric acid and 
-aminovaleric acid readily lose water and yield the corresponding 5- 
and 6-membered lactams (2). No corresponding polymers are formed or 
known. The reaction of f-aminoheptoic acid with itself is apparently 
exclusively intermolecular, for it leads to an undistillable product quite 
different in its properties from the known 8-membered lactam (3). Similar 
relationships are found among the hydroxy acids: 7- and 5-hydroxy acids 
yield lactones; e-hydroxy acids yield both lactones and polyesters (4) and 
the higher w-hydroxy acids yield only the polyesters (5) , although many of 
the higher lactones are known (6). Polyesters have now been studied in 
some detail (7), but (except for polypeptides) little attention has been de- 
voted to the polyamides. In this paper we record some observations 
concerning the polyamide derived from c-aminocaproic acid. 

Water is rapidly evolved when e-aminocaproic acid is heated above its 
melting point (e. g., to 210-220). The polymer and the lactam which re- 
sult are readily separated by allowing the latter to distil under diminished 
pressure, or by extracting it with boiling alcohol. In different experi- 
ments the yield of lactam was 20-30% and the polymer 80-70%. The 
same products were obtained by gradually heating the ethyl ester of the 
acid from 160 to 200 during six hours, and the yield of lactam was about 

The polymer is a hard gray wax, insoluble in most organic solvents, but 
soluble in hot formamide, from which it separates as a microcrystalline 
powder melting at 212-214. Its analytical composition agrees very 
closely with that required for the structural unit NH (CH^sCO . 
Anal. Calcd. for C 6 H n ON: C, 63.71; H, 9.73. Found: C, 63.35, 63.62; H, 
9.93, 10.07. A product from another preparation gave: C, 63.35, 63.45; H, 9.56, 9.43. 

It is quantitatively hydrolyzed in six hours by boiling concentrated 
hydrochloric acid to -aminocaproic acid (identified by conversion to the 
p-toluenesulfonyl derivative, melting point and mixed melting point, 

By partial hydrolysis, polyaminocaproylaminocaproic acids are formed. 
Thus, a sample of the polyamide was heated to boiling for one hour in con- 
centrated hydrochloric acid solution, and the solution was diluted with 
water to precipitate the unchanged polymer (about 50%). The ma- 
terial, remaining in solution, on being treated with alkali and />-toluene- 
sulfonyl chloride, gave a solid product which, after repeated crystalliza- 
tion, showed a neutral equivalent corresponding with the formula QH?- 
S0 2 -[NH(CH 2 ) 5 CO]3-NH(CH 2 ) 6 COOH (calcd., 607; found, 601). The 
structure of the polymer may, therefore, be represented by the formula 


? NH(CH 2 ) 6 CO [NH(CH 2 ) 6 CO]x~NH(CH 2 ) 6 CO ? 

It is a true condensation polymer and is formed directly from the amino 
acid by intermolecular reaction. This is proved by the fact that the lac- 
tam does not polymerize under the conditions of formation of the poly- 
amide either in the presence or absence of catalysts. Moreover, no lactam 
can be obtained from the polyamide by heating it to high temperature 
in high vacuum. Thus there is no such reversible relationship between 
lactam and polyamide as is found (7c, e) to exist between six-membered 
cyclic esters and corresponding polyesters. Lactam and polyamide re- 
sult from the amino acid by two independent and simultaneous processes. 
The lactam incidentally is really the 7-ring r-NH(CH 2 ) 5 CO-| (8). This was 

proved by identifying its hydrolytic product with c-ammocaproic acid 
through its p-toluenesulfonyl derivative by means of a mixed melting 

The polyamide is sufficiently soluble in phenol to permit a determina- 
tion of molecular weight by cryoscopy. Values obtained ranged from 800 
to 1200, and since the errors of this method are such as to lead to low 
results it may be assumed that at least 10 molecules of the amino acid 
must have participated in the formation of a single molecule of the poly- 

The question of the nature of the groups at the ends of the polyamide 
chains cannot yet be answered. The formation of large rings is very 
improbable on theoretical grounds (9), and in the formation of polyesters 
ring formation is known not to occur the hydroxyl or carboxyl groups 
still persist at the ends of the polyester chain (7b, c, d, 5c). However, no 
certain evidence of the presence of terminal amino or carboxyl group in 
the molecule of the polyamide could be obtained. The polyamide dissolves 
readily in cold concentrated hydrochloric acid, but when the solution is 
diluted with water it separates unchanged and free from more than traces 
of halogen. It is not soluble in hot or cold aqueous sodium hydroxide nor 
does it form any sodium salt. It reacts with molten m-bromobenzoic an- 
hydride and, in hot pyridine, with ^-bromobenzene-sulfonyl chloride. 
The products contain bromine in amounts which correspond with deriva- 
tives of minimum molecular weights about 1100 and 1500, respectively, 
but in view of the failure of other reactions these cannot be accepted as 
evidence of the presence of free amino groups in the polyamide. At pres- 
ent we are inclined to assume that the amino and carboxyl groups which 
would normally be present at the ends of the polyamide chains are lost by 
pyrolysis. (The evolution of carbon dioxide during the formation of the 


polyamide could not be detected, but the vapors evolved had a strong amine 


e-Aminocaproic acid (or its ethyl ester) on being heated reacts with 
itself both intra- and intermolecularly and yields the 7-ring lactam (ca. 
30%) and a polyamide (ca. 70%) . The formation of a molecule of the latter 
probably involves at least ten molecules of the amino acid. 

Bibliography and Remarks 

(1) v. Braun, Ber., 40, 1840 (1907); Gabriel and Maas, ibid., 82, 1266 (1899). 

(2) Gabriel, ibid., 22, 3338 (1889); Schotten, ibid., 21, 2240 (1888). 

(3) v. Braun, ibid., 40, 1834 (1907); Wallach, Ann., 312, 205; 309, 18 (1899); Manasse, Bfr., 
35, 1367 (1902). 

(4) Baeyer and Seuffert, ibid., 32, 3619 (1899); Blaise and Koehler, Compt. rend., 148, 1772 
(1909); Marvel and co-workers, Journ. Am Chem. Soc., 46, 2838 (1924). 

(5) (a) Blaise and Koehler, Ref. 4; (b) Chuit and Hauser, Helv. Chim. Ada, 12, 4634 (1929); 
(c) Lycan and Adams, Journ Am. Chem. Soc., 51, 625, 3450 (1929); (d) Blaise and Marcilly, Bull, 
soc. chim., 31, 308 (1904). 

(6) Ruzicka and Stoll, Helv. Chim. Ada, 11, 1159 (1928). 

(7) (a) Lycan and Adams, Journ. Am. Chem Soc , Ref. 5c; (b) Carothers and Arvin, ibid., 51, 
2560 (1929); (c) Carothers and Van Natta, ibid., 52, 314 (1930); (d) Carothers and Dorough, ibid., 
52, 711 (1930); (e) Carothers, Dorough and Arvin, ibid., 52, 3292 (1930); (f) Chuit and Hauser, 
Ref. 5b; (g) Blaise and Marcilly, Ref. 5d; (h) Bougault and Bourdier, /. pharm. chim., (6] 29, 561 
(1909); [6] 30, 10 (1909). 

(8) The shifting of a functional group down a chain in attempts to prepare large rings by bifunc- 
tional reactions has been reported by various investigators. See, for example Blaise and Koehler, 
Ref. 4. 

(9) Carothers, Journ. Am. Chem. Soc., 51, 2556 (1929). 

IX. Polymerization* 


/. Definitions 82 

1 . Current definitions 82 

2. Proposed definitions 83 

3. Linear and non-linear polymers 84 

4. Types of compounds capable of polymerizing 84 

5. Types of polymerization 85 

6. Condensation polymerizations and bifunctional reactions 85 

II. Condensation polymerization 86 

L Polyesters 86 

a. The self-esterification of hydroxy acids 86 

b. Polyesters from dibasic acids and glycols 90 

* Wallace H. Carothers; Chemical Reviews 8, 353-426 (1931); Communication No. 
55 from the Experimental Station of the E. I. du Pont de Nemours and Company. 
Received March 21, 1931. Published June 1931. 


2. Bifunctional Wurtz reactions and Friedel Crafts reactions 93 

3. Other bifunclional reactions 95 

a. Polyamides 95 

b. Polyamines 96 

c. Polyacetals 96 

d. Poly anhydrides 96 

e. Grignard reactions 96 

f. Sulfur and selenium compounds 98 

g. Miscellaneous 98 

4. Stereochemical factors involved in condensation polymerization. . . 100 

a. Large rings 104 

III. Polymerization involving cyclic compounds 106 

1 . Six-membered cyclic esters 106 

2. Adipic anhydride no 

3. Diketopiperazines and polypeptides Ill 

4. Ethylene oxide 113 

IV. Addition polymerization of unsaturated compounds 113 

1 . Ethylene and other olefines 114 

2. Vinyl compounds 115 

3. Dienes 119 

4. Aldehydes 120 

V. Poly functional reactions and non-linear polymers 121 

VI. Natural polymers 123 

1. The association theory versus the structural theory 123 

2. Cellulose 124 

3. Rubber 127 

VII. The physical properties of high polymers 129 

1. Solubility and colloidal properties 130 

2. Crystallinity 132 

3. Mechanical properties 134 

This review is concerned with reactions that result in the combination of simple 
molecules to form materials of high molecular weight, and especially with the nature of 
such reactions and the structure and properties of the products. For the sake of com- 
parison, the structures of certain natural polymers concerning which it is possible to 
make definite statements are briefly considered also. The literature on the subject of 
high polymers is most profuse, and from the theoretical side it has until very recently 
been contradictory and confused. For these reasons the present discussion is of necessity 
rather selective and critical. (For other general discussions of certain aspects of this 
field the reader may consult references 1 to 7 and 18.) It is necessary also to define 
especially the sense in which certain terms are to be used. 

I. Definitions 

1. Current Definitions. It is generally agreed by organic chemists (9) 
that polymerization is chemical combination involving the operation of 
primary valence forces, and that the term polymer should not be used 


(as it frequently is by physical and inorganic chemists) to name loose or 
vaguely defined molecular aggregates. Beyond this, however, there is 
not much agreement. The word polymer was introduced by Berzelius 
(12) nearly a century ago to recognize the fact that two compounds may 
have the same composition but different molecular weights, and he classi- 
fied polymerism as a special type of isomerism. The accepted meaning of 
these terms has subsequently undergone considerable change, but current 
textbook definitions always state that a monomer and its polymer have 
the same composition, and it is usually either stated or implied that the 
process of polymerization is peculiar to unsaturated compounds and con- 
sists in self-addition. But recently discovered facts show that these 
conditions really are not satisfied in many instances to which the terms 
polymer and polymerization are applied. The formula of a-polyoxy- 
methylene, for example, is not (CH 2 O)*, but (CH 2 O)*H 2 O (110). 

It is true also that many compounds which are not unsaturated are 
capable of reacting with themselves to form products of high molecular 
weight, and that such reactions are quite commonly called polymerization. 
For example, benzyl chloride in the presence of aluminum chloride: 

*C 6 H 5 CH 2 C1 -* (CyHs)* + *HC1 

One can assume that the first step in this reaction is the elimination of 
HC1 to yield the radical C6H 6 CH = , which, being unsaturated, is capable 
of reacting with itself by addition. On the other hand, it may be that this 
is simply a polymolecular Friedel Crafts reaction, the first step being the 
formation of C6H5CH 2 C6H 4 CH 2 C1, which is capable of reacting with itself 
by the same mechanism and finally yielding a very long chain in which the 
chlorine content is negligible. If the usual definitions are adopted, this 
reaction can properly be called polymerization only if it proceeds by the 
first mechanism. Curiously enough, one actually finds in the literature 
cases in which the fact that a reaction is called polymerization is used to 
prove that the first step must be the formation of an unsaturated inter- 
mediate capable of reaction with itself by addition. It is perhaps un- 
necessary to say that the questions of the composition of high polymers 
and the mechanism of their formation are frequently rather intricate and 
cannot be solved in advance by definition. 

2. Proposed Definitions. It is more practical and useful (and also 
more consistent with actual usage) to define polymerization as any chemical 
combination of a number of similar molecules to form a single molecule. 
A polymer then will be any compound that can be formed by this process 
or degraded by the reverse process: formaldehyde can be regenerated 


from polyoxymethylene by the action of heat; ethylene glycol can be ob- 
tained from polyethylene glycol by hydrolysis; cellulose can be hydrolyzed 
to glucose; and rubber can be formed by the reaction of isoprene with 

3. Linear and Non-linear Polymers. The simplest and perhaps the 
most numerous and important high polymers are characterized structurally 
by the fact that their molecules are long chains built up from a repeating 
radical or unit. This type of structure may be represented by the general 

..... XV XV XV XV XV XV XV XV XV ' ..... 

The repeating radical A is called the structural unit. The number 
and nature of these units determine the nature of the molecule. To 
complete this formula it is necessary to specify the disposition of the 
terminal valences; they may conceivably be mutually joined to form a 
cyclic structure or saturated by univalent groups. Whether the chain is 
open or closed, polymers of this class will be called linear polymers. It 
should be added that linear high polymers are usually mixtures containing 
chains of different lengths. 

If the polymer is derived from a single compound the structural units 
of the chain will in general be identical, but the mutual reaction of two or 
more compounds that are chemically similar but not identical may lead 
to chains made up from two or more different units. Such products will 
be called linear mixed polymers.* Many polypep tides and proteins be- 
long to this class. Non-linear polymers also exist; they can be formed, 
for example, by the cross linking of long chains into two- or three-dimen- 
sional structures. 

4. Types of Compounds Capable of Polymerizing. The step-by-step 
synthesis of long molecular chains containing a repeating unit is illustrated 
by Fischer's famous synthesis of polypeptides. Reactions of polymeriza- 
tion, however, lead to the formation of polymeric chains in a single opera- 
tion. The capacity for self -combination of this kind is found among simple 
molecules of three different types: (a) unsaturated compounds; (b) cyclic 
compounds; (c) polyfunctional compounds. These are illustrated by the 
following examples. 

(a) nCH2=O + H 2 O - HO CH 2 O CH 2 O . . . CH 2 O CH 2 O CH 2 O H 

Formaldehyde a-Polyoxymethylene 

(b) n[CH 2 CH 2 O] + H 2 O -> HO CH 2 CH 2 O CH 2 CH 2 O . . . CH 2 CH,O H 

Ethylene oxide Polyethylene glycol 

* The term heteropolymer has also been suggested (23). 


(c) w[HO CH 2 CH 2 OH] - (n - 1)H 2 O -* 
Ethylene glycol 

HO CH 2 CH 2 O CH 2 CH 2 O . . . CH 2 CH 2 O H* 
Polyethylene glycol 

In each of these examples the product molecule is made up of a repeat- 
ing series of identical units. Each unit corresponds with one molecule 
of the starting material. The latter, therefore, is called the monomer 
(mer = part). Ethylene glycol and ethylene oxide are both monomers 
since, although they differ in structure and composition, their molecules 
contain one CH 2 CH 2 O unit each. Similarly diethylene glycol, 
HO CH 2 CH 2 CH 2 CH 2 H, is a dimer. 

5. Types of Polymerization. Molecules can combine to form larger 
molecules either by addition or by condensation, f and two corresponding 
types of polymerization may be recognized. The formation of polyoxy- 
methylene from formaldehyde is addition or A polymerization. The 
formation of polyethylene glycol directly from ethylene glycol is condensa- 
tion or C polymerization.** This second type of polymerization, although 
its existence as a general phenomenon has only recently been recognized 
(18), is especially simple both practically and theoretically since it involves 
the known behavior of typical functional groups. 

6. Condensation Polymerizations and Bifunctional Reactions. A 
general class of condensation polymerizations is represented by the equa- 

x R y H- x R y + x R y -f -* x R z~R z R . . . 

In the formula x R y, R is a bivalent radical and x and y are functional 
groups capable of reacting with each other to form the known functional 
group z. Thus if x is HO and y is COOH, z will be CO O. The com- 
pounds x R y are called bifunctional compounds and their reactions, 
bifunctional reactions. Reactions of the type, x R x + y R' y ""* 
product, may be included in this class. 

* The direct polyintermolecular dehydration of ethylene glycol to polyethylene 
glycol not involving the intermediate formation of ethylene oxide is perhaps somewhat 
hypothetical. It is used here merely as a formal illustration. 

f The term condensation is used here to name any reaction that occurs with the 
formation of a new bond between atoms not already joined and proceeds with the 
elimination of elements (hydrogen, nitrogen, etc.) or of simple compounds (water, ethyl 
alcohol, ammonia, sodium bromide, etc.). Examples are the Wurtz reaction, Friedel- 
Crafts reaction, esterification, etc. See Kempf in Houben-Weyl, ref. 6, Volume II, p. 

** The term condensation polymerization was at one time applied by Staudinger 
(38) to addition polymerizations that involve the migration or displacement of a group 
or an atom, but its use in this sense has never become general. In view of its obvious 
propriety for the purpose, it was adopted by the writer (18) as a name for polyinter- 
molecular condensation. 


Bifunctional reactions present the possibility of following various 
courses, (a) They may be intramolecular at the first stage. The product 
will then be the cyclic monomer 

(i) Reaction may be intermolecular at the first stage and intramolecular 
at some subsequent stage. The product will then be a cyclic polymer 

[( R Z )]. 

(c) Reaction may be exclusively intermolecular. In this case the product 
will be an open chain of the type x R z R z .... R z R y. 
These possibilities may be illustrated by the hydroxy acids of the series 
H O (CH 2 ) W CO OH. The self-esterification of these might lead 
to the simple lactones 

[O (CH,) n CO], 

or to cyclic polyesters 

f[0 (CH 2 ). CO] P ] 

or to open chain polyesters 

H[O (CH 2 ) n CO] P OH. 
All three of these possibilities can be realized. 

II. Condensation Polymerization 

1. Polyesters. Bifunctional esterifications are especially suitable for 
the study of condensation polymerization, because esterification is a re- 
versible reaction, and it is entirely free from side reactions under conditions 
that are easy to realize. 

a. The self-esterification of hydroxy acids. It is well known that 
hydroxy acids react with themselves to form cyclic esters when there is the 
possibility of forming a 5- or a 6-membered ring: 7- and 5-hydroxy acids 
lead to the corresponding lactones, and a-hydroxy acids yield the cyclic 
dimers. Thus glycolic acid reacts with itself to form glycolide:* 

2HO CH 2 COOH -* [O CH 2 CO O CH 2 CO ] + 2H 2 O 

The 7-lactones are stable substances, but the 5-lactones and glycolide 
and its analogs on being heated are rapidly transformed into polyesters of 

* But it is by no means certain that the glycolide is ever formed through the steps 
glycolic acid -* glycolylgly colic acid -* glycolide. Dietzel and Krug (19) have shown 
that in the self-esterification of lactic acid the primary steps are: lactic acid -* 
lactyllactic acid -* polylactyllactic acids. Lactide when it is formed results from the 
depolymerization of these polylactyllactic acids. 


high molecular weight. This peculiar behavior of the 6-membered cyclic 
esters is considered in more detail in a later paragraph. 

No information is available concerning the behavior of the simplest 
c-hydroxy acid, hydroxycaproic acid, but the corresponding bromo acid 
when treated with sodium ethylate in alcohol solution gives a poor yield 
of the seven-membered lactone (37). The chief product is an undistillable 
material, undoubtedly polyester. c-Hydroxycaprylic acid, C 2 H 8 CH- 
OH (CH 2 ) 4 COOH is partly converted into the corresponding seven- 
membered lactone on being heated, but the next member of this series, 
C2H5 CHOH (CH 2 )5 COOH, under the same conditions gives only an 
undistillable residue (24). 

Larger lactones containing rings of fourteen to eighteen atoms are known 
(25), and also /3-lactones (4-membered rings) (26, 27). All of these are 
fairly stable compounds, but none of them has ever been prepared directly 
from the corresponding acid.* 

The attempt to prepare self -esters from simple /3-hydroxy acids results 
merely in dehydration to the unsaturated acid, but in hydroxypivalic 
acid, HO CH 2 C(CH 3 ) 2 COOH, this usual behavior is impossible 
because of the absence of any a-hydrogen. The self-esterification of this 
acid has been studied by Blaise and Marcilly (28). It occurs at 200C. 
or at lower temperatures in the presence of certain catalysts. The product 
is a microcrystalline powder insoluble in most organic solvents. It forms 
sodium salts insoluble in cold water, and it is readily hydrolyzed to hy- 
droxypivalic acid. These facts clearly indicate a structure of the type 

HO CH 2 C(CH 3 ) 2 COO[ CH 2 C(CH 3 ) 2 COO ] CH 2 C(CH 3 ) 2 COOH 

and on the basis of molecular weight and analytical data Blaise and Mar- 
cilly assign to n the value 4. 

In regard to the higher o>-hydroxy acids, Chuit and Hausser (29) have 
recently synthesized the entire series from HO (CH 2 ) 7 COOH to HO 
(CH 2 ) 20 COOH. All these acids readily undergo self-esterification on 
being heated. The products are not the corresponding lactones (which 
in several cases are known), but solid acidic materials whose properties are 

* Kerschbaum (36) records various attempts to prepare the lactone of ambrettolic 
acid (hexadecene-7-ol-16-acid-l) from the acid obtained by hydrolyzing the naturally 
occurring lactone. By heating the acid he obtained a small amount of oil which, from 
its odor, was inferred to contain lactone. The chief product, however, was a non- 
volatile material soluble in alkali. Neither Chuit and Hausser (29) nor Lycan and 
Adams (30) report any evidence for the presence of lactone in the self -esters prepared 
from higher co-hydroxy acids. Unpublished experiments made in this laboratory have 
failed to reveal the presence of any lactone among the products of the dehydration of 
co-hydroxypentadecanoic acid. 



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consistent with the general formula HO R CO O R CO . . . . 
O R COOH. Similar products obtained by the self-esterification of 
higher hydroxy acids have also been investigated by Bougalt and Bourdier 
(31) and by Lycan and Adams (30). 

The presence of a terminal carboxyl group in each of these products 
proves that the ester formation is exclusively intermolecular. It should 
be possible, therefore, by regulating the degree of completeness of the 
esterification to obtain molecules of various lengths and, in particular, to 
obtain exceedingly long molecules. Such molecules are of especial interest 
in connection with the study of high polymers, since the nature of the acid 
used absolutely determines the structural unit of the polyester. 

b. Polyesters from dibasic acids and glycols. The study of bifunctional 
esterifications from this standpoint was first undertaken by the writer 
and his collaborators (32 to 35). Dibasic acids and glycols were used as 
starting materials since these are more readily accessible than hydroxy 
acids, and they also permit more numerous structural variations in the 
ester product. 

Esters derived from dibasic acids and glycols can be prepared by any of 
the typical reactions ordinarily used in the preparation of simple esters, 
e. g.: 

(a) HO ROH + HOOC R' COOH -* O R O CO R' CO + H 8 O 

(b) HO ROH + EtOOC R' COOEt -* O R O CO R' CO -f EtOH 

(c) HO ROH + C 6 H 5 OOC R' COOC 6 H 6 -*> O R O CO R' CO h 

C 6 H 6 OH 

(d) HO ROH + C1CO R' COC1 -* O R O CO R' CO + HC1 

(e) Br R Br + AgOCO R' COOAg -+ O R O CO R' CO + AgBr 

In these equations O R O CO---R' CO represents the struc- 
tural unit of the product. The studies have proved that the nature of the 
ester is completely determined by the number of atoms in the chain of the 
unit: if this number is five, the product is monomeric and cyclic (i. e., it 
contains only one unit) ; if the number is six, the product can be obtained 
in both monomeric and polymeric forms and these are interconvertible; 
if the number is more than six, the product is exclusively polymeric. 

The preparation of an ester of this last class may be illustrated by the 
action of succinic acid on ethylene glycol.* Succinic acid mixed with a 

* The preparation of ethylene succinate by reaction between the acid and glycol 
was first carried out by Lourenco (17). Later Davidoff (40) and Vorlander (41) pre- 
pared it by various methods, and the latter investigator assigned to it the formula of 
a 16-membered cyclic dimer. His molecular weight determinations were apparently in 
error. The dimeric ester is now known (42, 34) and its properties are quite different 
from those of Vorlander's ester, which closely resembles the polymeric ester prepared 
from the acid and the glycol. 


slight excess of glycol is heated in a distilling flask. At about 160-175C. 
rapid esterification sets in, accompanied by the liberation of water. When 
no more water is evolved the temperature is raised to 220-250 C. and the 
pressure is reduced below 1 mm., whereupon most of the excess glycol dis- 
tils. The ester product is completely non-volatile and remains in the dis- 
tilling 1 flask in the form of an exceedingly viscous liquid. When this is 
cooled it solidifies to a hard, brittle, opaque, white mass. This can be 
dissolved in cold chloroform and precipitated as a granular powder by the 
addition of ether or benzene. It can be recrystallized from hot ethyl 
acetate or a large volume of hot acetone. The recrystallized ester melts at 
about 102C. A product having the same physical properties can be 
prepared in the same way by the action of diethyl succinate or diphenyl 
succinate on ethylene glycol. The reaction then consists in ester inter- 
change and the volatile product liberated is alcohol or phenol. 

By the method of direct esterification or by ester interchange the esters 
listed in Table I have been prepared. They are without exception highly 
polymeric, the molecular weights being on the average in the neighborhood 
of 8000. Molecular weights of several of the esters have been determined 
in a variety of solvents and by both freezing and boiling point methods, 
and within the limits of experimental error the same values have been 

These esters show considerable similarity in their physical properties. 
They are completely non-volatile and all of them dissolve quite readily in 
cold chloroform. Those derived from the higher glycols or acids also dis- 
solve in cold benzene, and in the solid state they are less hard and more 
wax-like than ethylene succinate. In spite of their high molecular weight 
and lack of complete homogeneity all these esters except the phthalates 
are crystalline. The melting points vary slightly from one preparation to 
another of a given ester and usually cover a range of about 2. 

The ethylene succinate described above is neutral, and its composition 
and molecular weight indicate the average formula* 

HO [(CH 2 ) 2 O CO (CH 2 ) 2 CO O ] 22 (CH 2 ) 2 OH, 

* The molecules of this product certainly do not all have the same length, but on 
the other hand it appears to be much more nearly homogeneous than any products ever 
obtained by addition polymerization (polystyrene, etc.). In its qualitative solubility 
behavior it resembles a chemical individual, and fairly elaborate fractional crystalliza- 
tions have failed to separate it into portions showing any considerable difference in their 
properties. The reason for its relatively great homogeneity lies in the conditions of its 
formation, which are such as to force much smaller molecules to react with themselves, 
but are not sufficiently drastic to cause the formation of much larger molecules. Under 
other conditions, polymeric mixtures having a wide range of molecular weights can bo 


i. e. y it is an open chain derived from twenty-two molecules of acid and 
twenty-three molecules of glycol. 

The hydroxyls at the end of this chain do not readily react with the 
usual reagents for alcohol groups; their sluggish behavior in this respect 
is characteristic of many high molecular weight materials. However, on 
being heated to a fairly high temperature with ^-bromobenzoic anhydride 
the ester yields a di--bromobenzoyl derivative identified as such by its 
bromine content. The ester also reacts with molten succinic anhydride 
and yields an acidic ester whose neutral equivalent and observed molecular 
weight agree with the formula I d. 

(I) HOCO (CH 2 ) 2 CO [ O (CH 2 ) 2 O CO (CH 2 ) 2 CO ] x OH 
I a, x = 6; I b, x = 9; I c, x = 12; I d, x 23. 

Similar acidic esters of lower molecular weight have also been obtained by 
partial esterification of glycol with an excess of succinic acid and fractional 
crystallization of the product. The observed molecular weights and 
neutral equivalents of the fractions corresponding to various values of x 
in formula I are indicated in Table II. 


Molecular weight 

Sodium salt 






Found by 








I b 


1 c 




The presence of these terminal groups proves that the esterification of 
succinic acid by ethylene glycol is intermolecular at every stage.* The 
reaction evidently involves a series of condensations resulting in the pro- 
duction of ester molecules of progressively greater length. The first prod- 
uct might be 

HO(CH 2 ) 2 OCO(CH 2 ) 2 COOH, 
and the second 

HO(CH 2 ) 2 OCO(CH 2 ) 2 COO(CH 2 ) 2 OCO(CH 2 ) 2 COOH. 

* Lycan and Adams (30) also present convincing evidence of the open chain struc- 
tures of polyesters derived from w-hydroxydecanoic acid. They isolated fractions 
whose molecular weights estimated by titration with alkali ranged from 1000 to nearly 
9000. All of these fractions formed potassium salts which were completely soluble in 
warm water. 


Moreover, since both glycols and dibasic acids have a tendency to esterify 
at both ends simultaneously, it is reasonable to suppose that similar chains 
will be present, some of which are terminated at both ends by carboxyl 
and others by hydroxyl. It is evident that an exceedingly large number of 
species may be involved in the ester equilibrium.* But as the amount of 
water participating in the equilibrium is diminished by its constant removal, 
the smaller molecules are forced to couple with each other until finally 
practically none of them remains. A further simplification of the kinds 
of molecular species present in the product is effected by using an excess of 
glycol, since this makes all the terminal groups alike. For this purpose, it 
makes no difference how large an excess of glycol is used , provided the reac- 
tion mixture is finally heated for some time in a high vacuum, since under 
these conditions the reaction can be propagated by ester interchange. 
Thus it is possible to isolate the monomeric ester, foV/3-hydroxyethyl- 

HO (CH 2 ) 2 O CO (CH- 2 ) 2 CO O (CH 2 ) 2 OH, 

and this on being heated to 200C. in a vacuum loses glycol and is converted 
into the neutral polyethylene succinate of molecular weight 3000 already 
described. In view of this fact it is obvious that the accidental mutila- 
tion of the terminal groups (e. g., the loss of OH or CC^) cannot in itself 
prohibit the progress of the coupling; the reaction can progress by ester 
interchange involving the last ester linkage. 

Apparently the ultimate factors that set the attainable limit on the 
length of polyester molecules are purely physical. As the molecular weight 
becomes greater, the reaction product becomes more viscous and the rate 
of diffusion of the volatile product (water or alcohol) to the surface becomes 
slower and slower, f Moreover as the reaction progresses, the mobility 
of the reacting molecules diminishes and the relative concentration of the 
reactive groups becomes smaller. All of these factors affect the rate and 
some of them affect the position of equilibrium, but the molecular weight 
at which a practical limit is reached will depend upon the temperature and 
pressure, the area and thickness of the reacting mass, etc., and by a suit- 
able adjustment of these factors it is possible to obtain polyesters having 
very much higher molecular weights than those listed in Table I (51). 

2. Bifunctional Wurtz Reactions and Friedel Crafts Reactions. The 
action of sodium on polymethylene bromides, Br(CH2)*Br, leads in certain 

* The kinetics of the reaction between phthalic anhydride and ethylene glycol have 
been studied by Kienle and Hovey (43). 

t At least in the crystalline state polyesters adsorb water and hold it very tena- 


cases to the formation of cyclic hydrocarbons. Cyclopropane, cyclopen- 
tane, and cyclohexane have been prepared in this way. On the other 
hand, the action of sodium on decamethylene bromide does not yield any 
cyclic hydrocarbons. 

In the presence of absolute ether this reaction proceeds very smoothly 
(44, 45). The product, which is for the most part insoluble in ether, con- 
sists of a complex mixture formed by the coupling of various numbers of 
molecules of the halide with each other, e. g., 

Br(CH 2 ), Br + 2Na + Br(CH 2 )ioBr + 2Na + Br(CH 2 ) 10 Br -> 
Br(CH 2 ) 30 Br + 4NaBr. 

At the same time, owing to the participation of the ether in the reaction, 
most of the terminal bromine atoms are replaced by hydrogen.* This 
reduction of the terminal groups can finally be carried to completion by the 
action of sodium in boiling butyl ether, and the product then consists of a 
mixture having the general formula 

H f(CH 8 )io]* H 

in which the values of x range from 1 to at least 10. The various members 
of this mixture up to and including w-C 7 oHi 42 can be separated and isolated 
in a state of purity by fractional evaporation in a molecular still followed 
by crystallization. The identity of these fractions is established by their 
melting points and their x-ray diffraction patterns. There is no evidence 
of the presence of any materials in the mixture not belonging to the series 

H [(CH 2 ) 10 ]* H 

About 25 per cent, of the total product consists of members standing above 
C7oHi42. These are not capable of being distilled, and they cannot be 
separated from each other by fractional crystallization. The average 
molecular weight of this material indicates that it must contain hydro- 
carbons at least as high as CiooH 2 o2. 

The following results of unpublished studies made by Dr. R. A. Jacob- 
son in this laboratory are more or less closely related to those described 
above. The action of metallic sodium on ^-dibromobenzene in absolute 
ether leads to a product corresponding in composition to the formula 
Br CeH 4 (CeH^e CeH 4 Br. When the reaction is carried out in boiling 
toluene the product formed corresponds approximately in composition and 

* In its mechanism the Wurtz reaction undoubtedly involves as its first step the 
formation of the sodium compound RNa (46). This normally couples with another 
molecule of the halide, but if ether is present it may be destroyed by the reaction 
RNa + (C ? H 6 ) 2 Q -* H + C 2 H 4 + C 2 H 6 ONa (47). 


molecular weight to the formula Br C 6 H 4 (CeH 4 )i 2 CeH 4 Br. Both of 
these products are readily soluble in benzene. ^-Xylylene bromide, 
BrCH 2 C 6 H 4 CH 2 Br, when treated with sodium in hot toluene, yields a 
very insoluble hydrocarbon which does not melt below 350C. Apparently* 
a very large number of ^-xylylene units participate in this coupling. 

Benzyl chloride in the presence of aluminum chloride or ferric chloride 
(see 10) reacts with itself to form resins of high molecular weight. This is 
evidently a bifunctional Friedel Crafts reaction involving the progressive 
coupling of successively longer chains with the elimination of hydrogen 
chloride. Depending upon the conditions of their formation these resins 
are fusible and soluble, or infusible and insoluble. Molecular weight values 
indicate that the fusible resins are formed from 14 to 25 molecules of benzyl 
chloride. The infusible resins are no doubt much more highly polymeric. 
Benzyl fluoride in the presence of a trace of acid reacts very vigorously 
with itself in a similar manner (11). 

3. Other Bifunctional Reactions. The number of possible types of 
condensation polymers is practically unlimited. Although very few of 
these possibilities have received any considerable study, the following ex- 
amples at least indicate that the formation of such polymers is the usual 
result of bifunctional reactions when structural features preclude the 
formation of a 5- or a 6-membered ring. 

a. Polyamides. The acids NH 2 (CH 2 ) 3 COOH and NH 2 (CH 2 ) 4 COOH 
are readily dehydrated to the corresponding monomeric lac tarns (48). 
There is no record of either of these being caused to polymerize. The 
next higher member yields two products (49, 50a). One of these (20-30 
per cent.) is the lactam (7-membered ring), a distillable crystalline mate- 
rial; the other is a poly amide, an undistillable hard, waxy material insol- 
uble in most solvents except concentrated hydrochloric acid, phenol, and 
hot formamide. It can be hydrolyzed quantitatively to the amino acid. 
Molecular weight determinations indicate the presence of at least ten 
structural units in its molecule. The formation of this polymer is due 
to direct intermolecular condensation and the formation of the lactam is 
an independent reaction, for the latter cannot be polymerized under the 
conditions that lead to the production of the former. The acid NH 2 (CH 2 ) 6 - 
COOH on being dehydrated yields exclusively a product that is polymeric 
(50), although the corresponding monomeric lactam has been prepared by 
another method and is a stable substance (164). The acid NH 2 (CH 2 )i - 
COOH also yields only poly amide (51). 

A number of polyamides have been prepared by Dr. J. E. Kirby in this 
laboratory by the action of dibasic acids on aliphatic diamines. These 


materials are all much less soluble, and when crystalline have much higher 
melting points than the analogous polyesters (51). 

b. Polyamines. v. Braun observed (52) that the compound NH 2 - 
(CH 2 )eCl, unlike its immediate lower homologs, when it reacts with itself 
yields only a very small amount of the volatile cyclic base. The chief 
product is an undistillable, waxy solid, but this has never been studied in 

The formation of quaternary ammonium salts from various compounds 
of the series (CH 3 )2N(CH 2 ) n Br has been studied (53). The products have 
been assumed to be cyclic monomers or dimers, but it still remains to be 
demonstrated that none of them are linear polymers. 

c. Polyacetals. The reaction between glycols and acetaldehyde (or 
acetylene) presents the possibility of forming cyclic acetals, 


CH 3 CH R 



or polyacetals 

HO R O CH O R O CH O R O CH O . . . 

I I I 

CH 3 CH 3 CH 3 

This reaction has been studied by Hill and Hibbert (8). Ethylene and 
trimethylene glycols gave in excellent yields the cyclic acetals which are 
5- and 6-atom rings. Tetramethylene glycol gave in poor yield a volatile 
compound which was apparently the monomeric cyclic acetal containing a 
7-atom ring. A considerable part of the product was an undistillable sirup. 
The products from octamethylene and decamethylene glycols were also 
undistillable sirups. No molecular weight determinations are recorded, 
but it may be assumed that the undistillable products are polymeric. An 
analogous compound prepared in this laboratory (51) from benzaldehyde 
and diethylene glycol had an apparent molecular weight of about 1370. 

d. Poly anhydrides. Dibasic acids of the series HOOC(CH 2 )*COOH 
are readily converted into the corresponding anhydrides. Malonic an- 
hydride, the first member of the series, is polymeric (54). The next two 
members, succinic and glutaric anhydrides, are known only as the mono- 
meric 5- and 6-membered rings. The anhydrides of all the higher acids 
are polymeric.* The monomeric anhydrides are macrocrystalline, readily 

* The known examples are anhydrides of adipic, pimelic, suberic, azelaic, sebacic 
(55) and hexadecamethylene dicarboxylic (51) acids. These if monomeric would be 
respectively, 7-, 8-, 9-, 10-, 11-, and 19-membered rings. Adipic anhydride can be ob- 
tained in both a monomeric and a polymeric form (56). 


distillable solids; the polymeric anhydrides are non-volatile microcrystal- 
line powders or waxes, and they are less soluble than the monomeric an- 
hydrides. Both types are very reactive, but qualitatively their chemical 
behavior is not the same. The monomers react with aniline to give pure 
monoanilide; the polymers give a mixture of acid, monoanilide, and di- 
anilide in the ratio 1:2:1. This is precisely in accordance with the cal- 
culated behavior of a very long chain having the general structure. 

CO R CO O CO R CO O CO R CO O , etc. 

e. Grignard reactions. Bifunctional Grignard reagents such as BrMg- 
(CH 2 )6MgBr are capable of reacting with bifunctional reactants (dialde- 
hydes, dike tones, simple esters, etc.). The products may be rings or long 
chains. Several 5- and 6-atom rings have been prepared in this way, and 
in poor yield one 7-atom ring (57). In this laboratory it has been found 
(51) that decamethylene dimagnesium bromide reacts readily with methyl 
formate : 

. . . (CH 2 ) 10 MgBr + O=-D OCH 3 + BrMg . . . -* 


. . . [ (CH*),- CH(OMgBr) ] x . . . 

The final product is a microcrystalline solid readily soluble in various 
organic solvents and melting at about 120C. Its chemical and analytical 
behavior shows that it is the expected linear poly alcohol containing about 
eight of the structural units (CH2) 10 CHOH . 

In the formation of bifunctional Grignard reagents some coupling always 
occurs: dibromides of the formula Br(CH 2 ) w Br yield considerable amounts 
of BrMg(CH 2 ) 2n MgBr and progressively smaller amounts of higher cou- 
pling products. It is a curious fact that this reaction does not occur intra- 
molecularly even in the case of pen tame thy lene bromide where it would 
lead to the formation of a 5-membered ring (154). 

The coupling of simple Grignard reagents can be effected by the action 
of iodine, and this method has been applied by Grignard and Tcheoufaki 
to acetylene dimagnesium bromide (155). Products having the following 
formulas were isolated : 

HC==C Cs=CI; HC^C C^C -CssC I; 

and also a form of carbon, which probably resulted from a continuation of 
the initial reaction in the same sense. 

The attempt to prepare a Grignard reagent from ^-xylylene bromide, 
BrCH 2 C 6 H4CH 2 Br, leads (51) to an insoluble hydrocarbon having the 


( CH 2 C 6 H 4 CH 2 ),. 

The value of x is probably very large. 

/. Sulfur and selenium compounds. The action of sodium sulfide on 
alkylene halides of the formula X(CHa) w X leads to considerable yields of 
the expected cyclic products (CH2) W >S only when these are 5- or 6-mem- 
bered rings (162). Ethylene bromide gives white amorphous insoluble 
products (161); when prepared under certain conditions these are capable 
of being depolymerized by heat to yield the cyclic dimer, diethylene di- 
sulfide. Monomeric ethylene sulfide has been prepared by Delepine (86) 
by treating ethylene thiocyanate with sodium sulfide. It polymerizes 
spontaneously on standing. The chief product of the action of sodium 
sulfide on trimethylene halides is an amorphous polymer, although a small 
amount of the cyclic trimethylene sulfide can be obtained under certain 
conditions (87). Hexamethylene iodide also yields an amorphous polymer 
as the chief product (163, 87). v. Braun suggests that its formula is 

I(CH 2 VS-(CH 2 ) fl . . . .S(CH,)J. 

but no estimates of its molecular weight have been presented. 

The amorphous material formed from ethylene chloride and sodium 
sulfide is capable of being molded into a product which is very resistant to 
solvents and somewhat resembles rubber in its properties. One may infer 
that its molecular weight is very high. In recent patents (159) it is claimed 
that similar products can be obtained from alkylene halides generally. 

Morgan and Burstall (160) state that the interaction of sodium selenide 
and the requisite alkylene dibromide leads readily to the production in good 
yields of cyclic seleno hydrocarbons containing 5- or 6-membered rings, 
but trimethylene bromide and sodium selenide give little cycloselenopro- 
pane. The main product is a sixfold polymer melting at 38-40 C. 

g. Miscellaneous. A curious spontaneous progressive coupling is found 
in the action of water on methyl orthosilicate. The reaction may be 
formulated as follows : 

OCH 8 OCH 8 

CH 3 O Si OCH 3 -f H 2 O -* CH 3 O Si OH 
OCH 3 OCH 3 


2CH 3 OvSi OH -* CH 3 O- Si OSi OCH S + H 2 O 



Partial hydrolysis and coupling of this dimeric product lead to a tetramer, 
and so on. By suitably adjusting the initial ratio of water to ester it is 
possible to obtain samples representing various degrees of polymerization. 
The lower members have been isolated as chemical individuals by fractional 
distillation (156). The intermediate members are viscous undistillable 
liquids soluble in organic solvents. Solutions of these polymers are used 
in the preparation of paints having unusual properties (60). Films pre- 
pared from these paints harden by a continuation of the initial progressive 
hydrolysis and coupling, which yields as the final product pure silica. This 
synthesis incidentally furnishes an elegant proof of the three-dimensional 
polymeric structure of silica. The OCH 3 groups are responsible for the 
coupling, which results in the formation of long chains from the mono- 
meric ester. 

OCH 8 OCH 3 OCH 3 OCH 8 OCH 3 

^i__ -Si-0~Si-0-Si-0-Si-0~ 

I 1 ^ ^ f ^ 

OCH 3 OCH 3 OCH 3 OCH 3 OCH 3 

Since each silicon atom in these chains still bears two OCH 3 groups, the 
coupling can continue in the other dimensions, yielding finally a 3-dimen- 
sional lattice in which each silicon atom is joined (through oxygen) to four 

Metallic sodium acts on compounds of the type R2SiCl2 yielding chain 
structures composed of SiR2 SiR2 groups (157). These products are 
for the most part amorphous and glue-like. 

The specially pronounced tendency toward the formation of 5- and 6- 
membered rings is frequently manifested in complex coordination com- 
pounds (148) and complex formation frequently fails when it would involve 
the formation of larger rings. Ethylene and trimethylene diamine yield 
crystalline nickelotriene succinimide compounds of the formula [Nien 3 ]- 
Sucj and [Nitrs] Suc2, but the compounds from tetra- and penta-methylene 
diamines are amorphous and, according to Tschugaeff (114), they must be 
represented as chains of unknown length: 

Sue Sue 

H 2 N(CH 2 )NH 2 . . .Ni. . .H 2 N(CH 2 ) n NH 2 . . .Ni. . . 
I I 

Sue Sue 

Bifunctional couplings probably occur in many reactions of oxidation 
and reduction, Busch and Schmidt h^ve shown (22) that the catalytic 


reduction of aryl halides under some conditions proceeds to a considerable 
extent as follows : 

2C 6 H 6 Br + 2H -* C 6 H 6 C 6 H 6 + 2HBr 

When the same reaction is applied to -diiodobenzene, terphenyl, CeHb'- 
CeH^CeHs, and ,'-diphenyldiphenyl are obtained. 

A preliminary stage in the oxidation of aniline to aniline black is the 
formation of long chains of the following type (20), 

In a similar way the oxidation of phenols may lead to the formation of 
polyphenylene ethers (21). Substituted polyphenylene ethers of high 
molecular weight are obtained by the condensation polymerization of silver 
salts of halogenated phenols (173). 

4. Stereochemical Factors Involved in Condensation Polymerization. 
Bifunctional reactions always present the possibility of following two 
courses they may lead to ring closure, or to progressive coupling but 
the very numerous studies of such reactions have been concerned almost 
exclusively with ring closure. The possibility of progressive coupling has 
frequently been ignored or rejected as unlikely, and highly improbable 
cyclic structures have occasionally been assigned to products on the 
ground merely that they were the result of bifunctional reactions. In 
other cases the actual products have been discarded simply because they 
were obviously not the expected cyclic products. This attitude has 
certainly been partly responsible for the comparative meagerness of the 
literature on condensation polymers. The examples cited in the previous 
paragraphs show, however, that bifunctional reactions may proceed in a 
strictly normal fashion by progressive coupling and result in the formation 
of large molecules, and that, considering any particular homologous series, 
this type of reaction is the rule and ring closure is the exception. 

This is not an especially astonishing conclusion. Intermolecular reac- 
tion is a perfectly general phenomenon; intramolecular reaction is a pecu- 
liar and special kind of happening. This is immediately evident from the 
consideration that if two groups are to react they must meet, i. e., they 
must approach each other very closely. If the groups are not present in 
the same molecules, such approach is always possible as long as the mole- 
cules are free to move about. But two groups, x and y, that are present at 
opposite ends of a single molecule are capable of approaching each other 
only if the architecture of the molecule permits, and even if such approach 
is permissible it does not necessarily follow that it will occur, or if it occurs 


that it will be effective. Meanwhile the x groups of this molecule are 
continually colliding with y groups of other similar molecules. From this 
standpoint progressive coupling in bifunctional reactions is inherently more 
probable, generally speaking, than ring closure, and the latter, if it occurs 
to the exclusion of the former, must be peculiarly favored by some special 

The reasons for the great difference in the relative ease with which dif- 
ferent types of rings are formed has been the subject of much speculation, 
and two factors that have frequently been assumed to be of self-evident 
importance are the relative energy content of the rings and their stability 
or degree of strain. There is, however, no theoretical justification for this 
assumption (58), and the facts show that neither of these factors can pos- 
sibly be decisive. Thus, the action of sodium on propylene bromide yields 
cyclopropane, not cyclohexane, although the latter has a much lower energy 
content per unit and is less highly strained than the former. 

On the other hand spatial relations in the reacting compound must of 
necessity have a great influence on the possibility of ring formation. The 
atoms of the benzene ring, for example, and all the atoms joined directly 
to it lie in the same plane (59), and to link the two para positions of the ring 
together through a chain of less than four or five accessory atoms would 
require a very improbable degree of distortion. Many attempts have 
been made to bring about such linkings, but the supposedly successful 
examples have proved on reexamination to be fictitious (62). The same 
thing is true (61) of alleged examples of ring closure through the p and p r 
positions of diphenyl. 

The stereochemistry of simple aliphatic chains is a rather complicated 
matter. Ruzicka's discovery that large aliphatic rings are no less stable 
than small ones is impossible to reconcile with the Baeyer strain theory, 
and the latter has been superseded by the Sachse-Mohr theory (66) which 
permits the existence of large rings in non-planar and strainless forms and 
is supported by a great deal of other evidence besides. The essential 
assumption in the Sachse-Mohr theory is simply the usual one that the 
four valences of the carbon atom are directed toward the corners of a 
tetrahedron and that there is free rotation about each single bond in a 
chain. Space models embodying these features can easily be made by 
joining small wire tetrahedra with rubber tubing in such a way that the 
arms that are being connected overlap. Rings constructed with such mod- 
els show graphically that a 5-atom ring is uniplanar and free from strain; 
a 6-atom ring is highly strained unless two of the atoms are allowed to lie 
in a different plane from the other four; larger rings if they are to exist 


must be multiplanar and strainless, and they possess a mobility which 
makes it possible for them to assume a great multiplicity of shapes.* 

The variety of possible configurations of an open chain is much greater 
than in a ring and it increases rapidly with the length of the chain. In 
the crystals of fatty acids and paraffins the chains probably are rigidly ex- 
tended into a zigzag structure (13); but a model of such a chain is ex- 
ceedingly flexible and mobile, and it seems very improbable that in the 
liquid condition or in solution the molecules retain their rigidly extended 
form. No doubt certain possible configurations are more probable than 
others, but there is at present no means of knowing just what these are.f 
It is clear, however, that the relative probability of close approach of the 
ends of a chain diminishes very rapidly as the length of the chain increases. 
Owing to the fewness of their separate points of rotation, 5- and 6-atom 
chains can assume relatively few configurations. In fact, if a space model 
of such a chain is supported at one of the bonds the entire structure can be 
rotated in such a way that the freely moving and unsupported ends collide 
at each rotation. This shows that there is a certain inevitability in the 
closure of such chains. Longer chains on the other hand can assume an 
extraordinary multiplicity of shapes without any close approach of the 
ends. The ends of the model can be brought together arbitrarily without 
any resistance, but as Mohr has pointed out (66) in the molecule itself one 
is dependent upon random collision,** "which will bring about a given form 
the more rarely the more forms are possible, i. e., the longer the chain." 

One obvious and important implication of this theory is that a cyclic 
structure for linear high polymers is very improbable. The formation of 
such polymers usually depends upon the absence of any tendency toward 
ring closure in the early stages of the coupling, and the probability of ring 
closure will diminish as the length of the chain increases. This implica- 

* Apparently the shape that they actually tend to assume involves the close ap- 
proach of opposite sides of the ring; the ring consists essentially of two parallel chains 
joined at the end (67, 144, 145). 

f Attempts to decide this question by studies of x-ray diffraction (88), electric mo- 
ment (63), ionization constant (65), and numerous other properties (143) have led to 
the following rather contradictory conclusions: (a) the chains are straight zigzags; 
(b) they are straight zigzags except for the 5-atom chain which is coiled ; (c) they have 
a helicoidal configuration which brings the first and fifth atoms very close together and 
introduces an anomaly in properties when the length of the chain is 5, 10 or 15 atoms. 
None of these configurations provides any mechanism for the close intramolecular ap- 
proach of the ends of chains longer than 6 atoms. 

** It seems quite possible that this dependence upon random collision is not in- 
herently necessary. It may be that some selective control over molecular form is possi- 
ble by the use of the orienting effects at surfaces, and such factors may perhaps come in- 
to play in the synthesis of large rings in nature. This suggests the possibility of some 
very interesting studies of a novel kind in surface chemistry. 


tion is also in accord with the facts. It is true that cyclic formulas have 
been assigned to various high polymers by Staudinger and by other investi- 
gators, but none of these formulas has been established.* On the other 
hand, in a variety of instances open chain structures have been experi- 
mentally proved. 

The presence of substituents and the nature of the terminal groups may 
be expected to modify the stereochemical behavior of chains. Thus, if 
two atoms of a chain are adjacent atoms of a benzene ring their position 
with respect to each other is fixed, and the chances of intramolecular reac- 
tion are greater than in an analogous simple chain of the same length. 
(The latter will have one more axis of rotation than the former.) It is 
not surprising therefore that the majority of the known 7- and 8-membered 
rings have at least two of their atoms members of a benzene ring. The 
compound NH 2 (CH 2 )6C1 reacts with itself intramolecularly only to a slight 
extent, while with <^NH 2 C6H 4 (CH 2 ) 4 C1 ring closure is almost quantitative 
(68). Other similar examples might be cited (50, 163). 

There is also some evidence to indicate that even simple substituents 
such as methyl groups on a chain may increase the tendency toward the 
formation of larger rings. Researches in the diphenyl series (69) have 
established that substituent groups suitably placed may completely inhibit 
rotation about a nearby single bond, and a similar effect in aliphatic chains 
is at least conceivable. Almost any restriction of the freedom of rotation 
of the atoms of a chain would, on the basis of the Sachse-Mohr theory, 
increase the chances of ring formation. 

The influence of the nature of the terminal groups is seen in the fact 
that co-hydroxy acids on dehydration give both lactone and polyester, 
while attempts to prepare 7-atom cyclic esters from dibasic acids and di- 
hydric alcohols yield only polyesters. Dilution also may be expected to 

* In this connection merely negative evidence, e. g. t the failure to detect terminal 
groups, is worthless. Terminal groups may become lost or mutilated, or they may be 
present and yet fail to react. High polymers are frequently very sluggish in their 
chemical behavior. 

The tendency to resort to cyclic formulas does not aid in clarifying the problems of 
high polymers. One can assume that the rubber molecule is an enormously large ring, 
but rubber is very susceptible to degradation by oxygen, heat, mechanical action, etc., 
and the first step in such degradation must cause the ring to be ruptured. This product 
then presents all those problems which the assumption of a cyclic structure was designed 
to evade. 

It seems evident that all polymerizations, whatever their mechanisms, must be 
progressive stepwise reactions. The simultaneous combination of 100 molecules presents 
insuperable kinetic difficulties. If this view is correct, ring closure can occur only as the 
last step in the reaction chain. But at this stage the ends of the chain must be more 
remote from each other than at any earlier stage, and the opportunities for ring closure 
must be at a minimum. 


favor ring formation as compared with progressive coupling (70), but all 
these effects are comparatively small. 

a. Large rings. The question arises whether large rings are ever formed 
as the normal primary products of bifunctional reactions. Meyer and 
Jacobson in their textbook (147) devote over fifteen hundred pages to 5- 
and 6-membered heterocyclic compounds and only 17 pages to all larger 
rings. Most of the examples of this class of compounds that they accept 
as authentic fall into one or more of the following types : the ring contains 
only 7 or 8 atoms ; two or more atoms of the ring are members of a benzene 
nucleus; the yields are very poor; the compound is formed by ring widen- 
ing. There remain a few examples (e. g., the cyclic duplomercaptal de- 
rived from acetone and pentamethylene dimercaptan) in which the evi- 
dence for a very large cyclic structure is good ; but this evidence is no better 
than that on which the 1 6-membered cyclic formula for ethylene succinate 
was based (41), and ethylene succinate when reexamined (32, 34) proved to 
be a linear polymer of high molecular weight. 

Concerning the identity of large rings described by Ruzicka, it is not 
possible to entertain any doubt at all, but the methods required for the 
preparation of these materials present some illuminating peculiarities. 
His lactones were obtained from the corresponding cyclic ketones by oxida- 
tion (25), 

r-i r\ 

(CH 2 ),CO -* (CH 2 ),CO 

,_! LJ 

i. e. y the ring was already established in the starting material. All at- 
tempts to prepare the lactones by a bifunctional reaction, e. g., from the 
hydroxy acids or from the silver salts of the bromo acids were unsuccessful. 
The ketones themselves (71) were obtained by heating the thorium salts 
of the dibasic acids to a very high temperature (400-500C.). These 
conditions of violence lead to thermal rupture, and, whatever the primary 
products may be, the ultimate products must, for the most part, be volatile 
materials of fairly low molecular weight. The cyclic ketones are found 
mixed with a great variety of other materials among these volatile prod- 
ucts. It is at least conceivable that the primary products in this reaction 
are linear polyketones and that the cyclic ketones are thermolysis products 
of the polymer. 

In this connection it is interesting that large heterocyclic rings can also 
be obtained by thermolysis of the polyesters. Thus when polyethylene 



O 1-1 


~ g 

s ^ 


W 04 
J U 
<3 t/3 

H W 





<fi bt 

d c 







r ^ 














, . 










2 OCO(CH 2 ) 2 CO 

4 OCO O(CH 2 ) 4 OC 

3 OCOCO O(CH 2 ) 3 

9 CO O(CH 2 ) 9 CO1 

H 2 ) 8 CO OCO(CH 2 



/ *e5' 

/ ~V1 


























































meric j 







succinate is heated to 300C. in a vacuum, it decomposes and yields a 
carbonaceous residue and a gaseous and liquid distillate. From the latter 
a small amount of the sixteen-membered cyclic dimer can be obtained. 
Similar cyclic products have been obtained from tetramethylene carbonate, 
from trimethylene oxalate and from the potassium salt of the acetyl de- 
rivative of hydroxydecanoic acid. The anhydride of sebacic acid can also 
be depolymerized under certain conditions to a cyclic dimer. The proper- 
ties of these compounds are listed in Table III. Their identity is estab- 
lished by analytical data and repeated molecular weight determinations. 
They are distinguished from the corresponding polymers by their definite 
macrocrystallinity and relatively great solubility. The attempts to obtain 
corresponding monomers which would be 7-, 8-, and 11-membered rings 
have been unsuccessful. It seems possible that this failure may be due to 
purely practical difficulties, but so far as the data go they are in agreement 
with the fact that Ruzicka found the yields of the cyclic ketones of 8 to 12 
atoms to be much less than the yields of the larger rings. This fact is not 
easy to explain on the basis of the simple Sachse-Mohr theory presented 
above. A possible explanation has recently been offered by Stoll and 
Stoll (67). 

III. Polymerization Involving Cyclic Compounds 

1. Six-Membered Cyclic Esters. Among cyclic esters the property of 
undergoing reversible polymerization is characteristic of and peculiar to 
the 6-membered rings. 7-Butyrolactone cannot be polymerized, and no 
corresponding polyester is known. Higher lactones, e. g. t 

[O (CH 2 ) 16 CO], 

are stable substances which show no tendency to polymerize spontaneously, 
although polymers are the only products of the self-esterification of the 
corresponding hydroxy acids. On the other hand, 6-valerolactone, a mo- 
bile liquid, gradually changes on standing to an opaque solid polymer, and 
from this the lactone can be regenerated by heating. 

Various isolated examples of the polymerization of the 6-membered 
esters have been reported, but only a few of them have been described in 
any detail. In this laboratory some study (mostly unpublished) has been 
made of the mechanism of this phenomenon. 

The esters now known to exhibit this behavior are listed in Table IV. 
(In most cases neither the molecular weights nor the melting points of the 
polymers can be regarded as very significant, since both are dependent upon 
the conditions under which the polyester is formed.) 



3 S. 








S 2 

SS S 5 

rH en 

s g 







i i 






r i 










o" 1 





* o 1 


















[OCH 2 CH(CH 3 











colic acid 


























! | 




























' S 


> t 




Lactide . 


Some of these examples present special features worthy of mention. 
Monomeric trimethylene carbonate is a very soluble, crystalline solid. 
If it is heated with a trace of potassium carbonate to 130C. for a few 
minutes, the mobile melt suddenly becomes very viscous and evolves a 
small amount of gas. The colorless sirup on being cooled solidifies to a 
stiff mass which shows an apparent molecular weight of about 4000. 
When heated in a vacuum this mass distils almost quantitatively, and the 
distillate consists of pure monomer. 

Ethylene oxalate is a solid crystallizing in transparent, flat diamonds, 
melting at 143C. These on standing for a few days in a stoppered con- 
tainer disintegrate to a powder, which consists of a mixture of polymers 
By extraction with cold solvents it can be separated into two definite frac- 
tions, one melting at 159C. and having an apparent molecular weight of 
about 3000, and one melting at 173C. and having an unknown but prob- 
ably much higher molecular weight. These fractions on standing for a 
few days lose their identity; they are partly converted into each other and 
into monomer. It is interesting to note in connection with the rapidity 
of these transformations that monomeric ethylene oxalate is exceedingly 
sensitive to hydrolysis. 

Drew and Haworth (74) have obtained the lactone of 2,3,4-trimethyl-/- 
arabonic acid in crystalline form (m. p. 45C.) and have observed that in 
the presence of traces of hydrogen chloride it is converted into a crystalline 
polymeric powder. This has a considerably higher melting point, a lower 
solubility, and a lower specific rotation than the lactone. Its molecular 
weight (about 2000) indicates that it is derived from about 10 molecules of 
the latter. At 175C. it distils completely in vacua t and the distillate con- 
sists of pure lactone. Drew and Haworth ascribed a linear polyester struc- 
ture to this polymer and were inclined to accept its crystallinity as evi- 
dence of its absolute homogeneity, but it seems much more probable that it 
is a polymeric mixture. 

The ease of polymerization of the 6-membered cyclic esters appears to 
be related to their susceptibility to hydrolysis. In general, substitution 
increases the resistance to hydrolysis and diminishes the tendency to poly- 
merize. Thus glycolide polymerizes spontaneously at the ordinary tem- 
perature, but lactide only on being heated or exposed to the action of cata- 
lysts. Attempts (51) to bring about the polymerization of analogs of 
glycolide derived from some of the higher a-hydroxy fatty acids have been 

These polyesters are formed from the monomers by a process of ester 


? ... O R CO O R CO O R CO R CO ... 

I ............. J L ............. J L ............. J L ............. J 

and the reverse transformation proceeds by a similar mechanism, as in- 
dicated by the arrows. Both transformations are catalyzed by acids and 
bases typical interchange catalysts. 

Direct evidence for this interchange mechanism is found by polymerizing 
5-valerolactone in the presence of various amounts of chloroacetic acid 
(51). This acid actually participates in the reaction, and according to the 
amount present it regulates the length of the chains produced. The effect 
of the acid may be compared with that which water might produce. One 
molecule of water with one molecule of the lactone would simply yield the 
hydroxy acid. A smaller amount of water would yield some hydroxy acid 
and this might react with the lactone to form a dimeric ester : 


If the amount of water were quite small the polyester molecule would 
have to be quite large. Apparently chloroacetic acid functions in precisely 
the same way, and a comparison of the halogen content and neutral equiva- 
lent of the polyesters produced under various conditions not only estab- 
lishes the open chain structure of the polymer, but also clearly indicates the 
mechanism of the reaction. 

It remains to explain why only the 6-membered esters are capable of 
undergoing reversible transformation of this kind. The presence of some 
strain in such esters is indicated by the great instability of 5-lactones as 
compared with y-lactones (78, 79). The Sachse-Mohr theory permits the 
existence of 6-atom rings in two isomeric strainless forms; but in simple 
rings these two isomeric forms have never been realized, and one is forced 
to conclude either that such rings are uniplanar and hence highly strained, 
or that the two isomeric forms are in dynamic equilibrium. Practically 
this amounts to the same thing, since at each conversion the molecule must 
pass through the uniplanar position of strain. These strains can be re- 
lieved by an ester interchange resulting in the formation of the polyester. 
The easy depolymerization of the resulting polyester is readily explained 
by the high degree of probability of the close approach of points 6 atoms 
apart in a chain. Polyesters whose structural units are longer than 6 
atoms are not readily depolymerized because of the improbability of close 
approach of the requisite atoms of the chain. Cyclic esters of 5 atoms or 


of more than 6 atoms are not polymerized because their cyclic systems are 
free from strain. 

In the formation of 6-membered cyclic esters from open chain compounds, 
either the monomer or the polymer may first be isolated according to the 
experimental conditions, but in either event it is not easy to prove that the 
form isolated is the primary product and not a polymerization or depoly- 
merization product of the primary product. It seems fairly certain that 
simple 5-lactones may be formed directly from the corresponding hydroxy 
acids. On the other hand there is evidence (19) that the self-esterifica- 
tion of lactic acid yields only polylactyllactic acids and that lactide, when 
it is formed, is produced from these by depolymerization. Since lactide 
can itself be polymerized to a polylactyllactic acid, the latter furnishes an 
example of a polymer that can be formed either by addition or by condensa- 
tion polymerization. In most cases, no doubt, either the cyclic ester or 
the polymer can be formed first depending upon the conditions. 

2. Adipic Anhydride. The behavior of adipic anhydride (56) illus- 
trates the fact that the property of undergoing reversible polymerization 
of this type is not peculiar to the polyesters. In this case it is the 7-atom 
ring that is unstable. Succinic and glutaric anhydrides, the 5- and 6- 
atom rings, cannot be induced to polymerize. Sebacic anhydride is poly- 
meric and cannot be depolymerized to a monomer. Adipic anhydride, 
when it is prepared from the acid, is also polymeric. It is a waxy solid 
which separates from solvents as a microcrystalline powder. On being 
heated in vacua it is depolymerized to a considerable extent, and distilla- 
tion occurs. The distillate is the monomeric adipic anhydride, a liquid 
freezing at about 21 C. In the presence of traces of moisture this very 
rapidly reverts to the waxy polymeric form. 

A direct chemical proof of the actual structural difference between the 
two forms of adipic anhydride is possible (56). Both forms react very 
rapidly with aniline. The monomer, since it contains in its molecule only 
a single anhydride linkage, can yield only the monoanilide and this is in 
fact the only product formed. 

fCO(CH 2 ) 4 CO O] + C6H 6 NH 2 -* HO CO(CH 2 ) 4 CO NHC 6 H 6 . 
But with the polyanhydride the nature of the final monomeric products 
will depend upon which side of the anhydride linkage is involved at suc- 
cessive steps : 

f 1 I ! I 





If the molecule is infinitely long, i. e. t if it is so long that the terminal groups 
can be ignored, considerations of probability indicate that the product will 
be 50 per cent, monoanilide, 25 per cent, dianilide and 25 per cent. acid. 
Within the experimental error these are the yields actually obtained. 
This indicates that the molecule must be made up of at least 10 or 15 
structural units. A cyclic polymer would of course give the same result, 
but a low molecular weight is excluded by the properties of the material, 
and a cyclic structure of any kind is made improbable by considerations 
already discussed. 

3. Diketopiperazines and Polypeptides. The curious behavior of the 
a-hydroxy acids is especially interesting in connection with the structure 
and formation of proteins, since these are for the most part derived from 
a-amino acids. If glycine were to behave like glycolic acid, the following 
transformations could be realized. 

-H,0 . / \ 

NH 8 CH a COOH <, . TT ^ * CO CH, heat 


H,0 \ 

I I <- 


... -NH-CH a -OD~NH-CHr-C-NH-<JHr-^0-NH-CHCX>- ... 

It appears that diketopiperazine can be polymerized, though with some 
difficulty (83), and polypep tides are readily obtained by the self -condensa- 
tion of glycine and its esters. 

Balbiano (80) in 1900 showed that when glycine is heated in glycerol 
solution it loses water and yields as the principal product a horn-like mass, 
together with a small amount of diketopiperazine. The horn-like mate- 
rial is practically insoluble in all solvents except hot concentrated hydro- 
chloric acid, and by hydrolysis it is converted to glycine. Later this reac- 
tion was studied in more detail by Maillard (81), who showed that accord- 
ing to the conditions and time of heating more or less of the dimeric an- 
hydride or the horn-like polymer can be obtained. He also isolated an 
intermediate individual, triglycylglycine, 


This polypeptide is soluble in water, and Maillard made the curious ob- 
servation that if its aqueous solutions contain diketopiperazine they deposit 
on standing an insoluble material. Analytical evidence indicated that this 


material is a hexapeptide. Except for the solubility effect in aqueous solu- 
tions there is no obvious reason why this coupling of the diketopiperazine 
with the polypeptide should stop at the hexapeptide stage. In glycerol 
solution one may suppose that it continues progressively and results in the 
building up of a very long chain. The final horn -like product would then 
be a polypeptide containing a very large number of structural units 
perhaps forty or more. Maillard assigns to this horn-like polymer the 
structure of a cyclopolyglycylglycine, but no convincing proof of the 
cyclic formula is presented, and on general grounds an open chain formula 
seems more probable. 

The production of glycine anhydrides in glycerol solution probably 
involves the formation of glyceryl esters as transitory intermediates, but 
diketopiperazines and polypeptides can also be obtained by heating amino 
acids in the absence of a solvent. Curtius and Benrath (82) state that at 
high temperature the chief product from glycine is a pentapeptide. 

The esters of glycine couple with themselves much more readily than the 
free acid. Curtius (84) showed that glycine ethyl ester loses alcohol even 
at the ordinary temperature. When water is present the chief product is 
diketopiperazine, but some polypeptide ester is formed at the same time. 
Under anhydrous conditions the latter is the chief product. When glycine 
ethyl ester is dissolved in a little dry ether and allowed to stand at the 
ordinary temperature, a crystalline precipitate gradually accumulates. 
This consists almost exclusively of the ethyl ester of triglycylglycine. 
The esters of such polypeptides are of course capable of coupling with 
themselves to form still longer molecules. Fischer (85) observed that 
when the methyl ester of alanylglycylglycine is heated to 100C. it yields 
some hexapeptide ester and some less soluble amorphous material. Curtius 
(84) found that when his triglycylglycine ester was heated to 100C. in 
vacua it was converted into an insoluble infusible material having the 
composition NHCH 2 CO . He assigned to this material the struc- 
ture of a cyclic octapeptide, but a much more highly polymeric open chain 
structure seems more probable. 

In connection with the polypeptides the behavior of the anhydrides of 
the N-carboxy amino acids may be mentioned (89). The compound 

CH 2 CO 


dissolves readily in water at 0C. At 15C. carbon dioxide is evolved and 
an aqueous solution of glycine results. If, however, the compound is 


rubbed with a little water at room temperature or is heated, carbon dioxide 
is lost immediately and an insoluble infusible material is the only other 
product. This has the composition NHCH^CO and on hydrolysis it 
yields glycine. It appears to be similar to the horn-like product obtained 
by Balbiano (80). It is quite probably a polypeptide of very high molecu- 
lar weight. The anhydrides of other N-carboxy a-amino acids behave 
in a similar fashion (90). 

4. Ethylene Oxide (14, 15). Ethylene oxide provides another ex- 
ample of the polymerization of a cyclic compound not involving double 
bond unsaturation. The polymerization is induced by various catalysts 
such as alkali metals, tertiary amines, and stannic chloride. Ultraviolet 
light or Florida earth is not effective. The product is a solid readily 
soluble in water and in most organic solvents except ether. Its properties 
vary somewhat according to the method of preparation. By fractional 
precipitation it can be separated into fractions ranging in apparent molecu- 
lar weight from about 400 to nearly 5000. The lowest member of this 
series is liquid and the highest one a solid melting at about 59 C. The 
polyethylene oxides are not depolymerized by heat; but above 300C. 
they decompose and yield a complicated mixture of products containing 
some acetaldehyde and acrolein. Roithner (15) assigns to polyethylene 
oxide the following formula, 

[O CH 2 -~ CH 2 O CH 2 CHir- O CH 2 CH 2 ] 

but the evidence for this structure is largely negative. Unpublished work 
in this laboratory has led to the detection of terminal hydroxyl groups, 
indicating that the molecule is an open chain. 

Ethylene oxide also polymerizes under certain conditions to yield dioxane 
(16). Apparently this shows no tendency to polymerize further as the 6- 
membered cyclic esters do. 

IV. Addition Polymerization of Unsaturated Compounds 

This is the only type of polymerization that is recognized by the usual 
definitions. Thus Cohen states (91) that "the property of undergoing 
polymerization is peculiar to unsaturated compounds, from a natural 
tendency to saturate themselves/' So far as the formation of materials 
of high molecular weight is concerned, such reactions are much less clear- 
cut than bifunctional condensations, for the latter involve only the applica- 
tion of the known reactions of typical functional groups, and the general 
structural plan of the product may be inferred directly from the structure 


of the starting materials. On the other hand, no clue to the intimate de- 
tails of the mechanism of self-addition can be found in the reactions of the 
compound concerned with any compounds other than itself. Ethylene 
oxide and certain cyclic esters and anhydrides already discussed poly- 
merize by self -addition, but these reactions in some respects are radically 
different from the polymerization of compounds containing multiple link- 

Under sufficiently drastic conditions almost any compound can be con- 
verted into a material of high molecular weight. Thus methane when sub- 
jected to the action of alpha particles or to the silent electrical discharge is 
partly transformed with loss of hydrogen into higher hydrocarbons (165), 
and benzene in the electrodeless discharge is converted into insoluble, 
amorphous products (166) . The reaction of acetylene with itself illustrates 
how complicated the polymerization of an unsaturated compound may be. 
Polymerization at elevated temperature in the presence of active charcoal 
leads to a complicated mixture of hydrocarbons containing considerable 
amounts of benzene and naphthalene (167); the use of copper- or mag- 
nesium-containing catalysts yields (168) a completely insoluble amorphous 
powder (cuprene) of unknown structure, and similar products are obtained 
by the action of ultraviolet light (169), cathode rays (170), or alpha parti- 
cles (171) on acetylene. On the other hand, the action of the silent elec- 
trical discharge (172) at low temperature leads to the formation of con- 
siderable amounts of liquid products which contain highly unsaturated 
open chain compounds. 

The brief discussion of addition polymerization contained in the follow- 
ing paragraphs is confined to a few of the simplest and most thoroughly 
studied cases. 

1. Ethylene and Other defines. The rather extensive literature on 
the polymerization of ethylene has recently been reviewed by Stanley (92) . 
Ethylene polymerizes less readily than most of its homologs and derivatives. 
The polymerization is accelerated by heat, pressure, ultraviolet light, the 
silent electric discharge, and by certain catalysts such as sulfuric acid 
(especially in the presence of salts of copper and mercury), zinc chloride, 
boron trifluoride, and aluminum fluoride. The products are usually oils 
having a wide boiling range. In general, they are not exclusively of the 
C n H2 W type; hydrogenation, dehydrogenation, and cyclization may occur 
at the same time. These products are commercially valuable, but too com- 
plicated to furnish any clue as to the mechanism of their formation. 

Mignonac and Saint-Aunay (93), however, have succeeded in isolating 
the first products formed in the action of the silent electrical discharge on 


ethylene. These products are butene-1 and hexene-1. Pease (94) has 
found evidence of the presence of butene-1 in the products of the thermal 
polymerization, and from a study of the kinetics has concluded that this is 
a chain reaction. In effect at least this reaction involves at the first step 
the addition of ethylene, as H + CH=CH 2 , to the double bond of another 
molecule of ethylene, and then a similar addition to butylene. 

Lebedev and his collaborators (95) have presented interesting data on 
the early stages of the polymerization of isobutylene. They used espe- 
cially Florida earth as a catalyst and obtained mixtures from which a whole 
series of polymers up through the heptamers was isolated. They found 
that the trimer was not polymerized under the conditions that lead to 
higher polymers from either the monomer or the dimer. Hence the higher 
polymers must be built up by the successive addition of monomer or dimer. 
As to the precise structure of these polymers very little is known. 

2. Vinyl Compounds. Substituted ethylenes of the type CH 2 = 
CH R, in which R is a negative group, polymerize much more readily 
than does ethylene itself. Examples are: 

Styrene (96) 

CH 2 =CH Cl(Br) Vinyl chloride (bromide) (97) 

CHr=CH O COCH 3 Vinyl acetate (98, 142) 

CH^CH COOH Acrylic acid (99) 

CH2=CH CHO Acrolein (100) 

Indene (101, 105) 

The most extensive and important studies of the polymerization of vinyl 
compounds are those carried out by Staudinger and his co-workers. The 
behavior of styrene may be taken as typical. On standing or on being 
heated, this mobile, volatile liquid first becomes more viscous, then changes 
to a more or less tough elastic jelly, and finally it may become converted 
into an exceedingly hard brittle mass. This change is powerfully catalyzed 
by light and by atmospheric oxygen, and it is inhibited by certain anti- 
oxidants such as hydroquinone. Other catalysts that are effective in ac- 
celerating the change are organic peroxides and certain metallic halides 
such as stannic chloride and antimony chloride. If a little stannic chloride 
is added to an alcohol solution of styrene, the solution becomes warm and 
the polystyrene quickly separates as an amorphous mass. 


Polystyrene is not a definite material having a constant set of properties. 
By whatever method it is prepared it can be separated by fractional ex- 
traction or precipitation into fractions having the same composition but 
different properties and molecular weights. The lower members are readily 
soluble in ether and the highest members are quite insoluble. The ap- 
parent molecular weights range from 1000 up to 25,000 or more (perhaps 
as high as 200,000). All these fractions are soluble in benzene, and the 
viscosity of the solutions increases progressively with increasing molecular 
weight. The average molecular weight of the crude polymer depends 
upon the conditions of its formation. Those formed very rapidly, e. g., 
by the action of heat at high temperature or by the action of stannic 
chloride at ordinary temperature, have relatively low molecular weights. 
Those formed more slowly, e. g., by spontaneous polymerization at room 
temperature, have much higher molecular weights. 

The polystyrenes having molecular weights above 10,000 show colloidal 
behavior; they swell before dissolving, and the viscosity of their solutions 
is very high. Nevertheless, there is considerable evidence to show that 
these solutions are true molecular dispersions (102), and that the molecule 
and the colloidal particle are identical. 

Chemically the polystyrols are completely saturated. They do not de- 
colorize permanganate or absorb bromine. Under sufficiently drastic 
conditions they can be completely hydrogenated (in the benzene nucleus) 
without any significant change in molecular weight (103). On being 
heated to about 320C. they revert to the monomer, styrene. It is doubt- 
ful if this reversion is ever quantitative. 

The polymerization of styrene evidently involves the disappearance of 
the double bond and the formation of very large molecules. The simplest 
and most probable structure of these large molecules is that suggested by 

. . . CH CH 2 CH CH 2 CH CH 2 CH CH 2 . . . 

I I I I 

C 6 H* C 6 H 6 C 6 H 6 C 6 H 6 

The polymerization of vinyl acetate is similar to that of styrene. In 
both cases the polymer first formed remains dissolved in the monomer, and 
the mixture of monomer and polymers can be obtained in the form of more 
or less tough, transparent, elastic masses. The chemical behavior of the 
polyvinyl acetate indicates that it is a mixture of molecules of various 
lengths built up according to the plan indicated in the formula 

. . . CH 2 CH CH 2 CH CH 2 CH CH 2 CH CH 2 CH-. . . 

I I I I I 

OAc OAc OAe OAc OAc 


The number of molecules of monomer involved probably ranges between 
forty and one hundred. By fractional precipitation, samples of different 
average molecular weights can be obtained, and these naturally differ 
somewhat in their physical properties. In general the polyvinyl acetates 
are soluble in organic solvents but not in water. As esters they are readily 
hydrolyzed. The products are acetic acid and polyvinyl alcohol: 



The latter as a polyhydroxy compound dissolves in water to form rather 
highly viscous solutions, but does not dissolve in organic solvents. 

Acrylic acid also polymerizes very readily, yielding a product to which the 
following formula may be assigned. 

. . . CH 2 CH CH 2 CH CH 2 -CH CH 2 CH . . . 

I I I I 


As a highly polybasic acid it forms a sodium salt which dissolves in water 
to yield very viscous solutions. The acid itself also dissolves in water 
(the most highly polymeric varieties swell very strongly and dissolve with 
difficulty), but does not dissolve in the typical organic solvents. On the 
other hand, polyacrylic esters prepared by polymerizing acrylic esters are 
insoluble in water, but soluble in organic solvents. 

The methyl ester of polyacrylic acid on being treated with methyl mag- 
nesium iodide yields a product corresponding approximately in com- 
position to the expected polytertiary alcohol, and this on reduction yields 
a hydrocarbon. Although the composition of this hydrocarbon does not 
exactly correspond to that required by the expected structure 
. . . CH 2 CH- CH 2 CH. . . 
(CH 3 ) 2 HC CH(CH 3 ) 2 

its high molecular weight proves that the ester from which it was derived 
is also really a material of high molecular weight, and that the units must 
be joined by carbon-carbon linkages. 

The problem of arriving at a definite mechanism for the polymerization 
of vinyl compounds is complicated by the fact that neither the formation 
of the polymers nor their chemical behavior furnishes any certain clue to 
their structure. The general formula 

-CH 2 CH 

CH 2 CH CH 2 CH CH 2 CH . . . 



is perhaps more plausible than any other, since, as Staudinger has pointed 
out, it best accounts for the fact that some poly vinyl compounds (e. g., 
polystyrene) are smoothly depolymerized by the action of heat: the re- 
curring substituent would weaken the linkages of the chain at the points 
indicated by the dotted lines. But depolymerization is never quantita- 
tive, and it is at least conceivable that a fraction of the units of each chain 
are arranged in the reverse fashion : 

. . . CH CH 2 CH 2 CH CH CH 2 CH 2 CH . . . 


R R R R 

It has already been mentioned that the first step in the polymerization 
of ethylene is the formation of butene-1 and the second step is the formation 
of hexene-1. Similar terminal unsaturations have been found by Whitby 
and Katz in a still longer series of polyindenes, and this suggests that the 
most plausible mechanism for the polymerization of vinyl compounds is 
"best represented as proceeding stepwise by the addition regularly of suc- 
cessive molecules of monomer to the double bond present at each stage of 
the polymerization immediately preceding" (105). This mechanism may 
be formulated as follows : 

RCH=CH 2 + RCH=CH 2 - RCH 2 CH 2 C(R)=CH 2 

RCH 2 CH 2 C(R)=CH 2 + RCH=CH 2 - RCH 2 CH 2 CH(R)CH 2 C(R)=CH 2 , etc. 

Staudinger disagrees with this view. He claims that the polyindenes 
do not have any terminal unsaturations, and from analogies based on the 
behavior of a-methylstyrene (104) he assigns a cyclic formula to poly- 
indene and polystyrene. a-Methylstyrene, CH 2 =C(CH 3 )C6H5, poly- 
merizes much less readily than styrene, but the reaction may be caused to 
occur quite rapidly by certain catalysts, e. g., stannic chloride (104). The 
principal product is the saturated dimer, a substituted cyclobutane. A 
smaller amount of a saturated trimer is formed at the same time, and in 
progressively smaller amounts saturated tetramer, pentamer, decamer, 
hexamer, heptamer, and octamer. On the basis of their physical and 
chemical properties Staudinger assumes that these higher polymers are 
respectively 6-, 8-, 10-, 12-, 14-, and 16-membered rings. 

It should be observed however that these products are quite different 
from those obtained from styrene under similar conditions ; moreover, the 
absence of the a-hydrogen makes it impossible for a-methylstyrene to 
polymerize by the mechanism suggested above for vinyl compounds 
generally. It is therefore scarcely permissible to conclude that the high 
polymers from styrene must be cyclic because the low polymers from a- 


methylstyrene are cyclic. Other more general grounds for rejecting cyclic 
formulas for linear high ploymers have been presented in previous para- 

The polymerization of vinyl compounds is enormously susceptible to 
catalytic and anticatalytic effects. Heat also accelerates the polymeriza- 
tion, and in general the more rapidly the polymer is formed, the lower is 
the average molecular weight of the product. Oxygen and peroxides are 
catalysts, and antioxidants act as inhibitors. Light, especially the shorter 
wave lengths of the visible spectrum, accelerates the polymerization. In 
certain cases the presence of oxygen inhibits this photochemical effect. 
With the aid of certain specific catalysts, it is possible to convert styrene 
and certain other vinyl compounds into dimers, but these are stable sub- 
stances that show no tendency to polymerize further, and they differ 
structurally from the dimers that have been hypothecated as intermediates 
in the formation of the high polymers. Under conditions that result in 
the formation of the latter, no polymers of very low molecular weight can 
be detected. All these facts indicate that the formation of the high poly- 
mer is a chain reaction. The collision of an activated molecule of mono- 
mer with another molecule of monomer yields an active dimer capable of 
coupling with another molecule of monomer, and the activating energy 
persists in the polymeric chain until it has been built up to a considerable 
length. Kinetic studies of the polymerization of vinyl acetate (106) and 
styrene (107) support the idea of a chain mechanism. 

3. Dienes. Butadiene and isoprene are of especial interest in con- 
nection with rubber. Nobody knows whether in nature rubber is ac- 
tually formed from isoprene or not; but it is true that rubber yields some 
isoprene on thermal decomposition, and that isoprene can be polymerized 
to a product which more or less resembles natural rubber. Similar prod- 
ucts can also be produced from butadiene and from some of its derivatives. 
Efforts to produce synthetic rubber have led to hundreds of patents and 
various other publications. This subject is reviewed in a book by Schotz 
(108). There is space here to mention only two or three points. 

Butadiene and isoprene polymerize much less readily than styrene, 
vinyl acetate, etc., but apparently they are subject to the same kind of 
accelerating influences. Among the catalysts that have been used are 
oxygen, peroxides, ozonides, alkali metals, alkali alkyls. When emulsi- 
fied, especially in the presence of oxygen, they polymerize more rapidly 
than otherwise. It is claimed in many patents that the presence of proteins, 
gums, etc., in such emulsions has a favorable effect on the course of the 
reaction and results in products more nearly resembling natural rubber, 


but it seems probable that the advantages of such additions are largely 
imaginary. It is possible to prepare from isoprene and butadiene high 
polymers that have very little resemblance to natural rubber, and the 
problem of preparing a synthetic rubber of good quality is enormously 
complicated and difficult. 

The structural unit in the polymeric chain from isoprene appears to be 

CH 2 C=CH CH 2 
CH 3 

In effect, the polymerization involves the union of a large number of these 
radicals derived from isoprene by the rearrangement of its bonds. Two 
such radicals might unite in three ways: 1,1; 1,4; 4,4. Midgley and Henne 
(109) have captured the first step in the reaction by carrying out the poly- 
merization in the presence of sodium and alcohol so that the terminal 
valences of the dimer are hydrogenated. The structures of the three prod- 
ucts, whose formulas are shown below, prove that all three types of com- 
bination occur. 

CH 3 ~-CH=C CH 2 CH 2 C=CH CH 3 

I I 

CH 3 CH 3 

CH 3 C=CH CH 2 CH 2 C=CH CH 3 

CH 3 CH 3 

CH 3 C=CH CH 2 CH 2 CH=C CH 3 
I I 

CH 3 CH 3 

This result indicates that the arrangement of the units in synthetic rubber 
formed under these conditions is less regular than in natural rubber, but 
it by no means proves that the actual mechanism of the reaction consists 
in the direct union of radicals corresponding in formula with the structural 

unit. The 1,4 addition of H 1 CH=C(CH 3 )CH=CH 2 to isoprene, 

for example, would lead to the same result. 

4. Aldehydes. The polymerization of formaldehyde under various 
conditions leads to polyoxymethylenes. These are microcrystalline pow- 
ders, which in general cannot be melted or dissolved in organic solvents 
without decomposition. They have been very elaborately studied by 
Staudinger and his co-workers (110). These studies have proved that the 
molecules of the polyoxymethylenes are long chains of the type 

. . . . O CH 2 O CH 2 O CH 2 O CH 2 O CH 2 . . . . 

The chains contain from forty to at least one hundred structural units. 
The different varieties of polyoxymethylenes are distinguished by the 


nature of the terminal groups: in the a-variety the terminal groups are 
OH ; in the /3- variety they are OCH 3 , and this variety is more inert chemi- 
cally than the a-variety. By the action of acetic anhydride, the poly- 
oxymethylenes are simultaneously degraded and acetylated. The product 
consists of a mixture of compounds of the series CH 3 CO (O CR^ X O 
COCHa. By distillation and crystallization, each individual of this series 
from x = 1 to x = 20 has been isolated in a fairly pure state. The melting 
points and boiling points of these individuals increase continuously with 
increasing molecular weight and their solubilities diminish. The successive 
members above x = 20 resemble each other so closely and their solubilities 
are so low, that they cannot be separated into pure fractions. 

Formaldehyde also polymerizes to yield the cyclic trimer, trioxymethyl- 
ene, and analogous trimers are the most common forms of the polymers of 
other aldehydes. It seems possible, however, that some of the metalde- 
hydes are linear polymers of high molecular weight analogous to the poly- 
oxymethylenes. Conant (111) has obtained from butyraldehyde by the 
action of very high pressure a solid polymer for which he suggests such a 
structure. This polymer is apparently stable only under pressure. Under 
the ordinary conditions it rapidly reverts to the monomer. 

The thio aldehydes and ketones show more tendency to polymerize than 
their oxygen analogs (158). Organic silicon compounds of the types 
R 2 Si=O, RSi(=O)OH, etc., are frequently incapable of being isolated in 
the monomeric condition, and they invariably polymerize very readily. 
It is interesting in this connection to compare carbon dioxide and silicon 
dioxide. The latter is probably a three-dimensional polymer. 

V. Polyfunctional Reactions and Non-linear Polymers 

The polymers discussed thus far, whether formed by condensation or 
by self -addition, are of a type that may be symbolized by the formula .... 
A A A A A A A . . * . Reactions of condensation are not 
limited to bifunctional compounds, however. If one of two reactants con- 
tains two functional groups and the other contains more than two, the 
product will be not a simple chain but a more complicated structure. 
Such reactions may be called bi-trifunctional, bi-tetrafunctional, tri-tri- 
functional, etc. (18). Reactions of this class are especially important 
technically in connection with the formation of synthetic resins. Two 
examples may be considered. 

The glyptal resins are formed by the action of phthalic acid on glycerol. 
The reaction first leads to the formation of a fairly soft, soluble, thermo- 


plastic resin, and this on being heated further yields a hard, insoluble 
resin which is completely lacking in thermoplasticity. The only reaction 
involved in the process is esterification (112), and the resin can be saponi- 
fied completely to yield phthalic acid and glycerol (51). Analysis of the 
resin just before the infusible stage is reached shows that the glycerol and 
the phthalic acid are far from having reacted completely with each other, 
that is, free carboxyl and presumably free hydroxyl groups are still present. 
The behavior of a dihydric alcohol such as ethylene glycol in this reaction 
presents quite a different picture from glycerol. With the glycol it is 
possible to obtain complete esterification, but however far the reaction is 
carried the product does not become infusible or insoluble. The reason for 
this difference is obvious. The polyester formed from the dibasic acid 
and the glycol is linear; as the reaction progresses the molecules grow, 
but the growth takes place only in one dimension. Similar chains formed 
in the reaction of glycerol with phthalic acid would bear hydroxyl groups 

HO -7- -y -y COOH 


By reaction of these groups with phthalic acid the chains would be linked 
together, and thus a very complicated 3-dimensional molecule would be 
built up. After a certain degree of complexity is reached, the possibility 
of molecular mobility no longer exists. It is conceivable that this cross- 
linking of the chains finally results in a mass that is essentially a single 
molecule. In any event it is easy to see how the possibility of further reac- 
tion disappears long before all the carboxyl and hydroxyl groups have a 
chance to participate. It is easy to see also why the action of any dibasic 
acid on glycerol always yields an amorphous resin, whereas the polyesters 
from dibasic acids and dihydric alcohols are frequently crystalline. 

The formation of Bakelite from phenol and formaldehyde may also be 
classified as a tri-bifunctional reaction. The formaldehyde behaves as 
though it were HO CH 2 OH, and it reacts no doubt largely at the o- 
and ^-hydrogens of the benzene nucleus. With phenol itself the number 
of possible products even of quite low molecular weight is so great that 
no intermediate polymeric individuals can be isolated. On the other hand 
p-cresol has only two readily reactive positions. Thus it is possible as 
Koebner has shown (113) to isolate the compounds indicated below 

CH 8 CH 3 


as crystalline individuals, by causing formaldehyde and ^-cresol to react 
in the appropriate ratios, and these can be used to build up still longer chains 
of the same series. The progressive hydrolysis of silicic esters to silicon 
dioxide, already discussed in a previous paragraph, furnishes another ex- 
ample of the formation of a 3 -dimensional polymer. 

Addition polymerization may also lead to the formation of 3-dimensional 
structures. Thus acetylene reacts with itself under the influence of cer- 
tain catalysts (not necessarily copper) with the formation of cuprene, an 
infusible and insoluble powder, and Staudinger suggests (2) that in this 
reaction the first step is the formation of unsaturated chains .... CH= 
CH CH=CH CH=CH .... which subsequently combine with one 
another. The vulcanization of rubber probably also involves the cross- 
linking of the long chains through the agency of the unsaturated linkages 
present (39). 

VI. Natural Polymers 

1. The Association Theory Versus the Structural Theory. The 

peculiar and difficult physical and chemical behavior of polymers has 
occasionally led to the suggestion that forces of a peculiar kind are involved 
in their formation. Thus Thiele in 1899 (132) suggested that perhaps in 
such materials as polystyrene the molecules of monomer are bound to- 
gether merely by partial valences. Rohm in 1901 (115) concluded that the 
transformation of monomeric acrylic esters into the h'ighly polymeric form 
is not chemical reaction but a kind of allotropic change. Later Schroeter 
(116, 117) suggested that the formation of dimeric ketene and of tetra- 
salicylide is due to the manifestation of an excess of peripheral external 
force about the monomeric molecules, and that the actual chemical struc- 
tures of the monomeric molecules are not changed in the process. These 
particular suggestions are not tenable in the light of chemical evidence now 
available, but the association theory of polymeric structure reappeared 
about 1924 and was widely accepted as an explanation of the peculiarities 
of natural high polymers (118). According to one form of this theory, 
cellulose, for example, might be an anhydroglucose having the molecular 
formula CeHioOs. This molecule, because of the unusual strain of its cyclic 
structure or for some other reason, is supposed to exhibit enormously ex- 
aggerated forces of association or residual valence, and hence to behave 
physically as though it were a material of very high molecular weight. 
In the same way proteins might be built up by the mutual association of 
various small units, e. g. t diketopiperazines. 

In support of this theory various investigators showed that it was pos- 


sible by freezing and boiling point methods to obtain small and rapidly 
shifting values for the molecular weights of polysaccharides, proteins, and 
rubber. Repetition of these determinations by other investigators proved, 
however, that the low results were due in most cases to errors in technique. 
Other support came from x-ray studies, which indicated that the unit 
cell of the crystal lattice of some high polymers is too small to contain a 
very large molecule. It was assumed at the time that a unit cell could not 
contain less than 1 molecule, but studies of known substances of high 
molecular weight proved that this assumption was incorrect. Meanwhile 
Staudinger's studies of synthetic materials repeatedly demonstrated that 
polymerization may lead to the formation of very long chains built up by 
real chemical forces in a regular fashion, and that such synthetic mate- 
rials often resemble natural high polymers in many significant physical 
and chemical properties. Studies made in this laboratory on high poly- 
mers formed by condensation reactions led to the same conclusion. The 
idea that natural high polymers involve some principles of molecular 
structure peculiar to themselves arid not capable of being simulated by 
synthetic materials is too strongly suggestive of the vital hypothesis, which 
preceded the dawn of organic chemistry, to be seriously considered. 

It should be emphasized in this connection that polymerization is riot 
peculiar to unsaturated compounds, and that a very high degree of mobility 
in the relation between a monomer and its polymer does not preclude the 
intervention of real primary valence forces in the process or the presence of 
a definite macromolecular structure in the polymer. This fact is illustrated 
especially by adipic anhydride and the 6-membered cyclic esters discussed 
in previous sections of this paper. These materials appear to exhibit all 
the supposedly diagnostic features of association polymerization; never- 
theless their transformation into polymers is a real chemical process, and 
the polymers are actually made up of large molecules. No example is 
yet known in which a small molecule of known structure simulates a mate- 
rial of high molecular weight without undergoing any change in structure. 

A return to the simple structural theory of organic chemistry and the 
application of modern tools have been responsible, during the past five 
years, for very rapid progress in the interpretation of the structure and 
properties of some of the simpler, naturally occurring high polymers. It 
appears that many naturally occurring macromolecular materials have a 
linear polymeric structure. The present status of this subject has been 
reviewed by Meyer and Mark (3), and in the following paragraphs only 
cellulose and rubber are briefly discussed. 

2. Cellulose. Chemical evidence for a linear polymeric structure in 


the cellulose molecule has long been available (125). More recently, 
Haworth's proof of the structure of cellobiose (120) and certain studies of 
the relation of glucose and cellobiose to cellulose (122) furnished the basis 
for tentative efforts to determine the nature of the units in the molecular 
chain (121). Sponsler and Dore (123) on the basis of x-ray evidence first 
put the linear polymeric structure in explicit form. This formula was 
further developed by Meyer and Mark (119), and it can now be said that 
the structure of cellulose, at least in its essential outlines, is definitely known. 
In its simplest chemical form the structural unit of cellulose may be repre- 
sented by the bivalent radical derived by the removal of water from glucose 

^ -jOHj 



A large number of these units are united chemically to form a long 
chain which constitutes the cellulose molecule. 

(For a more detailed picture of the spatial arrangements in this chain, 
including the atomic distances, see reference 3.) In the cellulose fiber 
(cotton, ramie, etc.) these long molecules are lined up parallel with each 
other along the fiber axis. 

To complete this formula it is necessary to specify the nature of the 
terminal groups and the length of the chain. The most reasonable assump- 
tion is that the chains are open and terminated by hydroxyl groups (alco- 
holic at one end and semi-acetal at the other), but no very definite experi- 


mental evidence on this point is available. Meyer and Mark estimate 
that the molecular chains are made up of from sixty to one hundred glucose 
units (molecular weight, 10,000 to 16,000), and they assume that the mole- 
cules are segregated into compact bundles (crystallites or micelles) from 
which the gross structure of the cellulose fiber is built up. Staudinger, 
however, has presented evidence (124) that the crystallites do not persist 
in dispersions of cellulose and its derivatives, and that the molecules have 
a weight much higher than 16,000. Stamm (126) has made direct measure- 
ments of particle size in ammoniacal copper dispersions of cellulose by the 
Svedberg ultracentrifugal method and obtained a value (on the copper-free 
basis) of 40,000 =*= 5 per cent. The molecules in a given sample of cellulose 
are probably not all of the same length, but Stamm 's data indicate a much 
higher degree of homogeneity than one might expect. 

According to this picture cellulose, like other high polymers, is not a 
chemical individual in the sense of being composed of identical molecules, 
and its structure cannot be completely specified by a single exactly defined 
formula. On the other hand, this picture accounts for the physical and 
chemical behavior of cellulose just as completely and satisfactorily as the 
formulas of simple compounds account for their properties. Cellulose 
fibers are very strong because of the parallel arrangement of the long molec- 
ular chains along the fiber axis. These chains adhere firmly to one 
another because of the cumulative force of association of the numerous 
hydroxyl groups. Strong aqueous alkali is able to penetrate this structure, 
and the resulting spreading apart of the chains causes lateral swelling. 
Other reagents (e. g., nitric acid) can penetrate the structure and esterify 
the hydroxyls without completely changing the apparent physical structure 
of the fiber. Cellulose is degraded much more rapidly by acids than by 
alkalies because the units are joined together by acetal linkages. The 
great length of the molecules accounts for the high viscosity of dispersions 
of cellulose and its derivatives. The very first stages of degradation 
greatly reduce the viscosity (124) because, for example, the hydrolytic 
absorption of 1 part of water in 2000 is capable of reducing by half the 
average size of the molecules. Undegraded cellulose has little or no re- 
ducing power (130) because it contains only one reducing group in an ex- 
ceedingly large molecule, but reducing power is manifested and increases 
progressively with hydrolysis (124, 127). Complete hydrolysis finally 
gives a quantitative yield of glucose (125) ; cellobiose, a triose, a tetrose 
(128) and higher polysaccharides are formed as intermediate products. 
The hydrolysis agrees in its kinetics with the theoretical requirements for 
the chain structure (129). This structure also accounts for the presence 


of three esterifiable hydroxyl groups for each Ce unit, and for the fact 
that completely methylated cellulose yields 2,3,6-trimethylglucose on 
hydrolysis (121). 

3. Rubber. It is well known that rubber hydrocarbon has the same 
empirical composition as isoprene (CsHs) ; that isoprene can be polymerized 
to yield a material resembling natural rubber; and that the behavior of 
rubber toward halogens, hydrobromic acid, and hydrogen indicates the 
presence of one double bond for each five atoms of carbon. Harries many 
years ago (131) showed that the degradation of rubber by ozone yields 
chiefly levulinic acid and aldehyde, and this fact indicates that the rubber 
molecules must be largely built up by the repetition of the unit 

CH 2 C=CH CH 2 

in a regular manner as indicated below. 

(II) . . . CH 2 CMe-=CHCH 2 ~-CH 2 CMe-=CHCH 2 CH 2 CMe=CHCH 2 . . . 

... CMe==CHCH 2 CH 2 CMe=-CH ... __gg omzatlon _ > CHOCH 2 CH 2 COMe 
Rubber Levulinic aldehyde 

Harries first assumed that the rubber molecule is an 8-membered ring. 
Later the discovery of larger fragments in the products of degradation by 
ozone led him to suggest a larger cyclic structure. The physical properties 
of rubber clearly indicate however that it is macromolecular. Pickles 
(133) suggested the more plausible linear polymeric structure indicated 
above, and Staudinger (134) has brought forward a large mass of evidence 
in favor of this structure. The best evidence available indicates that the 
average weight of the rubber molecule is exceedingly large perhaps in the 
neighborhood of 70,000 (135). It appears, moreover, that molecules hav- 
ing widely different sizes must be present in a given sample of rubber. 
Raw rubber is not homogeneous in its behavior toward solvents. When 
placed in contact with ether it swells and part of it diffuses into solution 
fairly rapidly. The action of fresh ether on the residue is much slower, 
and it is possible to carry the process of extraction so far as to obtain ulti- 
mately a residue that shows scarcely any tendency to dissolve. On the 
basis of such experiments it has been assumed that rubber is made up of 
two phases, sol and gel, and that the properties of rubber are due to the 
colloidal relationships of these two phases. In support of this idea there 
is the fact that colloidal dispersions of the jelly type frequently exhibit 
striking elasticity. This is true, for example, of polystyrene when it is 
swollen with unpolymerized styrene or with other hydrocarbon solvents. 


Whitby (137), however, has shown that sol-rubber (diffused) alone shows 
all the characteristic physical properties of raw rubber and differs from the 
latter only in degree. Moreover, it appears that sol- and gel-rubber are 
not distinct species and that neither of them is homogeneous. Rubber 
probably consists of a long continuous series of molecules of differing 
lengths. The smallest molecules dissolve quite readily and the largest 
ones only with difficulty. 

If the rubber molecule is built up uniformly according to the regular 
plan indicated in formula II it should yield levulinic acid and aldehyde as 
the exclusive products of degradation by ozone. The yields of these prod- 
ucts obtained by Harries accounted for only about 70 per cent, of the rub- 
ber. Pummerer (136) has recently undertaken a reexamination of the 
Harries method, using carefully purified rubber, and has been able to ac- 
count for 90 per cent, of the rubber. He finds significant amounts of suc- 
cinic acid, acetic acid, and acetone among the products. If the rubber 
molecule is an open chain, it might have an extra double bond at one end, 
and according to the disposition of this bond any of the above named by- 
products might be produced by ozonization. These products might also 
result from occasional irregularities in the arrangement of the units along 
the chain. The various possibilities have been outlined by Whitby (137). 

Although the examination of unstretched rubber by x-rays gives only 
an amorphous ring, stretched rubber gives a sharp fiber diffraction pattern 
(138). When unstretched rubber is cooled in liquid air and then frac- 
tured by impact, it breaks up into irregular fragments; stretched rubber 
treated in the same way breaks up into thin fibers along the axis of stretch 
( 1 39) . These facts show that stretched rubber is much more highly oriented 
than unstretched rubber. 

The structural unit of rubber contains a double bond and this brings 
about the possibility of stereoisomerism. The units may be in the cis 
or the trans form (or both). 

/ \ 

CH 2 H H CH 2 

v v 

it 4 

/ \ / \ 

CH 2 CH 3 CH 2 CH 2 

\ \ 

cis form trans form 

The x-ray data are said to favor the cis orientation (140). A spatial 
model of a long chain of such cis units is capable of being coiled up into a 


cylindrical spiral, and this spiral can be stretched out into a long chain. 
Various writers have suggested that the spiral model is capable of ex- 
plaining the reversible stretching of rubber. This model has recently been 
discussed in detail by Fikentscher and Mark (140). It is assumed that the 
residual valence forces at the double bonds are responsible for holding the 
spiral in its compressed form. When the rubber is stretched, work is done 
against these forces, and the molecules assume the chain-like form, where 
they are much more highly oriented with respect to one another. Vulcani- 
zation is assumed to involve the chemical linking of these spiral chains at 
occasional points through sulfur atoms. A very small amount of com- 
bined sulfur does not interfere with the stretching of the spirals, but it pre- 
vents the chains from slipping past one another or being torn apart; con- 
sequently vulcanized rubber is not plastic, and it is not dissolved but only 
swelled by rubber solvents. As the amount of combined sulfur increases, 
the entire structure becomes more rigidly linked together, the plastic prop- 
erties are completely suppressed, and the ability to imbibe solvent is lost. 
This picture is useful but it can hardly be said to account completely for 
the remarkable properties of rubber. 

VII. The Physical Properties of High Polymers 

Perhaps the most important result of the study of synthetic high poly- 
mers has been to establish the fact that such materials are actually made up 
of exceedingly large molecules in the sense of the ordinary structural theory 
of organic chemistry. This is a point on which considerable scepticism 
has prevailed in the past, and the attempt to evade or ignore the idea of 
the molecules in dealing with high polymers has led to much speculative 

It is true that synthetic linear high polymers are invariably mixtures 
whose molecules are chains of slightly differing lengths, and it is difficult 
to obtain reliable estimates of the average size of these molecules. Never- 
theless it must be admitted that a molecule does not lose any of its definite- 
ness as an entity either through the fact that it is exceedingly large or 
through the fact that it cannot be completely separated from other similar 
but slightly different molecules, and the properties of high polymers must 
ultimately be conditioned by the kinds of molecules which they contain. 

It would be beyond the intended scope of this paper to attempt a de- 
tailed discussion of the relation between the molecular structure and the 
physical properties of high polymers, but there are two or three points that 
deserve some mention. 


It is evident that in some respects the physical behavior of a molecule 
whose length is 100 times as great as its other dimensions must be pro- 
foundly different from that of a small compact molecule. Enormously 
long, flexible, and clumsy molecules must be very sluggish in their kinetic 
behavior, and it is not surprising that high polymers cannot be distilled or 
that they are never obtained in the form of thin mobile liquids. 

The cohesive forces which resist the separation of molecules from one 
another (as measured, for example, by the heat of vaporization) increase 
continuously with increasing molecular weight in a given series, and in high 
polymers they reach values greatly in excess of the energy required to 
rupture a primary valence linkage in a chain (141). For this reason high 
polymers cannot be distilled without decomposition ; indeed it appears that 
the upper limit of distillability may lie at as low a molecular weight as 1200 
to 1500 (44). 

1. Solubility and Colloidal Properties. High polymers are subject to 
the same rough qualitative solubility rules that apply to simple com- 
pounds: like dissolves like; polar compounds dissolve in polar solvents 
and non-polar compounds in non-polar solvents ; solubility in a given series 
diminishes with increasing molecular weight. Thus, rubber and poly- 
styrenes are soluble in benzene, but not in acetone; polyamides are not 
dissolved by the usual organic solvents, but are dissolved by hot form- 
amide; polyacrylic acid is soluble in water, while its esters dissolve in 
organic solvents but not in water; polystyrenes of low molecular weight 
(about 1000) are soluble in ether while polystyrenes of high molecular weight 
(about 20,000) are only slightly soluble in ether but are still dissolved by 

The solubility of high polymers is sometimes surprisingly great com- 
pared with that of analogous simple compounds of much lower molecular 
weight. The higher normal paraffin hydrocarbons (e. g., heptacontane) 
are practically insoluble in any solvents at the ordinary temperature, while 
polystyrene and hydrorubber, which are essentially very long paraffin 
chains substituted at intervals by phenyl and methyl groups, dissolve 
readily in benzene. One reason for this no doubt lies in the fact that most 
high polymers are mixtures of molecules of different lengths, and these are 
capable to a certain extent of behaving independently in their solubility 
behavior. Moreover, the crystal lattices of high polymers are not so well 
ordered and rigidly constructed as those of low molecular weight materials 
and they may, for this reason, be more susceptible to attack by solvents. 

The fact that the cohesive forces operating between large molecules are 
exceedingly high does not mean that polymers are incapable of forming 


molecular dispersions. Solubility depends upon specific affinities. Solu- 
ble linear polymers of relatively low molecular weight (e. g., 1000 to 5000) 
dissolve spontaneously and very rapidly in appropriate solvents and yield 
solutions which are not highly viscous. The osmotic unit in these solu- 
tions is the molecule, not an aggregate of molecules. For various poly- 
esters this has been proved (32, 34) by the fact that the same molecular 
weight values are obtained in a variety of solvents and by both freezing 
point and boiling point methods. Moreover, as the data of Table II show, 
direct chemical determinations of molecular weight give the same values 
as the osmotic methods. Polyesters having molecular weights consider- 
ably above 5000 dissolve in the same solvents as the lower polyesters, but 
the process of solution is slower and the solutions are more viscous (51). 
The same behavior is observed in other polymeric series. Polystyrenes 
having molecular weights of about 1000 dissolve instantly in benzene, and 
the viscosity of the solutions is low; polystyrenes having molecular weights 
above 10,000 swell before dissolving, and the solutions are highly viscous 
(102). These evidences of colloidal behavior are due simply to the fact 
that the molecules are exceedingly long. The probable mechanism of 
solution of certain polymers is best illustrated by a specific example. 

Rubber is made up of enormously long hydrocarbon chains ranging 
perhaps from 1000 to 10,000 A. in length. These chains have a high 
specific affinity for certain non-polar solvents such as benzene. In a mass 
of rubber, adjacent chains are firmly bound to one another by cohesive 
forces. The structure is not an entirely regular one and there is no doubt 
a considerable amount of purely mechanical entanglement. Benzene, by 
virtue of its specific affinity for the chains, is capable of penetrating into 
the mass, solvating the chains, and spreading them apart. The structure 
thus becomes swollen and more tenuous, and finally individual fragments 
are carried away into solution. The fragments may be single molecules 
or only incompletely disrupted aggregates, but finally, if sufficient solvent 
is present, the latter are broken down and what amounts to an actual mo- 
lecular dispersion results. This dispersion has a very high viscosity even 
when quite dilute, for the molecules are not only very large, but, owing 
to the fact that they are solvated and extended in only one dimension, they 
have an effective radius of action quite out of proportion to their size. 

The view that lyophilic dispersions of linear high polymers are usually 
true molecular dispersions, although it has not yet been generally ac- 
cepted by colloid chemists (see 174), has been supported by Staudinger 
with a large mass of evidence (146, 102, 124, 135), which in its cumulative 
force seems to the writer fairly conclusive. Reference may be made also 


to Stamm's determination with Svedberg's ultracentrifuge of the particle 
size of cellulose dispersions in copper-ammonia solutions (126). He ob- 
tained the value 40,000 =*= 5 per cent. It is quite certain on various 
grounds that the average molecular weight of cellulose cannot be less than 
about 16,000, so that if Stamm's particle is an aggregate it cannot con- 
tain more than two or three molecules. It seems highly arbitrary to 
assume that the solvent action of the dispersing agent, which depends upon 
a specific affinity for the cellulose molecules, should be capable of carrying 
off the molecules only as pairs or triplets and never as single molecules. 

It is of course not contended that association never occurs in lyophilic 
solutions of high polymers, but merely that association occurs only as the 
result of some appropriate peculiarity in the molecular structure of the 
polymer, e. g., through the presence of recurring amide or carboxyl groups. 

2. Crystallinity. Linear polymers, in spite of their lack of complete 
homogeneity and their high molecular weight, are by no means always 
amorphous. As indicated in Table I, all the polyesters derived from gly- 
cols of the series HO(CH2)*OH and acids of the series carbonic, oxalic, 
succinic, etc., separate from solvents in the form of powders which show 
quite definite melting points. On the other hand, similar esters derived 
from phthalic acid are invariably transparent, amorphous resins. The 
ability to crystallize appears to require a high degree of linear symmetry 
in the structural unit. The presence of side chains such as methyl or 
phenyl groups on the units, and the random mixing of structural units, 
which occurs, for example, when polymers are prepared from a single glycol 
and two different acids, diminish the tendency toward crystallinity. Thus, 
polyesters and poly amides derived from unsubstituted aliphatic com- 
pounds are crystalline, and so also are the polyoxymethylenes and poly- 
ethylene oxide, while polymers derived from vinyl compounds of the type 
XCH=CH 2 are usually amorphous. In these vinyl polymers the X group 
diminishes the linear symmetry of the chain ; moreover in the formation of 
such polymers occasional inversions of the order of the units probably 

The behavior of ethylene succinate (molecular weight, 3000) on crys- 
tallization appears to be typical of many high molecular weight materials. 
It separates from a melt or from concentrated solutions in chloroform as 
doubly refracting microscopic spherulites which grow to what appear to be 
star-like clusters of needles. Further growth leads to frost-like patterns. 
The melt finally solidifies to an opaque porcelain-like mass. From dilute 
alcohol solutions ethylene succinate separates in very thin, discrete needles, 
but these lose their identity as soon as the solvent has evaporated. This 


behavior is highly characteristic of very long chains. It is reproduced 
in all its details by such diverse materials as triacetylinulin (150), n- 
heptacontane (51) and trimethylcellulose. The photomicrographs of 
crystalline trimethylcellulose presented by Hess (149) would serve equally 
well to represent ethylene succinate. Linear polymers in the form of 
microcrystalline powders have a pronounced tendency to become elec- 
trified, and they strongly adsorb considerable amounts of water vapor 
even when they show scarcely any tendency to dissolve in water. 

The crystallization of linear polymers probably involves the parallel 
arrangement of the long chains into compact bundles, since this arrange- 
ment enables the molecules to exert their maximum cohesive force (151). 
Loose parallel swarms of molecules may also exist in melts or solutions of 
the polymers. Molecules of identical length might be arranged in bundles 
as shown in Fig. l(a), but especially with very long molecules, a less 
regular type of structure such as that shown in (b) might be produced. 
The arrangement shown in (b) has no sharp boundaries and this defect 

FIG. la FIG. Ib 

would be exaggerated if the molecules were not all of the same length. 
Since the molecules of a specimen of high polymer are very long and have 
not all the same length the lattice bundles first formed must more nearly 
resemble (b) than (a). In the presence of solvent, such crystals might 
persist as discrete particles, but in the absence of a solvent they would tend 
to coalesce and lose their identity, owing to the absence of sharp bound- 
aries and the incomplete neutralization of the residual forces of the pro- 
jecting molecules. Thus, it is never possible to isolate large discrete (un- 
solvated) crystals of high polymers. Moreover, though solid masses of 
crystalline high polymers may be either hard and brittle or very tough, or 
soft and wax-like, they never show any definite planes of cleavage. The 
coalescence of the initial crystallites, which occurs as a molten mass of 
polyester finally solidifies, must occur in a random and rather disordered 
fashion, and it is probable also that the crystallites are cemented together 
by molecules that have not succeeded in completely identifying themselves 
with any particular crystallite. 

The melting points of crystalline linear polymers show certain regulari- 
ties. For a given molecular weight the melting points increase with the 


cohesive force (polarity) of the structural units. Polyamides have much 
higher melting points than analogous polyesters. Polyesters derived 
from short chain dibasic acids melt higher than those derived from the 
longer chain acids; mixed polyesters melt lower than simple ones. For a 
given polyester the melting point usually increases with increasing molecu- 
lar weight up to a certain point, and after that it remains unchanged even 
though the molecular weight be increased many fold. Melting points are 
sometimes rather vague, but more frequently they are surprisingly sharp 
even when the molecular weight is so high that the molten polymer shows 
no sign of flowing and the only indication of melting is the disappearance 
of opacity. 

The question of the meaning of the term crystallinity in connection with 
high polymers is rather confused. Linear polyesters whose molecular 
weights lie below 5000 are definitely crystalline; they have sharp melting 
points and the crystals can actually be seen under the microscope. The 
evidences of crystallinity in polyesters whose molecular weights lie above 
10,000 are somewhat more vague, but even these materials furnish sharp 
x-ray powder diffraction patterns. Similar though less sharply defined 
patterns are obtained from a transparent sheet of regenerated cellulose. 
These patterns indicate that part at least of the molecules of such ma- 
terials must be definitely ordered with respect to one another. On the 
other hand, certain linear polymers, e. g., polystyrene, can be obtained in 
the form of white powders which show no microscopic or x-ray evidence 
of crystallinity. In these cases apparently the molecules tend to collect 
into discrete aggregates of some kind, but not in a sufficiently orderly 
fashion to exhibit any of the usual properties associated with crystals. 

3. Mechanical Properties. High polymers are very extensively used 
as structural materials in the construction of artifacts. One has only to 
mention cellulose, silk, and rubber to indicate the great economic im- 
portance of these non-chemical uses of organic materials. These uses 
depend upon such properties as mechanical strength, toughness, pliability, 
and elasticity. Such properties are found to a useful degree only among 
polymers of very high molecular weight. The synthetic materials of this 
class that have been most successfully used are 3 -dimensional polymers 
such as Bakelite and the glyptals. These materials have considerable 
strength, rigidity and toughness, but they are completely amorphous, 
and they are greatly inferior to natural fibers in breaking strength and 
pliability. The breaking strength of a flax fiber (100 kg. per sq. mm.) 
is of the same order as that of a good grade of steel (152). The qualities 
necessary for a useful fiber appear to be associated with a very high molecu- 


lar weight linear -polymeric structure and a certain degree of crystallinity 
or definite order in the arrangement of the molecules. The relation be- 
tween molecular structure and arrangement and the physical properties 
of fibers has been most clearly recognized and discussed by Meyer and 
Mark (3). In a natural cellulose or silk fiber the long molecular chains 
are arranged in an ordered fashion parallel with the fiber axis. This state 
of affairs is symbolized in Fig. 2. This arrangement provides the maximum 

Fig. 2. 

possible strength in the direction of the fiber axis since the mutual cohesive 
force of the long chains is fully utilized. To rupture the fiber it is necessary 
to cause the chains to slip past one another against this cohesive force as 
indicated in the dotted line. A transparent sheet of regenerated cellulose 
shows (by x-ray patterns) a certain degree of order in the arrangement of 
its molecules, but there is no general orientation. This state of affairs is 
symbolized in Fig. 3. This more or less random arrangement of ordered 
molecular aggregates can be brought into the more highly ordered state 
symbolized in Fig. 2 merely by the action of mechanical stress. Thus 
the strength of a sheet of Cellophane that initially has approximately the 
same strength in all directions can be so changed, merely by careful 
stretching, that its strength along the axis of stretch is increased several 
fold (153). At the same time its strength along the axis normal to its 
stretch is considerably diminished. The strength of a rayon filament can 
be increased several fold by the action of stress while it is in the spinning 
bath, and a comparison of the x-ray patterns shows a much higher degree 
of orientation along the fiber axis in the filament formed under stress. For 
a rough mechanical analogy of the mechanism of this process one may pic- 
ture a disordered mass of long straws (molecules) coated with a semi- 
fluid adhesive (cohesive force). The gradual application of stress to such 
a mass would finally bring the straws into parallel alignment where they 
would more strongly cohere and resist the further action of stress. 

The peculiarities of high polymers are nowhere more strikingly exempli- 
fied than in this curious ability to accept permanent orientations through 


the action of mechanical stress. The properties of simple organic com- 
pounds are, generally speaking, independent of their physical history; 
they are completely determined by the nature of the molecules. Very 
large molecules, however, are not capable of adjusting themselves instantly 
to any changes in physical environment, and the properties of a very high 
molecular weight material may vary over a wide range depending upon the 
physical treatment it has received. 

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X. The Reversible Polymerization of Six-Membered Cyclic 


This publication describes the reversible polymerization of six-membered 
cyclic esters t which has already been observed in a previous paper of this 
series, f 

Such behavior has been found in several cases by different authors and 
seems to be fairly common with six-membered cyclic esters or 8-lactones; 
it does not occur however with e-lactones nor with rings of a higher number 
of members. 

Different linear polymers of d-valerolactone were prepared up to molecular 
weights of 2240, at temperatures between 20 and 150, with and without 
catalysts. Their melting points ranged between 35 and 55. Poly- 
merization took place in 3 to 29 days. The samples represent crystalline 
powders; they are all acidic. Titration gave molecular weights which 
closely agreed with the boiling point values. 

Depolymerization takes place if the samples are heated and presumably 
consists of an ester interchange. The behavior of the six-membered rings 
is in accordance with the stereochemical theory of Sachse and Mohr. 

Reactions leading to the formation of high polymers have received con- 
siderable attention during the past few years both because of their inherent 
interest as representatives of a realm that has been relatively little ex- 
plored, and because of their bearing on the formulation of naturally 
occurring macromolecular materials. In this connection the behavior of 
six-membered cyclic esters is of peculiar interest : they combine with them- 
selves, in many cases spontaneously at the ordinary conditions, to form 
polymers of high molecular weight, and this transformation is reversible. 
Since these esters are generally free from unsaturation, their self-combi- 
nation must have its origin in some peculiarity of the heterocyclic system. 
Six-membered heterocycles are quite common among naturally occurring 
materials and are closely associated with natural polymers (sugars and 
polysaccharides, diketopiperazines and polypeptides) ; it is therefore rea- 
sonable to assume that the behavior of the cyclic esters lies closer to the 
natural processes that result in the formation of macromolecular substances 

* W. H. Carothers, G. L. Dorough and F. J. van Natta; Journ. Am. Chem. Soc., 
54, 761-72 (1932) ; Contribution No. 72 from the Experimental Station of E. I. du Pont 
de Nemours and Co. 

Received September 24, 1931. Published February 5, 1932. 

t W. H. Carothers and F. J. van Natta; Journ. Am. Chem. Soc., 52, 314 (1930); 
pages 29 to 42 of this volume and W. H. Carothers, J. A. Arvin and G. L. Dorough; 
Journ. Am. Chem. Soc., 52, 3292 (1930); this volume pages 54 to 62. 


than does the polymerization of unsaturated compounds, such as styrene, 
which have been more extensively studied. 

Several isolated examples of the reversible polymerization of six-mem- 
bered cyclic esters have been recorded in the literature but the question of 
the mechanism of this phenomenon has received very little attention. 
In the present paper, we report some observations bearing on this point. 

Previous Work. Fichter and Beisswenger (1) first reported that "5- 
valerolactone (I) is characterized by a remarkable and surprising phenome- 
non: it polymerizes after a short time; the initially mobile oil becomes 
gradually thicker and finally solidifies to a crystalline mass." The polymer 
after recrystallization melted at 47-48. Attempts to determine its mo- 
lecular weight gave values ranging from a five- to a seven-fold polymer. 
Later the same transformation was observed by Hollo (2), who also records 
the spontaneous polymerization of the lactone (II) of hydroxyethylglycolic 

Bischoff and Walden (3) in 1893 described the transformation of glycolide 
(III) under the influence of heat or a trace of zinc chloride into a polymeric 
solid melting at 220. On being distilled in a vacuum it was reconverted 
to the monomer, melting at 86-87. 

Drew and Haworth (4) have obtained the lactone (IV) of 2,3,4-trimethyl- 
/-arabonic acid in crystalline form (m. p. 45) and observed that, in the 
presence of traces of hydrogen chloride, it is converted into a crystalline 
polymeric powder which has a considerably higher melting point, a lower 
solubility and a lower specific rotation than the monomer. Its molecular 
weight (about 2000) indicates that it is derived from about ten molecules 
of the monomer. At 175 it distils completely in vacua, and the dis- 
tillate consists of pure lactone. Drew and Haworth (5) favored a linear 
polyester structure for this polymer. 

Trimethylene carbonate (V) was first described by Carothers and van 
Natta (6). The monomer is a very soluble crystalline solid melting at 47. 
If it is heated to 130 for a few minutes with a trace of potassium carbonate, 
the mobile melt suddenly becomes very viscous and evolves a small amount 
of gas. The colorless, viscous sirup on cooling solidifies to a stiff mass 
which shows an apparent molecular weight of about 4000. When this mass 
is heated in a vacuum it distils almost quantitatively, and the distillate 
consists of pure monomer. 

Monomeric ethylene oxalate (VI) was first described by Bischoff and 
Walden (7), who obtained it as a crystalline solid melting at 143 by distill- 
ing the product of the action of ethylene glycol on monoethyl oxalate. 
Later Bischoff (8) obtained less soluble, higher melting forms of ethylene 


oxalate, and showed that these on distillation were converted into the 143 
form. Bischoff also observed that the melting points of the ethylene 
oxalates change on standing, and suspected that these changes might be 
due to a reversible polymerization, but he reported no comparative mo- 
lecular weight data. The relation of the various forms of ethylene oxalate 
to one another has lately been studied by Carothers, Arvin and Borough 
(9). It was found that the flat, diamond-shaped crystals of the monomer 


R-dS'b R-dH O 

o ca o CH-R 


I R = H* II. R - H III. R - H 

la, R = n-C,H, Ha, R - C.H, Ilia, R - CH, 


<A> Ao 


\/ \/ \d 


IV V VI. R - H 

/ C \ 

I I /0-(CH,)-<X ^^^9 

CH, CH, (fo \ CO ((5H0.4 I 

H, CH, \0-(CH,V-0/ ^-O 


CO0-- (CH 2 )r-0 CO COO (CH,), O~CO 


\CO--O-~(CH 2 ), O (X) CO O (CH 2 )r-0--CO 


completely disintegrate during the course of a few days, yielding a micro- 
crystalline powder which consists of a mixture of polymers. From this 
mixture by careful extraction with cold solvents two definite fractions 
can be isolated. One of these melts at 159 and has an apparent molecular 
weight of about 3000; the other fraction, which is probably much more 
highly polymeric, melts at 173 and is too insoluble for molecular weight 
determinations. These isolated fractions on standing lose their identity; 


they revert spontaneously within a few days to more complicated mix- 
tures which usually contain appreciable amounts of monomer. The poly- 
meric ethylene oxalates can be converted into the monomer (with con- 
siderable loss) by vacuum distillation. Propylene oxalate (Via) (m. p. 
142) is converted by the action of heat into an insoluble, microcrystalline 
polymer melting at 178 (9). 

Generality of the Phenomenon. The examples cited above show that 
the capacity to undergo reversible polymerization is quite common among 
six-membered cyclic esters. The presence of substituent groups, however, 
has a considerable effect and, in general, a depressing effect, on this tend- 
ency. The unsubstituted esters I, II and VI polymerize spontaneously 
at ordinary temperature in the complete absence of any added catalyst; 
substituted esters on the other hand, such as la, Ila, IV and Via, require 
the action of heat, or catalysts, or both, and in some cases polymerization 
fails altogether. 

A number of substituted S-valerolactones (I) have been described in the 
literature, but without any record of their polymerization. We have 
therefore prepared a-w-propyl-S-valerolactone (la) as described in the 
experimental part. Although 5-valerolactone was found to be completely 
polymerized to a waxy solid after twenty-nine days at room temperature, 
the new a-w-propyl-6-valerolactone was still unchanged after twelve 
months, nor did it show any signs of polymerization after being heated to 
80 for one month. However, when heated to 80 for one month in the 
presence of a trace of potassium carbonate or zinc chloride, it became more 
viscous, and its apparent molecular weight rose to 1100-1200. 

Hollo (2) records the spontaneous polymerization of the lactone (II) of 
hydroxyethylglycolic acid, but makes no mention of the polymerization 
of its a-alkyl derivatives (II, R = C 2 H B , w-C 3 H 7 and is0-C 3 H 7 ) which he 
prepared at the same time. We have made attempts to polymerize the 
lactone (Ila) of hydroxy ethyl- a-hydroxybutyric acid. Sixteen hours of 
heating at 140-160 failed to produce any change in the viscosity or the 
color of the pure lactone. Under the same conditions in the presence of 
a trace of potassium carbonate, only a slight increase in viscosity occurred. 

Bischoff and Walden (10) state that lac tide (Ilia) does not polymerize 
under the conditions that result in the polymerization of glycolide (III). 
We find, however, that at 250-275 the polymerization of lactide is quite 
rapid : in two hours a sample was transformed into a resinous mass which 
showed an apparent molecular weight of about 3000. A similar effect 
can be obtained at much lower temperatures, e. g. t 140-150, if potassium 
carbonate is present. 


We have also examined some of the higher homologs of lac tide. The 
cyclic esters derived from a-hydroxycaprylic and from a-hydroxy palmitic 
acids were prepared and heated for various periods both in the presence and 
absence of catalysts. Evidences of polymerization could be found only at 
temperatures sufficiently high to produce considerable decomposition. 

Finally, it may be mentioned that although ethylene oxalate polymerizes 
spontaneously at the ordinary temperature, propylene oxalate undergoes 
a similar transformation only at considerably elevated temperatures (9). 

Peculiarity of the Phenomenon. Among cyclic esters the capacity to 
undergo reversible polymerization in the manner illustrated by the ex- 
amples cited above is peculiar to six-membered rings. The 7-lactones and 
other five-membered cyclic esters show no tendency to polymerize, and no 
corresponding polymers are known. Thus, we have heated samples of pure 
7-butyrolactone both with and without catalysts (zinc chloride, potassium 
carbonate) at 80 for twelve months; none of the samples showed any 
detectable increase in viscosity. We have also made various attempts to 
polymerize ethylene carbonate, but these attempts were all unsuccessful, 
although the corresponding six-membered ring, trirnethylene carbonate 
(V), is readily polymerized (11). 

It appears also that cyclic esters of more than six members show no tend- 
ency to polymerize spontaneously. Information on this point is some- 
what incomplete. No aliphatic cyclic esters of nine to thirteen atoms 
have been described. In fact no practical means for the preparation of 
such esters are yet known. In general, the attempt to prepare cyclic esters 
of more than six atoms from open chain compounds leads to linear polymers. 

Thus the six-atom ring trirnethylene carbonate (V) is readily obtained 
by heating diethyl carbonate with trimethylene glycol, and subsequently 
distilling the product in vacuo, but when diethyl carbonate is similarly 
treated with tetramethylene glycol, the product is a polymeric material 
that is not capable of being distilled in vacua, i. e., it cannot be depoly- 
merized; only at temperatures above 300, it undergoes complete thermal 
decomposition and yields a complicated mixture of products among which 
are found very small amounts of the dimeric fourteen-membered ring VIII 
(6). Attempts to detect the presence of the monomer VII have been 
unsuccessful. Similar behavior is observed with trimethylene oxalate (9) 
and ethylene succinate (12) . The preparation of these esters leads to linear 
polymeric products of high molecular weight. No smooth depolymeriza- 
tion of these polymers can be effected by the action of heat; but at high 
temperatures they undergo complete thermal decomposition, yielding a 
complicated mixture of gaseous, liquid and tarry products containing 


very small amounts of the cyclic dimers which are, respectively, fourteen 
and sixteen-membered rings (XI and IX) The corresponding mono- 
mers have never been obtained. 

The following observations indicate the relatively great stability of the 
large cyclic esters. Specimens of dimeric trimethylene oxalate (XI), 
dimeric ethylene succinate (IX), and exaltolide (X) have been preserved 
in the laboratory for two years without showing any signs of change. The 
last two of these compounds have been heated in sealed tubes at 170 for a 
considerable period of time. After twelve hours they were quite un- 
changed; after forty- three hours they had become slightly colored and their 
melting points slightly lowered, but signs of appreciable polymerization 
were absent (13). 

The peculiar position of the six-membered esters is shown in the follow- 
ing outline, in which, for simplicity, the starting material in each case is 
represented as an hydroxy acid. 

(A) The structural unit is five atoms long 

-H 2 O 

+H 2 ' > 

(B) The structural unit is six atoms long 

~H 2 O 

+H 2 1 I -H 2 

I \ 


(C) The structural unit is more than six atoms long 

+H 2 o1 ^-H 2 O 

Under B it should be mentioned that the reversible relation between 
the monomer and the polymer complicates the problem of deciding in a 
given case which of these is the real primary product of the dehydration 
of the hydroxy acid. In any event the conditions of the dehydration 
usually favor the polymerization of the monomer, so that the latter is 
isolated only by vacuum distillation of the actual reaction product. Diet- 


zel and Krug (14) have presented evidence to show that the self-esterifica- 
tion of lactic acid leads directly only to polylactyl lactic acids and that 
lactide, when it is produced, results from the depolymerization of these 
polyesters. On the other hand, it is established, at least in certain in- 
stances (4), that 5-lactones may be formed as such directly from the 
corresponding hydroxy acid. No doubt in most cases, depending upon 
the conditions, either the monomer or the polymer may be formed as 
the primary product. 

Under C it should be mentioned that although attempts to prepare 
seven-membered cyclic esters from dibasic acids and dihydric alcohols 
lead only to polyesters, e-hydroxy acids appear to be dehydrated with the 
simultaneous formation of polymer and monomer (15). There is, how- 
ever, no record of the interconversion of these two forms, and the higher 
a>-hydroxy acids yield linear polymers exclusively (16). 

Mechanism of the Phenomenon. The smooth reversibility of the 
polymerization of the six-membered cyclic esters and the absence of double- 
bond unsaturation at first suggested that here, if anywhere, might be 
found the missing models of that hypothetical phenomenon, association 
polymerization, to which for a time the peculiarities of natural polymers 
were widely attributed (17). This thought was considerably weakened, 
however, when Dr. J. W. Hill presented in the closely analogous case of 
adipic anhydride, a direct and decisive proof of the structural difference 
between monomer and polymer (18). No similar proof is possible in the 
esters under consideration; nevertheless, all the evidence together con- 
clusively favors the mechanism suggested by Carothers and van Natta (6), 
namely, that the polymers are linear polyesters and that both the poly- 
merization and its reversal proceed by a process of ester interchange. This 
is indicated in the equation 

O R CO + O R CO + O R CO + O R CO -f etc. > 

rr_ o R CO^O-~R-~CO--O-~R---- CO O R CO O R CO 

The peculiar position occupied by the six-membered cyclic esters may be 
explained, as we have already suggested (6), by stereochemical factors: 
rings of six atoms are strained (or may pass through positions of strain) ; 
rings of five atoms or more than ten atoms are strainless. In the cyclic 
esters the strain can be relieved by the process of ester interchange, which 
results in polymerization. The resulting polymers (from the six-membered 
esters) are easily depolymerized owing to the high probability of the close 


approach of atoms six atoms apart in a chain. Polyesters whose de- 
polymerization would result in larger monomeric rings are not depoly- 
merized; the probability of the close approach of the requisite atoms is 
too slight. 

The interchange mechanism for the polymerization of the six-membered 
esters is supported by the following facts: (1) The polymers have the same 
apparent analytical compositions (19), and saponification equivalents 
as the corresponding monomers, and they yield the same products on 

(2) The products of the polymerization of the six-membered cyclic esters 
closely resemble in their physical properties the polymers formed by 
the action of dibasic acids on glycols, or by the self-esterification of the 
higher w-hydroxy acids. It has been definitely established that these 
polymers are linear polyesters; the chains are open and are terminated 
by hydroxyl and/or carboxyl groups (12, 20). 

(3) Both the forward and the reverse transformations are catalyzed by 
acids and bases, typical ester-interchange catalysts. 

(4) The speed of the polymerization runs parallel with the susceptibility 
to hydrolysis. The much greater susceptibility of 5-lactones to hydrolysis 
as compared with analogous 7-lactones is well known. The hydrolysis 
constant for 5-valerolactone (I) is more than twice that of its a-methyl 
derivative (2), and the former polymerizes spontaneously while the latter 
does not. Similar relations hold for the lactone of hydroxyethylglycolic 
acid (II) and its a-alkyl derivatives (2). The hydrolytic constants for 
glycolide in its two stages are, respectively, 0.0179 and 0.119, while the 
corresponding values for lactide are 0.00313 and 0.0611 (21). The former 
polymerizes much more readily than the latter. Ethylene oxalate, which 
polymerizes with extraordinary facility, is so sensitive to hydrolysis that, 
like an acid, it can be titrated directly with moderately warm dilute alkali. 

Further Observations. Data on some polymers of 6-valerolactone are 
presented in Table I. 






M. p. of 
polymer, C. 

Mol. wt. of 
(boiling point in 




29 days 


1060 1060 




13 days 


1270 1330 


K 2 CO 3 


5 days 


1840 1820 


ZnCl 2 


5 days 


2230 1846 




10 hrs. 


2110 2240 


Over CHsCOCl 


3 days 


1720 1720 


The melting points are in general somewhat higher than those reported 
by Fichter and Beisswenger (1). The polymer is soluble in a variety of 
organic solvents and is readily crystallized, e. g., from a mixture of petro- 
leum ether and benzene. When allowed to solidify from a melt it forms 
an opaque, soft, waxy solid. The crystals are poorly developed; they ap- 
pear under the microscope as tiny irregular particles. 

Polymeric 5-valerolactone is acidic. In acetone solution it can be 
titrated with alkali to a sharp end-point which persists for several minutes. 
Sample 5 of Table I thus showed a neutral equivalent of 2153 and 2243 
in two determinations. The closeness of this value to the observed mo- 
lecular weights indicates the probable presence of one carboxyl group for 
each molecule. It is not quite certain that this agreement is not accidental. 
A product prepared by heating the monomer at 175 showed an equivalent 
weight of 6320 and a molecular weight of about 1500 (observed 1420 and 
1660), but the molecular weight values are under suspicion because of severe 
foaming. It seems certain however that at least part of the polyester 
molecules bear carboxyl groups. 

The polymeric 5-valerolactone is not homogeneous, but contains mole- 
cules of different sizes. Its sodium salts are not soluble in water. A 
sample of recrystallized polymer formed at ordinary temperature (m. p. 
53-56) showed by titration in acetone an equivalent weight of 3160. 
The acetone solution was evaporated and the resulting dry sodium salt 
taken up in hot absolute alcohol, filtered and cooled to crystallize. In this 
way fractions of different solubilities and sodium content were isolated: 
(1) 0.38 g. containing 0.22% Na; (2) 0.77 g. containing 0.49% Na; (3) 
0.14 g. containing 3.92% Na. The equivalent weights inferred from the 
sodium contents are: (1) 10,400, (2) 4700, (3) 590. Determination of the 
molecular weight of fraction 2 by the boiling point in benzene gave the value 

The polyester derived from the lactone (II) of hydroxyethylglycolic 
acid differs in its behavior from polymeric 5-valerolactone. It cannot be 
titrated to a sharp end-point with 0.1 N alkali. Attempts to prepare 
a sodium salt by treating it with sodium bicarbonate under various condi- 
tions have led to mixtures from which two distinct fractions are readily 
separated. One of these is unchanged polyester entirely free of sodium; 
the other is the pure sodium salt of hydroxyethylglycolic acid. Thus 
even so mild an alkali as sodium bicarbonate hydrolyzes the polyester, and 
it is impossible to determine whether the polymeric molecules bear car- 
boxyl groups. Ethylene oxalate (VI) shows a still more exaggerated sen- 
sitivity to alkalies. Trimethylene carbonate (V) appears to be neutral. 


In this case any terminal carboxyl groups that might arise during the poly- 
merization would be lost spontaneously since they would be linked to 
oxygen (R O CO OH). The momentary liberation of a small amount 
of gas at the end of the sudden thermal polymerization of trimethylene 
carbonate is perhaps due to loss of carbon dioxide from acid carbonic ester 
groups formed in this way. 

For 5-valerolactone, however, as already indicated, the presence of a 
terminal carboxyl group in the polymeric molecule is fairly clearly demon- 

It seems likely that the first step in the polymerization of the cyclic 
esters involves the intervention of a trace of the corresponding hydroxy acid 
or one of its derivatives. One can imagine that hydroxy valeric acid would 
thus react with valerolactone 

HO(CH 2 ) 4 COOH + O(CH 2 ) 4 CO > HO(CH 2 ) 4 CO O(CH 2 ) 4 COOH 

The dimeric acid would then react in the same manner with valerolactone 
to form a trimeric acid, and the reaction would continue in this sense until 
all of the lactone was exhausted or the chains became too long for further 
reaction. In this mechanism a foreign acid might equally well participate, 
and it would be bound in the product as the first unit of the polymeric 
chain. Experiments in this direction were made with chloroacetic acid. 

One gram of 5-valerolactone was heated with 0.1 g. of chloroacetic acid for fifteen 
hours at 150-160. The reaction mixture, which solidified on standing in a refrigerator, 
was recrystallized from alcohol and carefully dried. It melted at 42 and contained 
3.28% chlorine. It was triturated with an excess of sodium bicarbonate in the presence 
of a little water, and the product after drying was extracted with warm alcohol. The 
polyester which separated was thrice recrystallized from alcohol. It now melted at 
45-46 and contained 2.3% sodium. The molecular weights calculated respectively 
from the chlorine and sodium content, assuming a molecule having the formula XII, 
are 1008 and 1000. 

(XII) C1CH 2 CO [O(CH 2 ) 4 CO ]*OH 

In a similar experiment in which one gram of valerolactone was heated with 0.05 g. 
of chloroacetic acid the initial product melted at 48 and contained 2.06% chlorine. 
The sodium salt melted at 50-51 and contained 1.35% sodium. The molecular 
weights calculated from these data are 1720 and 1705. 

Chlorine-containing derivatives were also obtained by heating the polymeric 
lactone with chloroacetic acid. Two grams of polymer and 0.1 g. of chloroacetic acid 
were heated for ten hours at 150-160 . The product was thrice crystallized from alcohol. 
It contained 1.75% chlorine, and this corresponds to a molecular weight of 2030. Its 
neutral equivalent as indicated by titration with alkali was 2083. 

These results are at least consistent with the mechanism suggested above. A 
mechanism involving the mutual coalescence of two molecules of monomer to form a 


twelve-atom ring and the growth of this through the progressive absorption of more 
monomeric molecules seems less likely, but some observations lately made in this 
laboratory on the polymerization of cyclic anhydrides indicate that this mechanism 
is not at all impossible. 

Lactone (II) of Hydroxyethylglycolic Acid. Preparation: The method of Hollo (2) 
was slightly modified. The crude mixture containing the sodium salt of the hydroxy 
acid, sodium chloride and a little glycol was washed with acetone, suspended in abso- 
lute alcohol and treated with less than the calculated amount of hydrochloric acid. 
The precipitated sodium chloride was removed by filtration and the lactone isolated 
from the filtrate by distillation. In this way it was readily obtained free of chlorine. 

When allowed to stand it gradually solidified to a pasty mass. After five weeks 
the solid was crystallized five times from ethyl acetate. It then melted at 62-64 and 
showed a molecular weight in boiling benzene of about 460 (observed, 451, 471). This 
probably corresponds approximately with Hollo's polymer melting at 56-63. 

A polymer formed by heating the lactone at 150 for five hours after being crystal- 
lized four times from ethyl acetate melted at 87-89. It was very soluble in cold 
chloroform, hot ethyl acetate and hot alcohol, slightly soluble in ether, cold ethyl 
acetate and water. Molecular weight determinations in boiling benzene gave the values 
1647 and 1788. 

Hollo describes another type of polymer obtained when the sodium salt of the 
hydroxy acid is treated with an excess of hydrochloric acid. This melts at 66-68, 
boils at 216 to 220, and distils without change at atmospheric pressure. 

It seems quite impossible however that a polymer should boil only 5 higher than 
the monomer (210-215), and the explanation of Hollo's observation probably lies in 
the fact that the hydrochloric acid present in the polymer distils with the monomer 
during depolymerization and the polymerization of the distillate is so powerfully cata- 
lyzed that it occurs very rapidly. In our experiments a specimen of polymeric lactone 
originating from a preparation in which a slight excess of hydrochloric acid was used to 
liberate the hydroxy acid was observed to distil at 215-216 and the distillate solidified 
immediately on cooling. The solid product after three crystallizations from ethyl 
acetate melted at 85 to 87. 

This polymer distilled completely, but the distillate was not unchanged polymer. 
Molecular weight determinations on the distillate made immediately after the com- 
pletion of the distillation gave the values, 144, 140 and 121 (calcd. for monomer, 102). 
After two hours the distillate had already begun to crystallize. After twenty-four 
hours it had solidified, and the solid after recrystallization thrice from ethyl acetate 
showed a molecular weight of about 900 in boiling benzene (observed, 867, 931). 

Polymers originating from preparations in which an excess of hydrochloric acid 
was carefully avoided behaved in precisely the same way except that the reversion of 
the distillate was much slower; it remained fluid for twenty-four hours. The melting 
points of the purified polymers obtained from either source approached 89. 

tt-w-Propyl-5-valerolactone was prepared through the steps indicated in the follow- 
ing equations. 

C 6 HBOCH 2 CH 2 CH 2 Br + 

C 6 H 6 OCH 2 CH 2 CH 2 (C 3 H 7 )C(COOC 2 H 5 ) 2 --HBr > BrCH 2 CH 2 CH 2 CH(C 3 H 7 )COOH 

> BrCH 2 CH 2 CH 2 CH(C 3 H 7 )COONa - > OCH 2 CH 2 CH 2 CH(C 3 H 7 )CO 


phenoxypropyl-(w-propyl)-malonic diethyl ester: colorless liquid, b. p. (4 mm.) 
195-200; 4 1.0246; n 2 D 5 1.4820. 

Anal. Calcd. forCi 9 H 2 8O 6 : C, 67.85; H, 8.33. Found: C, 68.60, 68.04; H, 8.03, 

5-Bromo-a-tt-propylvaleric acid: b. p. (5 mm.) 148-150; d 1.3851, n 1.4730. 

Anal. Calcd. for C 8 Hi 6 O 2 Br: C, 43.05; H, 6.72; neutral equivalent, 223. Found: 
C, 44.33; H, 6.84; neutral equivalent, 220, 222. 

The high values for carbon are probably due to the presence of some phenol. 

a-n-Propyl-S-valerolactone: b. p. (10mm.) 118-120; </J 0.9929; w 2 D 1.4585. 

Anal. Calcd. for C 8 H 14 O 2 : C, 67.67; H, 9.85; saponification equivalent, 142. 
Found: C, 67.59, 67.50; H, 10.01, 10.18; saponification equivalent, 143, 144. 

Preparation of Lac tide. The preparation of lac tide by methods described in the 
older literature (22) gave poor yields, but excellent results were obtained by the follow- 
ing method based on recent patents (23). Two hundred grams of commercial lactic acid 
was heated at ordinary pressure in a Claisen flask by a metal bath at 120 until water 
ceased to distil. The temperature was then raised to 140 and the pressure reduced 
to 10 mm. During six hours water continued to distil slowly from the flask. The 
pressure in the system was reduced to 5 mm. and the temperature raised sufficiently 
to cause rapid distillation of the lactide, which solidified in the receiver. The yield 
of crude product was 125 g. It was purified by crystallization from ether; m. p. 128. 


Several isolated examples of the reversible polymerization of six-mem- 
bered cyclic esters have been briefly reported in the literature. Further 
examples are reported in the present paper, together with some experimen- 
tal data and speculations on the mechanism of the phenomenon. The 
following conclusions are reached. 

1. The ability to undergo reversible polymerization is generally char- 
acteristic of six-membered cyclic esters. 

2. Ester rings of five atoms or more than six atoms do not polymerize 
under the action of heat. 

3. The tendency of six-membered cyclic esters to polymerize is closely 
related to their great susceptibility toward hydrolysis; both tendencies are 
diminished by the presence of substituent groups. 

4. The polymers formed from six-membered cyclic esters are linear 
polyesters and, at least in certain instances, the chains are open and ter- 
minated by hydroxyl and carboxyl groups. 

5. Both the polymerization and the depolymerization consist essen- 
tially in a process of ester interchange. 

6. The peculiar position occupied by the six-membered cyclic esters is 
readily explained by stereochemical considerations based on the Sachse- 
Mohr theory. 


Bibliography and Remarks 

(1) Fichter and Beisswenger, Ber. t 36, 1200 (1903). 

(2) Hollo, ibid., 61, 895 (1928). 

(3) Bischoff and Walden, ibid., 26, 262 (1893). 

(4) Drew and Haworth, J. Chem. Soc. t 775 (1927). 

(5) Cf. Haworth, "The Structure of Sugars," Edward Arnold and Co., London, 1929, p. 78. 

(6) Carothers and van Natta, Journ. Am. Ghent. Soc., 52, 314 (1930). 

(7) Bischoff and Walden, Ber., 27, 2939 (1894). 

(8) Bischoff, Ber., 40, 2803 (1907). 

(9) Carothers, Arvin and Dorough, Journ. Am. Chem. Soc., 52, 3292 (1930); cf. Bergmann 
and Wolff, J. prakt. Chcm., 128, 229 (1930); and Carothers and van Natta, Ber., 64, 1755 (1931). 

(10) Bischoff and Walden, Ann., 279, 71 (1894). 

(11) 0-Lactones have not been specifically examined from this standpoint, but it appears likely 
in view of some observations recorded by Johansson [Lunds Universitets Arsskrift II, [2] 12, 3 (1916)) 
that simple lactones of this class undergo irreversible polymerization under the action of heat. 

(12) Carothers and Dorough, Journ. Am. Chem. Soc., 52, 711 (1930). 

(13) We do not intend to suggest that the polymerization of large cyclic esters is impossible. In 
fact, since such esters can be hydrolyzed to the corresponding hydroxy acids, it is evident from equation 
C that such a polymerization can probably be effected under some conditions. The process would 
consist in hydrolysis of the cyclic ester to the hydroxy acid and the dehydration of this to the poly- 
ester. Both of these steps might occur in a single operation under appropriate conditions, e. g., at 
elevated temperature in the presence of a small amount of water plus mineral acid. But the experi- 
ments described above do demonstrate that the large cyclic esters, at least qualitatively, are very 
different from the six-membered esters. 

(14) Dietzel and Krug, Ber., 58, 1307 (1925). 

(15) Marvel and Birkheimer, Journ. Am. Chem. Soc., 51, 260 (1929); Blaise and Koehler, Compt. 
rend., 148, 1773 (1909). 

(16) No exception to this statement has been observed among the numerous esters we have pre- 
pared from dibasic acids and glycols, but we have lately found that in the self-esterification of w hydroxy- 
decauoic acid in the absence of solvent, appreciable amounts of the cyclic dimeric ester are formed. 
This has already been prepared by a different method and described by Lycan and Adams [Journ. 
Am. Chem. Soc., 51, 3450 (1929)]. 

(17) Abderhalden, Naturwissenschaflen, 12, 716 (1924); Bergmann, ibid , 13, 1045 (1925), Pring 
sheim, ibid., 13, 1084 (1925); Hess, Ann., 435, 1 (1924). 

(18) Hill, Journ. A. Chem. Soc., 52, 4110 (1930). 

(19) But the values for carbon are usually somewhat low and the values for hydrogen somewhat 
high. This may signify that the chains are open, or it may mean merely that the polyesters in spite 
of the fact that they are insoluble in water are nevertheless somewhat hygroscopic and difficult to dry. 

(20) Lycan and Adams, Journ. Am. Chem. Soc , 51, 625, 3450 (1929). 

(21) Johansson and Sibelius, Ber., 52, 745 (1919). 

(22) Gay-Lussac and Pelouze, Ann., 7, 43 (1833); Bischoff and Walden, Ber., 26, 263 (1893) 

(23) Chemische Werke, French Patent 456, 824 (1913); Gruter and Pohl, U. S Patent 1,095,205 


XI. The Use of Molecular Evaporation as a Means for 
Propagating Chemical Reactions* 

A new design of molecular still is described, the aim being to remove a volatile 
reaction product of low vapor pressure from another product, which is 
formed simultaneously and having no volatility. The reaction mixture is 
heated in a flat pan, a high vacuum being maintained and a cooled con- 
densing plate being disposed in the immediate neighborhood of the evaporat- 
ing surface. 

Reversible reactions involving the simultaneous formation of a volatile 
and non-volatile product are often forced to completion by causing the 
volatile product to distil from the reaction mixture as fast as it is formed. 
The purpose of the present note is to call attention to the possibility of 
extending the application of this principle to instances in which the effective 
vapor pressure or escaping tendency of a volatile product or potential 
product is very small. In the molecular still (1) distillation or continuous 
evaporation can be effected even when the vapor pressure of the distilling 
substance is as low perhaps as 10~ 5 mm. The theory of the process has 
been discussed by Washburn (Ib), and we need only to mention that suc- 
cessful operation requires a highly evacuated system comprising a condenser 
placed very close to the evaporating surface. The temperature of the con- 
denser must be low enough to reduce the vapor pressure of the distillate 
to a negligible value. Under these conditions the mean free path of the 
molecules is less than the distance from the condenser to the evaporating 
surface; consequently most of the molecules that manage to escape from 
the evaporating surface are caught by the condenser with a negligible 
probability of return. 

Mercury and apparently cane sugar can be evaporated in the molecular 
still at room temperature (Ib) and w-heptacontane can be distilled without 
decomposition (2). The tendency to distil diminishes with increasing mo- 
lecular weight, and it appears that, so far as practically useful rates are 
concerned, the upper limit of distillability for paraffin hydrocarbons may 
lie at as low a molecular weight as 1100 or 1200. For other types of or- 
ganic compounds it will in general lie at still lower molecular weights (2). 
Thus all substances of high molecular weight are practically completely 
non- volatile. There are many instances of reversible reactions in which 

* W. H. Carothers and J. W. Hill; Journ. Am. Ghent. Soc. t 54, 1557-59 (1932); 
Contribution No. 74 from the Experimental Station of E. I. du Pont de Nemours and 

Received November 12, 1931. Published April 6, 1932. 


only one volatile substance is involved. If this substance has a very low 
vapor pressure, or if it is strongly held by other substances involved in the 
reaction, or if its equilibrium concentration is very low, then molecular 
evaporation may be capable of producing a result not even remotely ac- 
cessible with the aid of the usual distillation equipment and methods. 

Examples of the use of molecular evaporation as a means of propagating 
chemical reactions are provided in other papers of this series (3). The re- 
actions involved are reversible bifunctional 
condensations. The starting materials are 
linear polymers capable, by self-combination, 
of yielding a still longer molecule with the 
elimination of some volatile product. The 
size of the product molecules depends upon 
the completeness of the reaction. With the 
aid of the molecular still one can obtain prod- 
ucts of much higher molecular weight than 
with the ordinary distillation equipment. 
The same principles can doubtless be applied 
to other kinds of reversible reactions. 

The simple stills described by Washburn 
(Ib), function satisfactorily but they do not 
provide ready access to the residue from the 
distillation. For this reason we constructed 
the instrument shown in Fig. 1 and used it 
in carrying out some of the reactions referred 
to above. The new instrument has the ad- 
vantage that the flat pan containing the dis- 
tillation residue can be lifted out when the in- 
strument is open. 


1. Molecular still (Y 7 
natural size). 

Description of the Instrument 

The outer shell (A) of the apparatus is made up of 
two domes from pyrex vacuum distilling apparatus 

(Corning Glass Company Catalogue, Item No. 56). The ground surfaces are lubri- 
cated with a good vacuum grease. (B) is the condenser provided with water 
leads. (C) is the glass support for the heater (D) and the distilling pan (E) (4). 
The heater and distilling pan are contained in an outer copper pan as shown. The 
heater consists of nichrome wire wound spirally on a sheet of mica which is supported 
between sheets of thin asbestos board inserted below the distilling pan. The heater leads 
(F) are brought up through the support (C) and out as shown. Connection with the 
heater is made with spring clamps which make the heater readily demountable. The 
lower part of the apparatus is the regular distilling tube provided with the Pyrex 


vacuum distilling apparatus (Catalogue No. 56). The rubber stoppers at the top and 
bottom of the apparatus are well covered with a suitable wax such as picein. A 
thermocouple (not indicated in the drawing) is led in through the lower wax seal and 
the junction placed under the pan E. The slight bulges on the condenser and the pan 
support where they enter the apparatus are necessary to obviate the danger of these 
parts sucking in when the apparatus is evacuated. 


In reversible reactions involving the simultaneous formation of volatile 
and non-volatile products, the use of molecular evaporation makes it pos- 
sible to realize chemical effects that cannot be achieved with the aid of the 
usual distillation equipment. A new design of molecular still is described. 

Bibliography and Remarks 

(1) (a) Bronsted and Hevesy, Phil. Mag., 43, 31 (1922); (b) Washburn, Bur. Standards J. Re- 
search, 2, 476 (1929); (c) Burch, Proc. Roy. Soc, (London), 123, 271 (1929); (d) Hickman, Chcm. 
Ind., 48, 365 (1929). 

(2) Cat-others, Hill, Kirby and Jacobson, Journ Am. Chem. Soc., 52, 5279 (1930). 

(3) Papers XII, XIII, XIV and XVI. 

(4) This pan is conveniently made by cutting a pyrex beaker close to the bottom. The material 
of which the pan is composed has a considerable effect on the rate of the bifunctioual condensations 
described in Papers XII, XIII and XIV. No polyesterification could be effected in a copper pan. 
On the other hand, when superpolyesters and polyanhydrides are prepared in a glass pan the products 
adhere very firmly to the glass so that the dish is frequently shattered on cooling owing to the force 
of contraction. 

XII. Linear Superpolyesters* 

In previous papers^ polymerization up to molecular weights of about 5000 
has been investigated and compared with the tendency to the formation of 
low molecular rings. As a general rule it can be stated that if 5- or 6- 
rings can be formed by esterification, the intra-molecular type of reaction 
prevails; in the case of 6- and 7 -member ed rings both monomolecular rings 
and linear polymers of medium molecular weight are produced. 
From now on the main interest is concentrated on higher polymers, which 
are called superpolymers if the molecular weight exceeds 10,000. At the 
same time products with a molecular weight between 800 and 5000 are 
termed a-esters, while products above 5000 are called u-esters. 
In this paper a series of u-polyesters was prepared with the use of the mo- 

* W. H. Carothers and I. W. Hill; Journ. Am. Chem. Soc., 54, 1559-66 (1932); 
Communication No. 75 from the Experimental Station of E. I. du Pont de Nemours and 

Received November 12, 1931. Published April 6, 1932. 

I See Contribution I to XI of this series on pages 4 to 156 in this volume. 


lecular still described in a previous paper*. Linear a-esters are mostly 
microcrystalline powders (in some cases opaque, brittle waxes) with melt- 
ing points around 80. They are mostly quite soluble in chloroform and 
ethyl-acetate and on the average represent a polymerization degree around 

u-Superpolyesters are resins with considerable hardness and toughness. If 
placed in solvents, they first imbibe the latter and swell; later they give solu- 
tions of high viscosity. When heated they become transparent around 80, 
but do not yet flow at this temperature. Flow only takes place at consider- 
ably higher temperatures. In the solid state they give sharp X-ray powder 
diagrams, which closely resemble the corresponding patterns of the a-esters. 
A few degrees below the melting point, the rings lose their sharpness and at 
the melting point a diffuse halo characteristic of a liquid appears. Su- 
perpolyesters in the plastic state can be drawn out into very strong, pliable 
and as the X-ray investigation shows highly orientated fibers, f 
The following a-esters are described in this paper: 

u-trimethylene ester of hexadamethylene dicarbonic acid 
^-polyethylene succinate 
co-polyester from w-hydroxydecanoic acid 
(^-polyester from w-hydroxypentadecanoic acid 
u-ethylene sebacate 

In previous papers we have described polyesters having molecular 
weights ranging from 800 to 5000 derived from dibasic acids of the series 
HOOC(CH 2 )*COOH and glycols of the series HO(CH 2 )/)H (2). These 
esters are microcrystalline solids that dissolve readily in appropriate sol- 
vents. Their solutions are not highly viscous and they show no signs of 
inherently colloidal behavior. In the present paper we describe poly- 
esters of the same series having much higher molecular weights. They 
exhibit colloidal behavior and simulate to a remarkable degree some of 
the properties of certain naturally occurring high polymers. For the sake 
of brevity and convenience we use the designation a-ester or a-form for 
the polyesters having molecular weights ranging from 800 to 5000 and the 
designation co-ester or co-form for the new superpolyesters. 

Nature of the Polyesterification. The mutual esterification of dibasic 
acids and dihydric alcohols is a bifunctional reaction (3). As such it pre- 
sents the formal possibility of yielding (1) cyclic monomeric esters, (2) cyclic 

* See page 154 to page 156 of this volume. 

f Compare the next paper on pages 165 to 168 of this volume. 


polyesters, and (3) open chain polyesters. Studies already reported have 
proved that the first possibility is ordinarily realized only in those com- 
paratively rare instances where the nature of the starting material permits 
the formation of a ring of five or six atoms. Otherwise reaction generally 
is intermolecular and the product is of the third type. The polyesterifica- 
tion thus consists in a series of intermolecular couplings resulting in the 
formation of progressively longer chains as indicated by way of illustration 
in the following equations. 



(n + 1)HO R' O CO R COOH > 


The reaction is reversible and an exceedingly large number of entities are 
involved in the ester equilibrium. The equilibrium can be displaced by 
the removal of the liberated water, and the more nearly complete the re- 
action the longer are the molecules of the polyester product. The reaction 
becomes formally complete when all the molecules of the initial reactants 
are combined into a single molecule. 

A complicating circumstance arises from the fact that the glycol and the 
acid need not react in precisely equivalent ratio : at any particular stage 
n molecules of acid may be involved with n + 1 molecules of glycol or, 
conversely, n molecules of glycol may be involved with n + 1 molecules of 
acid. In the former case the product molecules will bear hydroxyl groups 
at both ends ; in the latter they will bear carboxyl groups at both ends. In 
either event no further progress is possible by esterification. Under favor- 
able conditions, however, coupling may continue nevertheless. Two mole- 
cules may unite by a process of ester interchange involving their terminal 
ester linkages. This mechanism is realized experimentally (2) when di- 
(/3-hydroxyethyl)-succinate (I) is heated in vacuo. Glycol is liberated 
and the final product of prolonged heating is the same as one obtained by 
similarly heating succinic acid with a very small excess of glycol. 

(I) HO(CH 2 ) 2 OCO(CH 2 ) 2 COO(CH 2 ) 2 OH + HO(CH 2 ) 2 OCO(CH 2 ) 2 COO(CH 2 ) 2 OH > 
HO(CH 2 ) 2 OCO(CH 2 ) 2 COO(CH 2 ) 2 OCO(CH 2 ) 2 COO(CH 2 ) 2 OH + HO(CH 2 ) 2 OH, etc. 

The only factor that could theoretically preclude the possibility of pro- 
ducing exceedingly long molecules as the result of bifunctional esterifica- 
tions is the loss of terminal groups through ring formation. Since in the 
polyesters thus far examined the terminal groups are still present, it ap- 
pears that such ring formation does not occur (4). 

Another factor that might be expected to have the same effect is the 


mutilation of terminal groups, e. g., through the loss of carboxyl as carbon 
dioxide. But our experience indicates that the loss of terminal groups in 
this fashion occurs only when patently inappropriate conditions are 
adopted. More important than this is the fact demonstrated by the be- 
havior of di-(/Miydroxyethyl)-succinate cited above that the coupling can 
progress through a process of ester interchange involving the last ester 
linkage. The possibility of further coupling therefore does not absolutely 
depend upon the integrity of the terminal groups as such. 

As a matter of fact, however, as such reactions have been carried out 
in the past an apparent limit is reached at a comparatively low molecular 
weight. Thus the glycol and the acid are heated together in a distilling 
flask provided with a receiver to collect the liberated water, and heating is 
finally continued at low pressure and considerably elevated temperature. 
The molecular weight of the polyester product increases during the progress 
of the heating and finally reaches a value of 3000 to 5000. This value is 
not noticeably increased if the period of heating is greatly prolonged. 

We ascribe the failure of the reaction to progress further under these con- 
ditions to the following factors. The concentration of reactive (terminal) 
groups in esters having molecular weights as high as 3000 is rather low; 
moreover, such esters are very viscous even at a temperature of 200. 
This implies a restricted mobility of the molecules, and a low rate of diffu- 
sion of the volatile products to the surface. Beyond this, macromolecular 
materials, perhaps because of their exaggerated molecular cohesions (5), 
have a very powerful tendency to retain dissolved or adsorbed liquids or 
vapors several days' heating in high vacuum are required to remove 
benzene completely from macromolecular polystyrenes or water from res- 
inous esters. All these factors affect either the position of the equilibrium 
or the rate at which the equilibrium is approached, and one may suppose 
that if some more drastic and effective means could be found for the re- 
moval of water or other volatile reaction products, it would be possible to 
force the reaction further. 

Methods of Producing Superpolyesters. The molecular still, first 
used by Bronsted and Hevesy (6) for the separation of the isotopes of mer- 
cury and later by Washburn (7) for the separation of petroleum hydrocar- 
bons, provides a means for continuously displacing physical equilibria 
involving very minute vapor pressures, and the application of this to the 
chemical equilibrium under consideration led to the desired result. 

Some of the experiments described below were made in stills of the type 
described by Washburn (7) and others in a modified still described in the 
preceding paper (8), where the principles involved are also briefly discussed. 


For reasons of a theoretical nature that need not be discussed, our first 
experiments were made with an ester which has not been described pre- 
viously, namely, the trimethylene ester of hexadecamethylene dicarboxylic 
acid. This was obtained from the acid and the glycol by the method used 
for other a-polyesters (2a). 

It was then placed in the molecular still and heated by a bath at 200 
for a total time of twelve days. During the first seven days a small amount 
of distillate collected on the condenser. No distillate was observed during 
the remaining five days. The viscosity of the molten polyester increased 
progressively during the heating, and the final product showed an apparent 
molecular weight of about 12,000. 

The same treatment applied to ethylene succinate, ethylene sebacate 
and to polyesters derived from co-hydroxydecanoic acid and from co-hy- 
droxypentadecanoic acid, led to similar transformations. 

A part of the effect of the molecular still treatment is evidently due to the 
greatly prolonged time of heating, and it was found later that a similar, 
if somewhat less pronounced, effect could be produced by merely heating 
polyesters such as ethylene sebacate in thin layers at ordinary pressure in a 
stream of nitrogen for a long period of time, or by bubbling a stream of 
nitrogen through the molten polyester. 

Physical Properties of Superpolyesters. Linear a polyesters are 
microcrystalline powders (in certain cases resins) that dissolve readily in 
certain solvents such as cold chloroform and yield solutions which are 
quite mobile. The crystalline esters melt fairly sharply and when molten 
they are exceedingly viscous. On cooling they solidify to opaque 
masses which range from rather hard, brittle, porcelain-like masses to soft, 
waxy solids as the length of the polymethylene chain in the acid or glycol 
from which they are derived increases. 

The superpolyesters in the massive state are harder and much tougher. 
They also dissolve in cold chloroform but the process is slow; they first 
imbibe solvent and swell, and their solutions are highly viscous. They 
separate from solution as more or less coherent powdery or curdy masses. 
When heated they melt at approximately the same temperatures as the 
a-polyesters from which they are derived. The phenomena associated 
with the melting, however, are quite different from those observed in the 
a-polyesters. The solid ester at the melting point suddenly becomes trans- 
parent, but it does not lose its shape ; a tendency to flow appears only at a 
considerably higher temperature. In the solid state the superpolyesters 
furnish sharp x-ray powder diffraction patterns that closely resemble similar 
patterns obtained from the corresponding a-polyesters. At a temperature 



a few degrees below the melting point the patterns lose their sharpness. 
At the melting point the pattern is a diffuse halo characteristic of a liquid. 
The most remarkable property of the superpolyesters is their capacity to 
be drawn out into very strong, pliable, highly oriented fibres, and this is 
discussed in detail in a subsequent paper (9). 

By way of summary, the properties of the a- and w-polyesters from tri- 
methylene glycol and hexadecamethylerie dicarboxylic acid are listed in 
Table I. Similar relations exist between the a- and the co-forms of other 
polyesters. One curious property of the co-esters deserves special mention. 
They adhere very firmly to the glass vessel (in the molecular still) in which 
they are prepared, and the force of contraction during cooling often causes 
the vessel to be completely shattered if the layer of ester is more than two 
or three millimeters thick. 



Apparent mol. wt. 

At 100 C. 

At room temperature 

Melting point, C. 


Viscous liquid 

White, opaque, brittle wax; 

d\l 1.061 

Very soluble in cold CHC1 3 . 
Readily soluble in hot 
ethyl acetate ; separates 
on cooling as a micro- 
crystalline powder 

8.6 units 



Soft, sticky resin 

Cream colored, opaque, 
horny, elastic; dgo, 1.058 

Becomes transparent at 75, 
but does not flow 

Swells in cold CHC1 3 , then 
dissolves. Swells and 
slowly dissolves in hot 
ethyl acetate; separates 
on cooling as a white, 
amorphous, curdy pre- 

166 units 

Relative viscosity of 7.3% 
weight solution in chloro- 
form (chloroform, 216 
seconds = 1 unit) 

Structure of the w-Polyesters 

The simplest and most reasonable assumption concerning the super- 
polyesters is that they are formed by a continuation of the same bifunc- 
tional esterification that first results in the formation of the a-polyesters, 
and that they are exceedingly long polymeric chains bearing hydroxyl or 
carboxyl groups at the ends. Their physical, chemical and analytical be- 
havior is completely consistent with this assumption, but no actual data 


on the presence of terminal groups are yet available. The average length 
of the molecules also is still very uncertain. The molecular weights 
quoted in Table I were obtained by ebullioscopy and in view of the fact 
that the observed elevations did not greatly exceed the experimental error 
they can be accepted as signifying only that the molecular weights are 
quite large at least as high as 10,000. Comparison of physical properties 
indicates that molecular weights of the co-polyesters probably lie in the 
neighborhood of 20,000. Further work on the determination of the mo- 
lecular weights of the superpolyesters is in progress. 

w-Tfimethylene Ester of Hexadecamethylene Dicarboxylic Acid. This ester was 
obtained in its a-form by heating the acid together with a 10% excess of the glycol in a 
distilling flask at 180-200 for three hours, and then heating the residue to 200 for 
six hours at a pressure less than 1 mm. It was thus obtained in the form of a very vis- 
cous liquid which on cooling solidified to a hard, waxy mass. It was readily soluble in 
cold chloroform and in hot ethyl acetate, from which it separated on cooling as a white 
microcrystalline powder. It melted rather sharply at 75-76. Its apparent molecular 
weight was about 3300. 

Anal. Calcd. for O (CH 2 ) 3 O CO (CH 2 ) 16 CO = C 21 H 38 O 4 : C, 
71.13; H, 10.81; saponification equivalent, 177.2; mol. wt. 354.3. Calcd. for 
H(~O (CH 2 ) 3 O CO (CH 2 ) 16 CO ) 10 O(CH 2 ) 3 OH = C 2 , 3 H 388 O 42 : C, 70.63; 
H, 10.80; saponification equivalent, 181; mol. wt. 3619. Found: C, 70.34, 70.40; H, 
10.60, 10.69; saponification equivalent, 175.2; mol. wt. (in boiling benzene), 3600, 3200. 

A sample (8.5 g.) of this recrystallized ester was placed in a molecular still of the 
type described by Washburn (10). The condenser was cooled with running tap water, 
and a trap between the mercury diffusion pump and the still was cooled with liquid air. 
The still was heated by means of a metal bath kept at about 200 . The pressure of the 
system as measured by a McLeod gage between the trap and the pump was always 
less than 10~ 5 mm. Heating was continued for seven days. During the first two days 
of heating, 0.4 g. of distillate collected on the condenser. This was removed and the 
heating of the residue was continued for five days more. No distillate collected during 
this period. 

The analytical composition and the saponification equivalent of the residual ester 
were the same as those of the starting material. Its molecular weight, however, was 
much greater. 

Anal. Calcd. for O (CH 2 ) 3 O CO (CH 2 ) 16 CO = C 2 iH 38 O 4 : C, 
71.13; H, 10.81; saponification equivalent, 177.2; mol. wt. 354.3. Calcd. for H (O 
(CHj)r O CO (CH 2 )i<rCO) 34 O (CH 2 ) 3 OH = C 7 i7H 13 ooO 2 6 8 : C, 70.89; H, 10.83; 
saponification equivalent, 178.2; mol. wt., 12,122. Found: C, 70.38, 70.52; H, 
10.56; 10.53; saponification equivalent, 178.4; mol. wt. (in boiling benzene) 12,100. 

The distillate was not homogeneous. By crytallizing it from alcohol there was 
obtained an apparently homogeneous fraction composed of needles melting sharply at 
124-125. It depressed the melting point of hexadecamethylene dicarboxylic acid 
(which melts at 122-123 ) and was neutral. A single ultimate analysis indicated that 
it was possibly a monomeric form of the trimethylene ester of hexadecamethylene di- 
carboxylic acid. 


Anal. Calcd. for O (CH 2 ) 3 O CO (CH 2 )i8 CO; C, 71.13; H, 10.81. 
Found: C, 70.06; H, 10.63. 

A sample of the a-ester and heated at 200-250 for thirty-two hours with a current 
of dry nitrogen bubbling through the molten mass. The viscosity of the molten polymer 
increased very greatly and the product exhibited the phenomenon of cold-drawing 
described in paper XV. 

co-Polyethylene Succinate. A sample of polyethylene succinate having the ap- 
proximate formula HO (CH 2 );r-O(CO (CH 2 ) 2 CO O (CH 2 )2 O) 2l H (11) was 
heated in the molecular still as described above for the trimethylene ester of hexadeca- 
methylene dicarboxylic acid. From the very small amount of distillate no homogeneous 
fraction could be isolated. The residue was a tough, horny, somewhat elastic mass 
(the original material was hard, brittle, rather porcelain-like). On being heated to 97 
is softened to a very viscous and somewhat rubbery mass. Fibers prepared from this 
material were short and rather brittle. 

Anal. Calcd. for C 6 H 8 O 4 : C, 50.00; H, 5.50. Found: C, 50.30; H, 5.70. 

w-Polyester from co-Hydroxydecanoic Acid. co-Hydroxydecanoic acid was prepared 
by the method of Lycan and Adams (12). Five grams was heated under a moderate 
vacuum for ten hours to bring about incipient polymerization. If this is neglected 
and the acid heated directly in the molecular still, it distils practically completely be- 
fore polymerization sets in. The material after the preliminary heating was transferred 
to the molecular still and heated at 150 for twenty-four hours. Two and four- tenths 
of a gram of distillate (m. p. 60-65) collected. This was not further examined. The 
residue was a hard brittle wax which was heated for four days more at 150 and then at 
200 for two days. The final residue was a light gray, translucent, tough, flexible mass. 
On being heated it suddenly became transparent at 65 but did not lose its shape. At 
higher temperatures it could easily be drawn into threads which could be cold drawn. 

Microscopic Observation at a Magnification of 180 Diameters. A thin hazy film 
was obtained by placing a small specimen of the ester on a slide and covering it with a 
cover glass. Between crossed nicols the field appeared as an aggregate of very small, 
irregular anisotropic particles, each of which showed two extinction positions. The 
slide was then warmed to melt the ester and the cover glass moved about 5 mm. The 
middle region of the specimen when cold was now transparent and homogeneous; the sur- 
rounding portions were unchanged. This effect is apparently due to the action of stress 
in moving the slide. The transparent section showed brilliant interference colors be- 
tween crossed nicols and extinction parallel and normal to the direction of the stress ap- 

Anal. Calcd.: C, 70.59; H, 10.59. Found: C, 70.24, 70.21; H, 10.74, 10.79; 
mol. wt. in boiling ethylene chloride, 24,600. 

co-Ester from co-Hydroxypentadecanoic Acid. One gram of "exaltolide" (the lactone 
of co-hyroxypentadecanoic acid) was refluxed with 0.5 g. of sodium hydroxide in 12 cc. 
of water and 5 cc. of alcohol for five hours. The solution was evaporated to dryness on 
a steam-bath, dissolved in 450 cc. of water, acidified with dilute sulfuric acid and ex- 
tracted with ether. The ether solution was dried over magnesium sulfate, filtered and 
evaporated to dryness; yield, 1.075 g. The product was recrystallized from benzene; 
m. p. 83-84. The acid was put in the molecular still and heated overnight at 150. It 
distilled completely except for a very small residue of hard wax, The distillate was 


heated for six hours at 150 at 1 mm. and again placed in the still and heated for seven 
days. A small amount of distillate which melted at 70-75 was not further examined. 

The residue consisted of 0.65 g. of a flexible, translucent mass. It became trans- 
parent at 95, yielding a stiff mass which held its shape up to 200. It could be drawn 
into thin fibers and cold drawn. 

Ethylene Sebacate. 101 g. (0.5 mole) of sebacic acid and 32.5 g. (0.525 mole) of 
ethylene glycol were heated together at atmospheric pressure in a Claisen flask at 175. 

The aqueous distillate was discarded. The residue was then heated at 250 for 
five hours at 2 mm. pressure. The light gray-buff product was a hard wax. Threads 
drawn from the molten ester, if pulled immediately while still warm, yielded transparent 
fibers. The product after recrystallization from ethyl acetate melted at 75 (cop- 
per block) and still exhibited the cold drawing phenomenon; molecular weight by 
cryoscopic method in benzene, 4800, 6000. 


By prolonged heating in a molecular still or in a stream of inert gas, 
the previously described linear polyesters derived from dibasic acids and 
glycols or from higher co-hydroxy acids are caused to react with themselves 
to produce polyesters of much higher molecular weight. The new super- 
polyesters (co-polyesters) are tough, opaque solids which exhibit sharp 
x-ray powder diffraction patterns and become transparent at a definite 
temperature. They dissolve in chloroform to form highly viscous solu- 
tions. The mechanism of the reaction is discussed. 

Bibliography and Remarks 

(1) The term "superpolymer" is applied to linear polymers having molecular weights above 

(2) (a) Carothers and Arvin, Journ. Am. Chem. Soc., 51, 2560 (1929); (b) Carothers and van 
Natta, ibid., 52, 314 (1930); (c) Carothers and Dorough, ibid., 52, 711 (1930); (d) Carothers, Arvin 
and Dorough, ibid. t 52, 3292 (1930). 

(3) Carothers, ibid., 51, 2548 (1929). 

(4) Valuable indications in this connection are furnished by Lycan and Adams' studies [Journ. 
Am. Chem. Soc., 51, 625, 3450 (1929)] of the polyesters derived from w-hydroxydecanoic acid. We 
should add that our generalization to the effect that no intramolecular reaction occurs in bifunctional 
esterifications involving compounds whose structural units are longer than seven atoms is not altogether 
free from exceptions. Dr. van Natta of this laboratory in preparing polyesters of co-hydroxydecanoic 
acid has recently observed the formation of appreciable amounts of the dimeric lactone already ob 
tained by Lycan and Adams by a different method. 

(5) Dunkel, Z. physik. Chem., Abt. A, 138, 42 (1928). 

(6) Bronsted and Hevesy, Phil. Mag., 43, 31 (1922). 

(7) Washburn, Bur. Standards J. Research, 2, 476 (1929); see also Burch, Proc. Roy. Soc. (Lon 
don), 123, 271 (1929); and Hickman, Chem. Ind., 48, 365 (1929). 

(8) Paper XI. 

(9) Paper XV. 

(10) Washburn, Bur. Standards J. Research, 2 480 (1929). 

(11) Carothers and Dorough, Journ. Am. Chem. Soc., 52, 711 (1930). 

(12) Lycan and Adams, ibid., 51, 625 (1929). 


XIII. Polyamides and Mixed Polyester-Polyamides* 

Above its melting point t-aminocaproic acid\ splits off water and gives 

a) a cyclic lactam, which is a seven-membered ring and 

b) a linear polyamide with a molecular weight around 3000, 

Linear super poly amides are harder, tougher, less easily fusible and much 
less soluble than the corresponding super poly esters owing to the higher 
molecular cohesion attributed to the CONH group. 
In the molecular still the polyamide obtained from e-amino caproic acid 
polymerizes further and exhibits a considerable change in physical 

By heating together e-aminocaproic acid with hexadecamethylene di- 
carboxylic acid and trimethylene glycol mixed poly ester -poly amides are 
obtained. They lie in their physical properties between the pure poly- 
esters and pure polyamides and can be drawn out into strong and fine 
transparent fibers . 

Above its melting point -aminocaproic acid undergoes dehydration 
with the simultaneous formation of two different products (1). One of 
these is the cyclic lactam, a seven-membered ring. The other is a poly- 
amide having the formula 

. . . NH (CH 2 ) 6 CO CH (CH 2 )5 CO NH (CH 2 ) 6 CO . . . 

This compound is analogous to the polyesters obtained by the self-esterifica- 
tion of higher hydroxy acids or by the action of glycols on dibasic acids. 
The binding atom between the units in the case of the esters is O and 
in the case of the polyamide NH . In comparison with the esters the 
polyamide is harder, tougher, less easily fusible and much less soluble. 
The difference lies in the direction required by the much higher molecular 
cohesions of amides as compared with esters (2). 

The polyamide prepared by dehydration of the amino acid at atmospheric 
pressure or under diminished pressure in the usual distillation equipment 
has an apparent molecular weight of about 1000 (the actual value probably 
lies somewhat higher than this, perhaps in the neighborhood of 3000). 
It corresponds, therefore, in molecular weight with the oi-polyesters that 
have been described in previous papers (3) . The polyesterification reaction 

* W. H. Carothers and J. W, Hill; Journ. Am. Chem. Soc., 54, 1566-69 (1932); 
Communication No. 76 from the Experimental Station of E. I. du Pont de Nemours and 

Received November 12, 1931. Published April 6, 1932. 

t Compare paper VIII of this series on pages 78 to 81 in this volume. 


can be forced further toward completion by the use of molecular distilla- 
tion, and this process leads to the formation of superpolyesters described 
in the preceding paper (4) . One of the objects of the experiments described 
in the present paper was to examine the possibility of forcing the amide re- 
action in a similar manner so as to obtain a polyamide of very high molecu- 
lar weight. 

The polyamide already described was placed in the molecular still and 
heated for forty-eight hours at 200. A very small amount of the crystal- 
line cyclic lactam distilled from the reaction mixture and the residue was 
considerably changed in its properties. It was harder and tougher than 
before and in thin sections was flexible and elastic. It softened at 210 
with considerable decomposition. Like the initial amide it was insoluble in 
common organic solvents with the exception of hot phenol and hot form- 

Anal. Calcd. for C 6 H U ON: C, 63.71; H, 9.73. Found: C, 63.94, 63.94; H, 9.72, 

The change in properties indicates a considerable increase in molecular 
weight but no actual measurements of molecular weight are available 
owing to the lack of any method of sufficient reliability. 

The superpolyesters described in the preceding paper are especially 
interesting because of their ability to furnish strong, pliable, highly oriented 
fibers. This property is discussed in detail in paper XV. As synthetic 
silk, however, these materials suffer from the defect of low melting point and 
considerable solubility in various organic solvents. On the other hand, 
the polyamide described above is too infusible and insoluble to allow a 
ready test of its ability to furnish fibers. In the hope of obtaining a com- 
promise between the properties of the polyesters and the polyamides we 
prepared mixed polyester-polyamides. These compounds were obtained 
by heating together trimethylene glycol, hexadecamethylene dicarboxylic 
acid and e~aminocaproic acid. The glycol was used in a 5% excess over 
the amount equivalent to the dibasic acid and the amino acid was varied 
in different experiments in the proportion of 1, 2, 3 and 5 moles per mole 
of dibasic acid. The mixtures were separately heated in a Claisen flask 
for three hours in a current of dry nitrogen by means of a bath kept at 
200-220. Heating was continued for five hours more at a pressure of 
1 mm. with the b^th at 250-260. The very viscous residue was then 
removed from the flask and transferred to the molecular still, where it 
was heated for three days at 200. So far as could be inferred from the 
physical appearance of these products the molecular still treatment pro- 


duced very little effect although in each experiment a small amount of the 
cyclic lactam from the amino acid was obtained as a distillate. 

The products were opaque or translucent solids, hard, very tough and 
in thin sections flexible and elastic. As the proportion of amino acid in- 
creased, the polymers increased in brittleness, hardness, transparency 
and melting point. The effect of the composition on melting point is indi- 
cated below 

1 mole of amino acid ca. 73 3 moles of amino acid 125 

2 moles of amino acid 100 5 moles of amino acid 145 

The melting points were determined on a copper block. In the two 
polymers of lowest amino acid content the temperatures at which the 
specimens became transparent were taken as the melting points. In the 
other two cases where the high degree of transparency when cold made this 
determination extremely uncertain the temperatures at which the sample 
first began to adhere slightly to the block were taken as the melting points. 

The solubility of the mixed polyester-polyamides also diminished with 
increasing amino acid content. The first member of the series was swelled 
by hot ethyl acetate and finally completely dissolved. It separated from 
the cold solution in the form of soft powder. The second was very slightly 
soluble in hot ethyl acetate, and the higher members were practically in- 
soluble. Since the polyamide derived from e-aminocaproic acid is not 
dissolved by hot ethyl acetate, the mixed polyester-polyamides are evi- 
dently not merely physical mixtures. From the nature of the reaction 
also it seems likely that all the components participate in forming each 
molecule of the product. The different units that may be present in the 
product molecules are 


which may be joined in any order that leads to ester or amide linkages. 

The mixed polyester-polyamides when molten can readily be drawn out 
into filaments by touching a specimen with a rod and drawing the rod away. 
The successful production of continuous filaments by this method requires 
a degree of plasticity in the molten mass that appears only at a temperature 
somewhat above the melting point. Filaments obtained in this way are 
brittle and opaque. When slightly warmed (e. g., to 40 to 50) they can 
be drawn out by the action of stress into transparent fibers which are ex- 
ceedingly pliable. Purely qualitative observations indicate that these 
fibers are considerably stronger than similar fibers derived from the co- 



The polyamide derived from -aminocaproic acid when heated in the 
molecular still undergoes a considerable change in its physical properties, 
indicating an increase in molecular weight. By heating together e-amino- 
caproic acid with hexadecamethylene dicarboxylic acid and trimethylene 
glycol, mixed polyester-polyamides are obtained. These materials in 
their physical properties lie between the polyesters and the polyamides 
and like the superpolyesters described in the preceding paper they can be 
drawn out into strong, pliable, transparent fibers. 

Bibliography and Remarks 

(1) Carothers and Berchet, Journ. Am. Chem. Soc , 52, 5289 (1930); Gabriel and Maass, Ber., 32, 
1266 (1899); Braun, ibid , 40, 1835 (1907) 

(2) Dunkel, Z. physik. Chem., Abl. A, 138, 42 (1928). 

(3) Carothers and Arvin, Journ. Am. Chem. Soc., 51, 2560 (1929), Carothers and van Natta, tbtd , 
62, 314 (1930). 

(4) Paper XII. 

XIV. A Linear Superpolyanhydride and a Cyclic Dimeric 
Anhydride from Sebacic Acid* 

The different products, obtained during the anhydrization of sebacic acid 
are described in this paper. 

a-sebacic anhydride is formed by the action of acetic anhydride on sebacic 
acid and has a melting point of 79-80. In the molten state it is very viscous, 
on cooling it crystallizes in needles; it has a molecular weight around 5200. 
p-sebacic anhydride, formed by treating the a-anhydride in the molecular 
still; is a definitely macrocrystalline solid melting sharply at 68. It dissolves 
readily in most of the common solvents and was identified as a twenty-two- 
membered cyclic dimer. 

y-sebacic anhydride is obtained by heating the ^-product to its melting 
Point, cooling down and heating again. Its melting point is 82, its molecular 
weight lies in the neighborhood of 600, which shows that it is formed from 
the ^-product by a process of polymerization. 

u-sebacic anhydride is a super polymer, with a rather high molecular 
weight;^ it is formed by heating a-sebacic anhydride in the molecular still. 
It is a hard and tough resin, which becomes transparent at 83 but remains 

* J. W. Hill and W. H. Carothers; Journ. Am. Chem. Soc., 54, 1569-79 (1932); 
Communication No. 77 from Experimental Station of E. I. du Pont de Nemours and 

t No definite figure can be given owing to the unsolubility of this material. 


still hard. At about 130 it gets soft and can be drawn into continuous, 
strong and pliable filaments. 
The reactions of the different anhydrides with aniline are described. 

In a previous paper (1) it has been shown that adipic anhydride as or- 
dinarily prepared is a linear polymer of the type formula represented by I 
and that under the action of heat in vacuo it is broken down to the cyclic 
monomer, a seven-membered ring. 
(I) ... O CO R CO O CO R CO O CO R CO. . . 

Observations have now been extended to sebacic anhydride, and it is 
shown that the anhydride (a-anhydride) prepared by the action of acetic 
anhydride or acetyl chloride on sebacic acid is also polymeric. No smooth 
depolymerization of this polymer can be effected under ordinary conditions, 
but in the molecular still (2) at elevated temperatures two processes occur 
simultaneously. The a-anhydride is transformed into a polyanhydride of 
much higher molecular weight (w-anhydride) and at the same time de- 
polymerization occurs with the formation of a crystalline product ({$- 
anhydride), which is shown to be not the eleven -membered cyclic monomer, 
but the twenty-two -membered cyclic dimer. At its melting point the 
dimer reverts to a higher polymer (7-anhydride). The anhydrides of 
different origins are arbitrarily assigned the prefixes a, 0, 7 and co to desig- 
nate them for the purposes of discussion. 

Sebacic a- Anhydride. Voerman (3) obtained this as a microcrystalline 
solid melting at 74.5 by the action of acetyl chloride on sebacic acid. He 
seemed unable to decide whether to regard it as monomeric or polymeric. 
Compared with succinic and glutaric anhydrides (known to be monomeric) 
it showed a diminished solubility and ability to crystallize, and molecular 
weight determinations in boiling benzene and boiling acetone gave ab- 
normally high values. The doubt concerning its polymeric character 
arose from the fact that freezing point determinations in phenol gave values 
agreeing with those calculated for the monomer, but Voerman overlooked 
the fact that sebacic anhydride reacts very rapidly with phenol to give 
phenyl esters of sebacic acid. 

We prepared the a-anhydride by heating sebacic acid with excess acetic 
anhydride, distilling off the volatile material, and precipitating a benzene 
solution of the residue with petroleum ether. The melting points varied 
somewhat from one preparation to another and a typical specimen melted 
at 79-80. The molten anhydride is exceedingly viscous; on cooling it 
crystallizes: minute doubly refracting spherulites first separate and grow 
to what appear to be star-like clusters of needles. This behavior is highly 


characteristic of crystalline linear high polymers and is observed with such 
diverse materials as polyesters (4), trimethylcellulose (5) and triacetylinu- 
lin (6). 

At a temperature of 200 and under a pressure of 0.1 mm. the a-anhy- 
dride shows no tendency to distil, and this behavior is also consistent with 
a highly polymeric structure. 

We tentatively assign to sebacic a-anhydride the formula II. 

(II) CH 3 CO [ O CO (CH 2 ) 8 CO ]* O-CO CH 3 

This represents it as a linear polymer and the chains are terminated by 
acetyl groups derived from the reagent used in bringing about the anhydride 
formation. It reacts with water to form sebacic acid and acetic acid. The 
latter may conceivably represent adsorbed acetic acid or acetic anhydride 
not removed in the purification process, but we are inclined to the view that 
it arises from acetyl groups that actually form a part of the polyanhydride 
molecule. This assumption together with the estimation by distillation 
and titration of the amount of acetic acid formed on hydrolysis furnished a 
value of 5260 for the molecular weight of a typical specimen of the polymer. 

A specimen of the recrystallized a-anhydride which had been stored for three days 
in a vacuum desiccator over phosphorus pentoxide, potassium hydroxide and paraffin 
was refluxed with 3 g. of sodium hydroxide in 40 cc. of water, diluted, acidified with 
sulfuric acid and distilled with the occasional addition of water. A total of 415 cc. of 
distillate was collected and titrated with 0.1 N sodium hydroxide. Sodium acetate 
was identified in the resulting neutralized solution by microscopic observation of the 
highly characteristic crystals of sodium uranyl acetate formed upon adding uranyl 
nitrate. We are indebted to Mr. W. D. M. Bryant for this identification. Since it was 
found that sebacic acid itself tends to steam distil very slowly, a correction was made by 
repeating the above determination using 6 g. of pure sebacic acid. 

Anal. Subs. 5.51 g.: 0.0999 N NaOH, 26.35 cc. Blank: 0.0999 N NaOH, 
5.35 cc. Found: (assuming two acetyl groups per molecule) mol. wt., 5260. 

A different though not strictly independent estimate of molecular weight 
is found in the quantitative analysis of the behavior of the polymer toward 
aniline, which is described in the next section, and this method gives a 
value of 5500. Attempted molecular weight determinations in boiling 
benzene gave much lower values than this (as low as 650), but their lack of 
constancy and reproducibility deprives them of any significance (7). On 
the other hand, the a-anhydride in its physical properties closely resembles 
polyesters derived from sebacic acid and known to have molecular weights 
in the neighborhood of 5000. 

The analytical data presented below are consistent with the suggested 
structure for the a-anhydride. 


c Calcd.,% H c Found, % ^ 

On 65.18 8.75 64.19 9.07 

CH 8 COO(CioHi6O 3 )3COCH3 62.35 8.31 64.22 9.01 

CHgCOCKCioHuCWaCOCHs 64 . 00 8 . 57 

CH 3 COO(CioHi6O 3 )3oCOCH3 64.84 8.71 

Behavior of Sebacic Anhydrides toward Aniline. Theory requires a 
difference, both qualitative and quantitative, between cyclic monomeric 
anhydrides and linear polymeric anhydrides in their behavior toward un- 
symmetrical reagents such as aniline. This requirement was first pointed 
out by one of us and experimentally verified in the case of adipic anhy- 
drides (1, 8). The analysis has now been extended and further observa- 
tions made with sebacic anhydrides. 

The monomeric anhydride (II) can react with aniline to form only a 
single product, namely, the monoanilide of sebacic acid. 

(II) CO (CH 2 ) 8 CO O j + CH 6 NH 2 > C.H 6 NH CO (CH 2 ) 8 COOH 

In the polymeric form, however, as soon as one anhydride linkage reacts 
with aniline, reaction at the next linkage will give different products de- 
pending upon which side of the oxygen is involved with the reagent. In 
III we symbolize aniline (RNH H) as b-a. 

(III) . . CO (CH 2 ) 8 CO O CO (CH 2 ) 8 CO O CO (CH 2 ) 8 CO O . . 


b-a > 


a-b > 


Considering the two linkages of an interior unit of the chain, if reaction 
occurs in the direction b a at the first one it may occur in the directions 
b a, and a b at the second; and if the direction is a b at the first it 
may be either b a or a b at the second. If we assume that reaction is 
random, i. e., that the direction of reaction at the second linkage is not 
affected by the direction that the reaction has already taken at the first 
linkage, then the probability of each of these four possibilities is the same, 
and the terminal groups attached to the reaction product derived from 
the unit under consideration are (1) a b, (2) a a, (3) b b and (4) b a. The 
first and fourth of these represent monoanilide, and the second and third 
represent, respectively, dibasic acid and dianilide. Thus each interior unit 
of the anhydride chain will yield 


l /4 molecule of dibasic acid 
1 /4 molecule of dianilide, and 
*/2 molecule of monoanilide 

and this total product will contain one atom of acid hydrogen. 

If the poly anhydride is cyclic, all the units are interior units, and the 
ratio of products indicated above will be maintained so long as the an- 
hydride is polymeric (i. e., not monomeric). But if the chains are open 
this ratio will be modified. 

We consider the case IV in which the chains are open and terminated by 
carboxyl groups. 

(IV) HOOC (CH 2 ) 8 CO O [ CO (CH,) 8 ~-CO O L_ 2 CO (CH 2 ) S 


The total number of units in the chain is x. Of these, the x-2 interior 
units will furnish acid, monoanilide and dianilide in the ratios already 
indicated, while the terminal units will each furnish one-half molecule of 
acid and one-half molecule of monoanilide. Thus the total products are 

(x 2)/4 -f 1 molecule of dibasic acid 

(x 2)/2 4- 1 molecule of monoanilide, and 

(.v 2)/4 molecules of dianilide 

and the number of atoms of acidic hydrogen in the product is 

[(* - 2)/4 4- 1]2 + (x - 2)/2 4- 1 = * 4- 1 

If M is the gram molecular weight of the polymer, then, since the molecu- 
lar weight of the structural unit is 184 

184* 4- 18 = M (1) 

and this weight after reacting with aniline will furnish x 4~ 1 mole of hy- 
drogen ion. Thus the weight of anhydride required to furnish one mole 
of hydrogen ion is 

M/(x 4- 1) (2) 

Let g represent the weight of a sample of polyanhydride of the formula 
IV which is treated with aniline and cc represent the volume in cubic 
centimeters of normal alkali required to neutralize the product. Then 
from (2) 

M/(x 4- 1) = 1000 g(/cc) (3) 

and from (1) 

x = (M - 18)/184 (4) 


Substituting and solving for M, we have 

M * 166,000 g/(184 cc - 1000 g) (5) 

If the terminal groups are acetyl as in V 

(V) CH 8 CO O [CO (CH 2 )8 CO O ] x COCH, 

M = 184 + 102, and this weight after reacting with aniline will furnish 

#/4 moles of dibasic acid 
#/4 moles of dianilide 
x/2 moles of monoanilidc 
1 mole of acetic acid, and 
1 mole of acetamide 

The moles of available hydrogen ion in this product are 

2*/4 + x/2 + 1 = x + I 
In this case equation (5) becomes 

M = 82,000 g/(184 cc - 1000 g) (6) 

The manner in which the calculated available acid from the aniline reac- 
tion product varies with the molecular weights of different sebacic an- 
hydrides is indicated in Table I. 

From the data of Table I it is evident that even if one allows an error 
of 2% in the titration it should be possible to distinguish an open chain 
terminated by carboxyl and having a molecular weight of 7500 or less from a 
cyclic anhydride. If the terminal groups of the chain are acetyl, a similar 
distinction is possible only if the molecular weight lies below about 3500. 


Millimoles of available H + in reaction 

Mol. wt. product from one gram of anhydride 

Structural (for cyclic Cyclic Chain terminated Chain terminated 

tinits anhydride) anhydride by carboxyl by acetyl 

1 184 5.44 9.9 6.95 

2 368 5.44 7.75 6.39 

3 552 5.44 7.02 6.12 

4 736 5.44 6.62 5.97 
6 1,104 5.44 6.24 5.82 

10 1,840 5.44 5.92 5.66 

15 2,760 5.44 5.76 5.59 

20 3,680 5.44 5.68 5.55 

40 7,360 5.44 5.56 5.49 

60 11,040 5.44 5.52 5.48 

100 18,400 5.44 5.49 5.46 

200 36,800 5.44 5.46 5.45 


The data from which the molecular weight of the a-polyanhydride 
described in the preceding section were calculated are as follows. 

Weighed samples of the reprecipitated anhydride, which had been stored for several 
days in an evacuated desiccator over solid potassium hydroxide, were added to 5 to 8 cc. 
of aniline and triturated with a stirring rod until a smooth cream was formed. The 
mixture was then diluted with about 30 cc. of 95% alcohol and titrated with 0.1 TV sodium 
hydroxide using phenolphthalein as the indicator. 

Anal. Subs. 0.5869, 0.4814, 0.5632: 0.0999 N NaOH, 32.40, 26.55, 31.12 cc. 
Calcd. by equation (6): M = 5532, 5891, 5248. 

Sebacic co- Anhydride. Attempts to depolymerize sebacic a-anhydride 
by heating it under greatly diminished pressure in a Claisen flask resulted 
merely in thermal decomposition with the formation of tarry products. 
But in the molecular still (9), at 200 change occurred simultaneously in 
two directions: a crystalline distillate gradually accumulated on the con- 
denser, and the residue became more viscous. The rate of distillation was 
about one gram in twenty-four hours (from 8 g. of a-anhydride, and an 
evaporating surface of about 40 sq. cm.). The final residue when cold was 
very much harder and tougher than the a- anhydride. It was opaque and 
when heated to about 83 it suddenly became transparent but remained 
very stiff. At 130 it was soft enough to be drawn into continuous fila- 
ments. These filaments exhibited the phenomenon of cold drawing 
described in a subsequent paper (10). The resulting fibers were lustrous 
and exceedingly strong and pliable, but after standing for a few days in a 
vacuum desiccator they became brittle and fragile. Massive specimens 
of the co-anhydride also become brittle under the same conditions and lose 
their capacity to be drawn out into filaments. No data concerning the 
molecular weight of co-poly anhydride are available, but its toughness and 
viscosity indicate a considerably higher molecular weight than for the co- 
polyesters described in paper XII. 

The brittleness and loss of tenacity that develop when the co-polyanhy- 
dride is stored are apparently due to hydrolysis. When a sample of the 
anhydride was stored at ordinary temperature in the still in which it was 
prepared, it retained its strength and toughness for six days. When the 
still was opened and the anhydride transferred to a desiccator containing 
anhydrous calcium chloride, brittleness developed within twenty-four hours. 

The co-anhydride adheres very firmly to the glass dish in which it is 
prepared, and if the specimen is more than 2 or 3 mm. deep the dish is com- 
pletely shattered by the force of contraction during cooling. The co-poly- 
esters manifest the same behavior. 

Anal. Calcd. for (Ci Hi 6 O 3 ) w : saponification equivalent, 92. Found; 92.0, 92.3. 


Sebacic ^-Anhydride. The distillate formed from the a-anhydride 
during the treatment in the molecular still is a definitely macrocrystalline 
solid melting sharply at 68. It dissolves readily in most of the common 
organic solvents with the exception of petroleum ether and ligroin. When 
recrystallized from a mixture of petroleum ether and benzene it separates 
in the form of fine needles and the melting point remains unchanged. The 
analytical behavior of this material identifies it as the twenty-two-mem- 
bered cyclic dimer. 

Anal. Calcd. for (Ci Hi 6 O 8 ) n : C, 65.18; H, 8.75; mol. wt., 368; saponification 
equivalent, 92. Found: C, 65.08, 64.85; H, 8.90, 8.77; mol. wt. in boiling benzene, 
386, 393; saponification equivalent 91.9. 

Still more decisive support for the cyclic dimeric structure for sebacic 
0-anhydride is found in the analysis of its behavior toward aniline when 
treated in the manner already described for the a-anhydride. 

Anal. Subs. 0.939 (after treatment with aniline): 0.0992 N NaOH, 51.2 cc. 

Thus one gram of the anhydride furnished 5.41 millimoles of hydrogen 
ion. The calculated value for a cyclic anhydride is 5.44, and, as the 
data of Table I show, this agreement demonstrates that the 0-anhydride 
cannot be an open chain unless its molecular weight lies above 3500. But 
the volatility of the compound indicates that its molecular weight must lie 
below 1000 (11). 

The neutral solution from the above titration contained both monoanilide and di- 
anilide. The alkali used in the titration was neutralized with an equivalent amount of 
hydrochloric acid; the alcohol was evaporated; the residue was treated with excess 
hydrochloric acid to dissolve the aniline, and filtered. The residue on the filter was 
repeatedly extracted with boiling water until no more material dissolved. The com- 
bined filtrates on cooling deposited handsome lustrous platelets of the monoanilide, 
m. p. 122-123. This compound appears not to have been described before. Re- 
crystallization from water did not raise its melting point. 

Anal. Calcd. for Ci 6 H 23 O 4 N: C, 69.26; H, 8.37. Found: C, 69.51, 69.58; 
H, 8.26, 8.48. 

The insoluble dianilide was transferred to a Gooch crucible and weighed: yield, 
0.450 g. ; theory requires 0.450 g. After recrystallization from alcohol it melted at 
201-202 (12). 

Anal. Calcd. for C^HafAN*: C, 74.96; H, 8.01. Found: C, 74.49, 74.67; 
H, 8.34, 8.05. 

The fact that dianilide was produced in this reaction demonstrates 
decisively that the anhydride is not monomeric, and the close agreement 
between the dianilide calculated and found indicates again that the 
structure is cyclic. 


The ^-anhydride is not present as such in more than traces in the a- 
anhydride before the molecular still treatment. The /3-anhydride is 
exceedingly soluble in cold carbon tetrachloride, whereas rough determina- 
tions indicated that the a-compound at 25 dissolves to the extent of 
only about 0.04 g. in 100 cc. of carbon tetrachloride. 

Sebacic 7-Anhydride. When the 0-anhydride is heated to its melting 
point, cooled and then heated again, its melting point is found to have 
changed from 68 to 82. This change is due to polymerization. Samples 
of the new anhydride were prepared by sealing the /3-anhydride in care- 
fully dried glass tubes and heating them in a water-bath. The specimens 
melted at 68 to an exceedingly mobile fluid which almost instantly became 
so viscous that it failed to flow when the tube was inverted. When cold, 
the product was an opaque (or translucent) macrocrystalline wax that 
resembled the a-anhydride. It melted at 82, and heating at 90-100 for 
five hours did not raise the melting point. 

When the 7-anhydride is heated in the molecular still it behaves in the 
same manner as the a-anhydride and is converted to the 0- and w-anhy- 

Determinations of the molecular weight of the 7-anhydride in boiling 
benzene gave values in the neighborhood of 600, but for reasons already 
discussed in connection with the a-anhydrides we think that these values 
merely signify that the material is not monomeric or dimeric. 

The behavior of the 7-anhydride toward aniline fails to reveal the pres- 
ence of terminal carboxyl groups in detectable amounts. 

Anal. Subs. 0.2234, 0.2707, 0.1925: (after treatment with aniline) 0.0999 N 
NaOH, 12.15, 14.70, 10.50 cc. These values correspond to 5.43, 5.43, 5.45 cc. of N 
NaOH per gram of sample. 

Thus one gram of the anhydride furnishes 5.44 millimoles of hydrogen 
ion, and from Table I it appears that if the polymer is an open chain bearing 
carboxyl groups at the ends, its molecular weight must lie above 7500. 


The a-anhydride probably bears terminal groups in the manner already 
indicated. We suppose that it is made up of chains of slightly differing 
lengths. Our idea in heating this product in the molecular still was that 
under these conditions it would condense or couple with itself by a process 
of dehydration or anhydride interchange involving the elimination of ter- 
minal groups between adjacent molecules. 

A similar mechanism had already been established (13) for the transfor- 
mation of the a-polyesters into co-polyesters. The available facts are all 


consistent with this mechanism for the transformation of the a-polyan- 
hydride into the co-poly anhydride. The molecules of the latter compound 
therefore are very long open chains. Their molecular weights lie perhaps 
in the neighborhood of 30,000, certainly at not less than 15,000. The 
transformation of this compound into the /3-anhydride evidently involves 
a process of anhydride interchange. 

*. CO-O-CO R CO 

t I t 


An analogous transformation occurs by heating polyesters derived from 
six-membered cyclic esters (14). But among polyesters smooth depoly- 
merization in this fashion can be effected only if the structural unit is six 
atoms long. Polyesters whose units are longer than six atoms yield cyclic 
degradation products only in small amounts and only when they are heated 
to a temperature sufficiently high to produce complete thermal decomposi- 
tion (15). The much smoother degradation of the anhydride is consistent 
with the much greater mobility and reactivity of anhydrides as compared 
with esters. 

A peculiarity of this transformation lies in the fact that the product 
is not the eleven-membered cyclic monomer but the twenty-two-membered 
cyclic dimer. A similar peculiarity exists in the nature of the cyclic 
degradation products from the polyesters (15). The significance of this 
fact will be discussed in connection with data concerning cyclic anhydrides 
from other dibasic acids which will be presented in a future publication. 

The formation of the 7-anhydride from the /^-anhydride apparently 
involves merely the mutual coalescence of the cyclic molecules of the latter 
compound. At any rate this transformation occurs under fairly rigor- 
ously anhydrous conditions, and, since no terminal groups could be de- 
tected in the resulting 7-anhydride, one is almost impelled to conclude that 
the 7-anhydride is initially a very large ring. This conclusion is a little 
difficult to reconcile with the fact that in the molecular still the 7-anhy- 
dride behaves just like the a-anhydride. According to our hypothesis the 
transformation of a-anhydride depends upon the presence in this compound 
of terminal groups capable of being eliminated by condensation between 
adjacent molecules. There is of course nothing theoretically to preclude 
the possibility of the mutual coalescence of very large anhydride rings to 
produce still larger rings. Moreover, it is possible that the co-anhydride 
derived from the ^-anhydride and that derived from the 7-anhydride are 
structurally different: the former may be an open chain and the latter a 


large ring. On the other hand, it appears that mere heating does not bring 
about the further polymerization of the 7-anhydride. The conditions 
of molecular distillation are required for this transformation. It is a 
little difficult to see just why the further growth of the 7-anhydride mole- 
cules, if they are rings, should occur only under conditions that favor the 
simultaneous elimination of smaller cyclic (^-anhydride) residues. Per- 
haps the resulting rupture leaves fragments (e. g., bivalent radicals) that 
are especially prone to unite with one another. 

We think it more likely, however, that traces of moisture inevitably ac- 
quired by the 7-polymer during its transfer to the molecular still may suffice 
to transform its molecules into open chains terminated by carboxyl groups. 
Coupling could then occur by the elimination of water between adjacent 

One further feature of the behavior of the a-anhydride in the molecular 
still remains to be mentioned. It has already been indicated that degrada- 
tion to the 0-anhydride and growth to the co-anhydride occur simultane- 
ously. The latter process however reaches an apparent limit after a short 
time, e. #., twelve hours. But this does not interrupt the formation and 
distillation of /3-anhydride, which continues at an only slightly dimin- 
ished rate until only a trace of residue remains. 


vSebacic a-anhydride formed by the action of acetyl chloride or acetic 
anhydride on the acid is a linear polymer. The chains are probably open 
and the molecular weights in the neighborhood of 5000. No smooth de- 
polymerization of this anhydride can be effected by ordinary vacuum dis- 
tillation, but in the molecular still two different transformations occur: 
(1) coupling of the a-anhydride with itself yields w-anhydride, a polymer of 
much higher molecular weight; (2) depolymerization of the a- and de- 
compounds, which occurs at the same time, yields the definitely crystalline 
volatile /3-anhydride melting at 68. The w-anhydride is very tough and 
hard and it can be drawn out into exceedingly strong, pliable, lustrous, 
highly oriented fibers. The /3-anhydride is a cyclic dimer (twenty-two- 
membered ring). When heated above its melting point it instantly poly- 
merizes, yielding a 7-anhydride which closely resembles the a-anhydride 
in its physical properties. The behavior of these anhydrides toward ani- 
line is described and discussed. The prefixes a, 0, 7 and o> used above 
have been adapted arbitrarily to designate the anhydrides of different 


Bibliography and Remarks 

(1) J. W. Hill, Journ. Am. Chetn. Soc., 52, 4110 (1930). 

(2) Paper XI. 

(3) Voerman, Rec. trav. chim., 23, 265 (1904). 

(4) Carothers and Arvin, Journ. Am. Chem. Soc. t 51, 2560 (1929). 

(5) Hess, "Die Chemie der Cellulose," Akademische Verlagsgesellschaft, Leipzig, 1928, p. 432. 

(6) Pringsheim and Hensel, Ber., 63, 1096 (1930). 

(7) The low and inconsistent molecular weight values obtained for the polymeric anhydrides in 
boiling benzene are probably due to progressive degradation by hydrolysis. Greer has lately shown 
[Journ. Am. Chem. Soc., 52, 4191 (1930)] that completely dry benzene seizes with great avidity suffi- 
cient moisture to provide 0.000229 g. of water for one gram of benzene. In a molecular weight deter- 
mination this would be sufficient to furnish more than one gram of water for each 100 g. of anhydride. 
The hydrolytic absorption of water in this proportion (1-100) would reduce the average molecular weight 
of the anhydride by 75% if the initial value were 5400 and the polymer were an open chain. If the 
initial value were 396 the reduction would be only 14%. If the polymer were cyclic its molecular 
weight might actually be slightly increased by partial hydrolysis. 

(8) We now find that Etaix [Ann. chim., [7] 9, 356 (1896)] has recorded the fact that the anhy- 
drides of adipic, suberic, azelaic and sebacic acids react with ammonia to give the diamides as well 
as the amic acids. Einhorn and Diesbach [Ber., 39, 1222 (1906)] have also observed that the anhy- 
drides of diethylmalonic acid react with ammonia and with diethylamine to form both diamides and 
amic acids. But apparently none of these investigators recognized the bearing of this behavior on 
the structure of the anhydrides. Incidentally, it seems certain in view of this behavior that Einhorn 
and Diesbach's tetrameric diethylmalonic anhydride is actually a sixteen-membered ring. On the 
other hand, their dodecamer is probably a high polymeric mixture rather than a definite individual. 

(9) Carothers and Hill, paper XI. 

(10) Paper XV. 

(11) Carothers, Hill, Kirby and Jacobson, Journ. Am. Chem. Soc., 52, 5284 (1930). 

(12) Pellizzari, Gazz. chim. ital., 15, 555 (1885), gives 198; Barnicoat, /. Chem. Soc., 2926 (1927), 
gives 200. 

(13) Paper XII. 

(14) Carothers, Dorough and van Natta, Journ. Am. Chem. Soc., 54, 761 (1932). 

(15) Carothers, Chem. Reviews, 8, 381 (1931). 

XV. Artificial Fibers from Synthetic Linear Condensation 


This publication contains a very interesting and complete description of the 
mechanical behavior of super polymers, when they are treated mechanically 
and especially when drawn into fine filaments .f The experiments refer to 
the u-ester of trimethylene glycol and hexadecamethylene dicarboxylic acid. 
The filaments can be obtained directly by drawing threads from the molten 
state with a rod; they can also be formed by dissolving the material in 
chloroform and dry spinning it like cellulose acetate rayon. Diameters 
less than Viooo inch can be obtained. 

* W. H. Carothers and J. W. Hill; Journ. Am. Chem. Soc., 54, 1579-87 (1932); 
Communication No. 78 from the Experimental Station of E. I. du Pont de Nemours and 

Received November 12, 1931. Published April 6, 1932. 

t It may be noted here that these investigations led finally to the development of 
an all synthetic fiber of very outstanding qualities, which has now been put on the mar- 
ket by E. I. du Pont de Nemours and Co. under the name of "Nylon." 


// no stress is applied during spinning, the filaments show properties simi- 
lar to those of the massive ester from which they are prepared (m. p. about 
75). If stress is applied, the fibers get transparent, lustrous, very strong, 
pliable and heat resistant. 

X-ray investigation shows that in the first case unorientated micellae are 
present, while the filaments drawn under stress exhibit a fiber diagram 
which shows that they are built up by a main valence chain lattice. 
The strength of such fibers ranges from 16 to 24 kg. per sq. mm., which is 
comparable with cotton (28 kg. per sq. mm.) and natural silk (35 kg. per* 

It is concluded from these observations, that in the case of hydroxydecanoic 
acid continuous filaments cannot be spun, unless the molecular weight lies 
above 7000\; the property of cold drawing appears above 9000 and a techni- 
cally useful degree of strength and pliability of the fibers requires a mo- 
lecular weight of at least 12,000. This corresponds to an average length of 
the chains of about 1000 A or 0.1 JJL and to a polymerization degree of about 

The linear condensation superpolyrners described in the three preceding 
papers of this series (1) can easily be drawn out into strong, pliable, trans- 
parent, permanently oriented (2) fibers. So far as we are aware no strictly 
synthetic material has hitherto been obtained in the form of fibers which 
thus simulate natural silk. 

The superpolyester derived from hexadecamethylene dicarboxylic acid 
and trimethylene glycol is for brevity referred to below as the 3-16 co-ester. 

Methods of Producing Filaments. Continuous filaments can be ob- 
tained from the molten 3-1 (i co-ester by touching it with a stirring rod 
and drawing the stirring rod away. More uniform filaments are ob- 
tained by dissolving the ester in chloroform and extruding the viscous 
solution through an ordinary rayon spinneret into a chamber warmed to 
permit the evaporation of the chloroform. The filaments can be picked 
up and continuously collected on a motor-driven drum at the bottom of 
the chamber. The production of filaments having a diameter of less than 
0.001 inch presents no difficulties. It is also possible to extrude the molten 
ester through a spinneret that is provided with a heating coil to maintain 
the ester in a sufficiently fluid condition. When this method is used it is 

* Meanwhile artificial superpolyamide fibers (Nylon) have been spun with tenaci- 
ties which considerably exceed the above figures. 

t This corresponds to a polymerization degree of about 40 and to a length of the 
extended chains of about 400 A or 40/zju. 



easy to apply considerable tension to the filaments as they are collected 
on the drum. 

Properties of the Filaments. The properties of the filaments produced 
by any of these methods de- 
pend upon the amount of ten- 
sion or stress applied during 
the spinning operation. If no 
stress is used the filaments 
closely resemble the massive 
ester from which they are pro- 
duced. They melt at 74-75 
and, like the initial mass, they 
are opaque and devoid of 
luster. But if sufficient tension 
is applied during the spinning 
operation the filaments are 
very different from the initial 
mass in their physical proper- 
ties. Instead of being opaque 
they are transparent, and they 
have a very high luster. They 

also have a very much higher breaking strength than the initial mass, and 
they are sufficiently pliable to be tied into hard knots, whereas the opaque 

filaments produced without 
stress are so fragile that 
they can hardly be bent 
without breaking. The 
opaque polyester and the 
transparent filaments pro- 
duced from it by spinning 
under tension furnish very 
different x-ray diffraction 
patterns. The pattern for 
the opaque ester (Fig. 4) in- 
dicates that it is crystalline, 
but there is no sign of gene- 
ral orientation. The trans- 
parent filaments on the 

Fig. 2. Same as Fig. 1 under crossed Nicols show- other hand f urnish a fiber 
ing parallel extinction of oriented section. pattern (Fig. 5) such as one 

Fig. 1. Filament from 3-16 co-ester showing 
opaque and transparent sections with boundary. 



obtains from certain natural silk and cellulose fibers and rayon filaments 
that have been spun under tension. The character of this pattern indi- 

Fig. 3. Same as Fig. 2, 45 to cross hairs. 

cates a very considerable degree of molecular orientation along the axis of 
the filament (3). 

Fig. 4. x-Ray diffraction pattern of 
unoriented 3-16 w-ester. 

Fig. 5. x-Ray diffraction pattern of 
oriented 3-16 w-ester. 

Cold Drawing. In connection with the formation of fibers the co-poly- 
mers exhibit a rather spectacular phenomenon which we call cold drawing. 
If stress is gently applied to a cylindrical sample of the opaque, unoriented 



3-16 co-polyester at room temperature or at a slightly elevated tempera- 
ture, instead of breaking apart, it separates into two sections joined by 
a thinner section of the transparent, oriented fiber. As pulling is con- 
tinued this transparent section grows at the expense of the unoriented 
sections until the latter are completely exhausted. A remarkable feature 
of this phenomenon is the sharpness of the boundary at the junction be- 
tween the transparent and the opaque sections of the filament (Fig. 1). 
During the drawing operation the shape of this boundary does not change; 
it merely advances through the opaque sections until the latter are ex- 
hausted. This operation can be carried out very rapidly and smoothly, 
and it leads to oriented fibers of uni- 
form cross section. The oriented and 
unoriented forms of the polyester are 
different crystalline states, and in the 
cold drawing operation one crystalline 
form is instantly transformed into the 
other merely by the action of very slight 
mechanical stress. 

Photomicrographs of filaments of 
the 3-16 co-ester are reproduced in 
Figs. 1, 2, 3 and 6. The opaque fila- 
ment shown in Fig. 1 was obtained 
by pulling out a specimen of the molten 
ester, and the transparent section 
attached to it was produced by cold 
drawing this filament. This figure 
shows clearly the sharpness of the 
boundary between the oriented and 
the unoriented sections. Figures 2 and 3 represent the same sample under 
polarized light. The oriented fiber is birefringent and shows parallel 
extinction. The opaque filaments of Fig. 6 were obtained by spinning a 
23 weight per cent, solution in chloroform of the 3-16 co-ester through a 
spinneret having 0.0045 inch holes into a chamber four feet deep heated to 
about 30. At the bottom of the chamber the filaments were caught and 
collected on a motor driven drum. The filaments had a diameter between 
0.001 and 0.002 inch. No attempt was made to stretch them during the 
spinning operation, and they were opaque, devoid of luster and very fragile. 
Nevertheless they furnished a fiber x-ray diffraction pattern practically 
identical with that shown in Fig. 5. Thus the x-ray data indicate a high 
degree of orientation while the physical properties indicate that scarcely 

Fig. 6. Filaments of 3-16 w-ester 
spun from chloroform solution showing 
transparent fibers produced by cold 
drawing (X 15). 



any orientation was effected during the spinning operation. When a 
bundle of the filaments was warmed to about 35 and pulled it was drawn 
out into thin, transparent fibers which were strong and pliable. Rough 
determinations of the tenacity of the partially oriented opaque filaments 
and the oriented transparent filaments prepared from them by cold draw- 
ing indicated that the breaking strength of the transparent filaments was 
about six times that of the opaque filaments. This specimen of oriented 
fibers was used in some of the physical tests described in the next paragraph. 
Tenacity and Elasticity. Rough determinations of the tensile strength 
of rather thick fibers of 3-16 co-ester prepared by pulling filaments from the 

molten ester and subsequently orient- 
ing them by cold drawing gave values 
ranging from 16 to 24 kg./sq. mm. 
(24,000 to 36,000 Ib /sq. in. These 
values compare well with those for 
cotton fibers (ca. 28 kg./sq. mm.) and 
silk (ca. 35 kg./sq. mm.). Under 
gradually increasing stress the fibers 
show a more or less definite flow point, 
and then, after considerable elongation, 
a point of increased resistance to stress. 
The fibers are sufficiently tough and 
flexible to be tied into hard knots ; thick 
fibers are sufficiently stiff to be bristle- 

We are indebted to Dr. W. H. 
Charch of the du Pont Rayon Company 
for a series of physical tests on fibers 
of the 3-16 co-ester spun from chloro- 
form solution and subsequently ori- 
ented by cold drawing. The observed tenacity was about 1.1 g. per 
denier, but this value is believed to be too low owing to the fact that no 
account was taken of the considerable number of broken filaments in the 
specimens tested. The wet tenacity is fully equal to the dry tenacity. 
(In the actual tests the fibers gave consistently higher values when 
wet than dry, but the differences probably lay within the experimental 
error.) A clump of filaments rolled into a small ball and compressed 
showed a remarkable springiness resembling wool. In their elastic proper- 
ties these fibers are very much superior to any known artificial silk. In 
Fig. 7 are presented some slightly idealized curves taken from drawings 

Fig. 7. x-Ray diffraction pattern of 
oriented ethylene sebacate. 


made by a modified Richards dynamometer. Bundles of filaments of vis- 
cose rayon, natural silk and fibers from the 3-16 co-ester were stretched for 
one minute and the curves show the extent and rate of recovery from 
stretch during the first minute after release from tension. These curves 
indicate that the synthetic product is even superior to natural silk, but it is 
possible that the silk specimen was somewhat deteriorated. 

Fibers from Other Linear Condensation Superpolymers. Fibers 
closely resembling those described in the preceding section have been ob- 
tained in the same way from a variety of other linear condensation polymers 
in the co- or superpolymeric state. The polyesters thus far examined are 

Fig. 8. Cold drawn fibers from co-anhydride of deca- 
methylene dicarboxylic acid. The lack of definition is 
due to specular reflection caused by high luster. 

those derived from w-hydroxydecanoic acid, co-hydroxypentadecanoic acid, 
ethylene glycol and sebacic acid, trimethylene glycol and adipic acid and 
ethylene glycol and succinc acid. All of these materials clearly exhibit 
the phenomenon of cold drawing. Wide differences in the nature of the 
structural unit naturally have a considerable effect on the capacity to 
form fibers and on some of the properties of the fibers. Thus ethylene 
succinate in its a-form has a higher melting point and is more brittle than 
most other a-polyesters derived from dibasic acids and glycols (4), and 
fibers derived from co-ethylene succinate are somewhat short and brittle. 
On the other hand, a-polyesters derived from adipic acid have rather low 
melting points, and w-polyesters derived from the same acid show a tend- 
ency to collapse at slightly elevated temperature. 

Polyamides such as that derived from e-aminocaproic acid can also be got 


in the superpolymeric state (5). Owing to the very high polarity of the 
amide group and the consequent high molecular cohesion, such polyamides 
are exceedingly hard and insoluble in most common solvents. They show 
signs of melting only at temperatures sufficiently high to produce considera- 
ble decomposition. These refractory qualities make it very difficult to draw 
satisfactory filaments from pure polyamides. On the other hand, mixed 
polyester-polyamides of the type already described (5) yield filaments that 
are easily cold drawn and, when oriented, have very high tenacity and pli- 

Polyanhydrides derived from dibasic acids of the series HOOCXCH^)*- 
COOH are especially easy to obtain in the superpolymeric state. The 
technique involved and the nature of the reaction is discussed in detail for 
sebacic acid (x = 8) in paper XIV. The behavior described there is char- 
acteristic (so far as the formation of superpolyanhydride is concerned) 
for those acids in which x is greater than 4. (Actual observations thus far 
include those in which x is 5, 6, 7, 8, 9, 10, 11, 12 and 16.) The fibers ob- 
tained from the anhydrides have an especially high degree of strength, 
pliability and luster (cf. Fig. 8) but they gradually disintegrate on stand- 
ing owing to hydrolytic degradation. 


Our studies of polymerization were first initiated at a time when a great 
deal of scepticism prevailed concerning the possibility of applying the 
usually accepted ideas of structural organic chemistry to such naturally 
occurring materials as cellulose ; and its primary object was to synthesize 
giant molecules of known structure by strictly rational methods. The 
use of synthetic models as a means of approach to this problem had already 
been undertaken by Staudinger, but the products studied by him (polyoxy- 
methylene, polystyrene, polyacrylic acid, etc.) although unquestionably 
simpler than naturally occurring polymers, were produced by reactions of 
unknown mechanism, and their behavior, except in the case of polyoxy- 
methylene, was not sufficiently simple to furnish an unequivocal demon- 
stration of their structure. On the other hand, the development of the 
principles of condensation polymerization described in preceding papers 
of this series has led to strictly rational methods for the synthesis of linear 
polymers, and the structures of the superpolymers III to VI follow directly 
from the methods used in their preparation. Meanwhile the weight of 
authoritative opinion has shifted; further evidence has been accumulated 
(6) and cellulose has been assigned a definite and generally accepted struc- 


ture (7). Like the synthetic products III to VI it falls in the class already 
defined as linear superpolymers. 

Cellulose (polyacetal) 

II ... NH R CO NH R' CO NH R CO NH R'-~ CO. . . 

Silk (polyamide) 

III ... 0-- R -CO O R CO O R CO O R CO O R CO . . 

Polyester (from hydroxy acid) 


Polyester (from dibasic acid and glycol) 

V ... O - R CO NH R'~ CO NH R' CO O R CO . . 

Mixed polyester-polyamide 

VI ... - O CO- R CO0 COR CO O CO R CO . . . 


Addition polymers of very high molecular weight have been synthesized 
in the past, but the capacity to yield permanently oriented fibers of any 
considerable strength has not been observed hitherto. Why is this, and 
what conditions of molecular structure are requisite for the production of 
a useful fiber? We regret that the limitations of space prohibit a detailed 
discussion of these and accessory questions, and permit only the baldest 
statement of our conclusions. 

We picture a perfectly oriented fiber as consisting essentially of a single 
crystal in which the long molecules are in ordered array parallel with the 
fiber axis (Fig. 10). (In actual fibers a considerable number of the mole- 
cules fail to identify themselves completely with this perfectly ordered 
structure.) The high strength in the direction of the fiber axis and the 
pliability are accounted for by the high cohesive forces of the long molecules 
and by the absence of any crystal boundaries along the fiber axis (7a). 
Fiber strength should depend upon molecular length, and recent work by 
Dr. van Natta in this laboratory indicates that it is not possible to spin 
continuous filaments from the polyester of hydroxydecanoic acid until 
its molecular weight reaches about 7000. The property of cold drawing 
does not appear until the molecular weight reaches about 9000. From 
these and other facts we conclude that a useful degree of strength and pli- 
ability in a fiber requires a molecular weight of at least 12,000 and a mo- 
lecular length not less than 1000 A. (The limits for polyamides may 
perhaps lie at somewhat lower values.) 

Besides being composed of very long molecules, a compound must be 
capable of crystallizing if it is to form oriented fibers, and orientation is 



probably necessary for great strength and pliability. Linear condensation 
polymers are quite generally crystalline unless bulky substituents are 
present to destroy the linear symmetry of the chains; addition polymers, 
especially those produced from vinyl compounds, are more rarely crystal- 
line. Possible reasons for this have already been discussed in part (8). 




* stl ! Ml^l 

Fig. 9. Elastic recovery of various fibers. 

Three-dimensional polymers are obviously unsuited for fiber orientation, 
and synthetic materials of this class are besides invariably amorphous. 
Glyptal resins belong to this class (9). It has been proposed (10) to use 
glyptal resins for the production of artificial silk, but our attempts to carry 
out the proposed process led to exceedingly fragile (though lustrous) 
threads which showed no signs of orientation when examined by x-rays. 

Although one will not ex- 

. / J ! I TT\"f"*. I 'r I J"TV? \ !.* * P e t OI "i en ted fibers to arise by 

1 * T .' ; ; j [ ;*''.. i ' * ' * T"T~ any process of spontaneous 

' ! j j 1 j I j V V i !' ! J J r I TT\ crystallization under the ordi- 

4 " ' r ' * f * " " " * T * -- * * nary conditions, the phenom- 

Fig. 10. Lattice from molecules of unequal enon of cold drawing is per- 

length. haps only accidentally asso- 

ciated with the capacity to 

yield oriented fibers; it apparently requires a certain degree of softness 
and suppleness in the molecules ; and its mechanism is doubtless analogous 
to that involved in the mechanical orientation of cellulose preparations (11). 
(Perhaps however, in the unoriented polyesters the molecules are in spiral 
form and become extended during orientation.) 

It has been suggested repeatedly by Staudinger that the great sensitivity 
of cellulose and rubber to degradation by heat and by certain reagents is 


due to the fact that the upper limit of thermal stability of the very long 
molecules lies close to room temperature. It is of interest in this connection 
that the synthetic linear superpolyesters, in spite of their very high mo- 
lecular weights, are formed at 200 to 250, and that they show no signs 
of being degraded by repeated exposure to elevated temperature. 


We are greatly indebted to Dr. W. H. Charch of the du Pont Rayon 
Company and to Dr. E. O. Kraemer of the Experimental Station for valu- 
able criticism and advice, and we have also to thank Dr. A. W. Kenney of 
the Experimental Station for the X-ray data. 


The linear condensation co -poly esters, polyanhydrides and mixed poly- 
ester-polyamides described in preceding papers are easily drawn out into 
very strong, pliable, highly oriented fibers which closely simulate natural 
silk and cellulose fibers. These materials also resemble cellulose and silk 
in the essential details of their molecular structure. The significance of 
these analogies is discussed. 

Bibliography and Remarks 

(1) Papers XII, XIII and XIV. 

(2) The orientation implied in this use of the term is general orientation, with reference to the 
fiber axis, of the molecules or of some ordered units involving the molecules. 

(3) According to some preliminary calculations made by Dr. A. W. Kenney of this laboratory 
the identity periods in the oriented fibers derived from ethylene sebacate and from the 3-16 ester lie 
within about 10% of the calculated lengths for one structural unit of the chain. 

(4) Carothers and Arvin, Journ. Am. Chem. Soc., 51, 2560 (1929). 

(5) Paper XIII. 

(6) Haworth, Long and Plant, J. Chcm Soc., 129, 2809 (1927) ; Freudenberg, Ber., 54, 767 (1921); 
Ann., 461, 130 (1928); Freudenberg and Brauri, ibid., 460, 288 (1929); Freudenberg, Kuhn, Durr 
Bolz and Steinbrunn, Ber., 63, 1528 (1930). 

(7) Sponsler and Dore, Colloid Symposium Monograph, 4, 174 (1926) Chemical Catalog Com- 
pany, New York; Sponsler, Nature, 125, 633 (1930); Meyer and Mark, Ber., 61, 593 (1928); Mark 
and Meyer, Z. physik. Chem , [B] 2, 115 (1929), Mark and Susich, ibid., 4, 431 (1929) 

(7a) Cf. Meyer and Mark, "Der Aufbau der hochpolyrneren organischen Naturstoffe," Akade- 
mische Verlagsgesellschaft, Leipzig, Germany, 1930. 

(8) Carothers, Chem. Reviews, 8, 415 (1931). 

(9) Ref. 8, page 402. 

(10) British Patents 303,867 and 305,468. 

(11) Ref. 8, p. 418. 


XVI. A Polyalcohol from Decamethylene Dimagnesium 


This is a short description of a polyalcohol made from decamethylene di- 
magnesium bromide. As this Grignard compound is a bifunctional re- 
actant it can build up a linear polymer. By reaction with methyl formate 
it yields a crystalline polydecamethylene carbinol. If this is heated in 
the molecular still a colorless, insoluble, tough, pliable mass of a very con- 
siderable molecular weight is obtained. 

The action of bifunctional reactants on bifunctional Grignard reagents 
(e. g., on BrMg(CH 2 ) A; MgBr) presents the possibility of producing cyclic 
compounds of various types. Cyclopentanone (1), cyclohexanone and 
methylcyclohexanol (2) have been prepared thus, and also 5- and 6-atom 
rings containing phosphorus, arsenic (3), lead (4) and silicon (5). Concern- 
ing larger rings it is recorded (2) that diacetyl reacts with pentamethylene 
dimagnesium bromide but yields very little of the product corresponding 
in composition with the expected 7-ring glycol. The same reagent is 
said (6), to react with dialdehydes and diketones to give ring compounds, 
though "not easily"; but none of these compounds are described. The 
rarity and obscurity of such allusions to reactions which might yield large 
rings is certainly not due to any difficulty of inducing reaction between 
MgX and carbonyl or other groups. On the other hand, bifunctional 
reactions are not obliged to choose between ring formation and frustration 
or abortion; they may be, and in fact when the possibility of forming a 
ring of 5 or 6 atoms is absent, they generally are intermolecular, and they 
thus lead to linear polymeric chains (7). The diversity of the Grignard 
reactions should permit the preparation of linear polymers of many differ- 
ent types. Complications due to the usual side reactions of the MgX 
group are multiplied when two of these groups are present in the same 
molecule. Nevertheless, as the following example shows, it is possible by 
such reactions to prepare linear polymers which are homogeneous in their 
analytical and chemical behavior. 

Methyl formate (0.2 mole) was added dropwise to an ethereal solution (250 cc.) 
of decamethylene dimagnesium bromide prepared from 0.2 mole of the bromide. Each 
drop of the formate immediately yielded a heavy, voluminous precipitate. After the 
addition of all the formate, the mixture was refluxed for one and one-half hours. A 

* W. H. Carothers and G E. Kirby; Journ. Am. Chem. Soc., 54, 1588-90 (1932); 
Communication No. 48 from the Experimental Station of E. I. du Pont de Nemours and 

Received November 12, 1931. Published April 6, 1932. 


color test (8) then showed that C MgX was absent from the ethereal layer but present 
in the insoluble precipitate. The mixture was treated with water and dilute hydrochloric 
acid, and the undissolved solid was separated by filtration. The yield was 88% of the 
theoretical. The product was completely freed of magnesium by triturating it with 
warm dilute hydrochloric acid. It dissolved readily in hot alcohol, acetic acid or ethyl- 
ene chloride, but not in acetone, ether or benzene. It melted at about 115-120. 
More sharply melting samples (e. g,, 120-121) were obtained by repeated crystalliza- 
tion from alcohol, and higher and lower melting samples were obtained in other experi- 
ments. The melting points all varied with the rate of heating. 

The analytical composition of this solid corresponds with the expected poly second- 
ary alcohol, and its apparent molecular weight indicates an average formula 
[(CH 2 )io CHOH] 6 . The equation for the reaction may be represented as 

. . . (CH 2 )ioMgBr + 0=C OCH 3 + BrMg . . . - > 


. . . [ (CH 2 ) 10 CH(OMgBr) ] . . . 

Anal. Calcd. for CiiH 22 O: C, 77.65; H, 12.94. Found: C, 77.35, 77.40; H, 
13.04,12.79. Mol. wt., calcd. for (C n H 22 O) 6 : 850. Found (in boiling ethylene chloride) : 
835, 879. 

It is undoubtedly mixed with traces of shorter and longer chains and with analogous 
derivatives of the coupling products BrMg(CH 2 ) 2 oMgBr, BrMg(CH 2 ) 3 oMgBr, etc., 
which are always formed in small amounts during the preparation of the reagent. The 
chains are probably all open, but there may be considerable variety in the nature of the 
terminal groups. However, its lack of homogeneity is probably no greater than that 
of a purified sample of poly vinyl alcohol (9), of which it is a homolog. 

The polyaclohol is readily converted into such acyl derivatives as bromobenzoyl, 
phthalyl, etc., but all of these products were obtained only in the form of oils or tars. 
The composition of an oily acetate prepared by refluxing the alcohol with acetic anhy- 
dride containing some sodium acetate confirms the empirical structure assigned to the 

Anal. Calcd. for (CH 2 ) 10 CH (OCOCH 3 ) = C 13 H 24 O 2 : C, 73.59; H, 
11.32; saponificatioti equivalent, 212. Found: C, 74.09, 73.61; H, 11.27, 11.33: 
saponification equivalent, 207, 223, 227. 

Experiments in other directions remain incomplete. 

The behavior of the polyalcohol in the molecular still is noteworthy. A sample 
melting at 110-113 was heated for a few hours at about 150 under a pressure less than 
10~ 6 mm. A trace of crystalline solid melting at about 80 collected on the condenser 
which was within 1 cm. of the evaporating surface. The residue was a colorless, semi- 
transparent, very tough pliable mass. It was insoluble in the common solvents. On 
being heated, it gradually became completely transparent between 60 and 100. At 
215-220 it had become somewhat soft and sticky without losing its shape. Above 
this temperature it first became slightly yellow and finally, at about 250, still darker 
with considerable decomposition. Its analytical composition was nearly the same 
as that of the alcohol from which it was derived. The change in properties indicates 
a considerable increase in molecular weight, and the absence of solubility and complete 
fusibility is consistent with the development of a three-dimensional polymeric molecular 
structure. This might readily occur, with very little change in composition, through 


cross-linking of the polyalcohol chains by occasional ether formation. When the 
polyalcohol is heated in the same way at ordinary pressure, it remains quite unchanged 
in its properties ; so this example provides an even more striking illustration than those 
previously recorded (10) of the powerfulness of the molecular still as a tool for displacing 
chemical equilibria. 

The description of these fragmentary experiments with bifunctional 
Grignard reagents is presented now because it has become necessary to 
suspend our work in this field. 


A crystalline polydecamethylene carbinol has been prepared by the 
action of decamethylene dimagnesium bromide on methyl formate. Its 
properties are described. It is converted into a colorless, insoluble, tough, 
pliable mass when it is heated in a molecular still. 

Bibliography and Remarks 

(1) v. Braun and Sobecki, Ber., 44, 1918 (1911). 

(2) Grignard and Vignon, Compt rend., 144, 1358 (1907). 

(3) Gruttner and Wiernik, Ber., 48, 1473 (1915). 

(4) Gruttner and Krause, ibid., 49, 2CG6 (1916). 

(5) Bygden, ibid., 48, 1236 (1915). 

(6) v. Braun, Chem. Zentr., II, 1993 (1909) 

(7) See Carothers, Journ. Am. Chem. Soc., 51, 2548 (1929), and subsequent papers of this series. 

(8) Gilman and Schulze, ibid., 47, 2002 (1925). 

(9) Herrmann and Haehnel, Ber., 60, 1658 (1927); vStaudinger, Frey and Starck, ibid., 60, 1782 

(10) Papers XI, XII, XIII and XIV. 

XVII. Friedel-Crafts Syntheses with the Polyanhydrides of 

the Dibasic Acids* 

This short note deals with the Friedel-Crafts reaction of polyanhydrides 
of dibasic acids. Such anhydrides react with benzene in the presence of 
anhydrous aluminum chloride to yield dibenzoyl-alkane, ^-benzoyl fatty 
acid and dibasic acid. The reaction has been actually carried out with 
benzene and polymeric adipic and sebacic anhydride; it provides a con- 
venient method of synthesis for u-benzoyl fatty acids and is presumably 
quite general. 

* J. W. Hill; Journ. Am. Chem. Soc., 54, 4105-06 (1932); Communication No. 100 
from the Experimental Station of E. I. du Pont de Nemours and Co. See also foot- 
note (1) in paper VI on page 63. 

Received June 15, 1932. Published October 5, 1932. 


It has been shown that the polymeric anhydrides of aliphatic dibasic 
acids react smoothly and normally as acid anhydrides with aniline (1), 
with phenol (la), with ammonia (2a) and with diethylamine (2b). In 
each reaction three products are formed. The course of the reactions is 
shown by the following scheme, which does not consider the end groups of 
the polyanhydride 

( OC(CHJ,,CO O), + RH ^*R OC(CH 2 ) M CO R + 

4 (a) 

*R OC(CH 2 ) n COOH + - A HOOC(CH 2 ), t COOH 
2 (b) 4 (c) 

where R is C 6 H<>NH , C 6 H 6 O , NH 2 or (C 2 H 6 ) 2 N 

It has been demonstrated on theoretical grounds that the products of types 
(a), (b) and (c) should be formed in the molecular ratio of 1:2:1. This 
has been verified quantitatively in the aniline reaction (1). 

The dibasic acid polyanhydrides have now been found to react typically 
with benzene in the presence of anhydrous aluminum chloride to yield 
dibenzoyl alkane, w-benzoyl fatty acid and dibasic acid. 

( OC(CH,) B CO O), + CeHo -f (A1CU) > 

~ C 6 H 6 CO(CH 2 )nCOC 6 H 6 -f o C 6 H 5 CO(CH 2 ) n COOH + - HOOC(CH 2 ) n COOH 
4 4 

This reaction has been carried out with polymeric adipic and sebacic an- 
hydrides and benzene. There is no reason to doubt that it is perfectly 
general and applicable to any polymeric dibasic anhydride and any suitable 
aromatic hydrocarbon. 

This reaction is particularly interesting in providing a convenient method 
of synthesis for the w-benzoyl fatty acids which have heretofore been pre- 
pared by more elaborate syntheses or in very low and uncertain yields as 
by-products in the Friedel-C rafts reaction of dibasic acid chlorides with 
aromatic hydrocarbons (3). 

Experimental Part 

Friedel-Crafts Reaction between Adipic Polyanhydride and Benzene. One hun- 
dred and forty-six grams (one mole) of adipic acid was refluxed with 400 cc. of acetic 
anhydride for six hours. The excess acetic anhydride and the acetic acid formed in the 
reaction were removed by distillation in vacua up to 120 bath temperature. The result- 
ing polyanhydride (la) was dissolved in 400 cc. of warm, dry benzene. Three hundred 
grams of anhydrous aluminum chloride was suspended in 1500 cc. of dry benzene con- 
tained in a 3-necked 3-liter flask fitted with a reflux condenser and a mechanical 

The anhydride solution was added with stirring over a period of one hour and the 
mixture allowed to stand overnight. The product was decomposed in ice and 250 cc. 


of concentrated hydrochloric acid added. The benzene layer was separated and ex- 
tracted with dilute aqueous sodium hydroxide. The water phase deposited a small 
amount of crystals on standing which were identified after recrystallization from water 
as adipic acid by a mixed melting point determination. 

The alkaline solution was acidified and the crystalline precipitate of cu-benzoyl- 
valeric acid filtered off: weight dry, 78 g.; yield, 75%. The product was recrystallized 
from a benzene-petroleum ether mixture, m. p. 70-71 (4). 

Anal. Calcd. for Ci 2 Hi 4 O 3 : C, 69.9; H, 6.8. Found: C, 70.1; H, 6.7. 

The extracted benzene was distilled to a small volume and chilled; 56. 5 g. of dibenzoyl- 
butane was obtained, yield 85%. Recrystallized from alcohol, it was slightly pink and 
melted at 105-106 (5). It was identified by means of a mixed melting point determina- 
tion with an authentic specimen. 

Anal. Calcd. for Ci 8 H 18 O 2 : C, 81.2; H, 6.8. Found: C, 80.6; H, 7.0. 

Friedel-Crafts Reaction between Sebacic Polyanhydride and Benzene. One 
hundred and one grams (0.5 mole) of sebacic acid was refluxed with 300 cc. of acetic 
anhydride for five hours. The excess anhydride and the acetic acid formed were dis- 
tilled off in vacua up to 120 . The residual polyanhydride was dissolved in 250 cc. of 
warm dry benzene and added to 150 g. of anhydrous aluminum chloride suspended in 
750 cc. of dry benzene as in the previous preparation. The products were worked up 
in the same way. The crude precipitated co-benzoylnonanoic acid was extracted with 
hot water to remove sebacic acid. Fifteen grams of sebacic acid separated from the 
washings and was identified by a mixed melting point determination with a known 
sample. The co-benzoylnonanoic acid weighed 50 g., a yield of 78%. Recrystallized 
from dilute alcohol, it melted at 77-78 (6). 

Anal. Calcd. for C 16 H 22 O 3 : C, 73.3; H, 8.4. Found: C, 73.3; H, 8.4. 

The dibenzoyloctane was recovered from the benzene solution and weighed 35 g., 
a yield of 86%. Recrystallized from alcohol, it melted at 92-93 (7). 

Anal. Calcd. for C 2 2H 26 O 2 : C, 82.0; H, 8.1. Found: C, 81.7; H, 8.1. 


The polymeric anhydrides of adipic and sebacic acids react with benzene 
in the presence of anhydrous aluminum chloride to yield mixtures of the 
appropriate dibenzoyl alkane, co-benzoyl fatty acid, and dibasic acid. The 
reaction is doubtless general. 

Bibliography and Remarks 

(1) (a) Hill, Journ. Am. Chem. Soc., 52, 4110 (1930); (b) Hill and Carothers, ibid., 54, 1569 (1932). 

(2) (a) Etaix, Ann. Chim., [7] 9, 356 (1896); (b) Einhorn and Diesbach, Ber., 39, 1222 (1906). 

(3) Auger, Ann. chim., [6J 22, 360 (1891); Etaix, ibid.. [7] 9, 391 (1896). 

(4) Grateau, Compt. rend., 191, 947 (1930), gives 71. 

(5) 6taix, Ann. chim., [7] 9, 372 (1896), gives 102-103. 

(6) Auger, ibid. t [6] 22, 364 (1891), gives 78-79. 

(7) Auger, ibid., [G] 22 363 (1891), gives 88-89. 


XVIII. Polyesters from w-Hydroxydecanoic Acid* 

A series of polyesters from u-hydroxydecanoic acid is described in this 
paper. It represents without doubt the best established set of synthetic 
high polymers concerning identity, uniformity and structure. 
The lower members were prepared by heating u-hydroxydecanoic acid at 
150 in a normal distilling flask; the higher ones were obtained in the 
molecular still previously described.^ 

The molecular weights range from 780 to 25,000 and were determined by 
titration of the terminal acid group; the highest member was also 
measured in the Svedberg Ultracentrifuge. The agreement is excellent 
(25,660 and 25,200). 

The polyesters below 10,000 dissolve rapidly and completely in cold chloro- 
form or benzene and in various hot organic solvents. The highest mem- 
bers show greatly diminished solubility and swell before going slowly into 
solution. The melting points increase with the molecular weight until 
about 1000 (being then around 70), which corresponds to a polymerization 
degree of jive and to a length of about 100 A for the extended chain. Be- 
tween 1000 and 25,000 there is no further increase of melting points. All 
esters are microcrystalline; the lower members being waxy and brittle, the 
higher ones hard, horny and very tough. 

The samples above 10,000 (polymerization degree 50, length of the extended 
cJiain around 1000 A) can readily be spun into fibers. The initial fila- 
ments are opaque and fragile; by stretching they become transparent, 
pliable and very strong. The x-ray diagram shows the presence of fiber 
orientation. Up to a molecular weight of 5700 (polymerization degree 28) 
no fibers can be obtained. The ester with m. w. 7330 furnished filaments 
but they could not be extended. The ester 9330 gave stretchable fibers 
but they were very weak. Above 10,000 strong extendable filaments are 

These esters have also been used to test the viscosity equation of Stand- 
inger.** It was found that this surprisingly simple relation between 
viscosity and molecular weight holds fairly well up to about 15,000. A hove 
this point the viscosities rise more rapidly than the equation requires, 
and the law loses its strict validity. 

* W. H. Carothers and F. J. van Natta; Journ. Am. Chem. Soc., 55, 4714-19 
(1933); Communication No. 129 from the Experimental Station of E. I. du Pont de 
Nemours and Company. 

Received August 8, 1933. Published November 7, 1933. 

t Compare paper XI, pages 154 to 156. 

** E. O. Kraemer and F. J. van Natta; /. Phys. Chem., 36, 3175 (1932). 


The obvious importance of simple synthetic models as an aid in studying 
macromolecular materials has been emphasized repeatedly by Staudinger 
(1), who has used polystyrene, polyoxymethylene, polyacrylic acid, etc., 
for this purpose. Our own researches on condensation polymers were 
started with the idea that the fact of a proposed model's being synthetic is 
of little value unless the method of synthesis is rational, i. e., unless it is 
sufficiently clear-cut to leave no doubt concerning the structure of the 
product. Polystyrene may, for example, serve as a simplified model of 
rubber, but it has the disadvantage that the method used in its synthesis (a 
spontaneous polymerization of unknown mechanism) furnishes no certain 
clue to its structure. The independent demonstration of its structure pre- 
sents the same difficulties as does rubber; in fact today the formula of 
rubber can be written with more assurance than that of polystyrene. 

In the first paper of this series (2) it was pointed out that bifunctional 
condensations frequently proceed by known mechanisms. In the second 
paper (3) it was shown for polyesters derived jointly from dibasic acids 
and glycols that the reaction consists exclusively in esterification while the 
average size of the product molecule increases progressively with the com- 
pleteness of the reaction. Rational, deliberate control over the average 
molecular weight is thus made possible, and, as was showu later (4), by 
using more drastic conditions molecules of average weight greater than 
20,000 can be obtained. Meanwhile a study of the self-esterification of 
co-hydroxydecanoic acid was presented by Lycan and Adams (5), who con- 
cluded that the products must be formulated like the polyesters referred to 

We have now greatly extended the range of polyesters derived from 
co-hydroxydecanoic acid and have obtained the series of fractions of differ- 
ent average molecular weights shown in Table I. The structure of these 
polyesters HO[ (CH 2 ) 9 CO O ] M (CH 2 ) 9 CO OH follows from the 
method used in their synthesis (see Experimental Part) and the fact that 
they can be hydrolyzed quantitatively to co-hydroxydecanoic acid. The 
only important uncertainty is the distribution, in a given sample, of the 
molecular weight about the observed average (i. e., the degree of homo- 
geneity). However, since these esters are all crystalline solids and since 
the mer or unit of the chain is quite long, the homogeneity is probably 
better than in the case of such materials as polystyrene which can be 
purified only by fractional precipitation or extraction. Moreover, a 
determination of the molecular weight of the highest member of the series 
by the Svedberg ultracentrifugal method indicates that the weight of most 
of the molecules lies fairly close to the observed average. It is indeed our 



opinion that this series of polyesters is more definitely and certainly es- 
tablished in identity and structure than any similar series of macromo- 
lecular compounds yet described. 


Mol. M. p., v 85 

wt. C. di n D Spinnability Tensile strength 

780 66-67 1.0957 1.4494 Absent 
1720 72-74 1.0935 1.4506 Absent 
3190 74-75 1.0877 1.4517 Absent 
4170 74-76 1.0814 1.4517 Absent 

5670 73-75 1.0751 1.4518 Very short fibers. No cold drawing 
7330 74-75 1.0715 1.4517 Long fibers, but cold drawing absent Very weak 
9330 75-76 1 .0668 1 .4518 Long fibers which cold draw Very weak 

16900 77-78 1 . 0627 Easily spins and cold draws 13.1 kg./mm. 2 

20700 77-78 1 . 0632 Spins with difficulty but easily cold draws 12.3 kg./mm. 2 

25200 75-80 1.0621 1.4515 Spins above 210 and cold draws 7.0 kg./mm. 2 


Observed tnol. wts. 
(by titration) 

A ^erage 

Calcul ted 
Observed mol. wts. length of 
(by other methods) molecule in A. 




930 (in boiling benzene) 





1620 (in boiling benzene) 





Not measured 





Not measured 





Not measured 





Not measured 





Not measured 



17110 16790 


Not measured 





Not measured 



25760 25660 


26700 (ultracentrifuge) 


Molecular Weights. Molecular weights were estimated by titration 
with standard alcoholic potash of the polyesters dissolved in a chloroform 
alcohol mixture. Phenolphthalein was used as an indicator. This method 
applied to pure lauric acid gave values agreeing sharply with the theoretical 
(200.4, 199.6 and 199.2 as against 200.2). Observed values for the poly- 
esters are shown in Table II. It is interesting to note that no difficulty 
was encountered in titrating the highest member of the series where the 
acid hydrogen is only one part in 25,000. 

The average equivalent weight measured by titration will of course be 
identical with the average molecular weight only if each molecule bears 
a terminal carboxyl in the manner required by the indicated structure. 


The accidental loss of terminal carboxyls under the conditions of prepara- 
tion used seems rather improbable, but indication of the absence of such 
loss is provided by independent estimations of molecular weight. The 
boiling point method furnished values for the first two members agreeing, 
within the probable experimental error, with the much more sharply re- 
producible values determined by titration. For compounds above mo- 
lecular weight 3000, in our experience, boiling-point or freezing-point 
methods are not self-consistent within 10%, and above 10,000 they are 
practically worthless. We were, however, very fortunate in being able to 
obtain a value for the highest polymer by the Svedberg ultracentrifugal 
method (6). The agreement is all that could be desired. 

Physical Properties. The polyesters below 10,000 dissolve rapidly 
and completely in cold chloroform or benzene and in hot acetone, ethyl 
acetate and acetic acid. They are practically insoluble in hot alcohol, 
ligroin or water. The highest members show diminished solubility in 
benzene and in hot acetone and ethyl acetate. They dissolve copiously in 
chloroform, but solution occurs only slowly and is preceded by some 
swelling. At ca. 110 the first member of the series is a highly viscous 
liquid while the highest member is a transparent resin that is too stiff to 
flow and sufficiently elastic to offer resistance to permanent deformation. 

Lycan and Adams have pointed out (5) that the melting point of self- 
esters of hydroxydecanoic acid increases with increasing molecular weight 
up to 1000, but changes little between 1000 and 9000. As the data of 
Table I show, there is also no further increase between 9000 and 25,000. 
The polyesters separate from solution in the form of white powders which 
give sharp x-ray powder diffraction patterns. At least in the lower mem- 
bers of the series crystallinity can also be demonstrated by microscopic 
observation. A very dilute solution of the 3190 ester in butyl acetate, 
when examined at a magnification of 400, showed the separation of tiny 
flat glittering plates. The molten esters in thin layers crystallize very 
rapidly, but with the lower members one can observe the crystallization to 
start with the emergence and growth of innumerable doubly refracting 
centers, apparently spherulites. At a magnification of 900 the growth ap- 
pears to involve the intermeshing of radiating clusters of needles. 

The masses resulting from crystallization of the molten polyesters are 
opaque solids. The lower members are waxy and brittle ; when fractured 
they show no planes of cleavage. The highest members are harder and so 
horny and tough that they can scarcely be fractured. 

In Table I are listed the densities of the solid esters at 25. The values 
diminish as the molecular weight rises, quite rapidly at first and then more 


slowly until at a molecular weight of 16,900 they become almost constant. 
This at first sight is very surprising. Polymerization usually involves an 
increase in density. There is, for example, an increase of 23% in passing 
from chloroprene to polychloroprene (7) The interpretation of the 
present case can scarcely be attempted in the absence of data on the 
molten esters, which are not yet available ; but it is easy to imagine that 
the longer the molecules are the more difficulty they will have in lining 
themselves up perfectly in the crystal lattice. The refractive indices of 
the molten esters at 85 are presented in Table I. There is a slight in- 
crease from 780 to 1720, a smaller increase from 1720 to 3190, and beyond 
that no further change. 

Fiber Formation. In a previous paper (8) it was shown that linear 
polyesters of molecular weight above 10,000, when melted or dissolved, 
can be spun into fibers. The initial filaments are opaque and fragile, but 
when stress is applied to them they are readily elongated several fold and 
then remain permanently extended. The stretching is accompanied by a 
loss of opacity, and an enormous increase in tensile strength and pliability. 
Examination by x-rays shows the presence of fiber orientation. 

These fibers are, we believe, the first examples of a synthetic material 
being obtained in the form of fibers having any considerable degree of 
strength, orientation and pliability. The analogies in chemical structure 
with cellulose and silk are especially interesting, and the esters of the present 
paper provide some data (Table I) on the relation between molecular 
weight and fiber-forming ability. Until a molecular weight of 5670 is 
reached, the viscosity and coherence of the molten esters are so low that 
they do not yield continuous filaments. The ester of molecular weight 
7330 furnished continuous filaments which, however, could not be stretched 
and oriented. The 9330 ester could be stretched, but the oriented fibers 
were too weak to permit the determination of tensile strength. Oriented 
fibers from the 16,900 ester had a tensile strength of 13.1 kg./sq. mm. or 
1.36 g. per denier, which is in the same range as good regenerated cellulose 
fibers. The value for the 20,700 ester (12.3 kg./sq. mm.) differs from that 
for the 16,900 ester by an amount that probably lies within the experi- 
mental error. The tensile strength of the 15,200 ester is, however, defi- 
nitely much lower. It is exceedingly improbable that the fiber strength 
of completely oriented molecules rises to a maximum and then falls off as 
the length of the molecule increases. On the other hand, the degree of 
orientation that can be produced by cold-drawing probably depends upon 
other factors besides molecular weight. An unusually high temperature 
(210) was required to soften the 25,200 ester sufficiently to permit spin- 


ning, and its relatively low tensile strength may therefore reasonably be 
ascribed to factors associated with orientation. It may be observed that 
the ability to form strong, highly oriented fibers does not appear until the 
molecular length reaches some value lying between 700 and 1300 A. 

Viscosity and Other Properties. The viscosities of dilute solutions 
of these polymers in tetrachloroethane have been carefully measured. 
The results already reported and discussed (9) show that Staudinger's 
simple empirical equations relating viscosity to molecular weight (1) are 
satisfied fairly well until the molecular weight reaches 16,900; beyond this 
point the viscosities rise more rapidly than the equations require. Staud- 
inger has assumed that the relations between viscosity and molecular 
weight established for compounds below 10,000 will hold for compounds of 
higher molecular weight. The data referred to above provide the first 
experimental test of this assumption, and for the polyesters under con- 
sideration it is shown to be not strictly valid. Molecular weights that 
have been assigned to cellulose, rubber, high polystyrenes, etc., on the 
basis of viscosity measurements must therefore be considered as subject to 
considerable uncertainty. 

Study of the physical properties of polyesters derived from hydroxy- 
decanoic acid is being continued, and further results will be reported in 
future papers. 

Experimental Part 

Preparation of Polyesters. The lowest members of the series were obtained by 
heating co-hydroxy decanoic acid at 150 in an ordinary distilling flask provided with a 
receiver. Diminished pressure and higher temperatures were used for higher members 
of the series; the highest members (above 10,000) were obtained by heating the hydroxy- 
decanoic acid or its lower polyesters in a molecular still consisting essentially of a suction 
flask into which a water-cooled test-tube was inserted to act as a condenser. The 
products were crystallized several times to increase their homogeneity. Typical pro- 
cedures are indicated below. 

Polyester of Molecular Weight 4170. Twenty grams of the acid was heated at 
150-175 under atmospheric pressure for one and one-half hours, then at 200 at 1 mm. 
pressure for eight hours. The resulting waxy product was crystallized by dissolving in 
a small amount of boiling chloroform and adding several volumes of hot acetone. The 
product had an apparent equivalent weight of 3906. A second crystallization from hot 
acetone by slow cooling raised the equivalent weight to 4170. The ester was obtained 
as a chalky powder, soluble in chloroform and benzene and in hot acetone, ethyl acetate 
and acetic acid. Hot alcohol, ligroin or water did not dissolve it appreciably. 

Anal. Calcd. for HO [(CH 2 ) 9 CO O] 24 H: C, 70.22; H, 10.66; mol. wt. 
4101; saponification equivalent, 170.8. Found: C, 70.35, 70.70; H, 10.69, 10.88; 
mol. wt. by titration, 4114, 4226; saponification equivalent, 170.7. 

Polyester of Molecular Weight 9330. Thirty-six grams of the acid was heated 


for seven hours at 120-175 at 1 mm. pressure, then for twenty-five hours at 225 at 1 
mm. It was crystallized several times from hot acetone by slow cooling and was ob- 
tained as a pure white powder. It did not differ appreciably from polymer 4170 in solu- 

Anal. Calcd. for HO[(CH 2 ) 9 CO O] 66 H: C, 70.39; H, 1066; mol. wt., 
9376; saponification equivalent, 170.5. Found: C, 70.56; H, 10.87; mol. wt. by titra- 
tion, 9335, 9331; saponification equivalent, 169.6. 

Polyester of Molecular Weight 20,700. Fifteen grams of hydroxydecanoic acid 
was heated at 230 in a molecular still for thirty hours. During the first few hours a 
considerable amount of distillate (dimer and unchanged acid) collected on the condenser. 
This was removed and the residue in the flask was stirred frequently. At the end of the 
reaction the mass was plastic when hot. It was purified by dissolving in a small amount 
of hot chloroform, then adding several volumes of acetone and allowing the product to 
crystallize by cooling. This polymer was less soluble than lower members in benzene 
or in hot acetone or ethyl acetate. Solution was slow and was preceded by some 

Anal. Calcd. for HO- [(CH 2 ) 9 CO O ]| 21 H: C, 70.47; H, 10.66; mol. wt.. 
20,605; saponification equivalent, 170.3. Found: C, 70.72, 70.29; H, 10.62, 10.47; 
mol. wt. by titration, 20,380, 20,930; saponification equivalent, 170.8. 

Saponification of the polyesters by alcoholic alkali always resulted in complete 
solution, indicating the absence of any appreciable amount of non-ester ingredient, and 
the following experiment showed that the product of saponification was the initial hy- 
droxydecanoic acid: 0.3097 g. of the 13,600 polyester was saponified with alcoholic 
sodium hydroxide; the alcohol was removed and the solution was acidified and extracted 
with ether. Evaporation of the ether yielded 0.34 g. (99.3% of the theoretical amount) 
of hydroxydecanoic acid. 

During the preparation of the polyesters a small amount of the cyclic dimeric lac- 

I n 

tone, a 22-membercd ring O (CH 2 ) 9 CO O (CH 2 ) 9 CO, was always formed. It 
was obtained in the form of needles from dilute alcohol; m. p. 95-96. 

Anal. Calcd. for C 2 oH 3 6O 4 : C, 70.58; H, 10.67. Found: C, 70.07, 70.04; H, 
10.53, 10.55. 

This lactone has already been prepared by Lycan and Adams (5) indirectly from the 
potassium salt of hydroxydecanoic acid and acetic anhydride followed by dry distilla- 
tion at 400-500 . 


A series of polyesters prepared from w-hydroxydecanoic acid and ranging 
in molecular weight from 780 to 25,200 is described. Strong, oriented 
fibers are obtained only from members having molecular weights above 
9330. The influence of molecular weight on some other physical proper- 
ties is discussed. 

Bibliography and Remarks 

(1) Staudinger, "Die hochmolekularen organischen Verbindungen," Julius Springer, Berlin, 1932. 

(2) Carothers, Journ. Am. Chem. Soc., 51, 2548 (1929). 

(3) Carothers and Arvin, ibid , 51, 25GO (1929). 


(4) Carothers and Hill, ibid., 54, 1559 (1932). 

(5) Lycan and Adams, ibid., 51, 625, 3450 (1929). 

(6) Kraemer and Lansing, Journ. Am. Chem. Soc., 55, 4319 (1933). 

(7) Carothers, Williams, Collins and Kirby, Journ. Am. Chem. Soc., 53, 4203 (1931). 

(8) Carothers and Hill, ibid., 54, 1579 (1932). 

(9) Kraemer and van Natta, J. Phys. Chem., 36, 3175 (1932). 

XIX. Many-Membered Cyclic Anhydrides (1)* 

This paper contains first some definitions: 

Linear polymers are those the molecules of which are built up from a 

recurrring bivalent structural unit. 

Superpolymers are linear polymers having molecular weights above 10,000. 

Unit length is the number of atoms in the chain of the structural unit. 

Macrocyclic compounds are rings of more than seven atoms. 

Monomers are compounds containing only one structural unit. 

Dimers contain two structural units. 

Bifunctional compounds are molecules bearing two groups capable of mutual 


Intramolecular reaction leads to ring closure, 

Intermolecular reaction to chain formation. 

a-Anhydrides] of a series of dicarboxylic acids were prepared by the action 
of acetic anhydride or acetylchloride on 

pimelic acid dodecandioic acid 

suberic acid brassylic acid 

azelaic acid tetradecandioic acid 

undecanedioic acid octadecandioic acid 

They are all linear polymers with molecular weights between 3000 and 
5000. When heated in the molecular still**, they disproportionate, yield- 

(a) <jo-anhydrides (residue). (These arc super polymers with very high 
molecular weight) and 

(b) ^-anhydrides (distillate), which are cyclic monomers or dimers. 

The latter are converted by heat or time into y-anhydrides, which are 
very similar to the a-modification. 

* J. W. Hill and W. H. Carothers; Journ. Am. Chem. Soc., 55, 5023-31 (1933); 
Communication No. 131 from the Experimental Station of E. I. du Pont de Nemours 
and Co. 

Received September 2, 1933. Published December 14, 1933. 

t Compare paper XIV on page 168. 

** See paper XI on page 154. 


Terminology. To clarify the text of subsequent papers the following 
definitions are presented. Some of the terms here defined were first intro- 
duced in previous papers of this series (2), but they have not yet gained 
very wide currency, while in some cases they have been used in a sense 
different from that suggested. 

Linear Polymers, e. g. 

. . . CH 2 CH 2 0CH 2 CH 2 O CH 2 CH 2 O CH 2 CH 2 O CH 2 CH 2 O CH 2 CHoO . . . 

are those whose molecules are built up from a recurring bivalent radical 
or structural unit, e. g., CH 2 CH 2 O . Linear polymers are not neces- 
sarily open chains; they may be rings. Super polymers are linear polymers 
having molecular weights above 10,000. The unit length is the number of 
atoms in the chain of the structural unit ; in polyethylene glycol (or oxide) 
the unit length is three. Macrocyclic compounds are rings of more than 
seven atoms. Monomers are compounds containing only one structural 
unit; ethylene oxide arid ethylene glycol are both monomers, since each 
has one CH^CHaO group. Dimers contain two units: dioxane and 
diethylene glycol are both dimers. Bifunctional compounds are chains 
bearing two groups capable of mutual reaction with the formation of a 
new bond. Bifunctional compounds always formally present two possi- 
bilities of self -reaction : intramolecular leading to ring closure, and inter- 
molecular leading to chain formation. 

Cyclic Anhydrides/ In previous papers, studies of adipic (3) and 
sebacic (4) anhydrides have been reported. The present paper describes 
the extension of these studies to the anhydrides of pimelic, suberic, azelaic, 
undecanedioic, dodecanedioic, brassylic, tetradecanedioic and octade- 
canedioic acids. The results closely resemble those already reported. 

The anhydrides obtained by the action of acetic anhydride or acetyl 
chloride on the acids are linear polymers of the type formula O CO 
R CO O CO R CO O CO RCO . . . These products are 
called a-anhydrides to distinguish them from other forms (/3, 7, cu) that 
originate in other ways. They are very reactive, microcrystalline powders 
which are only slightly soluble in organic solvents and have molecular 
weights in the neighborhood of 3000 to 5000. They react with aniline to 
yield a mixture of the dianilide, the monoanilide and the dibasic acid in 
the ratio 1:2:1 (3, 4). This reaction serves to distinguish any sort of 
polymeric dibasic acid anhydride from the cyclic monomer, which yields 
exclusively with monoanilide. 

When the a-anhydrides are heated in a high vacuum with a condenser 


placed close to the evaporating surface (molecular still) (5), they undergo 
the transformations summarized in the following scheme : 

a- Anhydride 0-Anydride (distillate) 

linear polymer > cyclic monomer or _ dimer 

w- Anhydride (residue) -y-Anhydride 

superpolymer < linear polymer (similar 

mol. wt. very high to a-anhydride) 

The a-anhydride yields /3-anhydride and co-anhydride simultaneously. 
As the 0-anhydride collects on the condenser, the residue increases pro- 
gressively in viscosity and molecular weight. The resulting co-anhydride 
when cold is a very tough, opaque, solid; it becomes transparent (melts) 
at a definite temperature without flowing; and at still higher temperatures 
it can be drawn into pliable, highly oriented fibers (6). The co-anhydride 
depolymerizes just as the a-anhydride does, and if heating in the molecular 
still is continued long enough complete conversion to ^-anhydride finally 
occurs. The /3-anhydrides on being heated or allowed to stand readily 
revert to a polymeric form, the 7-anhydride. In their physical properties 
the 7-anhydrides are generally practically indistinguishable from the a- 
anhydrides. However, the latter structurally are probably open chains 
terminated by acetyl groups while the former at least when freshly formed 
under anhydrous conditions are perhaps giant rings (4) . 

Adipic anhydride differs from the higher homologs in that the a-an- 
hydride depolymerizes relatively easily even in an ordinary distillation 
apparatus. The other anhydrides differ from each other only in the nature 
of the volatile jft-anhydride which they yield on depolymerization. The 
facts in this connection are shown in Table II. It will be observed that 
those anhydrides whose unit lengths are 7, 8, 10, 12, 14, 15 and 19 yield 
monomeric 0- anhydrides and that those whose unit lengths are 9, 11 and 
13 yield only dimeric /3-anhydrides. These relations are represented 
graphically in Table I. 

The three dimeric anhydrides (suberic, sebacic and dodecanedioic) are 
sharply crystalline solids sufficiently stable that their molecular weights 
can be determined cryoscopically in benzene. When allowed to react with 
aniline they furnish the dibasic acid, its monoanilide and its dianilide in 
the ratio 1:2:1. When heated above their melting points the dimeric 
anhydrides polymerize almost instantly to 7-anhydrides. 

The monomeric anhydrides are not sufficiently stable to permit cryo- 


scopic molecular weight determinations, but their identity as monomers 
is established by the fact that they react with aniline to yield pure mono- 
anilide. The first and last members of the series are low-melting crystalline 
solids; the others are all liquids. 

The monomers show characteristic differences in stability. Adipic 
0-anhydride, the seven-membered ring, polymerized completely in about 
seven hours at 100 under rigorously anhydrous conditions and fairly 
rapidly at room temperature in the presence of traces of moisture. The 
8-, 10- and 12-membered monomers polymerize so rapidly even at very 



stable up to Monomers 

melting point Unit length Unstable Extremely unstable 


7 - --- > 7 




10 ----- - -------------- ---------- > 10 



10 ._ ...... _ _______ y 19 

LA -- ~ ' " " ~ iA 



14 - ----- - - -- > 14 

(let radecanedioic) 

15 ---------- > 15 

19 ---- - - -- - ---- > 19 

low temperature that a special technique was required to demonstrate 
their temporary existence. When distilled in the molecular still onto a 
condenser cooled by tap water, they condensed as transparent fluid drops, 
which in the course of an hour or less became opaque, and in several hours 
set to hard waxes. Distillate collected under these conditions invariably 
contained polymer, since its reaction with aniline always yielded dianilide. 
In the method devised, the condenser was cooled by means of liquid air. 
At this temperature the monomers condensed as solids. When, after 
several hours, a sufficient quantity of distillate had collected, a small 



reservoir of aniline (Fig. 1) was connected with the vacuum system by 
opening a stopcock, and a layer of crystalline aniline was deposited by 
evaporation on top of the anhydride distillate. The condenser was then 
allowed to warm up gradually, and as soon as the aniline started to melt, 
was quickly removed from the still and the lower part, bearing the distil- 
late, was submerged in aniline. When the reaction was completed, the 
mixture was examined for dianilide and monoanilide. 

When this method was ap- 
plied to pimelic anhydride (8- 
atom unit) it yielded pure 
monoanilide. The next two 
members, azelaic and unde- 
canedioic anhydrides (10- and 
12-atom units) gave mixtures 
of acid, monoanilide and di- 
anilide. However, the pres- 
ence of some monomer in the 
distillate was established by 
the fact that the ratio of mono- 
anilide was considerally higher 
than that required by the 
Fig. 1 . theory for dimer or other poly- 

mer. Moreover, when the con- 
denser was operated at a higher temperature, these anhydrides first collected 
as transparent fluid drops with no evidence of the presence of crystalline 
material (dimer). It seems very probable therefore that the distillates 















Structural unit 

of depolymerization 

of anhydride 

and size of ring 

OC(CH 2 ) 4 CO--O 

Monomer 7 1 

OC(CH 2 ) 6 O>- O 

Monomer 8 1 

OC(CH 2 ) 6 CO O 

Dimer 18 

OC(CH 2 ) 7 CO O 

Monomer 10 ] 

OC(CH 2 ) 8 CO 

Dimer 22 

OC(CH 2 ) 9 CO 

Monomer 12 ] 

OC(CH 2 )i CO O 

Dimer 26 J 

OC(CH 2 ) n CO 

Monomer 14 1 

OC(CH 2 ) 12 CO O 

Monomer 15 1 

OC(CH 2 ) 16 CO 

Monomer 19 1 

Stability M. p., C. 

Unstable 20 

Extremely unstable Liq. 

Stable up to m. p. 56-57 

Extremely unstable Liq. 

Stable up to m. p. 68 

Extremely unstable Liq. 

Stable up to m. p. 76-78 

Unstable Liq. 

Unstable Liq. 

Unstable 36-37 


were initially pure monomers and that the presence of dianilide in the 
aniline reaction product was due to subsequent polymerization. 

The next three members of the monomer series were much more stable 
than those of 10- and 12-atoms. Brassylic anhydride (14-atom unit) 
yielded pure monoanilide when the condenser was cooled with solid carbon 
dioxide and acetone. The liquid distillates from the 15- and 19-membered 
anhydrides (tetradecanedioic and octadecanedioic) when condensed at 
the temperature of tap water yielded pure monoanilide if allowed to react 
with aniline within a few hours of distillation. These two monomeric 
anhydrides, like that of adipic acid, could also be preserved for several 
days at room temperature without complete polymerization. 

Experimental Part 

Preparation of a-Anhydrides. The a-anhydrides were prepared by the following 
method. A small quantity of the acid (5 to 20 g.) was refluxed with three parts by 
weight of acetic anhydride for four to six hours. In the case of the less soluble octa- 
decanedioic acid six parts of acetic anhydride was used. The volatile material was dis- 
tilled off under the vacuum of a water aspirator. The crude anhydrides were dissolved 
in hot dry benzene and precipitated, after nitration of the solutions, with petroleum 
ether. They were then preserved in a vacuum over potassium hydroxide, phosphorus 
pentoxide and paraffin. The a-anhydrides separate from solution as white micro- 
crystalline powders. In the molten state they are very viscous liquids which crystallize 
on cooling in the form of small radiating clusters of microscopic needles and finally 
solidify to hard waxes. 




C 7 H 10 3 
C 8 H 12 3 
C 9 H 14 3 
C n H 18 3 
C 12 H 20 3 

M. p., 


C H 

59.11 7.11 
61.50 7.75 
63.49 8.29 
66.60 9.18 
67.86 9.52 










Brassylic Ci 3 H 22 O 3 76-78 68.97 9.81 68.80 68.98 9.96 10.05 

Tetradecanedioic C 14 H 24 O 3 89-91 69.94 10.08 69.17 69.32 9.94 9.79 
Octadecanedioic Ci 8 H 32 O 3 94-95 72.90 10.90 72.69 72.02 10.69 10.49 

Reaction of the Anhydrides with Aniline. A small sample of the anhydride (ca. 
2 g.) was added to 5 to 10 cc. of aniline and triturated with a stirring rod until a smooth 
cream was formed. The mixture was then treated with 10% hydrochloric acid to dis- 
solve the excess aniline, cooled and filtered. The mixtures derived from the anhydrides 
below undecanedioic were separated by treatment with boiling water, in which the 
dianilides are completely insoluble. The monoanilides separated from the filtrate on 
cooling. Sufficient water was used to keep the dibasic acids in solution. The separa- 
tion of the mixtures from the anhydrides above sebacic was accomplished by means of 



dilute aqueous sodium hydroxide, which dissolved the monoanilide and the dibasic acids 
as sodium salts and left the dianilide. The monoanilide and the dibasic acid were pre- 
cipitated from the filtered solution by acidification, and separated by means of boiling 
water. In the cases of tetradecanedioic and octadecanedioic anhydrides, the alkaline 
separation was carried out hot, as the sodium salts of these monoanilides are difficultly 
soluble in the cold. Up to and including undecanedioic a-anhydride, the dianilide was 
estimated quantitatively and in each case was obtained in 25% yield, as required by 
theory. A clean separation was difficult with the higher homologs as the alkaline solu- 
tions were increasingly soapy. The properties of the anilides are given in Table IV. 






YO * 


M. p., C. 

Cryst. from 








H 2 O 










CnHi 9 O 3 N 








Dil. EtOH 

CuH 2 iO 3 N 











50% EtOH 

Ci7H 2 *O 3 N 











50% EtOH 












50% EtOH 












60% EtOH 

C 2 oH 31 3 N 











60% EtOH 



















155-156 MeOH-H 2 O CiH 22 OtN 2 73.50 

186-187 MeOH CsoH^Na 74.03 

186-187 Xylene C2iH 26 O 2 N 2 74.51 

160-161 EtOH C 23 H 30 O 2 N 2 75.35 

170-171 EtOH C 2 4H 32 O 2 N 2 75.73 

160-161 EtOH Q*H 3 4O 2 N 2 76.08 

169.5-170 EtOH C 2 6H 3 eO 2 N 2 76.41 

162-163 EtOH C 3 oH 4 4O 2 N 2 77.52 




8.27 75.44 75.32 

8.49 76.28 76.09 

8.70 75.86 76.16 

8.89 76.41 ... 

9.56 77.38 77.64 

73.16 ... 
73.87 73.95 
74.03 ... 

8.91 .. 
9.76 9.78 



Depolymerization. The molecular still (Fig. 1) used in these experiments was a 
cylindrical vessel, 18 cm. high and 5 cm. in diameter, with a rather flat curved bottom 
and an outlet to a high vacuum system. Into this vessel was fitted, by means of a large 
ground joint at the top, another cylinder 3 cm. in diameter, the rounded bottom of which 
was 3 cm. from the bottom of the larger vessel. This inner vessel constituted the con- 
denser. A sample of a-anhydride was introduced into the outer vessel, the condenser 
was placed in position and the apparatus was evacuated by means of a mercury diffusion 
pump backed by a Hyvac oil pump. A trap cooled by solid carbon dioxide in acetone 
was placed in the system. The still was heated by means of a metal bath. 

Pimelic Anhydride. The a-anhydride was heated at 150. The 0-anhydride 
collected on the water-cooled condenser as a dew which solidified to a pasty gel in about 
one hour and to a hard wax overnight (7-anhydride). The saponification equivalent 
of the wax was 70.7 [(C 7 HioO 3 ) n requires 71.0]. The experiment was repeated with 
liquid air in the condenser and the heating bath at 125. The heating was continued 
for six hours, after which the still was isolated from the vacuum system by closing a stop- 
cock and put in communication with a small evacuated bulb containing aniline by open- 
ing a stopcock (see Fig. 1). Aniline was distilled into the still until a layer of crystalline 
aniline completely covered the distillate on the condenser. To accomplish this, it was 
necessary to let the liquid air evaporate almost completely from the condenser. The 


liquid air was finally allowed to evaporate completely. As soon as the condenser had 
warmed up to a point where the aniline started to melt, the vacuum was relieved and the 
condenser removed and immersed in aniline to a level slightly above the coating of 
distillate. When the mixture had come to room temperature, it was treated with dilute 
hydrochloric acid. Complete solution indicated the absence of any dianilide in the 
reaction product. The solution was made alkaline and extracted three times with ether. 
It was then concentrated and acidified to Congo Red. The precipitate was filtered off 
and treated carefully with cold water to remove sodium chloride. The residue of mono- 
anilide was recrystallized from water; m. p. 107-108. It did not depress the melting 
point of an authentic sample of melting point 108-109. The absence of any dianilide 
in the aniline reaction product demonstrates that the 0-anhydride was monomeric. 

Suberic Anhydride. A sample of a-anhydride was heated at 160 in the molecular 
still. The )3-anhydride collected as a crystalline solid 011 the water-cooled condenser 
at the rate of about 0.1 g. a day. It melted at 55-57. This compound proved to be 
the cyclic dimer of suberic anhydride. 

Anal. Calcd. for Ci 6 H 2 4O 6 : C, 61.50; H, 7.75; mol. wt., 312.2; saponification 
equiv., 78.0. Found: C, 61.57, 61.67; H, 7.72, 7.89; mol. wt. (cryoscopic in benzene), 
343, 355, 346; saponification equiv., 77.5. 

It reacted with aniline to form the monoanilide (m. p. 125-127) and the dianilide 
(m. p. 186-187) of suberic acid, which were identified by mixed melting points. When 
heated above its melting point it rapidly polymerized to the 7-anhydride, a waxy solid 
melting at 65-68; mol. wt. observed in boiling benzene, 703, 718. 

Azelaic Anhydride. The a-anhydride was heated in the molecular still at 150. 
The /3-anhydride collected on the water-cooled condenser as a liquid and soon solidified 
to a gel and then in about twelve hours to a wax (7-anhydride) . The observed saponifica- 
tion equivalent of the wax was 84.8, and 84.5 [(CsHnOa).,. requires 85]. Two runs were 
made in which the condenser was cooled with liquid air and the cold distillate was 
treated with aniline as described under pimelic anhydride, but products free of dianilide 
were not obtained. The amounts of the two anilides were determined quantitatively, 
taking advantage of the insolubility of the dianilide in hot water and the insolubility of 
the monoanilide in cold. 

(1) Four hours: Dianilide, 0.18 g.; monoanilide, 0.35 g. A polymeric anhydride 
yields 2 moles of monoanilide for one ot dianilide ; a monomeric anhydride yields only 
monoanilide. The dianilide found indicates an amount of polyanhydride that could 
produce only 0.28 g. of monoanilide. The excess, 0.07 g., must arise from monomeric 
anhydride, and this amount corresponds with 11% of monomer in the total anhydride 

(2) Six hours: Dianilide, 0.22 g., equivalent to 0.34 g. of monoanilide (from poly- 
anhydride). Monoanilide found, 0.49 g. ; excess due to monomeric anhydride, 0.15 g., 
equivalent to 18% of monomer in the total anhydride sample. 

Since there was no evidence of any dimer (which should be a crystalline solid) in the 
distillate, the failure to demonstrate that the distillate was initially pure monomer was 
probably due to the fact that the latter polymerized to a considerable extent before the 
examination could be completed. 

The distillate if removed when still liquid or pasty, had a spicy, aromatic odor, 
This disappeared when the distillate solidified completely. 

1,11-Undecanedioic Anhydride. The a-anhydride was heated in the molecular 


still with a water-cooled condenser. The /3-anhydride condensed first as a dew which 
soon became pasty and finally after a few hours set to a hard wax, m. p. 85-88. Before 
complete solidification it had a spicy odor. The saponification equivalent of the wax 
was 98.8 [(CnHi 8 O 3 ) n requires 99.1]. The experiment was repeated using liquid air in 
the condenser and the aniline technique described under pimelic anhydride. As the 
reaction product was found to contain dianilide, the amounts of dianilide and mono- 
anilide formed were estimated quantitatively. The product from the aniline reaction 
was treated with a slight excess of warm dilute aqueous sodium hydroxide and the in- 
soluble dianilide was filtered off and weighed (0.079 g.). The filtrate was acidified, 
cooled and filtered (weight of crystalline precipitate 0.281 g.). This material was shown 
by a determination of the neutralization equivalent to be pure monoanilide (calcd. 291; 
found, 289). The dianilide found (0.079 g.) indicates an amount of polymeric anhydride 
capable of yielding only 0.125 g. of monoanilide. The excess of the latter (0.156 g.) 
must have come from monomeric anhydride, and calculation indicates 38% of mono- 
mer ic anhydride in the mixture. 

1,12-Dodecanedioic Anhydride. The a-anhydride was heated hi the molecular 
still at 110. A beautifully crystalline distillate of melting point 76-78 collected on 
the condenser at the rate of about 0. 1 g. in two days. This /3-anhydride was identified 
as the cyclic dimeric anhydride of dodecanedioic acid. 

Anal. Calcd. for C 2 4H 4 oOe: C, 67.86; H, 9.52; mol. wt., 424.4; saponification 
equiv., 106.1. Found: C, 67.15, 67.54; H, 9.46, 9.60; mol. wt. (cryoscopic in ben- 
zene), 496, 450; saponification equiv., 105.5. 

It polymerized at the melting point and then melted again at 85-87. It reacted 
with aniline in the usual way to yield monoanilide of m. p. 123 and dianilide of m. p. 

Brassylic Anhydride. The a-anhydride was heated in the molecular still for four 
hours at 150 with the condenser cooled with solid carbon dioxide and acetone. The dis- 
tillate of ^-anhydride was shown to be pure monomer by its reaction with aniline. The 
product was completely soluble in dilute aqueous sodium hydroxide. The monoanilide 
separated on acidification of the alkaline solution and was recrystallized from 50% 
alcohol. It melted at 118.5-119.5 and showed no depression when mixed with an 
authentic sample. 

When the depolymerization of the anhydride was carried out using a water-cooled 
condenser, the /3-anhydride collected as a liquid which changed in the course of a day to 
a pasty mass and in the course of two days to a hard wax (saponification equivalent 
found 112.7, calculated 113.1). The liquid was fragrant and aromatic and when al- 
lowed to react with aniline shortly after distillation yielded only a small amount of 
dianilide. The odor disappeared during the change to the wax. 

1,14-Tetradecanedioic Anhydride. The a-anhydride was heated four and one-half 
hours at 145-150 in the molecular still using solid carbon dioxide in acetone as the 
refrigerant in the condenser. The distillate of /3-anhydride was shown to be pure mono- 
mer by its reaction with aniline. The product was completely soluble in a large volume 
of warm, dilute sodium hydroxide and consequently consisted of monoanilide free of 
dianilide. The monoanilide separated on acidification of the alkaline solution and was 
recrystallized from 60% alcohol. It melted at 124-125 and did not depress the melting 
point of an authentic sample. 

When the depolymerization of the anhydride was carried out using a water-cooled 


condenser, the distillate, like that from brassylic anhydride, collected as a liquid which 
changed in the course of a day to a pasty mass and in the course of two to a hard wax, 
m. p. 88-91. The distillate, before it changed completely to the hard wax, possessed 
a strong odor like that of musk. This odor was completely lost on saponification 
(saponification equivalent found, 120.4, 119.1; calcd., 120.1) and on heating to 100 
which brought about the change to the 'y-anhydride instantly. 

1,18-Octadecanedioic Anhydride. The behavior of this a-anhydride was identical 
in every respect with that of tetradecanedioic anhydride. The fresh liquid distillate like- 
wise possessed the odor of musk but was rather fainter. It polymerized instantly on 
being heated to 100, to a wax of m. p. 98-100 (7-anhydride), losing its odor in the 
change. About 1 cc. of /3-anhydride was collected using a still adapted for the collection 
of liquids. It crystallized on chilling or seeding and melted at 36-37 (saponification 
equivalent found, 146.5; calcd. for CigHa^Oa, 148.1). After two weeks it had changed 
to the wax of m. p. 98-100. 


Data are presented on the anhydrides of dibasic acids COOH(CH 2 ) M - 
COOH where n is 4, 5, 6, 7, 8, 9, 10, 11, 12 and 16. The anhydrides are 
all linear polymers and when heated in a molecular still they are depoly- 
merized yielding volatiles products (0-anhydrides). The latter are either 
cyclic monomers or dimers depending upon the unit length, n + 3. The 
compounds thus obtained are rings of 7, 8, IS (dimeric) 10, 22 (dimeric), 
12, 26 (dimeric), 14, 15 and 19 atoms. The dimers are crystalline solids 
which polymerize instantly when heated above their melting points. 
The monomers are liquids or low melting solids which polymerize at lower 
temperatures than the dimers. The monomers of 8, 10 and 12 atoms are 
exceedingly unstable and polymerize rapidly even below room temperature. 

Bibliography and Remarks 

(1) An abstract of papers XIX, XX, XXI and XXII was presented at the Washington meeting? 
of the American Chemical Society, March 28, 1933. 

(2) Carothers, Journ. Am. Chem. Soc. t 51, 2548 (1929); Carothcrs, Chem. Rev., 8, 353 (1931); 
Carothers and Hill, Journ. Am. Chem. Soc., 54 1559 (1932). 

(3) Hill, ibid., 62, 4110 (1930). 

(4) Hill and Carothers, ibid., 54, 1569 (1932). 

(5) Cf. Carothers and Hill, Journ. Am. Chem. Soc., 64, 1557 (1932). The Washburn type of still 
was used in this work. 

(6) Carothers and Hill, ibid., 54, 1579 (1932). 


XX. Many-Membered Cyclic Esters* 

Only one general method of producing cyclic esters is described in the 
literature and this gives very low yields. In the present study a new and 
very effective way for the preparation of cyclic esters has been found. 
If linear polyesters are heated in vacuo and in the presence of a catalyst 
just below their point of thermal destruction, the monomer or the dimer 
cyclic ester is formed in good yield. Sodium or other ester interchange 
catalysts are most effective. No fewer than thirty new cyclic esters are 
described; in some cases (ethylene carbonate, decamethylene malonate, etc.) 
they were exclusively monomeric, in others (pentamethylene carbonate) they 
were only dimeric, in others again (tetramethylene carbonate, etc.) they 
were mixtures between monomers and dimers. The ratio in which the two 
forms are obtained is determined in part by the experimental conditions 
and in part by the nature of the ester, especially by its unit length. 

The only generally applicable method known for the synthesis of macro- 
cyclic esters consists in oxidation of cyclic ketones with Caro's acid. This 

""** (( r H>)n ? == 

I II (n = 12, 13, 14, 15, 16) 

method, discovered by Baeyer and Villiger (1), has been applied by Ruzicka 
and Stoll (2) to the synthesis of the lactones, II. 

The reaction is far from clean-cut, while the requisite ketones (I) are 
obtained in yields not exceeding 5% (3). The necessity for adopting this 
devious and extravagant method for synthesizing macrocyclic lactones is 
due to the fact, repeatedly illustrated in previous papers (4) of this series, 
that bifunctional esterifications involving unit lengths greater than seven 
yield almost exclusively linear polyesters instead of the desired cyclic 
esters. When linear polyesters of unit length greater than seven are heated, 
e. g., to 275, there is generally no evidence of depolymerization or distilla- 
tion (5), but at temperatures somewhat above 300 destructive general de- 
composition occurs, and from the complicated mixture of distillable prod- 
ucts in four cases (6) true depolymerization products have been isolated. 

* J. W. Hill and W. A. Carothers; Journ. Am. Chem. Soc., 55, 5031-39 (1933); 
Communication No. 132 from the Experimental Station of E. I. du Pont de Nemours 
and Co. 

Received September 2, 1933. Published December 14, 1933, 


These cyclic esters however are not monomers but dimers, and the yields 
are very poor (5% or less). 

When linear polyesters of unit length greater than seven are heated 
(e. g., to 200-250) under the conditions of molecular distillation (7), the 
long chains couple to form still longer chains (8). The products (super- 
polyesters) have molecular weights above 10,000 and are capable of being 
drawn out into tough, pliable, highly oriented fibers (9). 

It was recognized in advance that the conditions of the molecular still 
at high temperature would be likely to favor either this type of coupling 
or a depolymerization by ester interchange (10) : 


Evidences of such depolymerization, at least in traces, were in fact fre- 
quently observed in the formation of superpolyesters, and after methods 
had been developed for the smooth depolymerization of poly anhydrides, 
attention was again turned to the polyesters. The result was the develop- 
ment of the method presently described by means of which it became pos- 
sible to prepare with good yields and without unreasonable difficulty a 
whole series of macrocyclic esters, both monomeric and dimeric. Briefly, 
the method consists in heating linear polyesters in vacuo at a temperature 
just below the point of thermal destruction under conditions that permit 
any volatile product to be removed by distillation as fast as it is formed. 

The success of the new method, practically, depends upon numerous fac- 
tors not all of which have yet been completely isolated and defined; the 
two most important however are the identity of the polyester and the use 
of catalysts. On the first point it may be said that esters of carbonic acid 
and of oxalic acid depolymerize more readily and smoothly than any others 
yet examined. On the second point, ester interchange catalysts (e. g., 
sodium added as metal in the preparation of the initial polyester) are al- 
most indispensable. 

After some experimentation it was discovered that esters of carbonic acid 
and of oxalic acid can be depolymerized merely by heating them (with cata- 
lyst) under diminished pressure in an ordinary distilling flask. Most of 
the experiments now reported were however made in a simplified molecular 
still (described below) operated at pressures ranging from 0.1 to 2 or 3 mm. 

The depolymerization of the polyester in favorable cases progresses 
smoothly and fairly rapidly, the cyclic ester distilling to the condenser 
and being collected as it is formed. The residual ester at the same time 
progressively increases in viscosity (formation of superpolymer) ; if this 


increase in viscosity does not proceed too far, depolymerization still con- 
tinues and is ultimately almost complete and quantitative. In many 
cases, however, the residue, before complete depolymerization, becomes con- 
verted to a gel; even at 250 it is completely immobile and resembles a 
piece of porous, vulcanized, gum rubber. Depolymerization then practi- 
cally ceases. This residue incidentally is extraordinarily resistant to attack 
by solvents and chemical agents generally. It evidently results from a 
polymerization which has progressed beyond the linear superpolymer 
stage. It seems likely that some quantitatively minor side-reaction 
(dehydrogenation at a venture) occurs permitting accessory polymeriza- 
tion to form a three-dimensional macromolecule. 

The product of the depolymerization is generally a mixture of cyclic 
monomer and cyclic dimer although in many cases one of these forms pre- 
dominates to the practical exclusion of the other. The ratio of the two 
forms is in some cases quite sensitive to the experimental conditions 
(temperature, pressure, shape of apparatus) but the data on this point 
recorded in Table I were all obtained under closely similar conditions. 

Preparation of Polyesters. Polymeric carbonates derived from the 
glycols HO(CH 2 ) M OH where n is 3, 4, 6 and 10 and from diethylene glycol 
have already been described (11). The new polymeric carbonates pre- 
pared as intermediates in the present work include those derived from the 
glycols HO(CH 2 ) M OH where n is 5, 7, 8, 9, 11, 12, 13, 14 and 18, and from 
diethylene and triethylene glycols. The fact that these compounds are 
lacking in crucial physical properties (e. g. t sharply characteristic melting 
points) makes it unnecessary to describe them individually. They are all 
insoluble in water and soluble in certain organic solvents such as chloro- 
form. They separate from solvents in the form of soft powders (micro- 
crystalline) . The molten polyesters are very viscous liquids which crystal- 
lize on cooling in the form of small radiating clusters of microscopic needles 
or spherulites and finally solidify to more or less hard, tough waxes. The 
polymeric carbonate derived from triethylene glycol, however, failed to 
solidify and was obtained only as a thick sirup. 

The general method of preparation used for the new polycarbonates was the same 
as that already described (11), but, on account of the boiling points involved, dibutyl 
carbonate was found to be a more convenient source of the acid radical than diethyl car- 
bonate. To the mixture of glycol and alkyl carbonate a small amount of sodium was 
added; the mixture was heated at 170-220 until the distillation of alcohol ceased, and 
removal of the alcohol was completed by continuing the heating for two hours or more 
in vacuo. The residue was usually depolymerized without any purification. The 
alkylene oxalates and malonates were prepared from the glycols and the diethyl esters of 



^ ^ 

I H IQ O f^* 

liii iiil ill! 


- -i i-i O -OOO ^tHi-4*-* 



o i 



Q "^ 8 S 

i CO O ' 










a Ss 

to a < 



c* <M 

t~ ^ ^ 

7 crT d-^ d- 

S 3 33 S3 3 

I I 1 

* o-s-s h g 
s S o s S " 
8 ? ? ? I 4 S 


s lf 











the acids in the same manner as the carbonates. The other esters were prepared di- 
rectly from the acids and the glycols. 

Depolymerization. The apparatus used in the depolymerizations for which data are 
listed in Table I consisted simply of a 250-cc. Pyrex suction filter flask. Through a 
rubber stopper in the neck of the flask was inserted a test-tube about 1.5 cm. in di- 
ameter. Its bottom was about 3 cm. distant from the bottom of the flask. The outside 
of this test-tube served as the condenser and receiver; the inside was cooled with a 
stream of tap water (in some cases a mixture of solid carbon dioxide and acetone) ; the 
distillate collected on the outside. At intervals the test-tube was removed and the solid 
or pasty distillate was scraped from it. When the distillate was sufficiently fluid to flow, 
a small glass thimble was hung on the end of the test-tube to catch the drip. 

The side-tube of the flask was connected to a vacuum line. This line was connected 
to an oil pump and the pressure in the line was about O.I to 2 mm. No attempt was 
made to secure an exceedingly high vacuum. The flask was immersed to a depth of 
about 2 cm. in a metal bath which was kept at 210-240 by means of an electric heater. 

Nature of the Depolymerization Products. For the sake of complete- 
ness Table I includes the physical constants of ethylene and trimethylene 
carbonates. The former, a 5-atom ring, is known only as the monomer; 
the latter, which has a unit length of 6 atoms, can be obtained either as 
monomer or polymer, but the polymer depolymerizes so rapidly and 
smoothly on distillation that the use of a molecular still is quite unneces- 
sary (11). 

The rest of the compounds listed in Table I are obtained by the ordi- 
nary methods of preparation only as linear polyesters ; and the polyesters 
were all depolymerized in the apparatus and under the conditions de- 
scribed above. 

The behavior of tetramethylene carbonate (7-atom unit) was exceptional : 
the distillate was found to contain a considerable amount of tetrahydro- 
furan, which obviously might arise by the loss of carbon dioxide from the 
structural unit. The chief product, however, was the dimer, a 14-mem- 
bered ring which has already been described (11). No detectable amount 
of monomeric ester was formed. The products from the next five mem- 
bers of the series (unit lengths 8 to 12 atoms) also were chiefly dimeric. 
Pentamethylene carbonate (S-atom unit) gave no detectable amount of 
monomer. The presence of some monomer in the distillate from hexa- 
methylene carbonate was inferred from the odor, but the attempt to 
obtain isolable amounts of monomer by carrying out the depolymerization 
in ordinary distillation equipment gave only indefinite unsaturated prod- 
ucts. The odors of the distillates from heptamethylene, octamethylene 
and nonamethylene carbonates also indicated the presence of monomer, 
and in the last two of these cases monomer was actually isolated and id en- 


tified when the depolymerization was carried out in ordinary distillation 

The next member of the series, decamethylene carbonate, showed a sharp 
change in the nature of the depolymerization products: considerable 
amounts of the monomer were formed even in the molecular still. Undeca- 
methylene carbonate (14-atom unit) behaved similarly. Beyond this 
point the ratio was reversed: dodecamethylene carbonate (15-atom unit) 
gave only a small amount of dimer, and with higher members of the 
carbonate series the isolated products were exclusively monomeric. 

The oxalates (decamethylene and undecamethylene) showed a greater 
tendency to yield monomers than did the carbonates of the same unit 
length. Decamethylene malonate also gave almost exclusively the 

The rest of the compounds listed in Table I, compared with the car- 
bonates, exhibited a peculiar reluctance toward the formation of monomers. 
Even ethylene tetradecanedioate yielded a 36-membered dimer and no 
appreciable amount of the 18-membered monomer. Similarly co-hydroxy- 
pentadecanoic acid yielded the 32-membered dimer instead of the 18- 
membered monomer which has already been obtained by oxidation of the 
corresponding cyclic ketone (12). It should be added that the depoly- 
merization of all the esters referred to in this paragraph is especially slow 
and difficult. Depolymerization of carbonates, oxalates, and even malo- 
nates, proceeds much more rapidly and completely. 

Effect of Conditions on the Ratio of Monomer to Dimer. The data on 
the carbonates listed in Table I indicate that polyesters of this series on 
depolymerization yield dimers almost exclusively when the unit length is 
7 to 12 inclusive, both monomer and dimer when the unit length is 13 
and 14, and monomers almost exclusively when the unit length is more than 
14. It will be shown later that this characteristic change in the nature of 
the products under a given set of conditions is probably due to stereo- 
chemical factors inherently associated with the structure of the cyclic 
esters. It is also true, however, that the ratio of monomer to dimer can 
be controlled within certain limits by the experimental conditions. Thus 
octamethylene and nonamethylene carbonates in the modified molecular 
still yield almost exclusively the cyclic dimers, but when the depolymeriza- 
tion is conducted in an ordinary distilling flask the product is almost en- 
tirely the monomer. The distilling flask (unlike the molecular still) 
permits refluxing, so that while most of the monomer escapes most of the 
dimer is returned to the residue, where it is either cracked to monomer or 
converted to higher polymer. Limitations of this method of experimental 


control are indicated by the fact that when it was applied to hexamethylene 
carbonate no smooth depolymerization occurred and only indefinite un- 
saturated products were obtained. Esters of dibasic acids above malonic 
also depolymerize very slowly if at all in an ordinary distilling flask. 

The depolymerization of polymeric decamethylene carbonate in an 
ordinary distilling flask gave a much higher yield of monomer than dimer 
and the examination of a large sample of crude product showed the pres- 
ence of a by-product, decen-9-ol-l, CH 2 =CH(CH 2 )7CH 2 OH. The fol- 
lowing data are typical. 

The depolymerization was conducted in ordinary distilling flasks in batches of vari- 
ous sizes up to 100 g. and at (bath) temperatures up to 290. The product was a mix- 
ture of colorless liquid and crystalline material. The crystals (a mixture of dimeric 
carbonate and decamethylene glycol) were removed by filtration, and the filtrate (2425 
g.) was distilled, yielding 1276 g. of pure monomer, 400 g. of crystalline residue (chiefly 
dimer) and 742 g. of low-boiling material. The latter by redistillation was separated 
into four fractions of which the two largest were: A, b. p. 76-81 (2 mm.), 81%, and 
B, b. p. 87-89 (2 mm.), 9%. The latter was practically pure monomer. Analytical 
data for B indicated that it was a mixture of decamethylene carbonate (monomer) 
and decenol (apparently a constant-boiling mixture). The ester was destroyed by 
saponification with alcoholic sodium hydroxide, and the recovered alcohol was purified by 

Decen-9-ol-l. B. p. 85-86 (2 mm.); w 2 D 1.4480; 4 0.8446; M D calcd., 49.53; 
M D found, 49.45. 

Anal. Calcd. for C 9 H 18 O: C, 76.9; H, 12.8. Found: C, 76.51; H, 13.09. 

Its structure was established by the fact that it was oxidized to azelaic acid by neu- 
tral aqueous permanganate (in one experiment a small amount of suberic acid was found) 
and hydrogenated to decanol-1 which was identified by comparison of the phenyl- 
urethan with a known specimen (crys. from 80% alcohol, m. p. 61-62). 

Decen-9-ol-l yields a phenylurethan of m. p. 49-50 (crys. from 80% alcohol). 

Polymerization of Macrocylic Esters. Macrocyclic esters, unlike 6- 
membered cyclic esters (11), show no tendency to polymerize sponta- 
neously. They do, however, polymerize at elevated temperatures especially 
in the presence of catalysts for ester-interchange, and the following ob- 
servations are typical. 

Small samples of monomeric decamethylene, dodecamethylene, tridecamethylene, 
and tetradecamethylene carbonates were heated for twenty hours at 200 together with 
parallel samples each containing a trace of potassium carbonate. The uncatalyzed 
samples of tridecamethylene and tetradecamethylene carbonates showed a considerable 
increase in viscosity; the decamethylene and dodecamethylene carbonates a much larger 
increase. The catalyzed samples of decamethylene and dodecamethylene carbonates 
were completely changed to solid, waxy polymers; while the catalyzed samples of the 
other two esters were still semi-solid. 

Samples of dimeric hexamethylene and decamethylene carbonates, both with and 


CO O O CO 00 00 rl r-f -l ^1 

co -< c< >o eo r- co id 00 o> 

EC ^ ^ ^ ( 





I K Ti S 
J- O 3 2 . 


1^ ! I giBB SiT 8t| 


S S 8 a 



f|fi-si i l'lllllllll|5|l3^;:;-:;i2 





without traces of potassium carbonate, were heated for eight hours at 200. The 
catalyzed samples changed to very viscous liquids which solidified to hard waxes; the 
uncatalyzed samples were unchanged. 


By heating linear polyesters with catalysts under certain conditions it 
is possible in many cases to bring about a smooth depolymerization to the 
corresponding monomeric and/or dimeric esters. This method makes it 
possible for the first time to prepare macrocyclic esters in good yields. 
Thirty new macrocyclic esters are described, mostly esters of dibasic acids. 
The cyclic carbonates and oxalates are obtained most easily. The ratio 
in which the two forms, monomer and dimer, are obtained is determined 
in part by the experimental conditions and in part by the nature of the 
ester especially by its unit length. Monomers of 7 to 1 2 atoms are espe- 
cially difficult to obtain. 

Bibliography and Remarks 

(1) Baeyer and Villiger, Ber., 32, 3625 (1899). 

(2) Ruzicka and Stoll, Ilelv. Chim. Acta, 11, 1159 (1928). 

(3) Since this paper was prepared for publication Ziegler, Eberle and Ohlinger [Ann., 504, 94 
(1933)] have described an ingenious method based on Ruggli's dilution principle for the preparation 
of large cyclic ketones in good yields This principle has been referred to in previous papers of the 
present series [e g , Journ. Am Chem. Sac., 51, 2551 (1929)] and we hope to describe its application 
to the synthesis of cyclic esters in future papers. 

(4) Carothers, Journ Am Chem. Soc , 51, 2548 (1929); Carothers and Arvin, ibid, 52, 711; 
Carothers, Arvin and Dorough, ibid , 3292; Carothers, Chem. Reviews, 8, 353 (1931). Cf. Chuit and 
Hausser, Helv. Chim Acta, 12, 463 (1929), Lycan and Adams, Journ. Am. Chem Soc., 51, 625, 3450 

(5) Compounds of molecular weight higher than 1000 cannot be distilled even under the highest 
vacua, Carothers, Hill, Kirby and Jacobson, Journ Am. Chem. Soc., 62, 5279 (1931). 

(6) The cases referred to are ethylene succinate, Tilitschejew, J. Russ Phys -Chem. Soc , 57, 143 
(1925), Carothers and Dorough, Journ. Am Chem. Soc , 52, 711 (1930); tetramethylene carbonate, 
Carothers and van Natta, ibid , 52, 314 (1930), trimethylene oxalate, Tilitschejew, /. Russ. Phys - 
Chem. Soc , 58, 447 (1926); Carothers, Arvin and Dorough, Journ. Am. Chem. Soc , 52, 3292 (1930); 
self-ester of hydroxydecanoic acid, Lycan and Adams, ibtd , 51, 625, 3450 (1929). 

(7) Carothers and Hill, ibid , 54, 1557 (1932). 

(8) Ibid., p. 1559. 

(9) Ibid., p. 1579. 

(10) Cf. the discussion of 6-membered cyclic esters and their polymers in Carothers, Dorough and 
van Natta, ibid., 54, 761 (1932). 

(11) Carothers and van Natta, Journ. Am. Chem Soc , 52, 314 (1930). 

(12) Ruzicka and Stoll, Helv. Chim. Acta, 11, 1159 (1928). 


XXI. Physical Properties of Macrocyclic Esters and 
Anhydrides. New Types of Synthetic Musks* 

L. Ruzicka has shown that some large ring lactones have very valuable 
properties as perfume ingredients; this paper investigates the odor as a 
function of the molecular structure of multi-membered rings. 
Rings of 5 and 6 atoms smell like bitter almonds and menthol; from 9 to 
12 they exhibit the odor of camphor; 14 and 15 give musk, 17 and 18 civet. 
The molecular refractions and melting points of the carbonate and oxalate 
series are determined. 

Odors. The monomeric cyclic anhydrides and cyclic esters described 
in the two preceding papers have highly characteristic odors. In particular 
some of the higher members have odors closely resembling musk. This 
observation is especially interesting because of the bearing of odor on the 
general problem of macrocyclic compounds. 

The essential principles of musk and civet are the macrocyclic ketones 
I and II. Ruzicka's demonstration (1) of this fact was followed by the 
discovery (2) that the lactones III and IV are odorous principles of angelica 
oil and musk-seed oil. These materials are highly valued as perfume in- 
gredients. In spite of the great difficulties involved in their synthesis, a 
synthetic ketone ("Exaltone," cyclopentadecanone) and a lactone "Exalto- 
lide") have been placed on the market as substitutes for the natural musks. 

CH 3 CH (CH 2 ) 7V i O CH (CH 2 )g O 

I || >C=0 (CH,)u ' " ' 

CH (CH 2 )7 X 

1 - ic" 

CH CH 2 CH (CHi)?' ' CO CH (CH 2 ) 5 CO 

(CH 2 )i 2 CO 


The observations on the anhydrides and carbonates are assembled in 
Table I, together with Ruzicka's data on the ketones and lactones. It will 
be noted that in each of the four series, with the possible exception of the 
ketones, odors are rather vague and indefinite until the number of atoms 
in the ring reaches 9. A camphoraceous or minty note then appears, and 
at about 13 atoms a woody or cedar-like quality. Beyond this point there 
is a nuance of musk which however in the anhydride series does not be- 

* J. W. Hill and W. H. Carothers; Journ. Am. Chem. Soc., 55, 5039-43 (1933); 
Communication No. 133 from the Experimental Station of E. I. du Pont de Nemours 
and Co. 

Received September 2, 1933. Published December 19, 1933. 


come definite and pronounced until the 15-atom compound is reached. 
At 15 or 16 atoms the musk quality approaches a maximum of fullness and 
homogeneity. Tetradecamethylene carbonate (17 atoms) shows a remark- 
able resemblance to "Exaltolide" (16-atom lactone). Beyond 18 or 19 
atoms the odors in each series practically disappear. 

The structural feature common to the four series is the presence of one 
or more C=O groups as a member of a ring. It is rather extraordinary 
that the manner in which the C=O group is linked in the ring should make 
so little difference in the quality of the odor. On the other hand it appears 
that any modification of the C=O group itself completely changes the odor. 
Ruzicka, Schinz and Seidel report (3) that the alcohol corresponding to 
civetone has a faint and only slightly characteristic odor. 

The fact that the only requirement (there are no doubt other limits not 
yet known) for musk-like odor is the presence of a C=O group in a ring of 
approximately 15 atoms is further illustrated by the following compounds 
not included in Table I (other physical properties of these compounds are 
listed in Table I of Paper XX) 

L( CH 2 CH 2 O ) 4 CO O-" tetraethylene carbonate, 14 atoms, fresh, faint, musk-like 
L(CH 2 )io O CO CO OJ decamethylene oxalate, 14 atoms, fresh, musk-like 

L -(CH 2 )n O CO CO O^ undecamethylene oxalate, 15 atoms, musk-like 

i 1 decamethylene malonate, 15 atoms, faint, musk- 

L (CH 2 )io O CO CH 2 CO O- 1 like 

H 2 ) 8 CO O CH 2 CH 2 O-" ethylene sebacate, 14 atoms, musk-like 

L-CO(CH 2 ) 9 CO O CH 2 CH 2 OJ ethylene undecanedioate, 15 atoms, musk-like 

The first of these contains six oxygen atoms in the ring of which two are 
directly connected to the carbonyl group; the second and third have two 
adjacent carbonyls to which annular oxygens are attached, in the fourth a 
methylene has been interposed between the two carbonyls, and in the 
fifth and sixth eight and nine methylenes are inserted between the car- 
bonyls. It would be of great interest to have information on similar com- 
pounds in which the carbonyls are replaced by methylenes; unfortunately 
macrocyclic ethers are as yet entirely unknown. 

From the practical standpoint the cyclic anhydrides are of no interest 
as odorous materials since they polymerize spontaneously on standing and 
the odors then disappear. (This fact incidentally demonstrates that the 
odors of the cyclic anhydrides are not due to any traces of the corresponding 
cyclic ketones which might conceivably be present as impurities.) On the 


a 'a, 'a 
c/2 en en 

O 3 




other hand the cyclic esters, with the exception of the oxalates, are quite 
resistant to hydrolysis or polymerization and from the practical standpoint 
they have the advantage over the previously described ketones and lac- 
tones that they are formed in relatively high yields. 

Molecular Refractions. Ruzicka and Stoll have shown that the 
molecular refractions of macrocyclic lactones (4) and hydrocarbons (5) are 
lower by about 0.6 unit than the calculated values. The macrocyclic 
carbonates exhibit the same peculiar negative exaltation and to about the 
same degree. Typical data are assembled in Table II. 

E M D 

+ .15 

- .41 

- .43 

- .71 

- .54 

- .67 

- .63 

- .83 
+ .01 
+ .42 
+ .40 
+ .22 
4- .29 

The last four compounds of the table appear to have a positive exalta- 
tion, but the amounts of these materials available were quite small and 
their purity was somewhat uncertain. The compounds for which negative 
values are listed were for the most part purified by crystallization. No 
significance can be attached to the fluctuations in these negative values 
from one member of the series to the next since the molecular refractions 
were not all taken at the same temperature. 

Melting Points. The melting points of the known macrocyclic com- 
pounds show little regularity in the nature of the change produced by in- 
creasing molecular weight but in the range of 8 to 14 atoms there is generally 
an oscillation from one member to the next (5). The cyclic carbonates 
show a similar oscillation, the melting points and ring size for some of the 
monomeric polymethylene carbonates being 

11 12 13 14 15 16 17 

23 35 11 41 12 25 22 




in ring 

A/D calcd. 

MD found 

Ethylene carbonate 




Trimethylene carbonate 




Octamethylene carbonate 




Nonamethylene carbonate 




Decamethylene carbonate 




Undecamethylene carbonate 




Dodecamethylene carbonate 




Tridecamethylene carbonate 




Tetradecamethylene carbonate 




Octadecamethylene carbonate 




Tetraethylene glycol carbonate 




Decamethylene oxalate 




Undecamethylene oxalate 




Decamethylene malonate 






Macrocyclic esters and anhydrides have odors closely resembling those 
of the ketones and lactones of the same ring size. The rings in the neigh- 
borhood of fifteen atoms have musk-like odors. The molecular refractions 
show a negative exaltation. Melting points in the carbonate series oscil- 
late from one member to the next. 

Bibliography and Remarks 

(1) Ruzicka, Helv. Chtm. Ada, 9, 230, 716, 1008 (192G). 

(2) Kerschbaum, Ber. t 60, 902 (1927); Ruzicka and vStoll, Heh, Chim. Acta, 11, 1159 (1928). 

(3) Ruzicka, Schinz and vSeidel, Helv. Chim. Acta, 10, G95 (1927). 

(4) Ruzicka and Stoll, Helv. Chim. Acta, 11, 1159 (1928). 

(5) Ruzicka, Stoll, Huyser and Boekenoogen, ibid., 13, 1152 (1930). 

XXII. Stereochemistry and Mechanism in the Formation and 
Stability of Large Rings* 

The first systematic investigation of large rings was carried out by Ruzicka. 
His method gives low yields and does not allow a reliable insight into the 
mechanism of the reaction. The new data] on polyesters and anhydrides 
make it possible to discuss more thoroughly the formation of large rings. 
This is done in the present paper. 

First the thermalysis of thorium octadecanedioate was investigated as car- 
ried out by Ruzicka. During this reaction first a linear polymer a 
polyketone is formed; if this is heated in the molecular still, it cracks 
and gives appreciable amounts of the monocyclic ketone. As was 
found** in the case of the polyesters and polyanhydrides the formation 
of the macrocyclic product proceeds by way of the linear polymer. The 
cracking of the chains may be monomolecular or birnolecular. 
To explain the easiness of ring closure, the density of large ring com- 
pounds and the stability of the rings, a thorough discussion of the sterical 
conditions and interferences is made. If a radius of 0.77 A. is assumed 
for the carbon atom and a covering sphere with a radius of 1.1 A. for the 
hydrogen atoms attached to the carbons, models representing to an appre- 
ciable degree the actual conditions are obtained. There is a minimum of 

* W. H. Carothers and J. W. Hill; Journ. Am. Chem. Soc., 55, 5043-52 (1933); 
Communication No. 134 from the Experimental Station of E. I. du Pont de Nemours 
and Co. 

Received September 2, 1933. Published December 19, 1933. 

t Compare paper number XII on page 156. 

** Compare paper number XIII on pages 165 to 168. 


probability for the formation of rings between 10 and 17 carbon atoms 
owing to the fact that in such molecules a considerable strain is effected 
through the compression of the hydrogen domains inside the ring; if the 
rings get larger this stress decreases and the ease of formation increases. 

A great deal of discussion has been devoted to stereochemistry and 
mechanism in the formation of rings, but on account of the limited range of 
experimental facts available, fruitful discussion has been largely restricted 
to rings containing fewer than 8 atoms. The only important series 
exemplifying closure of long chains hitherto has been the macrocyclic 
ketones which Ruzicka obtained by heating salts of the higher dibasic 
acids. As a basis for theoretical inferences this general reaction suffers 
from two disadvantages. The macrocyclic ketones are formed in such 
small yields (0.1 to 5%) that they can hardly be regarded as major reac- 
tion products, and the nature of the reaction (thermal destruction at high 
temperatures) is such that its mechanism is inherently obscure. Data on 
the polyesters and anhydrides now provide the possibility for a much 
clearer insight into the mechanism of the formation of large rings. 

Following the demonstration of the stable existence of large carbon 
rings, Baeyer's theory of negative strain has been generally abandoned and 
replaced by the idea of Sachse and Mohr (1) that large rings may exist in 
strainless and non-planar forms. A carbon chain constructed from con- 
ventional wire tetrahedra in such a way as to allow free rotation about 
each single bond clearly shows that the chain can assume a great multi- 
plicity of shapes. If the number of atoms is 5 or 6, the model is readily 
rotated so that the ends collide once in each complete rotation. If the 
number is more than 6 the ends can be brought together arbitrarily 
without any difficulty to produce a non-planar ring, which is quite flexible 
and can be bent into a variety of shapes without any appreciable strain. 
Considering the chain model as a representation of the bifunctional mole- 
cule, it is evident why rings of 5 and 6 atoms are readily formed and stable. 
Larger rings should be equally stable, but it is obvious that their formation 
may present some difficulties. The chain can assume a great multiplicity 
of shapes; the particular configurations requisite for ring closure are rela- 
tively few. If then the molecule is placed under conditions where mutual 
reaction of its terminal groups can occur, the probability of iw/ennolecular 
reaction is very much greater than that of iw/ramolecular reaction. Hence, 
as has been demonstrated in previous papers of this series, reactions that 
might conceivably yield large rings from open chains almost invariably 
yield linear polymers instead. It is necessary again to emphasize this point 


since even some of the most recent writings on the subject of forming large 
rings still convey the impression that attempts to close long chains do not 
result in any clear-cut reaction at all. In fact, in most cases, reaction 
occurs perfectly smoothly without any difficulty but the reaction is inter- 
molecular not intramolecular. 

What means are available for controlling this situation? Obviously 
nothing much can be expected from any modification of the nature of the 
terminal groups. Even if these groups are of such a nature (e. g., NH 2 
and COOH) as to exercise a strong attraction for each other, this pre- 
existing attraction may itself be intermolecular rather than intramo- 
lecular (2). 

There is in fact no method known for controlling experimentally the 
shape that molecules assume. The possibility of such control may perhaps 
exist in the use of surface forces, but nothing is known about this matter. 
There are, however, two possible methods of controlling the result of the 
reaction. (1) High dilution will increase the relative probability of intra- 
molecular reaction (3). (2) If a series of mutually dependent and quan- 
titatively reversible reactions is involved, constant removal of any traces 
of cyclic product will cause a displacement of the equilibrium with the 
ultimate conversion of the entire sample into the cyclic product. This is 
the principle involved in the synthesis of the cyclic esters and anhydrides 
described above. 

In speculations devoted to Ruzicka's ketone synthesis it has been sug- 
gested (2, 4) that the peculiarity of the reaction which makes possible this 
very exceptional closure of large rings lies in the ability of the metal ion 
(e. g., thorium) to bring the ends of the chain into a position of close intra- 
molecular approach. But no reason is offered to explain why the metal 
ion should have such a peculiar effect; and the force of this suggestion is 
moreover considerably weakened by the claim (5) that macrocyclic ketones 
are obtained by thermal destruction not only from salts of dibasic acids 
but also from the acids themselves and their anhydrides. It appears 
to have been taken for granted in these speculations that reaction is intra- 
molecular and leads directly to the cyclic ketone. This is a point that 
appears to be open to test and the following experiment was accordingly 

Thermolysis of Thorium Octadecanedioate in Dixylylethane. Twenty-five grams 
of thorium octadecanedioate, prepared by the method of Ruzicka, and 150 cc. of un- 
syrnmetrical dixylylethane were placed in a 300-cc. flask fitted with a stirrer and ther- 
mometer. The mixture was heated by means of a metal bath to 325 . In the course 
of ten minutes the suspended solid became gummy and attached itself to the stirrer. 


At the end of fifty minutes the mixture had become homogeneous but very viscous. At 
the end of two hours, heating was stopped. On cooling, the melt solidified to a slightly 
elastic gel. Some solid material (apparently unchanged salt) around the top of the 
flask was discarded. The gel was continuously extracted with ether for eighteen hours, 
whereupon it disintegrated to a light gray powder. It was further extracted for four 
hours with benzene, dried, heated with concentrated hydrochloric acid for three hours, 
separated, ground in a mortar, extracted for three hours with hot hydrochloric acid, 
then twice with hot alcohol and once with ether. The dried residue (10 g.) was an almost 
pure white powder. It was practically free of ash and melted at 126-128; soluble in 
hot toluene, butyl alcohol and acetylene tetrachloride. 

Anal. Found: C, 78.22, 78.89; H, 12.29, 12.41. 

When triturated with aqueous sodium carbonate and thoroughly washed and dried 
it was converted to a salt (Na found, 1.16, 1.36%). The analytical data are consistent 
with the formula HOOC[(CH 2 )i 6 CO] 7 OH, the calculated values for C and H being 78.9 
and 12.4% while a monosodium salt would contain 1.25% Na. 

A sample of this material was placed in a molecular still and heated by a metal bath 
at 300-305. At the end of one day a film of white solid distillate had collected on the 
condenser. It had a very pronounced fragrant musky odor. 

Our interpretation of this experiment is that the thorium salt of the acid, 
as in other bifunctional reactions involving long chains, breaks down with 
the formation of a linear polymer 

. . . RCO RCO RCO RCO RCO . . . 

The resulting polyketone comprises the solid product remaining from the 
extraction. When the polyketone is heated in the molecular still, it cracks 
and appreciable amounts of the monomeric cyclic ketone (cyclohepta- 
decanone) are produced; the characteristic musk-like odor therefore first 
makes its appearance at this point in the experiment. 

The facts recited above place the macrocyclic ketones, esters (including 
lactones) and anhydrides on a common basis for discussion so far as the 
stereochemistry and mechanism of formation are concerned. 

The a-polyanhydrides for example may be represented by the formula 
. . .CORCOO CORCOO CORCOO CORCOO . . . . They are ap- 
parently open chains, and contain no more than traces of cyclic anhydrides 
of low molecular weight. The possibility of converting the polymeric 
a-anhydrides into ^-anhydrides of low molecular weight depends in the 
first place on the very great reactivity of carboxylic anhydrides and the 
fact that they are capable of reacting with themselves. A mixed anhy- 
dride such as that derived from acetic and butyric acids will, at least at 
elevated temperatures, pass into an equilibrium mixture containing a con- 
siderable proportion of the two simple anhydrides (6). The process is 
analogous to ester interchange and, like that, doubtless proceeds through 


the formation of an addition product involving the ether oxygen of one 
molecule and the carbonyl carbon of another (A) . A linear polyanhydride 


'/ '/ . . f\ /V f\ f\. "/V 'f\ | f\~ f\ f\ . . . 

R C O C R' A l ! 

A ! '! 

/It/ ... A- A A A A A-I-A-- A- -A- - . . 

R^_C O CR' i 

(A) (B) 

is not a chemical individual. It is a mixture of long chains of slightly dif- 
fering lengths each of which bears a series of anhydride linkages. It pre- 
sents a very complicated series of possibilities of reacting with itself. Ad- 
jacent chains can react with themselves to produce simultaneously longer 
and shorter chains (B). Reaction in a similar manner at two points will 
yield large rings and these may mutually coalesce by the same mechanism 
to produce still larger rings. Intramolecular reaction may result in the 
formation of cyclic monomers and dimers, etc. 

The potentialities of the situation are sufficiently complex to suggest 
that at equilibrium the number of entities involved will be limited only by 
the magnitude of the sample. No doubt a condition of genuine equilibrium 
is impossible to achieve; nevertheless, experiment shows that at elevated 
temperatures quite a considerable series of transformations occurs, and 
it leads chiefly to the formation of very large molecules. At the same 
time appreciable amounts of smaller cyclic fragments (/3-anhydride) are 
produced. The actual concentration of these /3-anhydrides present in the 
reaction mixture at any time must be small, since the /3-anhydrides poly- 
merize almost instantly at the temperature involved; and their rate of 
formation cannot be exceedingly great because even at elevated tempera- 
tures where the speed and amplitude of molecular vibrations are greatly 
increased, the relative probability of the configurations necessary for the 
formation of such cyclic compounds must be rather low. However, in 
the molecular still a mechanism is provided for removing and isolating the 
^-anhydrides as fast as they are produced. The equilibrium is thus con- 
tinuously displaced and the entire specimen is finally converted into the 

A precisely similar mechanism is unquestionably involved in the forma- 
tion of macrocyclic esters from polyesters and it is significant in this con- 


nection that smooth transformation requires the presence of ester-inter- 
change catalysts. 

The ketones obviously present a more difficult case. No mechanism cor- 
responding to ester interchange exists for the smooth rupture of the link- 
ages joining the structural units in a chain of the type . . . CO R 
CO R CO R . . . ; nor would one expect a macrocyclic ketone to 
polymerize with the formation of the corresponding polyketone. Since the 
reactions involved are not strictly reversible and there are many side reac- 
tions, the formation of cyclic ketones offers scarcely any possibilities for 
rational and deliberate control. The yields are therefore generally only a 
small fraction of those obtained with the cyclic carbonates, oxalates and 

Steric Interferences. The outline presented above is incomplete since 
the possible influence of atoms attached to the carbon chain has been ig- 
nored. Such atoms are capable of acting as obstacles to ring closure, and 
they may also introduce strains into rings. For the series under considera- 
tion hydrogen is the most numerous and important peripheral atom. The 
iiitermiclear C-H distance is known from spectroscopic data to be 1.08 
A., and since the (aliphatic) carbon radius is 0.77 A., the distance from the 
center of the hydrogen to the surface of the carbon is 0.31 A. This indi- 
cates an atomic diameter of 0.62 A. for hydrogen attached to carbon. 
But hydrogens not mutually joined will be expected to exercise a mutual 
repulsion preventing close approach, and data on the densities of hydro- 
carbons, collision areas, etc., show that the average distance between the 
centers of hydrogens belonging to separate molecules is always greater 
than 0.62 A. The combined hydrogen atom must therefore be assigned, 
in addition to its internal radius of 0.31 A., a larger external radius, which 
defines the closest average approach of other atoms (7). This external 
radius will vary with the compound and the conditions; it will decrease 
with increasing temperatures and will be larger in a crystal than a gas. 
The most elaborate experiments and speculations on the external radius of 
hydrogen are those of Mack (8), who in different cases uses values ranging 
from 0.49 to 1.26 A. Stoll and Stoll-Comte (9) had, however, already 
made specific application of the external domain of hydrogen atoms in 
connection with macrocyclic hydrocarbons. They pointed out that the 
cyclic paraffins exhibit two anomalies. (1) When density is plotted against 
number of CH2 groups a maximum appears in the range of 10 to 17 atoms, 
and (2) it is within this range that the yields of the cyclic ketones fall to an 
exceedingly low minimum. The explanation which they offer is very 
briefly this. Consideration of the densities of cyclic and open-chain hydro- 



carbons indicates a domain for the CH 2 group that can be accounted for by 
representing the hydrogen as spheres of diameter 2.3 A. (compared with 
1.54 A. for carbon). Rings of 5, 6 or 7 members therefore consist of an 
approximately flat cyclic chain of carbon atoms with a shell of much larger 
hydrogen atoms around the periphery. In rings of 8 to 15 atoms the 
geometrical limitations imposed by the valence angles of the carbon atoms 
force some of the hydrogens toward the center of the ring. The space 
available is not sufficient to receive them; the "domain" of the hydrogens 
(or of the CH2 group) is therefore reduced or compressed. The resulting 
strain explains the minimum in the 
yield of the cyclic ketones, and the com- 
pression explains the maximum in the 
density curve of the hydrocarbons. 
The turning in of hydrogens toward 
the center of the ring is illustrated for 
cyclononane by Fig. 1 in which the di- 
ameter of the hydrogens (1.18 A.) is 
much smaller than that assumed by 
Stoll and Stoll-Comte (2.3 A.). This 
configuration is quite rigid and it is 
evident that if the hydrogens were twice 
as large, the space available would not 
be adequate to receive them. As the 
size of the ring increases the number 
of hydrogens forced toward the center 
increases; above 15 atoms half of them 
are forced toward the inside of the 
ring, but there is sufficient space to 
accommodate them and no strains are 

This idea of steric interferences due to "external" radii is capable of 
many interesting applications to the cyclic esters and anhydrides. It is 
unfortunately for this purpose impossible to assign any exact value to the 
external radius of hydrogen. Results obtained in the diphenyl problem 
(10) suggest that the value implied in a spherical hydrogen of 2.3 A. is 
probably much too large; however, for immediate qualitative purposes the 
exact value adopted is not particularly important. The following discus- 
sion is based on observation of models in which the hydrogen is represented 
as a sphere of radius 2.2 A. 

When a zig-zag hydrocarbon chain is constructed with such large hydro- 

Fig. 1. Cyclononane (carbon, d = 
1.54 A.; hydrogen, 1.1). This model 
illustrates the cramped nature of the 
ring and the turning of hydrogen 
toward the center. 



o' 1.30 


rt 1.20 


8 1.15 


2 1.10 

x> x x 

g 1.05 

XX * N *^ 

Q 0.95 



gens, it is immediately evident that the mobility of the chain is greatly 
reduced. The hydrogens interfere with many rotational movements that 
would present no difficulty if the hydrogens were absent. The interfer- 
ences toward ring closure become exaggerated as the length of the chain 
is increased beyond 6 or 7 atoms. For a cycloparaffin of 9 atoms the inter- 
ferences are so serious that the model is not constructible. 

If an oxygen atom is inserted in the chain, the flexibility of the model is 
considerably increased, since this insertion is practically equivalent to the 
removal of two of the interfering hydrogens (11). Actually also oxygen 
probably presents less resistance to the deflection of its valence angles, 
although this is not illustrated by the models. In the anhydrides and esters 

under consideration each struc- 
tural unit contains in its chain 
at least one oxygen and at least 
one carbonyl carbon (which also 
bears no hydrogens). The re- 
sult is a great increase in the 
constructibility of the models. 
Construction of models of the 
cyclic hydrocarbons in the range 
of 9 to 15 atoms requires either 
a compression of the hydrogen 
spheres or a considerable deflec- 
tion of the angles. In the car- 
bonates and anhydrides the en- 
tire series of monomers from the 
5-atom ring up can be con- 
structed from spherical wooden 
atoms without more than very 

slight deflection of the angles. From this one may infer that no maxi- 
mum should appear in the curve relating density to ring-size of the car- 
bonates. The experimental curve is shown in Fig. 2. Unfortunately data 
are lacking in the range of 7 to 10 atoms, but the nature of the curve 
makes it very improbable that any maximum exists. 

Although it appears from the models that no compression of the hydro- 
gen domains is involved in any of the cyclic ester or anhydride molecules, 
the manipulation of the models shows that there is a great difference in 
ease of ring closure depending upon the length of the chain. In the range of 
8 to 14 atoms the interferences are so numerous that many trials and errors 
must occur before the particular configuration that permits ring closure is 

6 8 10 12 14 16 18 20 

Number of atoms in ring. 
Fig. 2. Densities of monomeric cyclic 
polymethylene carbonates at 20: O, values 
determined at 20; El, values obtained by ex- 
trapolation via molecular refractions from de- 
terminations of density made at 25; A, values 
obtained by extrapolation via molecular refrac- 
tions from determinations made at 50. 


found; and the ring when closed is then very rigid. In the carbonate 
series with the 15-membered ring the model acquires considerable flexi- 

The greater ease in constructibility of rings of 15 atoms or more ex- 
plains why in the preparation of carbonates in the range of 7 to 12 atoms 
the products are almost exclusively the dimeric forms. At equilibrium 
the concentration of dimer will be large compared with the concentration 
of monomer. But as the ring size increases not only does the ease of 
monomer formation approach (and ultimately exceed) that of the dimer, 
but the latter becomes relatively more and more difficult to remove by 
evaporation. The ring-size at which the two forms begin to appear in 
equal amounts will depend partly upon the experimental conditions; but 
it will also be controlled to a certain extent by the nature of the ring. It is 
interesting in this connection to compare the three 14-membered rings 
undecamethylene carbonate, tetraethylene glycol carbonate and deca- 
methylene oxalate. The first has two annular oxygens and one carbonyl, 
the second two oxygens and two carbonyls and the third five oxygens and 
one carbonyl. The first compound under the conditions used gave a mix- 
ture of monomer and dimer, the last two gave exclusively monomer. 

The alkylene esters of dibasic acids above malonic yield chiefly dimers 
even when the unit length is as great as 18. The models do not furnish any 
very clear explanation of this fact although it appears that some advantage 
in ease of ring formation may be expected if the oxygens and carbonyls 
are all adjacent (as in oxalic esters). 

The cyclic anhydrides present the same reluctance toward the formation 
of rings of intermediate size as do the esters, but they show some peculiari- 
ties. The products appear to be invariably either monomer or dimer, not 
mixtures of the two. The range of dimer formation is from 9 to 13 atoms 
(unit length). Very peculiar is the fact that the two even membered 
compounds (10 and 12 atoms) in this range appear to yield exceedingly 
unstable monomers rather than dimers. The result is the alternating effect 

shown graphically in Table I (p. 20o ). The manipulation of models 

in this range furnished the impression that even-membered rings were more 
readily constructed than odd, but the geometrical peculiarity responsible 
for this effect was not identified. All of the cyclic anhydrides are exceed- 
ingly unstable at the high temperatures used in their formation, and the 
nature of the /3-anhydrides produced might therefore be expected to be very 
sensitive to very slight differences in the ease of constructibility. 

Ring Stability. Ruzicka showed (12) that his macrocyclic paraffins 
were not destroyed by the action of phosphorus and hydriodic acid at 250 


and his cyclic ketones resisted the action of fuming hydrochloric acid at 
180-190. This was an indication that large rings are no less stable than 
those of 5 or 6 atoms and a refutation of the idea that large rings are 
strained. It appears, however, in view of the results obtained with the 
cyclic esters and anhydrides, that this conclusion requires some revision. 
The higher cyclic anhydrides all polymerize very readily. In this respect 
the large rings are very unstable as compared with those of 5 to 6 atoms, 
which do not polymerize at all. The instability rises to a maximum with 
the rings of 10 to 12 atoms, which polymerize rapidly even at temperatures 
below 0. At elevated temperatures (e. g., 200) the macrocyclic esters 
also polymerize. It is therefore very difficult to accept the conclusion that 
large rings are entirely free from strain. Our interpretation of the facts is 
as follows. 

Rings of 3 or 4 atoms have very large strains owing to the necessarily 
large deflection of the annular valences. Rings of 5 atoms are practically 
free of strain. Most rings of 6 atoms are strained. It is true that cyclo- 
hexane can be represented as existing in two strainless forms (cis and trans) 
but since it has not been possible to isolate two cyclohexanes, it seems 
likely that the two forms are in dynamic equilibrium. On each conversion 
of one form into the other the ring must pass through a nearly planar posi- 
tion of strain. The existence of such strain is indicated by the fact that all 
simple 6-membered cyclic esters polymerize very readily. (Glutaric an- 
hydride does not polymerize but the oxygen valences in this case may per- 
mit sufficient deflection to avoid strain.) Larger rings are all strained. 
In the range of 7 to 14 atoms where the models are very rigid the strain 
may be pictured as due to the mutual repulsion of non-linked peripheral 
atoms that are crowded against one another. In larger, more flexible 
rings the vibrations due to thermal agitation constantly present the possi- 
bility of introducing momentary strains. This effect is similar to that 
pictured for the cyclohexane ring but is probably less pronounced. At 
any rate, 6-membered cyclic esters polymerize more readily than macro - 
cyclic esters. 

The ease of polymerization of the macrocyclic anhydrides is unquestion- 
ably associated with the extraordinary reactivity of anhydrides generally. 
A facile mechanism for the rupture of the ring is provided by the nature 
of the anhydride linkage and hence very slight strains or distortions, which 
may arise merely through small interferences among substituent atoms, 
need not be tolerated. The macrocyclic esters polymerize by a similar 
mechanism (i. e., by ester interchange) but less readily because they are 
less reactive. The apparent high stability of the macrocyclic hydrocar- 


bons and ketones is due to the fact that they present no point of easy 
chemical attack; if a sufficiently delicate chemical probe were available 
they would probably prove to be somewhat less stable than those of 5 


It is shown that the formation of macrocyclic ketones from salts of the 
dibasic acids probably involves first a linear polyketone which is subse- 
quently cracked or decomposed. The ketones thus follow a course al- 
ready established for esters and anhydrides. The characteristic analogies 
and differences in the three series can be explained by taking into account 
the nature of the reactions involved and the steric effects of peripheral 
atoms. Rings of more than 5 atoms cannot be regarded as entirely strain- 
less. The probable nature of the strains in large rings is indicated. 

Bibliography and Remarks 

(1) Mohr, /. prakt. Chem., 98, 348 (1918). 

(2) The idea that such attractions may favor intramolecular reaction has been suggested by Mills, 
"Proceedings, Fourth International Solvay Conference," 1931, p. 20. 

(3) Cf. Ruggli, Ann., 392, 92 (1912). 

(4) Ruzicka, Stoll and Schinz, Helv. Chint. Acta, 11, 670 (1928). 

(5) Ruzicka, Brugger, Seidel and Schinz, ibid., 11, 496 (1928). 

(6) Cf. Autenrieth, Ber., 34, 168(1901). 

(7) Cf. the excellent review by Sidgwick in "Annual Reports of the Progress of Chemistry for 1932." 

(8) Melaven and Mack, Journ. Am. Chem. Soc., 54, 888 (1932); Sperry and Mack, ibid. t 904; 
Mack, ibid., 2141. 

(9) Stoll and Stoll-Comte, Helv. Chim. Acta, 13, 1185 (1930). 

(10) Cf. Adams and Yuan, Chem. Reviews, 12, 261 (1933). 

(11) This effect is probably considerably exaggerated in the models since the oxygens are repre- 
sented only by their internal diameters. 

(12) Ruzicka, Brugger, Pfeiffer, Schinz and Stoll, Helv. Chim. Acta, 9, 499 (1926). 

XXIII.* -Caprolactone and Its Polymers 

Bifunctional esterifications yield either cyclic monomers or linear poly- 
esters.^ Choice between these possibilities is controlled by 

(a) the unit length of the reactant, 

(b) the nature of the unit, 

(c) the experimental conditions (especially dilution). 

* F. J. van Natta, J. W. Hill and W. H. Carothers; Journ. Am. Chem. Soc., 56, 
455-7 (1934) ; Communication No. 118 from the Experimental Station of E. I. du Pont 
de Nemours and Co. 

Received October 20, 1933. 

f Compare paper No. XII on page 156. 


In the present paper these conditions are studied during the self esteri- 
fication of i-hydroxycaproic acid. Under certain experimental condi- 
tions the principal primary product is the monomeric lactone; a trace of 
dimeric ester is formed, but little if any of the high polymers. 
Under the action of heat (about 150) the lactone polymerizes to a linear 
polymer with a molecular weight around 4000. 

Bifunctional esterifications (1) generally yield either cyclic monomers 
or linear polyesters. Choice between these possibilities is controlled by 
(a) the unit length of the reactant, (b) the nature of the unit and (c) the 
experimental conditions (especially dilution). Factor (a) is generally by 
far the most important and its effect has been illustrated in previous 
papers. Unit lengths of 7 and 8 however constitute transition cases and 
factor (b) may here become the controlling one. It is well known that 
substitution (e. g., by methyl) favors ring closure but otherwise the effect 
of variations in the nature of the unit are not easy to foresee. The sim- 
plest possible structural situation for self-esterification is found in the o>- 
hydroxy acids, HO(CH 2 ) W _ 2 COOH, and information concerning them is 
available for all values of n from 3 to 22 (2) excepting 7 and 8. These 
acids have never been isolated or their self-esterification studied (3). 

The experiments presently reported were prompted by the considerations outlined 
above and by the fact that several grams of -hydroxycaproic acid had become avail- 
able as a by-product in the preparation of hexamethylene glycol by the reduction of 
diethyl adipate. The acid could neither be distilled nor crystallized, and it was therefore 
impossible to isolate it as such in a state of purity. It was however found possible, 
as described in B (below), to isolate an oil composed essentially of the acid (80%) and 
its lactone (20%). This mixture was dehydrated by heating it in a distilling flask at 
150 to 210. The residue was completely volatile, and, when purified by redistillation, 
it was found to consist of the lactone of -hydroxycaproic acid, a colorless liquid having 
a pleasant, spicy odor. It crystallized when strongly cooled and melted at about 5. 
The yield of purified lactone was about 63% of the theoretical. The only other product 
of the reaction consisted of a very small amount (1%) of a volatile crystalline solid melt- 
ing at 111 to 113. This was identified as the dimeric, cyclic self-ester of e-hydroxy- 
caproic acid, a 14-membered ring. 

The monomeric lactone showed no appreciable tendency to polymerize spontane- 
ously when allowed to stand at the ordinary laboratory conditions; but when heated at 
150 in a sealed tube it gradually became more viscous and, after twelve hours, when 
cooled to room temperature, it solidified to an opaque mass. When a trace of potassium 
carbonate was added to the lactone the same result was obtained in five hours at 150. 
The product crystallized from alcohol as a white powder melting at 53 to 55. It was 
very soluble in ethyl acetate, acetone and benzene but only slightly soluble in alcohol 
or ether. The analytical composition and chemical behavior of this material demon- 


strated that it was a linear polyester, and molecular weight determinations in freezing 
benzene gave values about 4000. 

Anal. Calcd. for HO [ (CH 2 )6 CO O ] 35 H: C, 62.83; H, 8.84; mol. 
wt., 4008. Found: C, 62.15; H, 8.86; mol. wt., 3660, 4300. 

All three of the above esters (lactone, dimer and polyester) were actually deriva- 
tives of e-hydroxycaproic acid, and no shift of the hydroxyl oxygen was involved in their 
formation (4), since they all yielded the same hydrazide when treated with hydrazine 
hydrate. This hydrazide was also obtained from the ethyl ester described in C. 

To test its susceptibility to depolymerization the polyester was placed in a molecular 
still and heated by a bath at 250 and at a pressure of 1 to 2 mm. for ninety hours (5). 
During this time only a very small amount of distillate was collected a viscous, dark- 
colored oil containing a few minute crystals of the cyclic dimer. The polymeric resi- 
due was darker in color but otherwise appeared to be unchanged. 

Conclusions. Under the conditions described, the principal primary 
product of the self-esterification of -hydroxycaproic acid is the monomeric 
lactone; a trace of the dimeric ester is formed at the same time but little 
if any of the higher polyester. Like 6-membered cyclic esters, the lactone 
of -hydroxycaproic acid is polymerized by the action of heat. (Appar- 
ently it polymerizes only slightly less readily than 5-valerolactone.) The 
polyester thus obtained can be depolymerized only with great difficulty 
under the action of heat. In this respect it differs from the polyesters 
that result from 6-membered cyclic esters, and resembles polyesters de- 
rived from the higher co-hydroxy acids. 

The following comparisons with other compounds having 7-atom units 
are also of interest. Tetramethylene carbonate and trimethylene oxalate 
are obtained only in the form of linear polyesters. These are depolymer- 
ized with great difficulty and the only products identified are the cyclic 
dimers. The lactone of 3,7-dimethyl-6-hydroxyoctanoic acid is reported 
(6) to exist in two forms, a liquid and a solid. The authors suggest that 
these are stereoisomers, but no molecular weight determinations are re- 
corded, and it seems more likely that they represent a monomeric and a 
dimeric form. Another e-lactone results from tetrahydrocarvone by oxi- 
dation with Caro's acid (7). It is hydrolyzed to the acid, HO CH- 
(CH 3 ) (CH 2 ) 2 -~CH(C 3 H7) CH 2 COOH, from which the lactone is 
regenerated by the action of heat. A simple lactone is also obtained from 
e-hydroxyoctanoic acid by direct distillation (5). When -aminocaproic 
acid is heated it is partly converted to the monomeric lactam (about 30%) 
and partly to polyaniide (about 70%). These are not directly intercon- 


Preparation of -Hydroxycaproic Acid. The acid was isolated from the product 
obtained by reducing a large amount of ethyl adipate. After the hexamethylene glycol 


had been removed by ether extraction of the completely saponified reaction mixture, the 
latter was acidified with sulfuric acid and continuously extracted with ether. From the 
concentrated ethereal extract, a part of the adipic acid was removed by filtration. At- 
tempts to remove the e-hydroxycaproic acid from the filtrate by distillation were un- 
successful. The mixture was therefore acetylated (8), and the acetate of the hydroxy 
acid was isolated by distillation (b. p. 134 to 145 at 2 mm.). This was saponified, 
acidified with dilute sulfuric acid, extracted with ether and concentrated in vacua. 
The acetic acid was removed by evaporation in vacua at room temperature in an all 
glass apparatus, the condenser bulb being cooled with carbon dioxide snow and acetone. 
After four days the bulbs were opened. The distillate consisted of dilute acetic acid. 
The residue was a pale yellow, viscous liquid having a slightly rancid odor. It solidified 
to a glass when cooled with solid carbon dioxide. Titration indicated that it consisted 
of a mixture of e-hydroxycaproic acid and its self -ester containing about 80% of the 
free acid. 

Anal. Calcd. for hydroxycaproic acid: neutral equivalent or saponification equiva- 
lent, 132. Found: neutral equivalent, 164.1, 164.3; saponification equivalent, 130.5. 

-Caprolactone. The lactone prepared as described in A was very soluble in alcohol, 
benzene, ether, ethyl acetate and water, but insoluble in petroleum ether; b. p. 98 to 
99 at 2 mm.; 4 4 1.0698; w 2 D 4 1.4608; M R calcd., 29.44; MR found, 29.15. 

Anal. Calcd. for C 6 Hi O 2 : C, 63.11 ; H, 8.84; mol. wt., 114; saponification equiva- 
lent, 114. Found: C, 62.81, 63.39; H, 8.68, 9.07; mol. wt. (in freezing benzene), 120, 
122; saponification equivalent, 113.4, 113.3. 

Dimeric e-Caprolactone. Obtained as described in A: granular crystals; soluble 
in most organic solvents, but insoluble in petroleum ether or water; m. p. 112 to 113. 

Anal. Calcd. for Ci 2 H2oO 4 : C, 63.11; H, 8.84; mol. wt., 228. Found: C, 62.98, 
63.39; H, 8.84, 9.09; mol. wt. (in freezing benzene), 243, 226. 

Hydrazide of c-Hydroxycaproic Acid. The monotneric and dim eric lac tones as 
well as the polyester described in A, and the ethyl ester described in C (below) were 
separately warmed with hydrazine hydrate for several hours on a steam-bath. In 
each case the mixture solidified on being cooled; and after crystallization from ethyl 
acetate or alcohol, the hydrazide was obtained in the form of white crystals melting at 
114 to 115. It was very soluble in water. 

Anal. Calcd. for C 6 HuO 2 N 2 : N, 19.18. Found: N, 18.68, 18.74. 

Attempts to Prepare -Caprolactone from -Bromocaproic Acid. Prior to the ex- 
periments described above, attempts had been made to prepare -caprolactone by the 
action of sodium thylate on -bromocaproic acid. By this method Marvel and Birk- 
himer (9) obtained a small amount of material "which seemed to be slightly impure 
c-caprolactone," but its identity was not established. 

e-Bromocaproic acid with an equivalent amount of sodium ethylate in absolute 
alcohol was refluxed for eight hours. The mixture was made very slightly acid with a 
few drops of hydrochloric acid, then filtered, concentrated and distilled in vacuo. 
After a small preliminary fraction, a halogen-free product distilling at 104 to 106 at 
4 mm. was obtained in 37% yield. 

This,, presumably, is similar to the product described by Marvel and Birkhimer, 


but it proved to be the ethyl ester of c-hydroxycaproic acid. Unlike e-caprolactone it 
was insoluble in water; and it did not crystallize on being strongly cooled but merely 
became very viscous. It was soluble in most organic solvents; d? 5 0.9944; w 2 D 5 1.4381; 
MR calcd., 42.32; MR found, 42.24. 

Anal. Calcd. for C 8 Hi 6 O 3 : C, 60.0; H, 10.0; mol. wt., 160; saponification equiva- 
lent, 160. Found: C, 59.98; H, 10.08; mol. wt. (in boiling acetonitrile) 151, 154; 
saponification equivalent, 158.7, 158.8. 

When treated with hydrazine hydrate it yielded the hydrazide already described 
in B (above). 

The residue from the distillation of the ethyl ester was distilled further and yielded 
a small fraction boiling at 110-200 at 4 mm. From this halogen-free distillate granular 
crystals of the caprolactone dimer described in B separated (identified by mixed melt- 
ing point). 

The residue still remaining (41%) solidified to a paste on standing, and was ob- 
tained as a light colored powder melting at 51 to 53 after several crystallizations from 
alcohol. It was similar to the polyester described in A, but it had a somewhat lower 
apparent molecular weight. 

Anal. Calcd. for HO [ (CH,), CO O ] ir - H: C, 62.7; H, 8.8; mol. wt., 
1842. Found: C, 63.56; H, 9.08; mol. wt. (in freezing benzene) 1980, 1660. 

When treated with hydrazine hydrate it yielded the hydrazide already described. 

From the above described reaction of sodium ethylate on bromocaproic acid no 
rnonomeric lactone was isolated. It is, however, not permissible to infer that no capro- 
lactone was formed, since hydrochloric acid was present, and this, doubtless, would 
strongly catalyze the polymerization of any caprolactone that might have been formed. 


-Caprolactone has been prepared for the first time. It is the principal 
product of the self-esterification of the corresponding acid. A small 
amount of the cyclic dimeric ester (14-membered ring) is formed at the 
same time. Under the action of heat c-caprolactone is converted to a poly- 
ester of high molecular weight. The process is not easily reversible. This 
behavior is compared with that already observed for other cyclic and poly- 

Bibliography and Remarks 

(1) For terminology. 

(2) Chuit and Hausser, Helv. Chim. Ada, 12, 463 (1929); Bougault and Bourdier, Compt. rend , 
147, 1311 (1908); Lycan and Adams, Journ. Am. Chem. Soc., 51 625, 3450 (1929). 

(3) Some of their derivatives are meagerly described in the following references: Baeyer and 
Villiger, Ber., 33, 863 (1900); Helferich and Malkomes, ibid., 55, 704 (1922); Marvel and Birkhimer, 
Journ. Am. Chem. Soc., 51, 260 (1929). 

(4) Cf. Blaise and Koehler, Compt. rend., 148, 1772 (1909). 

(5) Under these conditions polyesters having 6-atom units depolymerize very rapidly and smoothly 
Those having longer units depolymerize very slowly and incompletely if at all, although in many such 
cases a smooth depolymerization can be effected in the presence of an ester-interchange catalyst. 

(6) Baeyer, Ber. t 32, 3619 (1899); Baeyer and Villiger, ibid., 3628. 

(7) Baeyer, ibid., 29, 27, 30 (1896); Baeyer and Vill^er, ibid., 32, 3629 (1899). 

(8) Cf. Chuit and Hausser, Helv. Chim. Acta, 12, 463 (1929). 

(9) Marvel and Birkhimer, Journ. Am. Chem. Soc., 51, 260 (1929). 


XXIV. Cyclic and Polymeric Formals* 

Polymerization and ring formation follows the general scheme, f 


heat and action of 
a dehydrating medium 

linear polymers of molecular wt. between 1000 and 5000 

heating in the molecular still 


^-product; cyclic monomer < 

or dimer heat 



co-product; linear polymer ofmol. 
wt. up to 20,000. 

y-product; similar or identical 
with a-product. 

It is to be expected that a similar scheme holds for the acetals derived 
from glycols. In the present paper the action of glycols onformals is de- 

A new class of linear polymers and large rings is obtained. 
Heating a glycol with dibutylformal and an acidic catalyst at about 150 
yields an a-polyacetal. The molecular weight is around 2200. When 
a-polyacetals are heated at 230 to 250 in the molecular still, b- and 
(^-products are formed. The cyclic monomers (^-products) are liquids 
with characteristic odors fitting completely in the scheme developed by 

* J. W. Hill and W. H. Carothers; Journ. Am. Chem. Soc. t 57, 925-8 (1935); Com- 
munication No. 152, from the Experimental Station of E. I. du Pont de Nemours and 

Received March 21, 1935. 

t Compare paper No. I on page 4 and No. XII on page 156. 


Ruzicka and discussed in a previous paper.* The ^-polymers are 
hard and tough microcrystalline masses, which can be drawn out into 
strong, pliable, silk-like filaments : X-ray diagrams indicate high orien- 
tation. Depolymerization was studied. The relative rate of hydrolysis 
of the ethylene acetals derived from formaldehyde, acetaldehyde and ace- 
tone is approximately 1:4000:44,000. 

Macrocyclic esters and anhydrides can be obtained by depolymerizing 
the corresponding linear polymers (1). The various relations involved 
are shown in the following diagram. 

Reagents > a 
mol. wt 

polymer of 
. 1000-5000) 
in vacuum 


/3-product distillate (cy- 
clic monomer or dimer) < 

i . 

co-polymer residue (lin- 
ear polymer of mol. wt. 


7-polytner (similar to or identical 
with a-polymer) 

Heating the a-polymer causes (a) formation of traces of cyclic monomer 
and dimer derived from one and two structural units of the molecular 
chain and (b) coupling of the a-polymer chains to form the still longer 
chains of co-polymer. Linear polyesters and polyanhydrides result from 
reversible bifunctional condensations. In the ideal case, it would be pos- 
sible to establish an equilibrium among all the species of a-, ft- and co-forms 
and constant removal of the /3-forms by distillation should result in a com- 
plete transformation of the sample into cyclic monomer and/or dimer. 
This ideal is closely approached in some cases, the chief critical factors in- 
volved being inherent mobility of the link, catalysis, temperature and speed 
with which possible volatile products are withdrawn. 

The fact that acetal interchanges are smoothly reversible reactions sug- 
gests that it should be possible to realize all these transformations among 
the acetals derived from glycols. For formals this is indeed true, and the 
compounds now reported constitute a new family of linear polymers and 
large rings. Here also, as with the esters and anhydrides, a single member 
may be obtained in the form of a macrocrystalline solid, an odorous (e. g., 

* Compare paper No. XX on page 212 and No. XXI on page 221. 


musk-like) liquid, or a tough microcrystal line mass capable of being drawn 
into strong, pliable, highly oriented, silk-like filaments. 

Preparation and Behavior of Formals. Heating of a glycol with dibutyl 
formal and an acidic catalyst at about 150 results in acetal interchange 
with the distillation of butanol 

HO(CH 2 ) n OH + BuOCH 2 OBu > . . . CH 2 O(CH 2 )nO . . . + 2BuOH 

Trimethylene or tetramethylene glycol thus yields a mobile liquid easily 
distilled in vacuo, and the major primary product is, therefore, presumably 
the cyclic monomer. When the glycol is pentamethylene, or a higher one 
the residue remaining from the removal of the alcohol is a viscous liquid. 
To ensure complete removal of volatile products, it is heated to 200-220 
at low pressure. The remaining non-volatile product is then the a-poly- 
acetal. That derived from decamethylene glycol, for example, is a waxy 
solid which crystallizes as a powder from hot ethyl acetate and then melts 
at 56.5-57. Its observed apparent molecular weight was 2190. Similar 
a-polyformals were obtained from pentamethylene, hexamethylene, tetra- 
decamethylene and octadecamethylene glycols as well as from triethylene 
glycol. The last gave a sirupy product which showed 110 tendency to 
crystallize; the others were all solids. 

When the a-polyformals were heated to 230-250 at low pressure in a 
still provided with a condenser placed close to the evaporating surface (3), 
conversion to the ft and w forms occurred. Compared with the polyesters 
derived from carbonic acid, the rate of distillation was quite slow; in this 
respect, the polyformals resembled the previously described (2) polyesters 
derived from the higher dibasic acids such as sebacic; the tendency toward 
co-polymer was greater than toward the ft forms, and as the viscosity of the 
residue increased, the rate of distillation became less. The ultimate yield 
of distillate was, therefore, relatively small, and, as in the case of those 
polyesters that manifested a similar behavior, the distillates were largely 
the cyclic dimers which were without exception definitely crystalline solids 
of sharp melting points. The presence of cyclic monomer in the distillate 
could be inferred from the characteristic odor, and the monomer from 
pentamethylene formal was obtained in sufficient quantity to permit iso- 
lation and purification. The a-polyformal from triethylene glycol differed 
from the other formals of similar unit length. It was a viscous liquid and 
when heated in vacuum at 200-250 it depolymerized very rapidly. The 
product apparently consisted essentially of monomer, which was isolated 
in a state of purity and in considerable quantity by fractional crystalliza- 
tion of the distillate. 


Polymerization of Cyclic Formals. The possible polymerization of 
cyclic acetals has occupied an important place in speculations concerning 
cellulose, starch, etc. (3), but no clear example of a polymerization originat- 
ing in an acetal linkage per se has been adduced. Among cyclic esters, 
rings of more than 5 atoms generally polymerize on being heated with 
catalysts and those of 6 atoms occupy a peculiar position because this 
transformation occurs with especial facility and is easily reversible (4). 
It is of interest in this connection that trimethylene formal (6-ring) could 
not be induced to polymerize. The monomeric tetramethylene, penta- 
methylene and triethylene glycol formals, however, quickly became more 
viscous when heated (e. g., at 150) in the presence of a trace of sulfonic 
acid. Dimeric decamethylene formal (26-ring) when treated in this man- 
ner was converted to a microcrystalline solid having approximately the 
same melting point and molecular weight (2500) as the a-polymer from 
which the dimer was originally derived. Results obtained with monomers 
are indicated in the table. 


Successive Time of 

Formal Ring size heating intervals flow, sec. 

Trimethylene 6 Initial 4.5 

+2hrs. at 100 4.5 

-fl.Shrs. at 150 4.5 

Tetramethylene 7 Initial 4.5 

+2hrs. at 100 4.5 

+Q.5hr. at 150 35 
+ 1.0 hr. at 150 

Pentamethylene 8 Initial 4.5 

+2hrs. at 100 13 

4-0. 5 hr. at 150 16 

Odors. The remarkable parallelism between the odors of macrocyclic 
ketones, lactones, carbonates, malonates, oxalates, sebacates, etc., of simi- 
lar ring size previously reported (5) suggested that for musk-like odor the 
only requirement is a C=O group in a ring of suitable size. Meanwhile, 
Ruzicka (6) has extended his own researches and observed that the cyclic 
imine | (CH 2 )ie NH | ^as a musk-like odor, thus demonstrating that 

the presence of the carbonyl group is not necessary. The odors of the cyclic 
formals indicated in Table I now point to the same conclusion. Only the 
compounds marked* were actually isolated, but there is no doubt that the 
characteristic odor of the crude distillate was in each case due to cyclic 
monomer. The odors of the formals I were so strikingly similar to those 


of the corresponding carbonates II as to be almost indistinguishable from 

(ino-iH, W<Uc 

(I) (ID 


Glycol from which Ring 

formal is derived size Odor 

*Tetramethylenc 7 Sweet and penetrating 

*Pentamethylene 8 Minty, camphoraceous 

Hexamethylene 9 Minty 

Nonamethylene 12 Earthy, camphoraceous 

Decamethylene 13 Cedar, camphoraceous 

Tetradecamethylene 1 7 Musk-like 

Octadecamethylene 21 No odor 

*Triethylene glycol 11 Faint, flowery 

Fibers from co-Polyformals. w-Polyesters and polyanhydrides can be 
drawn out into continuous filaments which readily accept permanently a 
high degree of orientation along the fiber axis and have very good strength 
and pliability (7). These materials are in fact the only truly synthetic 
fibers for which any measured strengths have been reported although it 
appears that continuous oriented filaments can also be drawn from poly- 
oxymethylene and polyethylene oxide (8) when their molecular weights 
are sufficiently high. In all these classes of compounds as in cellulose it- 
self, very long molecules are 
made up of units joined through 
C-O bonds. In the co-polyfor- 
mals as in cellulose, this linkage 
is actually an acetal linkage. 
It is not surprising then that 
the co-polyformals can also be 
drawn into oriented filaments. 
X-ray diagrams for unoriented 
and oriented w-polydecamethyl- 
ene formal are shown in Figs. 1 
and 2. 

Fig. 1. x-Ray diffraction pattern of w-deca- Ease of Depolymerization. 

methylcnc formal. The most obvious factor in- 



fluencing ease of depolymerization is the reactivity characteristic of the type 
of linkage involved. Polyanhydrides are depolymerized much more readily 
than polyesters. The relative ease of hydrolysis of the ethylene acetals de- 
rived from formaldehyde, acetaldehyde and acetone is approximately 
1:4000:44,000 (9). That the alkylene formals depolymerize with dif- 
ficulty is therefore not surprising. One might expect that it would be 
very much easier to form macrocyclic acetals from other aldehydes and 
ketones; unfortunately, various other complications arise in these cases. 

Rings of 9 to 12 atoms are 
especially difficult to form and 
in some cases exceptionally un- 
stable. This may be attributed 
to repulsions arising from meth- 
ylene hydrogens whose ex- 
ternal radii are forced by the 
shape of the molecule into a 
space too small to receive them 
(10). As we have already 
pointed out (11), oxygens in a 
chain may relieve strains of this 
type since they carry no hydro- 
gens and their valences are prob- 
ably more flexible than those of 

carbon (12). Further presumed illustrations of the oxygen effect: poly- 
formals (III) are definitely more difficult to depolymerize than are poly- 
carbonates (IV), and trie thy lene glycol formal (V) is very much more 
easily depolymerized than alkylene formals of similar unit length. 

. . (O R O CH 2 )x (O R O CO)* . . . 

(HI) (IV) 

. . . (OCH,CH 2 OCH a CH20CH,CH,OCH 2 ) x --. . . 

Experimental Part 

Tetramethylene Formal. Difficultly separable mixtures were formed when the inter- 
change method was applied to this compound. The most convenient method was to dis- 
til the glycol (30 g.) with trioxymethylene (10 g.) and a trace of camphor sulfonic acid 
(bath temperature, 210). Fractionation of the distillate gave some tetrahydrofuran, 
and then a liquid boiling at 112-117 which was washed with strong caustic and redis- 
tilled at low pressure. It had dj 1.0022; w 2 D 1.4310; M D found, 26.34; M D calcd., 

Decamethylene Formal. Dibutyl formal (0.2 mole) with a 5% excess of decamethyl- 
ene glycol and 0.1 g. of ferric chloride gave fairly rapid reaction at 165 (bath). The 

Fig. 2. x-Ray diffraction pattern of cold-drawn 
o)-decamethylene formal. 



temperature was raised to 200 during two hours and heating continued for one and one- 
half hours in a good vacuum. The distillate was 98% of the theoretical calculated as 
butyl alcohol; the yield of residual a-polymer was 103%. When cold, it was a light 
brown rather hard wax. When dissolved in hot ethyl acetate (150 cc. for 17.5 g.), it 
separated in the form of a microcrystalline powder; soluble in chloroform, benzene, car- 
bon tetrachloride and xylene; insoluble in alcohol, ether, petroleum, hydrocarbons and 

0-Product. Eight grams of the crude a-polymer was heated in a 250-cc. suction 
flask provided with a test-tube through which water could be circulated to act as an in- 
ternal condenser (13) ; temperature, 230-250 (bath) ; pressure, about 1 mm. After forty- 
eight hours 2 g. of distillate had collected. It was a pasty mixture of liquid and crystals 
having a pleasant camphoraceous odor. The solid portion after crystallization from 
alcohol was odorless and melted at 93-94 . It was the cyclic dimer. 

w-Polymer. The residue from the above was a hard, very tough, opaque, leather-like 
mass. It melted (became transparent) at 58-63 , but at this temperature it was too 
stiff to flow and showed considerable resistance to deformation. At slightly higher 
temperatures, it could be drawn out into thin strips or filaments which could be stretched 
and cold-drawn (14). The product then showed fiber orientation (Fig. 2) and also ex- 
hibited parallel extinction between crossed Nicols. The cold-drawn material was ex- 
ceedingly strong, tough and pliable. 


Name of formal 

Decamethylene, a- 


Decamethylene, dimer 
Pentamethylene, dimer 


Hexamethylene, dtraer 
Nonamethylene, dimer 


Triethylene glycol, 


In boiling benzene; 

7-Polymer. The cyclic dimer (0.5 g.) with a trace of camphor sulfonic acid heated 
at 150 soon became very viscous and the characteristic odor of the cyclic monomer ap- 
peared. After an hour, the melt when cooled set to a hard wax, easily electrified when 
powdered. Purified out of benzene, it separated as a microcrystalline powder, m. p. 58- 
59; molecular weight observed in freezing benzene, 2580. 

Other Formals. The other a-polyformals were for the most part brittle waxy solids, 
and the observed melting points were: pentamethylene 38-39, hexamethylene 38, 
nonamethylene 54-55, tetradecamethylene 68-69, octadecamethylene 71-72 and 



i cm ua 

t jj , 
Mol. wt. in 

M. p., C. 









, p. 112-117 

C 6 HioO 2 






59 6 




(CuHOj) x 





(186) x 












70 8 


, 1 



(CeHi20 2 ) 2 











. p. 40-44 









(11 mm.) 


(C7Hl 2 2 )2 











(CioH 2 o0 2 )i 






69 8 



334 a 








73 . 1 











74 4 
















triethylene, sirup. The 0-polymers were generally pasty solids having the odors indi- 
cated in Table I and, when purified by crystallization from alcohol, yielded the odorless 
crystalline dimers whose melting points are shown in Table II. Pentamethylene formal 
depolymerized more readily than its higher homologs, and the monomer was isolated 
from the 0-product as a colorless liquid of camphoraceous odor; b. p. 40-44 at 11 mm. 
Triethylene glycol formal depolymerized more readily than any of the others and gave a 
70% yield of 0-product which apparently consisted largely of monomer melting at 18- 


A new class of linear polymers represented by the general formula 
. . . [CH 2 O R O] x . . . is obtained by the action of alkyl formals on 
the higher glycols (above tetramethylene). These a-polyformals can in 
part be depolymerized to the j3-forms (cyclic monomer and dimer) and in 
part converted to the higher co-polyformals. The latter can be drawn out 
into strong, pliable, highly oriented fibers. The /3-forms constitute a new 
type of large rings; the monomers have odors scarcely distinguishable 
from the corresponding carbonates j O R O CO j and in particular 

tetradecamethylene formal, the 17-membered ring, has a musk-like odor. 
The dimers are odorless crystalline solids. Trimethylene formal (6-mem- 
bered ring) does not polymerize, and no polymeric form is known, but 
tetramethylene formal (7-membered ring) and the higher ones polymerize 
rapidly (e. #., at 150) when catalyzed by acid. 

Bibliography and Remarks 

(1) Hill and Carothers, ibid., 64, 1569 (1932); 55, 5023, 5031 (1933). 
.(2) Ibid., 55, 5035 (1933). 

(3) See, for example, Hibbert and Timm, Journ. Am. Chem. Soc., 45, 2433 (1923); Hill and 
Hibbert, ibid., 3108 3124; Helferich and Sparmberg, Ber., 64, 104 (1931); Bergmann and Miekeley, 
ibid., 62, 2297 (1929). 

(4) Carothers and van Natta, Journ. Am. Chem. Soc., 52, 314 (1930); Hill and Carothers, ibid., 
55, 5037 (1933). 

(5) Ibid., 55, 5039 (1933). 

(6) Ruzicka, Salomon and Meyer, Helv. Chim. Ada, 17, 882 (1934). 

(7) Journ Am. Chem. Soc., 64, 1579 (1932). 

(8) Staudinger, "Die hochmolekularen organischen Verbindungen," Julius Springer, Berlin, 1932, 
p. 262; Sauter, Z. physik. Chem., 21B, 161 (1933). 

(9) Leutner, Monatsh., 60, 317 (1932). 

(10) Stoll and Stoll-Comte, Helv. Chim. Acta, 13, 1185 (1930); Hill and Carothers, Journ. Am. 
Chem. Soc., 55, 5023, 5031 (1933). 

(11) Journ. Am. Chem. Soc., 55, 5050 (1933). 

(12) Cf. Sidgwick, "Annual Reports of the Progress of Chemistry for 1932," p. 73. 

(13) Journ. Am. Chem. Soc. t 55, 5035 (1935). 

(14) Ibid. t 54, 1580 (1932). 


XXV. Macrocyclic Esters* 

Cyclic esters can be prepared^ with alkaline catalysts from linear poly- 
esters derived from carbonic, oxalic or malonic acid, but not from higher 
members of the homologous series. 

It has been found that by depolymerization at exactly 270 at a pressure 
below 1 mm. Hg in the presence of hydrous metal chlorides, such as SnCl 2 -- 
2H 2 O, KnCl 2 -4H 2 O, FeCl 2 -4H 2 O, or of magnesium powder good yields 
of macrocyclic esters are obtainei. 

Thirty -six new esters of this type partly monomer ic, partly dimeric 
derived from succinic acid and from other, higher dibasic acids, have been 
prepared and are described. 

The molecular refractions of large rings are generally considerably less than 
the calculated values, the depression, ranging from 0.4 to 0.7, being most 
marked in the region between 8 and 15 carbon atoms. This is attributed 
to the strain and interference of the peripheral atoms described in a pre- 
vious paper** and connected with the relatively large sphere of interference 
attributed to the hydrogen atoms of the diain. 

The depolymerization of linear polyesters derived from carbonic oxalic, 
or malonic acid to the corresponding cyclic monomers and dimers proceeds 
smoothly and rapidly in the presence of alkaline catalysts (1), but the same 
conditions give very poor results when applied to polyesters derived from 
succinic acid or from higher acids of the series. We have, however, now 
found a number of catalysts that act effectively in these cases. The result 
is a considerable extension of the useful range of the interchange method 
in the preparation of macrocyclic esters. Since n different glycols and an 
equal number of dibasic acids may give 2n 2 different cyclic monomers and 
dimers, the number of possible compounds is very large. In this paper, 
we describe thirty-six new macrocyclic esters. Their properties are listed 
in Table I and are discussed in a later paragraph. Details of procedure are 
presented in the Experimental Part, but points of essential importance are 
as follows. 

The depolymerizations are carried out in the glass apparatus depicted 
in Fig. 1 (2). The outer flask contains a liquid whose refluxing vapors 
heat the inner chamber where the depolymerization occurs. This makes it 

* E. W. Spanagel and W. H. Carothers; Journ. Am. Chem. Soc. t 57, 929-34 (1935) ; 
Communication No. 153 from the Experimental Station of E. I. du Pont de Nemours 
and Co. 

Received March 21, 1935. 

t Compare paper No XX on page 212. 

** Compare paper No. XXII on page 22.~>. 



possible to control the temperature very exactly and ensures an adequate 
heat input. The optimum temperature appears to lie in the neighborhood 
of 270; the pressure should be about 1 mm. or lower. Materials that 
act as effective catalysts with esters from higher acids are SnCl2.2H 2 O, 
MnCl 2 .4H 2 O, FeCl 2 .4H 2 O, MgCl 2 .6H 2 O, CoCl 2 .6H 2 O, MnCO 3 , MgO, Mg- 
CO 3 and Mg powder. 

By way of specific example, it may be mentioned that 20 g. of polymeric 
hexamethylene sebacate with 0.2 g. of MgCl 2 .(>H 2 O, in two hours gave 15 
g. of distillate from which 11.4 g. of pure crystalline monomer was isolated. 
In the preparation of other esters listed in Table I, the crude yields gen- 
erally ranged from 40 to 85%, while in favorable cases the yields of pure 
monomers approached 70%. Presum- 
ably both monomers and dimers are 
always present in the crude depoly- 
merizate. In fact, for unit lengths of 
8, 9, 10 and 11, the dimers tend to 
predominate whereas for unit lengths 
above 13 very little dimer, if any, is 
ordinarily isolated. The ratio of dimer 
to monomer can be controlled within 
certain limits since if refluxing in the 
depolymerization vessel is permitted 
relatively little dimer can escape. 

Physical Properties. The molecu- 
lar refractions of large rings are 
generally considerably less than the 
calculated values (3), the depressions 
commonly ranging from about 0.4 to 
0.7. This peculiarity is, however, 
for the most part restricted to rings of 8 to 15 atoms, and some of the largest 
rings (e. g., 30-34 members) even show considerable exaltations (3). In 
Table I molecular refractions for 26 new macrocyclic esters are listed. The 
average depression is 0.32. The values fluctuate rather widely, and this 
may be due in part to experimental error since the measurements had to be 
made at a relatively high temperature. It must, however, be regarded 
as a significant fact that, in general, the largest depressions are found for 
the smaller rings (9 to 14 atoms) where the strains or interferences of the 
peripheral atoms are greatest, while depressions very close to zero are found 
only in three cases. These are listed below together with one of the cyclic 
carbonates (4) which shows a similar peculiarity. 

Fig. 1. Apparatus for depolymerizing. 



Ring size 





, 0(CH 2 )i 8 OCO i 

, Q(CH a ) l4 OCO(CH 8 )aCO | 

r (OCH 2 CH2),OCO(CH 8 ) 4 CO [ - .09 

4- .03 

The first two of these are larger rings than any other esters for which values 
are available. The other two have in the ring extra oxygens which are 
capable of relieving steric interferences (5). 

Densities of the new esters for 
which values are available are 
plotted in Fig. 2. The curve 
shows no maximum, and the den- 
sities of isomers lie quite close 

The odors of the macrocyclic 
esters are especially interesting, 
but characterizations are not 
sufficiently exact to justify a de- 
tailed discussion. It may be 
pointed out, however, that Table I 
includes 3 new rings of 14 atoms, 
3 of 15, 4 of 16, 7 of 17, 2 of 18, 
and one each of 19 and 20 atoms. 
Those of 18, 19 and 20 atoms are 
practically odorless; the odors 
most prominent in the others are 
woody or cedary, earthy, cam- 
phoraceous and musk-like. The 
musk-like odor is very pronounced 
in all of the 17-atom rings. 

Data on melting points are rather incomplete, but here, as in the series 
previously examined, there is little to indicate any regular relation between 
ring size and melting point. Values for succinates are shown in Fig. 3. 
As the ring size increases, the melting point first falls to a minimum, then 
it rises to a maximum, falls again and becomes almost constant. The 
jsomeric 17-membered rings have the general formula i OCO(CH 2 ) n COO- 

(CH 2 )m | w j iere n = 17 ( m 4. 4). The melting points for various 
values of n are 






a 1.06 



20 22 

12 14 16 18 
Atoms in ring. 

Fig. 2. Densities (60/4) of cyclic esters: 
A, succinates; O, other cyclic esters. 











Ease of Ring Formation. As measured by yields, it has been found for 
every series thus far examined that rings of 9 to 12 atoms are more difficult 
to form than rings of 15 to 20 atoms. Stoll and Stoll-Comte (6) have sug- 
gested that this is due to a type of steric hindrance caused by mutual re- 

10 12 14 16 18 20 22 24 

Atoms in ring. 
Fig. 3. Melting points of monomeric alkylene succinates. 

pulsions of peripheral hydrogens which are crowded against one another 
in rings of intermediate sizes. The position of the minimum in the yield 
vs. ring-size curve is not the same in different series. For cyclic ketones it 
lies at 10 to 11 atoms; for esters derived from dibasic acids, it apparently 
lies at 7 to 8 atoms. At any rate, monomers of this size have not yet been 
obtained among the dibasic esters, while, for example, the 7-atom lactone 
is obtained as a major primary product from c-hydroxycaproic acid (7). 

Another point of interest is that in the intermediate range odd-mem- 
bered rings are relatively more difficult to produce than the adjacent even 
ones. An alternating effect of this kind was first observed in the dibasic 
acid anhydride series (8). Although rings of 8. 10 and 12 atoms were 
formed, those of 9, 11 and 13 atoms were not (the corresponding dimers 
were formed instead). More recently, an alternation in yield of cyclic 
ketones in passing from one member to the next has also been observed by 
Ziegler and Aurnhammer (9) in syntheses by the dilution method. The 
type of alternation observed in the present study is illustrated by Fig. 4 
where the yield of dimer is plotted against unit length for glycol esters of 
succinic acid. The material not dimer is mostly monomer, and so by in- 
verting the ordinate scale, a rough plot of monomer yield is obtained. The 



alternating effect is more clearly brought out by the inset of Fig. 4 which 
shows the rate of change of dimer yield with increasing unit length. One 
is tempted to compare this behavior with the well-known alternation in 
physical properties of certain series of open chain compounds, but the 
responsible factors are probably quite different in the two cases. Ex- 
amination of models (with large hydrogens) shows that the interferences to 
ring closure in chains of 9 to about 13 atoms appear to be greater for the 


h iU .U 1-t lo Irt 2U 

Atoms in cyclic monomer. 

Fig. 4. Yield vs. ring size in the preparation of alkyl- 
ene succinates. 

odd members (10), arid it is probable that these interferences are still opera- 
tive after the chains are closed so that the odd-membered rings are less 
stable than the even ones. 

Since the rates at which different rings are formed are sure to be dif- 
ferently affected by changes in conditions, the inherent relative ease of ring 
formation is a concept to which no quantitative significance can be at- 
tached. Nevertheless, the effect of structure and particularly of chain 
length is so pronounced that even crude data permit a qualitative compari- 



son of the inherent relative ease of forming rings of different sizes. A 
comparison of this kind for glycol esters of dibasic acids is shown in Fig. o, 
and it includes a similar plot for the stability of cyclic anhydrides. Ease 
of ring formation and stability will, in general, run parallel, but 6-membered 
cyclic esters furnish a notable exception to this rule. They are both more 
easily formed and less stable than larger ester rings. 

Methods of Ring Formation. For the synthesis of large rings, there 
are now available two methods based upon rational principles : the dilution 
method first explicitly formulated and applied by Ruggli in 1912 (11) and 
the interchange method described above. 

Ruggli's dilution method was cited in early papers of this series (12) 
where the general theory of bifunctional reactions was developed, but it 




16 18 

8 10 12 14 
Atoms in ring. 
Fig. 5. Ease of formation and stability vs. ring si/A'. 

received no further published attention until very recently. Ziegler and 
his co-workers (13) have now applied it in a very ingenious fashion to the 
synthesis of cyclic ketones in good yields, and further successful applica- 
tions have been made by various investigators to the preparation of macro - 
cyclic amides (14), imines (15) and lactones (1(5). 

In a recent discussion of ring formation (17), Ziegler suggests that the 
interchange method will be severely restricted in the range of its applica- 
bility. On the contrary, as we have shown in this and the preceding paper, 
it presents extensive possibilities, and it may be appropriate to point out 
some of the peculiar advantages of this method. 

It is true, of course, that the interchange method is inherently restricted 
to reversible bifunctional condensations, but almost all condensations are 




O t< 


><5 O 

CO >O -i CO ^ ** t- ' 

I I I I I I J I I 


i 00 O3 O 

I t 1 I I I I I I I I I + I I I I 

) oo oo < 

I CO r- ! 


' - 5 
s o c 


to o co co o 

l^ CO 10 O 00 

"H CO Q CO i i 

o o o o o 



"S fi 

<N W - O *-< 

o w S S 

J CM t-" 


01 M (N (M 


cc a o o o> o o> 
04 ^ececioio cc 



S) ; I 

"2 S fi 







Cyclic ester 

% Si d 

U*{,u. l <U4,rtrt-rJ.2 -^ 

s 1 1 1 3 * 3 1 -S I i i .& 5 


Tetramethylene azelate 

Hexamethylene azelate 

Ethylene sebacate 

Trimethylene sebacate 










Pentamethylene sebacate 

Diethylene glycol sebacate 
Hexamethylene sebacate 

Ethylene decamethylene dicai 

Ethylene brassy late 
Decamethylene octadecanedic 








reversible to a certain extent. We have already presented some evidence 
(18) indicating that even polyke tones can be depolymerized and that this 
is what is involved in Ruzicka's thorium salt method for the synthesis of 
large carbon rings. Practically, the interchange method will be restricted 
to those bifunctional condensations that are easily reversible, but, as the 
present paper shows, much can be accomplished in that direction by the 
choice of suitable catalysts, and the formation of esters and anhydrides by 
no means represents the limit of the possibilities. 

One other point that deserves mention is this: there are two general 
types of bifunctional reactions symbolized in the two formulas 

x R y > products, and 

x R x + y R' y > products 

In the first type (simple bifunctional), both the mutually reactive groups 
are present on the same molecule; in the second (bifunctional) (12) the 
mutually reactive groups are initially present on separate molecules. An 
example of the first type is an hydroxy acid, of the second type, a dibasic 
acid plus a glycol. The second type is much the more numerous, and it is 
one to which the theoretical advantages of the dilution method are not 
applicable with full efficiency since the initial step toward ring formation 
necessarily involves an intermolecular reaction. Practical advantages of 
the interchange method in connection with such problems as the prepara- 
tion of lactones and Brings will be illustrated in future papers. 

Experimental Part 

The apparatus used in the present work is shown in Fig. 1 . The flask B contains a 
liquid whose refluxing vapors heat the chamber A. This method makes it possible to 
control the temperature very exactly and ensures an adequate heat input. The de- 
phlegmator C may be cooled with steam if reflux is desired to restrict the distillation of 
dimer, or it may be omitted entirely. For amounts of polymer in the neighborhood of 
50 g., the chamber A should be of about 800 cc. capacity since considerable foaming 
occurs at the start of the depolymerization. 

Preliminary experiments were made with decamethylene carbonate using a sodium 
catalyst. The importance of low pressure was soon demonstrated, and in further work 
a pump with high capacity was used and the pressure kept at 1 mm. or below. The 
rate increased, but the purity of the distillate dropped off somewhat as the temperature 
was raised above 250. A temperature of 270 appeared to be about the optimum, and 
it was conveniently supplied by boiling 0-chlorodiphenyl. These observations were 
applied to the preparation of monomeric tetradecamethylene carbonate on a one hundred 
gram scale using an alkaline catalyst; a 93% yield of monomer having a purity of 97% 
was obtained in less than two hours. 

These conditions, however, gave no satisfactory results with polyesters derived from 
higher dibasic acids, and attention was therefore turned to a further exploration of 


catalysts. The polyesters used were prepared from the appropriate dibasic acid and a 
slight excess of glycol by heating at 200 (bath) finally at a pressure of 2-3 mm. Prod- 
ucts obtained in this manner were gray waxy masses of varying degrees of hardness. 
One of the first catalysts tried was stannous chloride, and the following results are 


Mol wt. 

f Calculated 

C, % H, % Mol. , Found 


Trimethylene succinate, Monomer 
Trimethylene succinate, Dimer 
Tetramethylene succinate, Monomer 
Tetramethylene succinate, Dimer 
Pentamethylene succinate, Monomer 
Pentamethylene succinate, Dimer 
Hexamethylene succinate, Monomer 
Hexamethylene succinate, Dimer 
Heptamethylene succinate, Monomer 
Heptamethylene succinate, Dimer 
Octamethylene succinate, Monomer 
Octamethylene succinate, Dimer 
Nonamethylene succinate, Monomer 
Decamethylene succinate, Monomer 
Dodecamethylene succinate, Monomer 
Tridecamethylene succinate, Monomer 
Tetradecamethylene succinate, Monomer 
Octadecamethylene succinate, Monomer 
Decamethylene glutarate, Monomer 
Hexamethylene adipate, Monomer 
Triethylene glycol adipate, Monomer 
Nonamethylene adipate, Monomer 
Heptamethylene suberate, Monomer 
Ethylene azelate, Monomer 
Ethylene azelate, Dimer 
Tetramethylene azelate, Monomer 
Hexamethylene azelate, Monomer 
Ethylene sebacate, Monomer 
Trimethylene sebacate, Monomer 
Tetramethylene sebacate, Monomer 
Pentamethylene sebacate, Monomer 
Diethylene glycol sebacate, Monomer 
Hexamethylene sebacate, Monomer 
Ethylene decamethylene dicarboxylate, Monomer 
Ethylene brassylate, Monomer 
Decamethylene octadecanedioate, Monomer 

Fifty grams of polymeric hexamethylene succinate with 0.5 g, of vSnCl 2 .2H 2 O was 
heated at 270 at 1 mm. or less. The polymer became soft and bubbled, and after 
about ten minutes distillation started. The rates observed were : after fifteen minutes, 
12 drops/min.; after thirty minutes, 14 drops/min. At this point the residue became a 
stiff porous gel occupying about half the volume of the flask, but distillation continued : 
after forty-five minutes, 10 drops/min. ; after sixty minutes, 8 drops/min. ; after seventy- 
five minutes, 3 drops/min. The ultimate residue was a light porous mass of tough, in- 


\_ttiu uiaicu * 

% H, % Mol. 


% H, % 


53 . 











53 . 





53 . 
















55 . 




















































































03 . 





















05 . 






















































































































































































































































soluble, yellow resin. The distillate (40 g.), consisting of a light yellow liquid and a 
white solid, was filtered. The solid on recrystallization from alcohol yielded 8 g. of 
dimer, white plates melting at 110. Any catalyst present in the monomer was removed 
by dissolving the filtrate in ether and washing with water. The dried ether solution was 
concentrated and distilled. The yield of cyclic monomeric hexamethylene succinate 
boiling at 108-110 (2 mm.) was 21 g. This general procedure was used in isolating 
other cyclic esters except that when the monomer was a solid it was usually purified by 
crystallization from alcohol at low temperature. Yields of crude distillate varied from 
40 to 85%. 

For further study of catalysts, experiments were made with hexamethylene sebacate 
since its monomer is a readily purified crystalline solid. The polyester (20 g.) was heated 
with 0.2 g. of the possible catalyst in a glass still at 270 (1 mm ) for two hours. The 
crude distillate was dissolved in 50 cc. of alcohol, cooled, and the pure monomer isolated 
by filtration. Results are shown in Table III. 

monomer, % 








Distillate, % 

SnCl 2 .2H 2 O 


MnCl 2 .4H 2 O 


FeCl 2 .4H 2 


MgCl 2 .6H 2 


CoCl 2 .6H 2 


Co(N0 3 ) 2 .6H,0 


Mg (powd.) 


MnCO 3 




MgC0 3 


PbCl 2 


FeCl 3 


T1 2 C0 3 


SbCl 3 


Th(NO 3 ) 4 .12H 2 O 


None (control) 


The following substances were also tried but gave very small distillates, from which 
no monomer could be isolated: NiCl 2 .6H 2 O, FeSO 4 , TiCl 4 , tin dust, Mg 3 (PO 4 ) 2 , CrCl 3 , 
CaCO 3 , CrCl 2 , ZnCl 2 . 

Typical yields of cyclic esters obtained at 270 /l mm. using 1 to 3% SnCl 2 .2H 2 O 
as catalyst are shown below. 

Losses in purification are due in part to polymerization during redistilla- 
tion, and these can be largely avoided by first washing out the catalyst. 
Considerable improvements can also be effected by suitable precautions in 
preparing the polymer. 





Ester length 

Ethylene succinate 8 

Trimethylene succinate 9 

Tetramethylene succinate 10 

Pentamethylene succinate 1 1 

Heptamethylene succinate 13 

Octamethylene succinate 14 

Nonamethylene succinate 1 5 

Octadecamethylene succinate 24 

Decamethylene glutarate 17 

Nonamethylene adipate 1 7 

Heptamethylene suberate 1 7 

Ethylene azelate 13 

Trimethylene sebacate 1 5 

Tetramethylene sebacate 16 

Pentamethylene sebacate 17 

Hexamethylene sebacate 18 

Diethylene glycol sebacate 17 

Ethylene brassylate 1 7 



isolated as 
monomer dimer 






































The optimum conditions for the depolymerization of linear polyesters 
have been more clearly defined and new catalysts have been found which 
make it possible to apply this method generally to the rapid preparation of 
macrocyclic esters in good yields. Thirty-six new cyclic esters, monomeric 
and dimeric, derived from succinic acid and from other higher dibasic 
acids are described. Conclusions developed in the study of other macro- 
cyclic compounds have been confirmed and extended. 

Bibliography and Remarks 

(1) Hill and Carothers, ibid , 55, 5031 (1933). 

(2) This design was suggested to us by Mr C H Greenewalt. 

(3) Ruzicka and Stoll, Helv. Chim. Acta, 11, 1159 (1928), Ruzicka, Stoll, Huyser and Boeke 
noogen, ibid , 13, 1152 (1930), Ruzicka and Boekenoogen, ibid., 14, 1319 (1931); Ruzicka, Hurbin, 
and Furter, ibid , 17, 78 (1934), Hill and Carothers, Journ. Am. Chem. Soc. f 55, 5042 (1933) 

(4) Journ Am. Chem Soc , 55, 5042 (1933) 

(5) Hill and Carothers, ibid , 57, 925 (1935); 55, 5049 (1933) 

(6) Stoll and Stoll-Cornte, Helv. Chim. Acta, 13, 1185 (1930) 

(7) van Natta, Hill and Carothers, Journ. Am. Chem Soc , 56, 455 (1934). 

(8) Hill and Carothers, Journ Am Chem. Soc , 55, 5025 (1933) 

(9) Ziegler and Aurnhammer, Ann., 513, 43 (1934) 

(10) Hill and Carothers, Journ. Am Chem Soc , 55, 5050 (1933). 

(11) Ruggli, Ann., 392 92 (1912) 

(12) E f> , Journ Am Chtm Soc , 51, 2551 (1929) 


(13) Ziegler, Eberle and Ohlinger, Ann, 504, 94 (1933); Ziegler and Aurnhammer, ibid., 513, 
43 (1934) 

(14) Reid and Lippert, St. Petersburg Meeting of the A. C. S , March, 1934 

(15) Salomon, Helv. Chim. Ada, 17, 851 (1934), Ruzicka, Salomon and Meyer, ibid , 17, 882 

(16) Stoll, Rouve and Stoll-Comte, ibid , 17, 1289 (1934). 

(17) Ziegler, Ber., 67, 139 (1934). 

(18) Joitrn Am. Chem Soc., 55, :>04, r > (1933). 

XXVI. Meta and Para Rings* 

This paper deals with rings closed through the m- and p- positions of the 
benzene nucleus. Six meta rings were prepared by combining resorcinol 
diacetate with 

ethylene glycol 
trimethylene glycol 
tetrametkylene glycol 
hexamethylene glycol 
nonamethylene glycol and 
decamethylene glycol 

Five para rings were obtained from hydroquinone diacetate with 

ethylene glycol 
trimethylene glycol 
tetramethylene glycol 
hexamethylene glycol and 
decamethylene glycol 

Again there is formed first a polyester, which gives the required ring by 
depolymerization. Mela rings much smaller than 13 atoms and para 
rings much smaller tlian 16 atoms are not likely to be obtained. All 
samples prepared were pure white sharply melting macrocrystalline 
solids. They are easily soluble in most organic solvents. 

The closure of rings through the m- and /^-positions of the benzene nu- 
cleus is one of the conventional problems of organic chemistry that long 
resisted solution. As soon as a rational theory of ring closure had been 
developed (1), it became apparent that success might easily be achieved 
by the use of either the dilution or the interchange principle. Meanwhile, 

* E. W. Spanagel and W. H. Car-others; Journ. Am. Chem. Soc., 57, 935-6 (1935); 
Communication No. 154 from the Experimental Station of E. I. du Pont 4e Nemours 
and Co. 

Received March 21, 1935, 


Ruzicka, Buijs and Stoll (2) applied their thorium salt method to the acids 
p-C6H 4 [(CH2) B COOH]2 and m-C 6 H 4 [(CH2) 6 COOH] 2 and from the latter 
obtained a small amount of the 16-membered cyclic ketone, while Ziegler 
and Luttringhaus (3) have applied the dilution method to the nitriles m- 
and />-C8H4[O(CH 2 )eCN]2. The cyclic iminonitriles of 18 and 10 atoms 
were obtained in good yields. 

In the experiments presently reported, the acids m- and -C 6 H4(OCH 2 - 
COOH) 2 were estcrified with glycols of the series HO(CH 2 ), 7 OH and the 
resulting polyesters depolymerized. In this manner, we have obtained 
w-rings (I) of 13, 14, 15, 17, 20 and 21 atoms, in yields ranging from 10 to 
35%. In the ^-series (TI) none of the 14- or 15-membered ring was ob- 
tained, but those of 1G, IS and 22 atoms were isolated in yields of 12 to 




-O -CH 2 CO - 

(I) (ID 

Data concerning yields and properties arc shown in Table I. 


Poly- Crude Pure M. p of Ring size 

ester, Heating, yield, yield, monomer, of 

Polyester diacetate K hrs. % % C monomer 

Ethylene resorcinol 23 2.5 30 21 100 13 

Trimethylene resorcinol 29 3.5 41 24 134 14 

Tetramethylene resorcinol 22 5 23 16 112 15 

Hexamethylene resorcinol 22 3 45 35 115 17 

Nonamethyleiie resorcinol 20 5 45 35 86 20 

Decamethylene resorcinol 20 5 55 35 86 21 

Ethylene hydroquinone 48 14 9 

Trimethylene hydroquinone 35 65 10 

Tetramethylene hydroquinone 28 4 42 12 140 10 

Hexamethylene hydroquinone 50 7 30 12 124 18 

Decamethylene hydroquinone 50 7 38 18 58 22 

These results demonstrate clearly the formation and stable existence of 
a 13-membered w-ring, and they suggest that in the ^-series a ring of 16 
atoms is the smallest possible, but caution must be used in extending or 
generalizing these conclusions. On the one hand, it has already been 


shown (4) that the replacement of CH 2 by O - in a chain consider- 
ably increases the ease of ring formation; thus, in the carbocyclic series 
the limits may lie at values different from those obtained for rings contain- 
ing two ester groups and two ether oxygens. On the other hand, the fail- 
ure to obtain a ring by no means demonstrates that its formation is im- 
possible or that it would be very unstable if formed. Nevertheless, in 
view of the observed facts and the implications of space models, it seems 
that m-rings much smaller than 13 atoms and ^-rings much smaller than 16 
atoms are not likely to be obtained. 


Preparation and Properties of Polymers. Polymeric resorcinol diacetatcs were 
prepared by heating equivalent amounts of the acid and glycol at 19O 200 for about 
three hours and finally heating the residue to 210 in vacua one or two hours. 
The polymers were light brown viscous resins. Nonaniethylciie resorcinol diacetate, 
however, crystallized on long standing; the melting point of the crude polymer was 

Owing to the high melting point of the acid, polymeric hydroquinonc diacetatcs were 
prepared by heating equivalent amounts of the glycol and the ester (ethyl hydroquinone 
diacetate) with a small crystal of stannous chloride to 190 for about three hours. Final 
traces of alcohol were removed by heating the mixture in a vacuum to 210. Ethylene 
and trimethylene hydroquinone diacetates are low-melting brown glassy resins. The 
remaining polymers were solids which had the following melting points: 

Tetramethylene hydroquinone diacetate 45-50 

Hcxamcthylene hydroquinone diacetate 50-55 

Decamethylene hydroquinone diacetate 00-65 

Depolymerization. The polyesters were depolymerized at 270 (1 
mm.) in an 800-cc. vapor-heated still or clepolymerizer of the type described 
in the preceding paper (5), using 1 to 2% of SnCl 2 .2H 2 O as catalyst. 
Rates, yields and melting points are given in Table I. The monomers, 
purified by crystallization from alcohol, are all pure white sharply macro- 
crystalline solids. They are very soluble in ethyl acetate, benzene, acetone 
and ether; moderately soluble in petroleum ether and carbon tetrachloride. 
In the latter solvent, the higher members are more soluble than the lower 
ones. The compounds are all neutral (i. e., they do not decompose po- 
tassium carbonate) , and they react only slowly with a carbon tetrachloride 
solution of bromine. Two of the monomers, hexamethyletie hydroquinone 
diacetate and hexamethylene resorcinol diacetate, were saponified; the 
corresponding dibasic acids were obtained in good yields and identified by 
mixed melting points. 


Polyesters prepared from the acids m- and ^-CeH^OCI^COOH^ by 

causing them to react with glycols HO(CH 2 ) n OH have been depolymerized. 
Cyclic monomers were thus obtained in the w-series, having rings of 13, 
14, 15, 17, 20 and 21 atoms. In the ^-series none of the 14- or 15-mem- 
bered rings could be isolated, but those of 16, 18 and 22 atoms were readily 


C H Mol. wt. C H Mol wt rt 

Ethylene resorcinol diacctate Decamethylene resorcinol diacetate 

Calcd. for Ci,H 12 O 6 57 .14 4 70 252 Calcd. for C 20 H, 8 O fi 65 . 92 7 . 69 364 

Found 57.37 4 76 252 Found 66.02 7.75 364 

Trimethylene resorcinol diacetate Tctramethylene hydroquiuone diacctale 

Calcd. for C,,H u Oc 58.64 5.96 266 Calcd. for C 14 H 16 O 6 60.00 5.71 280 

Found 58.69 5.38 260 ^.M 586 268 

Tetrarricthylene resorcinol diacetate 

r* 1 A f r* ri r\ I'M MM - ~ i OOM Hexamethylcnc hydroquinone diacetate 
Calcd. for Ci4Hi 6 O 6 ()() 00 o./l 280 J J H 

Found 6004 5.65 290 Calcd. for C,H 20 O 6 6233 6.49 308 

Found 62.40 6.84 307 

Hexamethylcnc resorcinol diacetate 

Calcd. for CieH^Oe 62 33 6 49 308 Decamethylene hydroquinone diacetate 

Found 61.96 629 302 Calcd. for C 20 H 28 6 65.92 7.69 364 

Nonamethylene resorcinol diacetate Found ^ 7 ' 84 356 

Calcd. for Ci 9 H 26 O 6 65 .14 7 . 43 350 
Found 65 02 7 31 360 

a Determinations made by freezing point in benzene. 

Bibliography and Remarks 

(1) Journ Am. Chem Soc , 51, 2548 (1929); 54, 1569 (1932); 55, 5043 (1933) 

(2) Ruzicka, Buijs and Stoll, Helv. Chtm. Ada, 15, 1220 (1932). 

(3) Ziegler and Luttringhaus, Ann., 511, 1 (1934). 

(4) Journ Am. Chem. Soc., 55, 5050 (1933). 

(5) Ibid , 57, 929 (1935, this volume, page 248). 


XXVII. Polydecamethylene Oxide* 

^During the course of the investigations which belong to this series it was 
observed] that the depolymerization of polydecamethylene carbonate 
gave a rubbery residue. This material was hydrolyzed and gave deca- 
methylene glycol, decene-9-ol-l and polydecamethylene oxide. The 
latter has a molecular weight of about 1200. 

In a previous paper of this series (1) it was shown that polyesters, when 
heated under appropriate catalytic conditions in an evacuated vessel 
designed for the quick and irreversible removal of volatile material, undergo 
the series of transformations represented in the scheme 

cyclic monomer 

and/or dimer 


a-polyester / A 

molecular weight ca. \ | 

2000-5000 \ | 

w-polyester (residual) 

molecular weight above 


In the case of the polymethylene carbonates, the end-products under 
favorable circumstances were almost all obtained as volatile depolymerizate 
and very little residue remained. However, in certain instances in which 
it is now believed that the pressure rose too high on account of inadequate 
pumping, the residue became rubbery early in the experiments and distilla- 
tion soon practically ceased. During the course of a large number of 
experiments on the depolymerization of polymeric decamethylene carbon- 
ate, a considerable amount of these residues accumulated, and it was 
thought worth-while to attempt the recovery of decamethylene glycol from 
them by hydrolysis. Comparatively little glycol was recovered in this 
way. The major product isolated from the hydrolysis mixture was a 
wax-like material which examination showed to be polydecamethylene 
oxide (2) 

. . . (CH 2 ) 10 O (CHa)ia O (CH 2 )io O (CH 2 ) 10 O. . . 

The hydrolysis of the rubbery residue was carried out by means of 
alcoholic potassium hydroxide. The resulting solution was filtered from 
a small amount of insoluble matter. Most of the alcohol was distilled and 

* J. W. Hill; Journ. Am. Chem. Soc., 57, 1131-2 (1935); Communication No. 156 
from the Experimental Station of E. I. du Pont dc Nemours and Co. 
Received April 26, 1935. 
t Compare paper No. Ill on page 29. 


replaced by hot water. The insoluble upper layer was allowed to solidify 
and separated. It was then heated under vacuum in a Claisen flask until 
the distillation of decamethylene glycol ceased. The residue was re- 
peatedly extracted with boiling water until no more glycol could be re- 
moved in this way. It was then recrystallized once from acetone, once 
from alcohol, dried and analyzed. 

Anal. Calcd.for(CH 2 )i O: C, 76.92; H, 12.82; mol. wt., 150. Found: C, 7(5.18, 
76.31; H, 12.44, 12.37; mol. wl. (cryoscopically in benzene), 1200, 1200. 

It was soluble in benzene, carbon tetrachloride, chloroform and con- 
centrated sulfuric acid, insoluble in cold alcohol, ethyl acetate and acetone, 
and insoluble, hot or cold, in petroleum fractions. From recrystallizing 
solvents, it separated as a microcrystalline powder of melting point 58- 
60. From a melt it was obtained as a soft wax. 

The probable course of the reaction leading to the polyether may be 
represented by the following scheme in which it is supposed that the poly- 
ester undergoes scission and gives carbon dioxide and decamethylene 
oxide radicals. These may rearrange to decene-9-ol-l or combine to give 
the polymeric ether. 

-C0 2 

( O (CH 2 ) 1(r - OCO ) H > 

a-poly decamet hy lene 

w[ (CH 2 ), O] > ( (CH 2 ) 10 0-) M 


CH 2 CH (CH 2 )iOH polydecamethylene oxide 

Support for this mechanism is the fact that in the depolymerization ex- 
periments yielding considerable polyether residues, the distillates were 
always contaminated with decene-9-ol-l (3). 

The constitution of the material was established by its analyses and 
molecular weight and by its transformation by the following reactions to 
decamethylene glycol. 


( (CHOur-O ) > I(CH 2 ) 10 I > 

AcO(CH 2 )i OAc > HO(CH 2 ) ]0 OII 

The intermediate compounds were not purified. 

Twenty grams of recrystallized polymer, 40 cc. of glacial acetic acid, and 
75 cc. of constant boiling hydriodic acid were refluxed together for six 
hours. The resulting dark red lower layer was separated and refluxed 
briefly with a suspension of calcium carbonate which removed the color. 


The heavy oil was recovered by means of an ether extraction and then re- 
fluxed with 40 g. of powdered freshly fused potassium acetate, 15 cc. of 
glacial acetic acid and 3 cc. of acetic anhydride for four hours. This mix- 
ture was drowned in water and the product recovered by ether extraction. 
This was then hydrolyzed by refluxing for ten hours with a solution of 15 
g. of sodium hydroxide in 150 cc. of 50% alcohol. The alcohol was re- 
moved by steam distillation which was continued for several hours to re- 
move a very small amount of oil, which smelled like decene-9-ol-l. The 
oil on top of the residual liquid solidified and from it was isolated, by two 
crystallizations from benzene, 5 g. of pure decamethylene glycol, m. p. 
71-72. It showed no depression in melting point when mixed with a 
known sample of decamethylene glycol. 


The depolymerization of polydecamethylene carbonate, under certain 
experimental conditions, pursues an abnormal course and yields a distillate 
contaminated with decene-9-ol-l together with a large amount of residue. 
This residue has been examined and found to consist largely of a poly- 
meric substance which has been characterized as polydecamethylene oxide. 
A mechanism for the side reaction has been suggested. 

Bibliography and Remarks 

(1) Journ. Am. Client Sac., 55, 5031 (1933) 

(2) It is of interest to mention that Franke \Monatsh , 53-54, .177 (1929), and earlier papers] 
obtained, by the dehydration of decamethylene glycol with sulfuric acid, not a decamethylene oxide, 
but tt-amyl pentamethylene oxide. 

(3) Journ. Am. Chem. Soc., 55, 5037 (1933) 

XXVIII. Preparation of Macrocyclic Lactones by 
D epolymerization * 

The depolymerization method^ is used in this publication to prepare 
macrocydic lactones. A yield of 70% of pure monomer could be obtained 
from the polyester of hydroxy tetradecanoic acid. Depolymerization 
was carried out with the aid of catalysts, mostly hydrous magnesium 
chloride at 270 in vacua. The lactones of the following acids were pre- 

* E. W. Spanagel and W. H. Carothcrs; Journ. Am. Chem. Soc., 58, 654-6 (1936); 
Communication No. 16(5 from the Experimental Station of E. I. du Pont de Nemours 
and Co. 

Received January 18, 1936. 

t Compare e. g. paper No. XXV on page 248. 


14-hydroxy-12-oxatetradecanoic (mono and dimer) 
1 6-hydroxy-l 2-oxahexadecanoic (monomer) . 

Lactones of the higher co-hydroxy fatty acids were first successfully syn- 
thesized by oxidizing the corresponding cyclic ketones (1). Preparation 
by the conventional methods is impossible because the acids tend to react 
intermolecularly yielding linear polyesters. Once this fact was clearly 
recognized it became evident (2) (p. 8 . . .) that direct lactonization might 
be favored by application of the dilution principle first utilized by Ruggli 
(3) ; and Stoll and Rouve (4) have showed experimentally that this is in- 
deed the case. 

A third method for the preparation of large ester rings consists in de- 
polymerizing the corresponding linear polyester. This was first realized 
with esters of carbonic and oxalic acids (5). Later it was shown that, by 
the proper control of temperature and selection of catalysts, excellent re- 
sults could also be obtained with glycol esters of other dibasic acids (0). 

We have now applied this method to the preparation of several lactones. 
The possibilities are indicated by the fact that a 70% yield of pure mono- 
mer was obtained from the polyester of hydroxytetradecanoic acid. Stoll 
and Rouv (4) in a similar case report yields by the dilution method con- 
siderably higher than this, but their scale of operation was small (10 g.), 
the volume large (10 liters) and the time long (six days). Our depoly- 
merization required only three and one-half hours for 32 g. of polyester. 
It appears therefore that this method has some advantages over the use of 
high dilution. 

One of the objects of the present work was to obtain some lactones which 
had not been reported previously. Meanwhile Stoll and Rouve have pub- 
lished a much more extended study (4) of lactonization by the dilution 
method covering in part the same ground. The overlapping results of the 
two investigations are in good agreement, but in several cases we are able 
to report slightly higher melting points than were found by Stoll and 

As new compounds we report 

r O(CH 2 ) 2 0(CH 2 ) 10 CO- 1 
j-O(CH 2 )4O(CH 2 )ioCO- 1 and 
r O(CH 2 ) 2 O(CH 2 ) 1 oCOO(CH 2 ) 2 0(CH 2 ) 10 CO^ 

The first two are rings of 15 and 17 atoms and their odors are definitely 
musk-like. These compounds are also of interest because the required hy- 


droxy acids are relatively easily accessible from undecylenic acid through 
the addition of hydrogen bromide followed by reaction with the sodium 
derivative of the glycol. 

The greater ease of forming rings of large size Compared with inter- 
mediate size is again illustrated in the present study by the fact that hy- 
droxytetradecanoic acid gave monomer and dimer in the ratio of 15/1 
while for hydroxyclecanoic acid the corresponding ratio was 0.17/1. Fur- 
ther evidence of specificity in the catalysis of ester depolymerizations was 
also found. Whereas in most of the cases previously examined, stannous 
chloride and various magnesium salts were almost equally effective, in the 
present work magnesium chloride was much more effective than any other 
material tested (cf. Table I). 



MgCl 2 .6H 2 O 

MgCl 2 .6H 2 O 


MnCl 2 .4H 2 O 

SnCl 2 .2H 2 

CoCl 2 .6H 2 

Mg (powd.) 

No reflux 

Steam cooled reflux 
No reflux 
No reflux 
No reflux 
No reflux 
No reflux 




























Temp., C. Time, hours % crude yield % dimer 

270 2 73 26 

260 2 30 12 

260 5 58 15 

250 2 28 10 

250 5 52 12 

Experimental Part 

Polyesters were prepared by heating the hydroxy acids at 180-250 for three or four 
hours in a Claisen flask without added catalyst but with diminished pressure during the 
last hour. 

Depolymerizations were carried out in the vapor heated still already described (6) at 
a pressure of 1 mm. or less. In some experiments a dephlegmator cooled with steam was 
inserted at the top of the still. It lowered the rate of distillation and increased the ratio 
of monomer to dimer. 

Catalysts were explored using polyester (15 g.) from hy droxy undecanoic acid with 
0.2 to 0.3 g. of the proposed catalyst at 270 during two hours. The distillate was dis- 




rt rj 

tn rt fl 



-s 65 


w rt O 


O5 CD O I- 



o co 

iO iO 


I I 




O 1- CD 

o o 

t- 10 


O t"- 



p ^ > 


^ o 




s l 

o KL; 



10 rN, 




6 1 



tt ^| Q 

O clo 




O O Q 


't CO -t 

~t CO 

fc *S.s 


. p 

W g WJ K 

6 tf 





o- rt 

O h" <c ' 






^ ^ ^o 

CD 0-1 





d C/3 




o 9. Q. 

O O 

S S Ifv 

S ' 




O ^ 


hH W W 


PQ c/3 Q 
< W *o 



Q ~ v 



CD tD 

H P 

1^ ^ 

>4 -2*O 


C ?l 

<M -1 


^ co 

M r^ rt 


n o o 



^ CO 



U b/Q bC 

be be 



"<2 rH 


co S S 

^ ^ 

< OP 

^ T _ H 




^ ^~^ 


C o 

o" -\ 

rH t^. 







. ^ 



rH C^-l 



r^ ~ 

^i W) 

rH rH 



' r ~ | 

_. in 


LO (M O 
O5 CO l^~ 


" t/ 





^ ^ 






cT rt" 

. G o 



P *~* 

s l 




i ^ 


tf) 1 o 

rP C 

s r! w 

h-" hr< 

o a ' c a 

rH <M rt< 




2 ^ 

Polyester from 

10-Hydroxydecanoic acic 

13-Hydroxytridecanoic a 
HO(CH 2 ) 2 O(CH 2 ) )0 COO1 

HO(CH 2 ) 3 O(CH 2 )i COOI 
HO(CH 2 )4O(CH 2 ) 10 COOI 

Monomeric lactone from ri 


acid 1 
1 1-Hydroxyundeca- 
noic acid a - c ] 
noic acid ] 

tetradecanoic acid ] 

pentadecanoic acid c \ 

hexadecanoic acid ] 

C/. Stoll, Rouve and 5 
50 (1929); Carothersand 


solved in 50 cc. of alcohol and the crystalline dimer which separated on cooling was 
filtered off and weighed. The accumulated nitrates on distillation were shown to be 
largely monomer, and in the last column of Table I the indicated yields are based on the 
assumption that all not dimer was monomer. 

The effect of temperature is indicated in Table II. In each case 20 g. of polyester 
from hydroxyundecanoic acid was heated with 0.5 g. of MgCl 2 .6H 2 O. The ratio of 
monomer to dimer rises as the temperature falls, but the rate falls off quite rapidly at the 
same time. 

The results for the depolymerization of polylactone derived from 10-hydroxy- 
decanoic, 13-hydroxytridecanoic, 14-hydroxytetradecanoic acid and the ether acids are 
listed in Table III. Properties and analyses are indicated in Tables IV and V. 


C H Mol wt " 

Lactone of 10-hydroxydecanoic acid 

Calcd. for C,oH,,sO> 70.58 10.58 170 

Found 70.31 10.13 162 

Dimcric lactone of 13-hydroxytridecanoic acid 

Calcd. for C 26 H 18 O 4 73 24 11.26 426 

Found 73.46 11.30 407,432 

Dimeric lactone of 14-hydroxytetradecanoic acid 

Calcd. for C 2 sH fi2 O, 74 33 11 .50 452 

Found 74.29 11.79 414 

Lactone of 14-hydroxy-12-oxatetradecanoic acid 

Calcd. for CiaHajOs 68.42 10.52 228 

Found (38 57 10.17 232 

Dimeric lactone of 14-hydroxy-12-oxatetradecanoic acid 
Calcd. for C>6H 4 sO 6 68 . 42 10. 52 456 

Found 68.16 10.36 466 

Lactone of 15-hydroxy-12-oxapentadecanoic acid 

Calcd. for Ci 4 H 2l O 3 69 . 42 10 . 74 242 

Found 69.76 11.00 234 

Lactone of 16-hydroxy-12-oxahexadecanoic acid 

Calcd. for Ci6H 2s O3 70.3 10.93 256 

Found 70.31 11.01 258 

a Determinations made in freezing benzene. 

Preparation of Materials 

11 -Hydroxyundecanoic was prepared from undecylenic acid and hydrobromic 
acid following Ashton and Smith (7) . Our best yields were 72 % of the theoretical. Con- 
version of this to 11 -hydroxyundecanoic acid was carried out as follows: a solution of 
60 g. (1.5 rnolcs) of sodium hydroxide and 132 g. (0.5 mole) of the bromo acid in one liter 


of water was refluxed for five hours. The filtered solution was cooled to about 5 for 
several hours where upon the sodium salt of the acid separated. It was filtered off and 
redissolved in hot water. Acidification liberated the hydroxy acid as an oil which 
crystallized when cooled. One recrystallization from benzene gave 74 g. (72% of the 
theoretical amount) of fine crystals melting at 70. 

The Ether Hydroxy Acid HO(CH 2 )2O(CH 2 )i COOH. One mole of sodium (23 g.) 
was dissolved in 248 g. of ethylene glycol. The mixture was then heated to 110-115 
under reflux and stirred while 133 g. (0.5 mole) of 11-bromoundecanoic acid was added 
in four portions during one hour. Stirring and heating were continued for four hours 
and most of the glycol was removed by further heating in vacua . The residue was dis- 
solved in water, acidified and extracted with ether. The crude acid after crystallization 
from petroleum ether was obtained as a white powder melting at 48-50. 

Anal. Calcd. for Ci 3 H 26 O 4 : C, 63.41; H, 10.56; neut. equiv., 246. Found: C, 
63.27; H, 10.36; neut. equiv., 244. 

The ether hydroxy acid HO(CH 2 )3O(CH 2 )ioCOOH was obtained similarly, m. 
p. 51. 

Anal. Neutral equivalent calcd. for Ci 4 H 2 8O 4 : 260. Found: 263. 

The ether-hydroxy acid HO(CH 2 ) 4 O(CH 2 ) 10 OOH, m. p. 53. 

Anal. Calcd. for Ci 5 H 30 O 4 : C, 65.69; H, 10.95; neut. equiv., 274. Found: C, 
65.39; H, 11.01; neut. equiv., 281.5, 281.3. 


The interchange method for preparing macrocyclic esters is shown to be 
applicable to the preparation of many-membered lactones and to have ad- 
vantages in speed and simplicity over the high dilution method. 

Three new large cyclic esters are described: the monomeric and dimeric 
lactones of 14-hydroxy-12-oxatetradecanoic acid and the monomeric lac- 
tone of 16-hydroxy-12-oxahexadecanoic acid. 

Bibliography and Remarks 

(1) Ruzicka and Stoll, Helv Chim. Acta, 11, 1159 (1928). 

(2) Carothers, Journ. Am. Chem. Soc., 51, 2548 (1929). 

(3) Ruggli, Ann., 392, 92 (1912). 

(4) Stoll and Rouv, Helv. Chim. Acta, 17, 1283 (1934). 

(5) Hill and Carothers, Journ. Am. Chem. Soc., 55, 5031 (1933). 

(6) Spanagel and Carothers, ibid., 57, 929 (1935). 

(7) Ashton and Smith, J. Chem. Soc., 1308 (1934). 





I. Introduction 

This introduction, by drawing to the reader's attention the salient and 
most significant features of Carothers' series of papers on Acetylene 
Polymers and their Derivatives, will, it is hoped, add to his satisfaction 
in perusing the papers themselves, which are reprinted hereafter. 

The outstanding element of interest and importance in the work de- 
scribed in the series is the discovery of a synthetic rubber, polychloroprene, 
which has been manufactured since 1933 and represents one of the most 
striking achievements of industrial chemistry in recent years. In addition 
however, to providing the basis for this industrial development, the work 
also makes a brilliant and important contribution to our knowledge of 
(1) the influence of substitution on the polymerization of conjugated 
systems and (2) the chemical behaviour of conjugated systems in which 
an ethylenic and an acetylenic bond are disposed in a conjugated relation- 
ship to one another. 

In the first paper of the series Acetylene Polymers and their Derivatives, 
by Nieuwland, Calcott, Downing and Carter (1) there is described the 
discovery that, by means, as a catalyst, of cuprous chloride (preferably 
together with ammonium chloride) acetylene can be polymerized in high 
yield to (1) a dimer, vinyl acetylene, CH 2 :CH.C:CH, b. p. 5 and (2) a 
trimer, divinylacetylene, CH 2 :CH.C:C.CH:CH 2 , b. p. 83.5. With 
these two highly reactive substances thus made readily available, Carothers 
and his co-workers proceeded to an extensive study of their chemistry, 
the results of which are described in subsequent papers of the same series. 


Of outstanding interest was the discovery that 2-chloro-l,3-butadiene, 
CH 2 : CC1 . CH : CH 2 , obtainable by the action of aqueous hydrogen chloride 
on vinyl acetylene under appropriate conditions in the presence of cuprous 
and ammonium chlorides, polymerizes readily to a rubber-like product 
(Paper II, p. 281). By analogy with isoprene, the compound is commonly 
referred to as chloroprene. It polymerizes far more rapidly than does 
isoprene, thanks to the activating effect of the /3-chlorine atom, and, con- 
publications listed in the Bibliography, p. 423, el seq., are referred to by Reference 
numbers in large Roman corresponding to the Serial Numbers of the papers.) 


trary to what might perhaps have been expected (by, e. g., analogy with 
poly vinyl chloride) the product turns out to be highly elastic and indeed 
similar in elastic properties to vulcanized natural rubber. By inter- 
rupting the polymerization before it is complete, it is possible to isolate 
an intermediate polymeric material (the so-called a-polymer) which, 
being plastic, can be handled by much the same techniques as raw rubber 
and its stocks are handled, and which having thus been fabricated into 
rubber goods of various shapes, can be converted into the final elastic 
material (the so-called ju-polymer) by the further application of heat, 
which completes the polymerization. 

Polychloroprene is now being manufactured on a substantial scale, be- 
cause, despite the fact that it is more costly than natural rubber, it has 
special properties which make it better adapted than the latter to certain 
applications. Chief of these properties is its lower imbibition of aliphatic 
hydrocarbons, which, presumably, is dependent on its high content of 
chlorine. It is, further, more stable than vulcanized rubber. This sta- 
bility is exhibited in its resistance to deterioration on aging, by heat, by 
ozone, etc., and is probably due largely to the fact that in it the poly- 
merization process to which it owes its formation is substantially complete, 
whereas vulcanized rubber is susceptible to further polymerization or if 
views now in vogue as to the nature of vulcanization are adopted to 
further cross-linking. 

Under certain special conditions chloroprene yields a polymer of different 
properties, distinguished as the w- or granular polymer. It is lacking in 
plasticity and imbibes organic liquids hardly at all. It can be produced, 
for example, by exposing chloroprene vapour to light from a mercury arc, 
and after its formation has been thus initiated, it will without further 
irradication continue to grow in the vapour or in liquid chloroprene. 
Carothers (2) in some interesting comments calls attention to the curious 
and perhaps suggestive analogy of such a process to vital growth. 

Influence of Substituents on Polymerization 

Carothers' studies afford much valuable information as to the influence 
of substitution in the butadienoid system on the readiness with which 
polymerization occurs and the degree to which it proceeds. Considering 
first halogen substitution: chloroprene, with chlorine in the 2-position, 
polymerizes about 700 times as quickly as isoprene and yields a product 
of excellent rubber-like properties; the analogous bromo derivative 
(Paper V, p. 314) polymerizes somewhat more rapidly, but the product, 
although having good extensibility and strength, shows, presumably owing 


to the high percentage by weight of halogen in the molecule, less com- 
pletely recovery from extension than does polychloroprene, and the lower 
limit of the temperature range within which it is elastic is higher than for 
polychloroprene; 2-iodo-l,3-butadiene (Paper XV, p. 361) polymerizes at 
a markedly higher rate than either of the preceding, but the product 
(which contains 70 per cent of iodine) is rubber-like only under certain 
conditions (3). 

The introduction of a second chlorine atom to one of the interior carbon 
atoms of the butadienoid system yields a compound CH 2 : CC1 . CC1 : CH 2 , 
which polymerizes even more rapidly than chloroprene CH 2 : CC1 . CH : CH 2 
(Paper XI, p. 340). The product, either because of the high degree to 
which polymerization has proceeded or because of its high chlorine content, 
is a hard tough mass lacking in rubber-like properties. (The high speed 
of polymerization of CH 2 :CC1. CC. :CH 2 is perhaps somewhat unexpected, 
if one were to judge by analogy with the effect of chlorine substitution in 
the ethenoid system, for, whereas here the introduction of one atom of 
chlorine, which renders the system highly polar, gives a compound (vinyl 
chloride) which is highly polymerizable, the introduction of a second atom 
of chlorine, symmetrically disposed with respect to the first, gives a com- 
pound (sym.-dichloroethylcnc) in which the ability to polymerize has been 
lost. It is, however, to be noted that in 2,3-dichloro-l,3-butadiene un- 
substituted terminal :CH 2 groups are present and that the presence of 
such may well be a condition for chlorine to exert an " activating" effect 
on polymerizability.) The substitution of chlorine in the terminal posi- 
tions of the butadiene has no such striking effect on polymerization as its 
substitution in the interior positions. l-Chloro-l,3-butadiene, C1CH:- 
CH.CH:CH 2 , polymerizes not much more quickly than isoprene and the 
product has only inferior rubber-like properties (Paper XI, p. 340) (3). 
Similarly, l,2,3,-trichloro-l,3-butadiene, C1CH : CC1 . CC1 : CH 2 polymerizes 
much more slowly than 2,3-dichloro-l,3- butadiene, CH 2 : CC1 . CC1 : CH 2 
(Paper XI, p. 340). 

Observations on a number of chlorine compounds prepared from divinyl- 
acetylene by steps involving the addition and the removal of the elements 
of hydrogen chloride provide further data on the influence of chlorine- 
substitution on the polymerization of conjugated systems (Paper XIII, 
p. 348). The compounds in question, each containing a conjugated 
system, are as follows: 

1. CH 2 :C:CC1.CC1:C:CH 2 
: 2. CH 2 :C:C:CC1.CH:CH 2 


3. Cl CH 2 . CH : C : CC1 . CH 2 : CH 2 

4. C1CH 2 .CH:CC1.CC1:C:CH 2 

5. Cl CH 2 . CH : CC1 . CC1 : CH . CH 2 C1 

Of these, 1 showed a strong tendency to polymerize: it changed to a 
hard resin when allowed to stand for 24 hours; 2 and 3 polymerized only 
slowly (3 changed to a viscous syrup on standing for three months); 4 
and 5, in which one end or both ends of the molecule carries chlorine and 
in which a terminal vinyl group is lacking, showed no tendency at all to 

The effect of introducing other groups into the 2-chloro-butadienoid 
system, especially their introduction into the terminal positions, is to 
depress the poly merizability . 3-Methyl-2-chloro- 1 ,3-butadiene, CH 2 : - 
CC1.C(CH 3 ):CH 2 , polymerizes somewhat more slowly than chloroprene, 
and the polymer is somewhat inferior to polychloroprene in rubber-like 
properties: although tough and elastic, it has a rather low extensibility 
(Paper XXI, p. 384). The introduction to the last-mentioned butadiene 
of a second, terminal methyl group has a more profound effect, 3-4-di- 
methylchloroprene, CH 2 :CC1.C(CH 3 ) :CH.CH 3 , polymerizing not much 
more rapidly than isoprene and yielding a produce which, although elastic, 
recovers only sluggishly from deformation (Paper XXI, p. 384). As con- 
trasted with the relatively slight effect of a 3-methyl group on the poly- 
merization of chloroprene, a terminal methyl group, as in 1-methylchloro- 
prene, CH 3 CH : CC1 . CH : CH 2 , reduces the speed of polymerization greatly 
and detracts markedly from the rubber-likeness of the polymeric product 
(Paper IX, p. 331). Increase in the magnitude of the terminal alkyl 
group magnifies the effect in the same direction, w-heptylchloroprene, 
CyHis . CH : CC1 . CH : CH 2 , yielding on polymerization only a sticky solid 
with very slight elasticity (Paper IX, p. 331). Such observations are in 
accord with previous findings that the introduction of terminal alkyl 
groups to butadiene reduces polymerizability and that, the larger the 
alkyl group, the greater is the reduction (4). 

In papers of the Acetylene Polymers series published subsequent to 
Carothers' association with it, the influence of certain other substituents 
on the polymerizability of chloroprene is recorded. The introduction of 
a dialkylmethylamino group in a terminal position depresses the poly- 
merization of the chloroprene system very greatly, the 1- (dialkylmethyl- 
amino) -chloroprenes, R 2 N . CH 2 . CH : CC1 . CH : CH 2 , polymerizing very 
slowly indeed and yielding no rubber-like product (5). Alkoxy-methyl or 
alkoxy- (methyl) methyl groups in the terminal position have some but a 


far smaller effect on the polymerizability of chloroprene, CHj>O.CH 2 .- 
CH : CC1 . CH : CH 2 polymerizing at one-sixth the rate of chloroprene and 
yielding a product with somewhat rubber-like properties (6). 

In view of the profound effect on polymerization of the presence of 
chlorine in the 2-position of the butadienoid system, the effect of other 
substituents in this position was examined. As compared with isoprene, 
which has a methyl group in the 2-position, 2-heptyl-l,3-butadiene was 
found to polymerize rather less readily and to yield a weaker and softer 
rubber (Paper XVI, p. 368). 2-Phenyl-l,3-butadiene was found to poly- 
merize considerably more rapidly than isoprene but to yield under ordinary 
conditions only a dimeric product (Paper XVI, p. 368). In continuation 
of Carothers' studies, the effect of introducing (a) alkoxy groups, (b) ali- 
phatic acid residues with the 2-position has been examined. 2-Ethoxy- 
1,3-butadiene, CH 2 :C(OC 2 H5).CH:CH 2 , was found to polymerize at 
about one-third the rate of isoprene and to give a product of inferior 
rubber-like properties (7) . 2-Acetoxy-l,3-butadiene, CH 2 . C(O . COCH 3 ) . - 
CH:CH 2 , and analogous compounds on the other hand were found to 
polymerize at a rate not vastly different from that of chloroprene and to 
yield rubber-like products (8). 

4-Cyano-l,3-butadiene, CH 2 :CH.CH:CH .N;C, despite the fact that 
the substituent group is in a terminal position, has been found to polymerize 
about twenty times as quickly as isoprene (9). Perhaps the extra conju- 
gation which the cyano-group provides is responsible for this result. The 
final product of polymerization, although rubber-like, is markedly inferior 
to polychloroprene or polyisoprene : it could be stretched 1300 per cent 
without breaking, but ten minutes after release still had an extension of 
200 per cent. 

Chemical Behaviour of Vinylacetylene and Divinylacetylene 

Turning attention now to the acetylene polymers, vinylacetylene and 
di vinylacetylene, note may be made of the salient features of observations 
regarding (a) the polymerization of these substances, (b) their addition 
reactions other than self -addition (i. e., polymerization), (c) their other 

Polymerization. In the presence of cuprous chloride, vinylacetylene 
gives a liquid dimer, which is probably octatriene-l,5,7-ine-3, CH 2 :CH.- 
CiC.CH:CH.CH:CH 2 (10, 11), and which, on exposure to air, becomes 
converted into an insoluble product which explodes violently when 
heated (11). When heated in the absence of a catalyst, as, e. ., at 105 
for six hours (11) or in the presence of peroxide catalysts (10), vinyl- 


acetylene polymerizes first to viscous oils and finally to hard, resinous 
solids. The available evidence (11) suggests that these polymers, which 
contain acetylenic hydrogen, are cyclobutane and cyclobutene derivatives, 
as follows : 

: C.CH . CH.C: CH, CH: C.CH . CH. 


CH 2 CH 2 CH 2 .CH 2 

/C : CH\ 
(I I ) 

\CH 2 CHA.CiCH, 

their formation involving only the ethylenic linkages of vinylacetylene. 

When heated in the presence of acids, acid anhydrides or phenols, vinyl- 
acetylene, in addition to forming resinous products, yields, unexpectedly, 
styrene in amount corresponding to about one-half of the vinylacetylene 
used (11). Styrene, C 8 H 8 , in this connection figures as a dimer of vinyl- 
acetylene, C 4 H 4 . Its formation can be represented as, analogously to the 
dimerization of butadiene, involving first the addition, in a 1,4-sense, of 
the acetylenic portion of one molecule of vinylacetylene to the conjugated 
system of another, and then a rearrangement of bond. Thus: 


^ \ / \ / \ 


+ > \ / * \ / 


I I I 

CH=CH 2 CH CH 2 CH=CH 2 

Divinylacetylene, when heated in the absence of air (in the presence of 
air it absorbs oxygen and becomes dangerously explosive) polymerizes to 
first a viscous liquid and finally a hard, brittle, insoluble resin. The inter- 
mediate, oily material, when exposed as a film to air, takes up oxygen and 
"dries" to a hard, insoluble product (12) (Paper XIX, p. 378). It 
has been referred to as a "synthetic drying oil." From the inter- 
mediate, oily polymeric material it was possible to isolate a dimer 
of vinylacetylene, which was shown to be divinylethinyl-l,2-cyclobutane, 
CH 2 : CH . C : C . CH . CH . C i C . CH : CH 2 (Paper XIX, p. 378), its formation 

I I 
CH 2 CH 2 

depending as in the case of the analogous dimer of vinylacetylene (supra) 
on the mutual addition of an ethylenic part of each of two molecules of the 

The work provides some information regarding the polymerization of 
certain substitution products of vinyl- and divinyl-acetylene. l-Halo-2- 
vinyl acetylenes, CH 2 :CH.CH:C Hal., on standing for several months, 
polymerize to black, highly explosive solids (Paper XVIII, p. 375). 


l-Alkyl-2- vinyl acetylenes, CH 2 : CH . CH ; CR, polymerizes only slowly; 
they become viscous syrups after 2-3 months' standing (Paper VIII, p. 329) . 
Replacement of the acetylenic hydrogen of vinylacetylene by a secondary 
carbinol residue has a favourable effect on the speed of polymerization, 
the tertiary carbinols all polymerizing spontaneously with considerable 
rapidity (Papers VI, p. 321; VII, p. 323). Thus, e. g. t (vinylethinyl) 
methylethyl carbinol, CH 2 :CH.C:C(OH)(C 2 H 5 )CH3, on standing in a 
stoppered bottle, had become a thick syrup in one week, then passed 
through a tough elastic, rather rubberlike stage, and in six weeks had 
become a very hard mass (Paper VII, p. 323). 

Addition Reactions. Prior to Carothers' study of the chemistry of 
vinyl- and divinyl-acetylenes little was known about the additive be- 
haviour of -enine systems or, alternatively, enyne systems in which 
contiguous carbon atoms carry respectively double and triple bonds, thus : 
-.C:C.C : . C. Can such a system of fonds behave as a conjugated 
system? Yes; for it was shown rather conclusively that the first result 
of treating vinylacetylene with hydrochloric acid is to form 4-chloro-l,2- 
butadiene (P^per III, p. 306). 

HC;C:CH:CH 2 + HC1 > H 2 C : C : CH . CH 2 C1 

This compound, which can readily be isolated, can be put through a series 
of methathetical reactions without any rearrangement of the bonds 
(Paper XV, p. 361). Thus, e. g., by means of hot water, preferably in the 
presence of a mild alkali, the chlorine is replaced by a hydroxyl group 
(Paper XV, p. 361; U. S. Patent 2,073,363, p. 431; 2,136,178, p. 432). 
It readily rearranges, however, under many conditions of treatment, espe- 
cially in the presence of cuprous chloride, and gives 2-chloro-l,3-butadiene, 
CH2 : CC1 . CH : CH 2 , i. e., chloroprene, the allene arrangement changing 
to the more stable conjugated one, in which the chlorine atom, attached 
to a doubly bonded carbon, has lost its mobility (Paper III, p. 306). 
Carothers discusses the mechanism of the a,7-shift involved and concludes 
that it is dependent, not on ionization, but on the formation of a complex 
(coordination compound) and chelation (Paper XV, p. 361). 

In the presence of a little sodium alcoholate, alcohols readily react with 
vinylacetylene to yield acetylene ethers of the type, RO . CH 2 . C : C . CH 3 
(Paper XX, p. 381). This reaction is considered to involve first 1,4- 
addition, and then a rearrangement similar to the rearrangement of mono- 
substituted allenes to disubstituted acetylenes which Favorsky found to 
be produced by sodium alcoholates. Thus : 

CH 2 :CH.C'CH, i. e., .CH 2 . CH:C:CH. > (RO.CH 2 . CH:C:CH 2 ) > 

RO.CH 2 .C:C.CH 3 


When two atoms of chlorine are added to divinylacetylene, the product 
is reasonably concluded to have the structure C1CH 2 . CH : C : CC1 . CH : CH 2 , 
and hence to have been formed by 1,4-addition. When to this product 
two more atoms of chlorine are added, the tetrachloride formed has ap- 
parently the structure C1CH 2 . CH : CC1 . CC1 : CH . CH 2 . Cl, and to be found 
by a further 1,4-addition (Paper XIII, p. 348). This last conclusion 
makes it appear that in a system . C:C:C.C:C. a carbon atom with 
twinned double bonds can act as one of the ends of a conjugated system 
(cf. the remarks on the polymerization of chlorine compounds derived 
from divinylacetylene, p. 378). 

It is to be remarked that some of the chlorides derived from divinyl- 
acetyelene and also certain derivatives of vinylacetylene, despite the fact 
that they contain conjugated double bonds, fail to react with maleic 
anhydride or naphthoquinone (Paper XIII, p. 348). 

Although in most of their addition reactions vinyl- and divinylacetylenes 
behave as conjugated systems, two exceptions were encountered. (1) In 
reaction with thiophenols, only the ethylenic bond of the -enine system is 
involved, and in view of this Carothers made considerable use of thio-p- 
cresol in studying the constitution of acetylene polymers and their deriva- 
tives. Divinylacetylene readily adds, at room temperature, two molecules 
of thio-/?-cresol to give a crystalline product (Paper XII, p. 344), thus: 
CH 2 :CH.CiC.CH:CH 2 ->C 7 H 7 S.CH 2 .CH 2 .C:C.CH 2 .CH 2 S.C 7 H 7 . (2) 
A reaction dependent on addition to the acetylenic bond of vinylacetylene 
is the formation of 1,3-butadienyl esters by treating vinylacetylene with 
an aliphatic acid in the presence, as catalyst, of a mercuric salt or boron 
fluoride (13). Thus: 

CH 2 :CH.C;CH + R.COOH > CH 2 :CH.C(O.COR) :CH 2 

Other Reactions. Reactions of vinylacetylenes dependent on the re- 
activity of the acetylenic hydrogen (CH 2 : CH . C : . CH) are as follows: 

(a) With Grignard reagents, vinylethinyl magnesium halides, e. g., 
CH 2 : CH . C i CMgBr, are formed, and the latter react in a way typical 
of organo-magnesium compounds, giving, with acetone, a carbinol, 
CH 2 :CH.C:C(OH)(CH 3 ) 2 , and with carbon dioxide, an acid, vinyl- 
propiolic acid, CH 2 :CH.C : -C.COOH (Paper VI, p. 321). 

(b) With sodium or sodamide, there is formed sodium vinylacetylide, 
CH 2 : CH . C : CNa, which is very reactive; giving with aldehydes secondary 
carbinols, CH 2 :CH.C;C. (OH)CHR; with ketones, tertiary carbinols, 
CH 2 :CH.C:C(OH)CRR / (Paper VII, p. 323), and with alkyl halides, 
alkyl-vinylacetylenes, CH 2 :CH.C:CR (Paper VIII, p. 329). 


(c) With potassium mercuric iodide or with mercuric acetate in acetic 
acid, vinylacetylene yields di-vinylethinyl mercury (CH 2 :CH.C:C)2Hg 
(Paper XVII, p. 372). 

(d) Treatment of vinylacetylene with alkali hypohalides replaces 
the acetylenic hydrogen by halogen and yields the 1 -halo-vinylacetylenes, 
CH 2 :CH.C;C Hal. (Paper XVIII, p. 375). 

(e) By treating vinylacetylene with formaldehyde and a secondary 
amine, the acetylenic hydrogen is replaced by a secondary-amino-methyl 
residue (14,15), thus: 

CH 2 :CH.C;CH + CH 2 O + HNR, > CH 2 :CH CiC.CH 2 NR 2 + H,O 

Bibliography and Remarks 

(1) J. A. Nieuwlancl, U. S. Pat. 1,811,959 (1931), Journ Am. Client. Soc , 53, 4197-4202 (1931) 

(2) W. H. Carothers, Trans. Farad. .Sac., 32, 39 (193(5). 

(3) W. H. Carothers, Ind. Eng. Chem , 26, 30 (1934). 

(4) Whitby and Gallay, Can. Jour Res., 6, 280 (1932); Whitby and Cro/ier, ibid., 6, 203 (1932) 

(5) D. D. Coffman, Journ. Am. Chem. Soc., 57, 1978 (1935) 

(6) H. B. Dykstra, ibid., 58, 1747 (1936) 

(7) H. B. Dykstra, ibid., 57, 2255 (1935). 

(8) J. H. Werntz, ibid , 57, 204 (1935) 

(9) D. D. Coffman. ibid., 57, 1981 (1935) 

(10) Nieuwland, Calcott, Downing and Carter, ibid., 53, 4197 (1931). 

(11) H. B. Dykstra, ibid , 56, 1025 (1934) 

(12) Nieuwland, Calcott, Downing and Carter, ibid , 53, 4197 (1931) 

(13) J. H. Werntz, ibid., 57, 204 (1935). 

(14) D. D. Coffman, ibid., 57, 1978 (1935); also II V S Patent 2,136,177 
CIS) Journ. Am Chem. Soc., 53, 4197 (1931). 

G. vS. W. 

II. A New Synthetic Rubber: Chloroprene and Its Polymers* 

Study of the reactions of vinylacetylene, a compound which has be- 
come available through discoveries described in the preceding paper (1), 
has led to the synthesis of a series of new analogs arid homologs of isoprene. 
The present paper is concerned with one of the simplest of these, namely, 
chloro-2-butadiene-l,3 (I). This compound is especially interesting for 
the following reasons. It is easily prepared in quantity in a state of purity; 
it differs structurally from isoprene only in having a chlorine atom instead 
of a methyl group; like isoprene it reacts with itself to yield a synthetic 

* W. H. Carothers, I. Williams, A. M. Collins and J. E. Kirby, Journ. Am. Chem. 
Soc. t 53, 4203-25 (1931) ; Contribution from the Experimental Station of the Central 
Chemical Department (Communication No. 83) and from the Jackson Laboratory of 
the Dyestuffs Department (Communication No. 26), E. I. du Pont de Nemours and Co. 

Received October 3, 1931. Published November 5, 1931. 


rubber; but the transformation occurs with much greater velocity than 
in the case of isoprene; and the product is distinctly superior to natural 
rubber in some of its properties. 

In order to recognize the analogy in structure and behavior which ex- 
ists between isoprene and chloro-2-butadiene-l,3, we call the latter com- 
pound chloroprene, and this name also serves to distinguish it from other 
chlorobutadienes that will be described in future papers. 

Preparation of Chloroprene. Chloroprene is obtained by the addition 
of hydrogen chloride to vinylacetylene 

CHEEEC CH=CH 2 + HC1 > CH 2 =C CH=CH 2 

Other products, which will be described in future papers, are also formed 
by the action of hydrogen chloride on vinylacetylene under certain condi- 
tions; but under the conditions indicated in the following example, chloro- 
prene is practically the only product. 

Fifty grams of cold vinylacetylene is placed in a pressure bottle containing a 
thoroughly chilled mixture composed of 175 g. of coned, hydrochloric acid (sp. gr. 1.19), 
25 g. of cuprous chloride and 10 g. of ammonium chloride. The bottle is placed in a 
water-bath the temperature of which is held at approximately 30, where it is shaken 
for a period of four hours. The contents of the bottle are placed in a separatory funnel, 
the lower aqueous layer is drawn off and the oily layer is washed with water, dried 
with calcium chloride, mixed with a small amount of catechol or pyrogallol and dis- 
tilled in vacuo through an efficient column provided with a refrigerated dephlegmator 
and receiver. Pure chloroprene is thus obtained in yields of about 65% of the theo- 
retical based on the vinylacetylene applied. Some vinylacetylene is recovered in the 
distillation. The yields can be considerably improved if the chloroprene is separated 
from the reaction mixture by steam distillation in vacuo (100-250 mm.). 

Physical Properties and Analysis. Chloroprene is a colorless liquid 
with a characteristic ethereal odor, somewhat resembling ethyl bromide. 
It is miscible with most of the common organic solvents, but only slightly 
soluble in water. v Some of its other properties are boiling point (2), 59.4 
at 760 mm., 46.9 at 500 mm., 40.5 at 400 mm., 32.8 at 300 mm., 6.4 
at 100 mm.; vapor pressure, logio v. p. (mm. Hg) = l545.3/T(abs) + 
7.527; calculated molal latent heat of evaporation, 7090 cal.; density, 
d% 0.9583; refractive index, nf, 1.4540, n 1.4583, n, 1.4690. 

Found M c 25 . 06 MD 25 . 26 

Calcd. 24.67 24.66 

Difference 0.39 0.60 

Viscosity at 25, 0.394 centipoise. 


Anal. Calcd.forC^HfiCl: C, 54.25; H, 5.69; Cl, 40.06; mol. wt., 88.5. Found: 
C, 54.37; H, 5.95; Cl, 39.51, 38.81; mol. wt. (in freezing benzene) 89.0, 89.0. 

Chemical Properties and Proof of Structure. The structure of chloro- 
prene as chloro-2-butadiene-l,3 is established by its analytical composi- 
tion and by the following reactions. 

It reacts readily with maleic anhydride and yields, after hydrolysis 
with water, a crystalline product to which we assign the structure chloro-4- 
tetrahydro-l,2,3,G-phthalic acid (II). The chlorine atom of this product, 
in accordance with its assumed structure, is very resistant to the action 
of concentrated boiling alkali, and oxidation with boiling nitric acid 
yields a crystalline compound identical in melting point and composition 
with the known acid, butane-a,/3,7,5-tetracarboxylic acid (III). 


I + || )>0 > || I 


VHj x CHo/ 





Chloroprene further reacts readily with naphthoquinone, and the pri- 
mary product, which presumably has the structure chloro-2-tetrahydro-l,- 
4,4a,9a-anthraquinone-9,10 (IV), is smoothly oxidized by air in the presence 
of alkali to 0-chloroanthraquinone (V). The identity of this is established 
by the method of mixed melting point. 

/CHfv /COv / 


II I . 

CHv /CHs 


CH 3 CC1=CH CH 2 C1 

In view of the recent studies of Diels and Alder (3) this result decisively 
demonstrates the presence in chloroprene of a pair of conjugated double 
bonds, and it fixes unequivocally the position of the chlorine atom. 


In chloroform solution chloroprene readily adds approximately two 
atoms of bromine before substitution begins. It rapidly decolorizes 
alkaline permanganate solution. In the presence of cuprous chloride it 
reacts with aqueous hydrochloric acid. The reaction consists in 1,4- 
addition and the product is dichloro-l,3-butene-2 (VI), which will be de- 
scribed in a future paper. The chlorine atom of chloroprene is very 
firmly bound. Only traces of chloride ion appear on boiling with alcoholic 
silver nitrate, alcoholic sodium hydroxide, or pyridine. 

Reaction of Chloroprene with Maleic Anhydride. Preparation of Chloro-4- 
tetrahydro-l,2,3,6-phthalic Acid (II). 21.2 g. of chloroprene was warmed with 19.6 g. 
of maleic anhydride. The anhydride dissolved readily and when 50 was reached 
sufficient heat was developed from the reaction to maintain this temperature for some 
time. After standing overnight the reaction product was boiled with 200 cc. of water to 
remove unchanged chloroprene and filtered. On cooling stout rectangular plates sepa- 
rated; yield 31.5 g. or 77% of the theoretical; m. p. 171-172 (corr.). Recrystalliza- 
tion from water raised the melting point to 173-175. 

Anal. Calcd. for C 8 HO 4 C1: C, 46.94; H, 4.43; Cl, 17.34; neutral equivalent, 
102.3. Found: C, 47.54; H, 4.78; Cl, 17.24, 17.71; neutral equivalent, 103.5. 

The acid was boiled with 25% potassium hydroxide for three hours. No significant 
quantity of chloride ion was produced. This indicates that the chlorine is attached to a 
double-bonded carbon atom. 

Oxidation to Butane-a,/3,7,S-tetracarboxylic Acid (III). A sample (8.2 g.) of the 
above-described acid was warmed with 30 g. of 70% nitric acid until a rather violent 
reaction took place with the evolution of nitrogen oxides. The unused nitric acid was 
removed in vacuo and the partly crystalline residue taken up in a small volume of hot 
water. On cooling a thick mass of flat needles with square ends separated After a 
second crystallization, the product melted at 192-193 with effervescence. 

Anal. Calcd. for CsHioO 8 : C, 41.05; H, 4.27; neutral equivalent, 58.5. Found: 
C, 41.32, 41.46; H, 4.66, 4 67; neutral equivalent, 58 8. 

This acid has already been described by Auwers and Jacob (4) and also by Farmer 
and Warren (5). 

Action of tt-Naphthoquinone on Chloroprene. Conversion to 0-Chloroanthra- 
quinone. To 10 g. of naphthoquinone dissolved in benzene 12 g. of chloroprene was 
added. The mixture was refluxed for three hours and then allowed to stand overnight. 
The benzene was removed under reduced pressure, and the residual mass was dissolved 
in warm alcohol and cooled. A considerable amount of unchanged naphthoquinone 
separated. This was filtered off. Dilution of the mother liquor with water gave a solid 
which separated from alcohol in fine needles melting at 76 . It was still contaminated 
with naphthoquinone. It was suspended in alcohol containing a little sodium hydrox- 
ide, and air was bubbled through the suspension for twenty minutes. The suspended 
solid was crystallized three times from amyl alcohol; small needles, m. p. 209.5; mixed 
melting point with /3-chloroanthraquinone, 209.5. 

Spontaneous Polymerization of Chloroprene. The following example 
is typical of the spontaneous polymerization of chloroprene. 


About 40 cc. of chloroprene is placed in a 50-cc. bottle of soda glass, 
closed with a cork stopper, and allowed to stand at the laboratory tem- 
perature (about 25) in the absence of direct light. After twenty-four 
hours the viscosity of the sample has considerably increased; after four 
days it has set to a stiff, colorless, transparent jelly, which still contains 
a considerable amount of unchanged chloroprene. As the polymerization 
proceeds further this jelly contracts in volume and becomes more tough 
and dense. After ten days all the chloroprene has polymerized. We 
call this product ju-polychloroprene to distinguish it from other chloro- 
prene polymers that will be described later in this paper. 

Properties of ^-Polychloroprene. The product of the above de- 
scribed reaction is a colorless or pale yellow, transparent, resilient, elastic 
mass resembling a completely vulcanized soft rubber. Its density at 
20 is about 1.23, and its refractive index (n%) is about 1.5512. It has 
a tensile strength of about 140 kg./sq. cm. and an elongation at break 
of about 800%. It is not plastic; that is, it does not sheet out smoothly 
on the rolls of the rubber mill nor break down on continued milling. It 
is not thermoplastic. It swells strongly but does not dissolve in carbon 
tetrachloride, carbon disulfide, benzene, nitrobenzene, pyridine, aniline, 
ethyl acetate and ether. Compared with natural rubber the tendency 
of this material to imbibe gasoline and lubricating oil is very slight. When 
a stretched sample is immersed in liquid air for a moment and then struck 
with a hammer it shatters into fibrous fragments (6). 

The properties of the ju-polymer vary somewhat depending upon the 
conditions under which it is formed. When the chloroprene has access 
to large amounts of air or oxygen during the polymerization the product 
is dark in color and harder and stiffer than otherwise. The polymer 
formed at elevated temperature is inclined to be soft and it has a distinct 
terpene-like odor. This is due to the presence of the volatile ^-polymer, 
which will be described in a subsequent paragraph. 

x-Ray Diffraction Pattern. Although the x-ray diffraction pattern 
of unstretched rubber shows only a single diffuse ring, characteristic of a 
liquid or an amorphous solid, stretched rubber shows a point diagram (7). 
On the other hand, according to Mark (8), all synthetic, polymerized 
products from isoprene or other unsaturated hydrocarbons so far investi- 
gated have given, even when stretched, a diffraction pattern analogous 
to that of a liquid. 

It is therefore a matter of considerable interest that the ju-polychloro- 
prene described above (as well as the cured plastic polymer described in 
a later paragraph), when stretched about 500%, exhibits a fully developed 


fiber diagram showing a number of definite layer lines. One of these 
diffraction patterns is reproduced in Fig. 1, together with a diagram of 
stretched rubber (Fig. 2). The identity period along the fiber axis is 
4.8 A. This length corresponds rather closely with the calculated length 
for one chloroprene unit. The agreement is better if one assumes a trans 
instead of a cis configuration since the calculated identity period in a cis 
polyprene chain is about 2 X 4.1 A., whereas in a trans chain it is about 
4.8 A. (9). Incidentally, 4.8 A. is exactly the identity period observed for 
0-gutta percha by Hauser and v. Susich (10). Unstretched samples of 
poly chloroprene give an amorphous ring (Fig. 3) entirely like natural 
rubber. The spacing corresponding to this ring is 4.86 A. We are in- 
debted to Dr. A. W. Kenney for these observations. 

Chemical Properties of M-Polychloroprene. The /i-product has the 
composition required for an addition polymer of chloroprene. 

Anal. Calcd. for (CJIsCl),: C, 54.25; H, 5.69; Cl, 40.06. Found: C, 53.74, 
54.83; H, 5.70, 5.93; Cl, 40.06, 39.32. 

Molecular weight determinations are not possible on account of its 
lack of solubility. 

The /i-polymer is unsaturated toward bromine but no quantitative 
data on this point are yet available. The chlorine atoms are very firmly 
bound. Only slight traces of chloride ion are liberated when the compound 
is heated for six hours in boiling alcoholic potash or boiling pyridine. This 
fact suggests that the chlorine atoms of the polymer are still attached to 
carbon atoms bearing double bonds. 

The oxidation of the jit-polymer with hot nitric acid leads to the isolation 
of succinic acid. 

No attempts have yet been made to degrade the /^-polymer with ozone 
completely, but when stretched it is much more resistant than natural 
rubber to the deteriorating effect of ozone-containing air. 

It is well known that purified rubber hydrocarbon is very susceptible 
to autoxidation. A similar and perhaps more exaggerated sensitivity is 
characteristic of synthetic rubbers derived from diene hydrocarbons. 
In this respect ju-polychloroprene appears to be considerably more resistant. 
Nevertheless it does not long remain completely unaltered when freely 
exposed to air and light. It gradually becomes darker in color and finally 
after two or three weeks is dark brown. At the same time, it becomes 
harder, especially on the surface. These changes are accompanied by 
the liberation of traces of hydrogen chloride. The autoxidation can be 
suppressed by treating the polymer with small amounts of antioxidants. 


Chemical Structure of /n-Polychloroprene. The molecules of natural 
rubber are long chains built up from the unit (VII) derived (formally) 
from isoprene (VIII). This structure follows from the fact that the ozoni- 
CH 2 C=CH CH 2 CH 2 =C CH=CH 2 

CH 8 CH S 


zation of rubber (IX) leads to levulinic acid and levulinic aldehyde (X) 
as the principal products (11). 


. . .CH 2 C=CH CH 2 CH 2 C=CH CH 2 . . . > 

I I 

CH 3 CH 8 OCH CH 2 CH 2 COCH 3 


In their behavior toward ozone the so-called normal synthetic rubbers from 
isoprene very closely resemble the natural products (12), and their mole- 
cules must therefore for the most part be built up on the same general plan. 
Analogy suggests that the molecules of the chloroprene polymer are 
similarly built up from the units (XI) derived from chloroprene (XII). 

CHj C-=CH~-CH 2 CH 2 = C CH CH 2 

Cl Cl 


The resulting chains would have the formula XIII. 

. . .CH 2 C=:CHCH 2 CH2C=-CHCH2CH 2 C=CHCH 2 . . . > HOOC(CH 2 ) 2 COOH 

Cl Cl Cl 


This formula readily accounts for the fact that oxidation of the ju-poly- 
chloroprene yields succinic acid (XIV) . It also explains why the chlorine 
atom is very resistant to the action of alkalies : as in vinyl chloride, the 
chlorine is attached to a carbon atom bearing a double bond. This situa- 
tion is changed however by autoxidation. This must lead to some such 

'. CC1 CH 

grouping as \ o / m which the chlorine atom would be^xceedingly 


Some evidence for this formula is also found in the physical properties 
of the /x-polychloroprene. The molecular refractivity calculated for this 
formula (22.95) agrees exactly with the experimental value (22.95). The 
x-ray diffraction pattern indicates an identity period of 4.8 A., which 
corresponds quite well with that calculated for one chloroprene unit. 


It appears to be generally true that the presence of a chlorine atom at 
a double bond decreases the tendency of the double bond to react with 
ozone, and in this connection it is significant that /i-polychloroprene is 
much more resistant than natural rubber to the deteriorating action of 

In the formulas IX and XIII the isoprene and chloroprene units have 
been represented as being united regularly in 1,4-1,4- . . . order. The 
units however are not symmetrical arid in joining of two units one or both 
of them might be inverted. This would lead to the arrangements 1,4- 
4,1- . . . and 4,1-1,4- .... It has been demonstrated that such inver- 
sions (13) do occur in the polymerization of isoprene in the presence of 
sodium and alcohol. On the other hand isoprene rubber formed by ther- 
mal polymerization, since its behavior toward ozone is normal (14), must 
be free from any considerable proportion of such inversions in its molecules. 

The oxidation of /x-polychloroprene to succinic acid gives no indication 
as to whether the arrangement of the units is normal as represented in 
XIII or inverted as in XV and XVI, since the latter would also yield suc- 
cinic acid. 

. . CH*CC1=CII CHjr CH 2 CH= CCi CH 2 . . . XV 

. . CH 2 --CH=CC1 CHr CH 2 - CC1=CH CH 2 . . . XVI 

On the other hand the fact that ju-polychloroprene like natural rubber 
yields a sharp x-ray diffraction pattern whereas this property is absent 
from other synthetic rubbers perhaps indicates that the polychloroprene 
is freer from irregularities in the structure of its molecules than other 
synthetic rubbers. 

The spontaneous polymerization of isoprene requires several years for 
its completion ; with chloroprene the transformation is complete in a few 
days. The great difference in speed may be ascribed, in part at least, 
to the activating influence of the chlorine atom. The methyl group is too 
feebly polar to exert any such effect. A similar difference exists between 
vinyl chloride and propylene in their tendency to polymerize. The 
chlorine atdm here functions not only to powerfully activate the double 
bond, it also exerts a greater effect than methyl on the direction of addi- 
tion reactions at the double bond : it more effectively controls the polarity 
of the molecule. A similar directive effect in chloroprene is demonstrated 
by its behavior toward hydrogen chloride. Thus the relation between 
chloroprene and isoprene may be symbolized by the formulas XVII and 


2 = C-CH=CH 2 * ~ CH 2 =CCH=CH 2 

CH 3 
xvn xvin 

It seems highly probable therefore on theoretical grounds not only that 
the polymerization of chloroprene will proceed much more rapidly than 
the polymerization of isoprene, but that there will be much less chance 
of inversions of the units in the polychloroprene chains. 

Formula XIII adequately represents the chemical behavior of /x-poly- 
chloroprene, but is not sufficiently complete to account for the remarkable 
physical behavior of this material. The difficulties in this connection 
are precisely the same as those presented by natural rubber. The linear 
polymeric structure partially represented in formula IX furnishes a suffi- 
cient basis for describing the chemical behavior of rubber. It is known 
further that the molecular weight of rubber is exceedingly high perhaps 
in the neighborhood of 70,000 (15); but no entirely adequate explanation 
of the elastic properties of rubber in terms of this structure has been 
offered. It seems scarcely necessary to review the numerous speculations 
that have been devoted to this subject (16). We merely point out that 
the units in a substituted polyprene chain (e. g., rubber or polychloroprene) 
present the possibility of geometrical isomerism and that they may be 
arranged in cis-cis-cis- . . . order or trans-trans-trans- . . . order; or both 
arrangements may be present in a single chain. The molecules may be 
coiled into spirals rather than rigidly extended. On these questions we 
have no data concerning polychloroprene (see however the above paragraph 
entitled x-Ray Diffraction Pattern). Moreover, we have no direct infor- 
mation concerning the molecular weight of polychloroprene. It seems 
certain, however, that the chains must be very long. 

We observe finally that ju-polychloroprene resembles vulcanized rather 
than unvulcanized rubber. It is not plastic; it does not become plastic 
when heated; and it does not dissolve but merely swells in rubber solvents 
such as benzene and chloroform. This behavior is much more consistent 
with a three-dimensional polymeric structure than with a simple linear 
structure. We assume therefore that in /i-poly chloroprene chains of the 
type already described are chemically linked together at occasional points. 
The resulting structure as we conceive it may be symbolized by formula 
XIX in which A stands for the structural unit. The cross-linking may 
occur through the mediacy of oxygen atoms, but it seems more likely that 
the double bonds of parallel chains mutually saturate one another directly. 
Since the chains are exceedingly long they need to be linked together 
only at occasional points to produce a non-plastic structure. 


XIX . . . A A A A -A A A A . . . 


A A A A A A A A 

. . XX XX XX XV XX XX XX XX . . . 

I I 

. . . xx A xx x\~ xx xx A A . . . 

/3-Polychloroprene. When chloroprene is polymerized at elevated 
temperature, e. g, t at 60 especially in the absence of air, the product is 
somewhat less dense and softer than that formed at ordinary temperature, 
and it has a pronounced terpene-like odor. This odor is due to the presence 
of volatile chloroprene polymers (/3-polymer) . This product can be pre- 
pared in quantity by storing or heating chloroprene in the presence of sub- 
stances such as pyrogallol or trinitroberizene that inhibit the transfor- 
mation of chloroprene into the ^-polymer. By distillation the ^-polymer 
can be separated into two fractions, one boiling at 92 to 97 at 27 mm. 
and the other boiling chiefly at 114 to 118 at 27 mm. The odor of the 
fractions is very similar fragrant and terpene-like. The constitution 
of these products has not been determined, but there is little doubt that 
they are cyclic dimers of chloroprene. It is already known that analogous 
products are formed from isoprene (17), butadiene (18) and dimethyl- 
butadiene (19). The /3-polychloroprenes are stable substances, which show 
no tendency to polymerize further. They have therefore no direct bearing 
on the formation of the rubber-like polymers, and they are relatively un- 
important, especially since they appear in significant amounts only under 
rather exceptional conditions. 

Influences Affecting the Polymerization of Chloroprene. A few typical 
data are assembled in Table I. 

(a) Catalysts. Oxygen is an exceedingly powerful catalyst for the 
transformation of chloroprene into the /-i-polymer. Samples of chloro- 
prene distilled in high vacuum and sealed off in glass tubes without ex- 
posure to the air show an appreciable increase in viscosity only after a 
period of one or two months, and the transformation to /^-polymer is still 
incomplete after twelve months. From some recent studies of the behavior 
of isoprene (20) it appears that the active catalyst in such transformations 
is a volatile peroxide. Since it is practically impossible to prepare a sam- 
ple of chloroprene without exposing it to air at some time during the course 
of its preparation, it seems probable that even the products distilled in 
high vacuum are not altogether free from catalyst. 

The amount of oxygen necessary to produce an optimum catalytic 
effect is quite small. At ordinary temperatures the presence of a volume 
of air equal to about 10% of the volume of the chloroprene sample causes 
the transformation to /^-polymer to be completed in about eight to ten 





days Character of product 

8 Colorless, strong, tough 

400 Soft, low density, strong odor of dinier 

2 Strong, tough, slight odor of dimer 

10 Strong, tough, odor of dimer 

< 1 Semi-fluid, black, much dimer 

<2 Transparent, rather hard, very tough 

0.7 Transparent, rather hard, very tough 

2 Rather soft, odor of dimer 

1 Product variable 

1 Strong, tough, colorless 
vStill fluid after G5 days 
vStill fluid after 13 months; strong 
odor of dimer 


, Pressure 

Other added 


















































1 . 5% Benzoyl 






0.5% Benzoyl 






0.1% Catechol 





1.6% Thiodi- 


days. If the volume of air is much smaller than this the time required 
for the transformation is somewhat greater, but much larger ratios of air 
or oxygen do not greatly increase the velocity. Large quantities of oxygen 
do however affect the character of the final product. In general they 
lead to a product which, instead of being colorless or only slightly yellow, 
is dark brown and considerably harder and stiffer than the usual products. 

Peroxides such as benzoyl peroxide also function as catalysts. Their 
use, however, presents no practical advantages, and is in fact somewhat 
hazardous. In samples containing benzoyl peroxide there is frequently 
a considerable induction period, and then reaction suddenly starts at 
some point in the sample and spreads very rapidly through the mass. 
The heat of reaction may be sufficient to char the sample. 

The rate of formation of /3-polychloroprene is not appreciably acceler- 
ated by oxygen or peroxides. 

(b) Temperature, Pressure and Light. Catalytic and anticatalytic 
effects in the polymerization of chloroprene are very powerful and difficult 
to regulate exactly ; for this reason it is impossible to obtain precise quanti- 
tative correlations concerning the effect of temperature, pressure and 
light on the reaction velocity. It appears, however, that in the presence 
of air the transformation of chloroprene into the ^-polymer occurs about 
four times as fast at 62 as at 25. Thus the temperature coefficient of 
the catalyzed reaction is abnormally low. The relative rate of the for- 


mation of 0-polymer, which is negligible at ordinary temperature, is more 
strongly affected by rise in temperature. Polymers produced at tempera- 
tures above 50 contain appreciable proportions of /3-polymer. These 
proportions are still further increased if the reaction is carried out in the 
absence of air since this greatly reduces the speed with which ju-polymer 
is formed without affecting the formation of /3-polymer. In the absence 
of air the effect of increased temperature on the rate of transformation 
of chloroprene into /z-polymer appears to be much greater than in the 
presence of air. 

When chloroprene is polymerized at temperatures above 80 (in the 
absence of solvents) considerable decomposition occurs with liberation 
of appreciable amounts of hydrogen chloride, and the product is dark in 
color and tarry in consistency. 

At a pressure of 6000 atmospheres the polymerization of chloroprene 
occurs about ten times as rapidly as at ordinary pressure. 

Light has a considerable accelerating effect on the transformation of 
chloroprene into the //-polymer. The rate of formation of /3-polymer 
is not affected. The active wave lengths He in the blue, violet and near 

(c) Inhibitors. -Substances that generally function as antioxidants 
act as powerful inhibitors for the transformation of chloroprene into the 
ju-polymer. Under ordinary conditions a sample of chloroprene will set 
in four days to a stiff jelly containing about 40% polymer but the pres- 
ence of 0.1% of catechol will permit the sample to remain fluid for several 
months. This fact confirms indications already mentioned that the spon- 
taneous transformation of chloroprene into ju-polymer is normally de- 
pendent upon the presence of traces of autoxidation products of the chloro- 
prene. The formation of /3-polymer is not thus dependent upon oxida- 
tion. Samples of chloroprene containing inhibitors, after several months 
at the ordinary temperature, are found to contain several per cent, of 
jS-polymer, and frequently they are quite free of ju-polymer. A sample of 
chloroprene containing 0.2% of pyrogallol yielded 49% of crude 0-polymer 
after being heated for forty days at 62. 

The following types of compounds generally function as inhibitors: 
phenols, quinones, amines, mercaptans, thiophenols, aromatic nitro com- 
pounds, halogens. Some compounds in each of these classes function as 
powerful inhibitors, others have a feebler effect. It is somewhat sur- 
prising to find aromatic nitro compounds in this list. Trinitrobenzene 
is among the most powerful of the inhibitors. 


In the presence of relatively feeble inhibitors or small amounts of the 
more powerful inhibitors the polymerization of chloroprene can be effected 
at a somewhat diminished rate. The non-volatile polymer formed under 
these conditions, however, differs very considerably in its properties from 
that formed in the absence of inhibitors. 

(d) Solvents. The polymerization of chloroprene can be effected in 
the presence of solvents. If the solvent is one such as benzene, toluene, 
ethylene chloride, or carbon disulfide that powerfully swells M-poly chloro- 
prene, the resulting polymer remains dissolved. The polymerization of 
solutions containing as much as 50% by volume of chloroprene leads to 
the formation of stiff jellies. Even as little as 10% of chloroprene leads 
to highly viscous solutions. The solutions are generally colorless and 
transparent. The polymers contained in these solutions are somewhat 
different in their properties from the /i-polychloroprenes produced under 
ordinary conditions. They are softer, and unless the solution is very 
old they can usually be redissolved in benzene. 

The presence of the solvent considerably diminishes the rate of the 
polymerization, and dilute solutions polymerize more slowly than con- 
centrated ones. Solvents also frequently exert a specific effect. Chloro- 
prene dissolved in benzene polymerizes very much more rapidly than 
chloroprene dissolved in ether or pyridine. 

Chloroprene may also be polymerized in the presence of non- volatile 
solvents, inert fillers and foreign materials of various kinds. 

Granular Polymer (co-Polymer). The polymerization of chloroprene 
occasionally leads to a coherent mass of glistening, hard, rubbery granules 
or globules (co-polymer). This material is non-plastic and it shows scarcely 
any tendency to imbibe solvents. The conditions favoring its formation 
are not very clearly understood, since it occasionally appears under the 
most diverse conditions. It seems certain, however, that its formation 
is autocatalytic. When a speck of this polymer appears in a sample of 
chloroprene during the early stages of its polymerization the granular 
growth continues to spread through the whole sample. Because of its 
cell-like structure it occupies more volume than the same amount of M~ 
polymer, and if the growth begins to spread laterally through a sample 
it may burst the walls of a heavy Pyrex container even when the total 
volume of the container is much greater than the volume of the product. 
The presence of metallic sodium especially favors the formation of the 
granular polymer. It frequently appears under other conditions that 
result in very slow polymerization. 

According to experiments made by Dr. H. W. Starkweather the forma- 


tion of the co-polymer is initiated (or accelerated) by light of 3130 A. wave 
length. The following observation is especially interesting. 

Chloroprene containing pyrogallol to inhibit polymerization was placed 
in the bottom of a long Pyrex tube. The chloroprene was cooled to 80, 
and the tube was evacuated and sealed off. The lower half of the tube 
was covered with black friction tape to exclude light and the upper part 
was exposed to light from a mercury arc. During the exposure the lower 
part of the tube was kept in a bath at 10 ; the upper half was at 00-65. 
After twelve hours there was a white deposit at the top of the tube. This 
deposit gradually increased during two and one-half days. Exposure 
to the light was then discontinued. The solid deposit, however, continued 
to form at the top of the tube as a white, crinkly mass, until the liquid 
in the bottom of the tube was completely exhausted. 

It is interesting to observe that products similar to this granular poly- 
mer have been obtained from other dienes. Kondakow (21) observed that 
dimethylbutadiene in a closed flask in diffused daylight is gradually 
transformed into a white, insoluble mass, and Harries (22) obtained a 
similar product by the action of ultraviolet light on isoprene. 

The great resistance of the co-polychloroprene as compared with the 
co-polymer to the swelling action of solvents indicates a considerably 
higher degree of cross-linking of the chains in the co-compound. If such 
a cross-linking should occur in a sufficiently regular fashion, it would lead 
to a three-dimensional primary valence lattice, a type of structure which 
is illustrated by the diamond, but is not known among synthetic organic 
compounds. The conditions under which the co-polymer is formed are 
such as might be especially favorable to the development of a regular 
three-dimensional structure. The formation of co-polymer is catalyzed 
by an co-polymer surface ; the process is one of heterogeneous autocatalysis. 
It seems most probable that the function of the co-polymer surface in this 
connection is not (or at least not wholly) to activate adsorbed molecules 
of the monomer, but rather to orient the adsorbed molecules into a con- 
figuration favorable for mutual union. The incidence of activating energy 
could then bring about the combination of a very large number of mole- 
cules in a single act. 

As a matter of fact the granular polymer at first sight gives the im- 
pression of being definitely macrocrystalline, but on closer observation 
the crystals turn out to be globules. x-Ray examination gives only an 
amorphous pattern. 

It is interesting to note that the granular polymer never appears in 


samples of a-poly chloroprene (described later) that contain phenyl-/3- 

Progressive Changes during the Spontaneous Polymerization of 
Chloroprene. The following table illustrates in more detail the changes 
in properties and composition that occur when a sample of chloroprene 
is allowed to stand under ordinary conditions in the presence of a little air. 


Time, Viscosity in 

days Polymer, % Density centipoises 

0.952 0.4 

1 4 .. 6.0 

2 14 0.98 550.0 

4 45 1.06 vSt iff jelly 

10 99-h 1.23 Non-plastic 

The polymer formed during the early stages of the reaction can be 
isolated by precipitation with alcohol, or by distilling off the unchanged 
chloroprene in vacuo. This material is very different in its properties 
from the final product, the ^-polychloroprene already described. It is 
soft, plastic, and completely soluble in benzene. We call this plastic 
polymer a-poly chloroprene. When allowed to stand at the ordinary 
temperature it slowly reacts with itself and in the course of a day or two 
is transformed into a product apparently identical with the ju-polymer. 

Mechanism of the Formation of a- and ju-Polymers. The isolation 
of the a-polymer demonstrates that the transformation of chloroprene 
into the /x-polymer is a step-wise reaction. Some of the facts concerning 
the two polymers and their relation to each other are best correlated by 
a brief discussion of mechanism. 

The transformation of chloroprene into the a-polymer is evidently a 
chain reaction. It is enormously susceptible to catalytic and anticatalytic 
effects; it is accelerated by light; and although a large number of mole- 
cules is combined to form a single larger molecule, the apparent order of 
the reaction is low. The reaction probably first involves the coupling 
of an activated molecule of chloroprene with another chloroprene mole- 
cule. The activating energy persists in the polymeric chain until it has 
been built up to a considerable length. The molecules of a-polymer 
thus formed doubtless have the linear structure already suggested in 
formula XIII. The formation of a molecule of a-polymer involves a 
series of separate acts, but these follow one another in very rapid succession 
Under ordinary conditions the a-polymer present when 4% of a sample 


of chloroprene has polymerized is indistinguishable from the polymer 
present when 20% of the chloroprene has polymerized. 

The transformation of the a-polymer into the ju-polymer consists in the 
cross-linking of the long chains into a three-dimensional structure of the 
type represented in formula XIX. In a sample of chloroprene under- 
going spontaneous polymerization this process becomes noticeable when 
the concentration of polymer has reached about 25%. It is marked by 
an abrupt change in properties. The viscosity increases very rapidly 
and the sample soon sets to a stiff jelly. If the polymer is isolated just 
before this point is reached, it is found to be soft and plastic. Polymer 
isolated just after this abrupt change is still soft, but the manner in which 
it resists permanent deformation indicates the presence of a considerable 
proportion of the ju-polymer. 

The reactions, (1) chloroprene > a-polymer, and (2) a-polymer >* 
^-polymer, are not merely two stages of a single process, but are different 
reactions. The polymerization of chloroprene is rather strongly inhibited 
by primary aromatic amines, such as aniline, the naphthylamines, and 
benzidine; but these same compounds when mixed with isolated a-poly- 
mer accelerate its conversion into /z-polymer. The temperature coefficients 
of the two reactions are different. For reaction (2) the ratio of the velocity 
constants for a temperature increase of 10 is about two; for reaction (1) 
the ratio is considerably less than two. 

Other Polymers; Balata-like Polymer. The a-, M- and co-types arc 
not chemical individuals but rather qualitatively different species of poly- 
meric mixtures. The properties of each type may vary over a considerable 
range and in practice no doubt one generally has to do not with a pure 
species but with a mixture in which one of the species may preponderate. 
A consideration of the formulas assigned to the a- and /z-polymers will 
suggest some of the complications that might arise. The molecules of 
the a-polymer are no doubt chains of very great length, but in the poly- 
merization of chloroprene under certain conditions, e. g., at elevated 
temperature, the process of cross-linking may set in before the chains 
have attained the usual length of a-polymer chains. One will then have 
a product very different in its properties from that produced by the vul- 
canization of a-polymer. Stereochemical factors (e. g., cis-trans isomerism) 
may also produce great variations in the character of the products. 

The a-y 0-, /A- and co-polymers by no means exhaust the different types 
of polychloroprenes. Anything that influences the velocity of the poly- 
merization has some effect on the properties of the product, and the modifi- 
cations produced by inhibitors and catalysts are especially marked. The 


phenomena in this connection are very complex. It would be useless to 
attempt to recognize as distinct species all the distinguishably different 
polymeric products derived from chloroprene. 

There is, however, one type of product that appears to be qualitatively 
different from the a-, /3-, ju- and w-polymers. This material rather closely 
resembles balata in its properties. It is obtained more or less contami- 
nated with the other types of polymers under various conditions, but es- 
pecially by the polymerization of chloroprene in the presence of inhibitors 
such as iodine or the tetraalkyl thiuramdisulfides. 

A typical specimen of this balata-like material when cold is a hard, 
amorphous, non-brittle mass. When warmed to 60 it becomes soft 
and plastic. At higher temperatures it is quite sticky. When heated 
under vulcanizing conditions the plasticity is partly lost, but the trans- 
formation to the elastic condition is very incomplete. We make no at- 
tempt to suggest a structure for this material. 

Conditions for the Isolation of a-Polymer. Owing to the effect of 
changing concentrations, the rate of formation of a-polymer progressively 
decreases during the polymerization of chloroprene, and the rate of the 
conversion of a-polymer into ^-polymer progressively increases. For 
this reason if pure a-polymer is to be obtained, the reaction must be in- 
terrupted before all of the chloroprene has polymerized. 

Under the most favorable conditions the concentration of the a-polymer 
in the polymerizing mixture can be built up to 30 or 40% before any ap- 
preciable transformation to ju-polymer occurs. The reaction is best con- 
ducted in glass vessels under strong illumination from a Mazda lamp or 
a mercury arc in glass. The most effective wave lengths lie in the long 
ultraviolet, but the use of a quartz container with mercury arc radiation 
is not advisable on account of the danger of forming granular polymer. 
The temperature should be kept in the neighborhood of 35. Under these 
conditions about 30% of the chloroprene is polymerized in sixteen to 
twenty-four hours. The product is a thick, colorless, transparent sirup. 
If this sirup is poured into a large volume of alcohol, the a-polymer sepa- 
rates as a colorless mass and the unchanged chloroprene remains dis- 
solved in the alcohol. The a-polymer can also be separated by allowing 
the unchanged chloroprene to distil out of the mixture under diminished 
pressure. The mixture is preferably stirred during the distillation. 

Properties of the a-Polymer. In density and refractive index the 
a-polymer lies very close to the /^-polymer. The a-polymer resembles 
milled smoked sheets in its physical properties and mechanical behavior. 
It is plastic and it dissolves completely in benzene to form highly viscous 


solutions. It can be calendered into thin sheets or extruded with the 
usual rubber machinery 

At 30 the a-polymer loses its plastic properties and becomes completely 
changed to the elastic form (ju-polymer) in about forty-eight hours. At 
130 the transformation is complete in about five minutes. This process 
corresponds to the vulcanization of natural rubber, but sulfur is not 
needed and when present it takes no part in the process. The speed of 
this transformation can be greatly modified by the addition of various 
substances, some of them materials that are used in the vulcanization 
of natural rubber. 

Zinc oxide brings about the vulcanization of a-polymer in eight to ten 
hours at 30. Zinc chloride, zinc butyrate and ferric chloride arc even 
more active catalysts. The most effective organic catalysts are primary 
aromatic amines such as aniline, the naphthyl amines and benzidine. 
Diphenylguanidine, which is a relatively active vulcanization accelerator 
for natural rubber, is a mild accelerator for a-polymer. On the other 
hand mercaptobenzothiazole and tetraalkylthiuram sulfides, which are 
active natural rubber accelerators, have no accelerating action on the 
a-polymer. Basic inorganic materials such as lime and magnesium oxide, 
which may accelerate natural rubber through their action with sulfur, 
have a slight retarding influence on the vulcanization of a-polymer. 
Strong acids and acidic materials that retard the vulcanization of natural 
rubber have no influence on the curing of plastic polymer. 

Secondary aromatic amines such as phenyl-0-naphthylamine power- 
fully inhibit the vulcanization of a-polymer at ordinary temperature. 
This fact is of considerable practical importance since it brings about the 
possibility of storing the plastic polymer over long periods of time. The 
phenyl-jft-naphthylamine also acts as an antioxidatit arid confers age- 
resisting properties on the final product. The inhibiting effect of the 
phenyl-j3-naphthylamine on the curing of the a-polymer largely disappears 
above 100. 

Behavior of a-Polymer in Compounding and Properties of the Cured 
Rubber. The compounding ingredients that can be used with a-poly- 
mer are similar to those used with natural rubber, but there are a number 
of important additions. Materials such as ground leather and cork which 
strongly retard the vulcanization of natural rubber act as inert ingredients 
in a-polymer. Carbon black and zinc oxide act as reinforcing agents as 
they do in natural rubber and impart good abrasion resistance. In con- 
trast to their action in rubber, whiting and clay are perfectly wet by a- 
polymer, and they produce compounds having good tear resistance. Cot- 


ton and other vegetable fibers are also much more perfectly wet by a- 
polyrner than by rubber. Most plasticizing and softening materials 
such as mineral oil, stearic acid and pine tar are insoluble in a-polymer 
and have little true softening action. Milling also produces little soften- 
ing other than a temporary thermal effect. Mineral rubber and similar 
asphaltic materials act as diluents with little effect on the physical proper- 
ties of the vulcanized material. 

Natural rubber can be successfully milled into a-polymer, although 
there is little affinity between the two. Sheets of the two rubbers before 
vulcanization may be firmly pressed together and easily separated. A 
reasonably firm union can be obtained between natural and chloroprene 
rubber when they are vulcanized together under sufficient pressure. Ben- 
zene solutions of rubber and a-polymer are not compatible; when thor- 
oughly mixed they quickly separate into two layers. 

Table III gives the composition of compounded stocks prepared from 
a-poly chloroprene and from smoked sheets. These were used for a series 
of parallel tests to compare the behavior of the two rubbers. All the 
processing and testing were carried out with the usual rubber laboratory 




Amount in grams 

Compound from Compound from 

Materials a-polychloroprene smoked sheets 

aj-Polychloroprene 100 

Smoked sheet rubber . 100 

Zinc oxide 10 10 

Sulfur ... 3 

Stearic acid 2 2 

Diphenylguanidine ... 1 
Benzidine (accelerator) 0.5 

Phenyl-/3-naphthylamine 1 1 

The effect of different times and temperatures of vulcanization on the 
physical properties of the two compounds is shown in Table IV. The 
chloroprene rubber reaches a maximum tensile strength after only five 
minutes at 140 ; but the strength is not adversely affected if the curing 
time is extended to sixteen hours. In contrast to this, the natural rubber 
compound vulcanizes more slowly and softens rapidly under the action 
of prolonged vulcanization. The maximum tensile strength for the chloro- 
prene compound is slightly lower than that for the natural rubber com- 



pound. In contrast to the smoked sheet compound the chloroprene 
compound shows no reversion and the load supported at 500% elongation 
continues to increase slowly throughout the curing range. When the 
polychloroprene compound is heated longer than sixteen hours at 140 
a material resembling hard rubber is formed. 






Load at 500% elongation, 
kg. per sq cm. hr 
Chloroprene vStnoked Chi 
rubber sheet r 


29 9 

Pensile strength at 
eak, kg. per sq cm. 
oroprene Smoked 
ubber sheet 

!(>(> 9 

109 3 
70.5 . . 
70 5 

% Elongation 
at break 
Chloroprene Smoked 









































1 1(>7 









2 1(57 








42.0 181 









1 05 










9 151 






The chloroprene compound is very resistant to the action of ozone. 
The two compounds were stretched about fifteen per cent, and exposed 
to ozone-containing air. The natural rubber compound was ruptured 
in three minutes; the chloroprene compound was not detectably affected 
during an exposure of three hours. 

Table V shows the results of artificial aging tests on the two compounds 
of Table III. The chloroprene compound was cured for fifteen minutes 
and the natural rubber compound for sixty minutes at 140. The tests 
were carried out at 70 in oxygen at twenty atmospheres. Under these 
conditions twenty-four hours is generally considered to approximate one 
year of natural aging for rubber. These data indicate that the chloro- 
prene rubber is considerably more resistant to oxidation than natural 
rubber. Natural aging tests have not been carried out for a length of 
time sufficient to confirm this conclusion, but samples of the chloroprene 
rubber compound that have been kept for one year show no deterioration. 

Chloroprene rubber is much more resistant than the natural product 
to the action of solvents and many chemicals. After seventy- two hours 
the chloroprene compound had increased 7% in weight by immersion in 




Tensile strength, 

kg. per sq cm. % Elongation at break 

Days in oxygen Chloroprene Smoked Chloroprene Smoked 

bomb at 70 rubber sheets rubber sheets 

172.2 232.0 890 720 

1 212.6 195.0 845 660 

2 181.0 163.4 820 660 

3 193.3 159.9 805 670 
8 209.1 116.1 720 595 

14 165.2 ,54.5 690 510 

light machine oil and 25% by immersion in kerosene, and it had retained 
more than half of its original tensile strength in each case. The tensile 
strength of the natural rubber was destroyed under these conditions. 
In contrast to natural rubber, Chloroprene rubber is not attacked by hy- 
drogen chloride, hydrogen fluoride, sulfur chloride, ozone and many other 
chemicals. The high chlorine content of the chloroprene rubber also 
renders it very resistant to combustion. Measurements of diffusion of 
both hydrogen and helium through a poly chloroprene membrane show it to 
be only 40% as permeable as natural rubber. The absence of water- 
soluble materials in chloroprene rubber makes it very resistant to pene- 
tration by water. 

Synthetic Latex. Chloroprene is readily emulsified by shaking or 
stirring it with water containing an emulsifying agent such as sodium 
oleate. The resulting emulsion polymerizes very rapidly and completely. 
The polymer remains suspended or emulsified and constitutes an artificial 
latex. When the water is allowed to evaporate from a layer of this latex, 
a thin coherent, strong, elastic film remains. This film in its physical 
and mechanical properties very closely resembles the ^t-polychloroprene 
already described: it is strong, extensible, elastic, resilient, non-plastic 
and not thermoplastic, and it is swelled but not dissolved by benzene. 

The following example illustrates the preparation of a synthetic latex. 
Four hundred grams of chloroprene is slowly added with vigorous stirring 
to 400 g. of water containing 8 g. of sodium oleate in a wide-mouthed 
bottle. A smooth emulsion results. After a time (usually about thirty 
minutes) the temperature of the mixture begins to rise and it may quickly 
reach the boiling point of the chloroprene unless cooling is applied. After 
standing for two to eight hours at room temperature the polymerization 
is complete. The mixture is then practically odorless. 

More uniform products are obtained if the temperature is carefully 


controlled during the emulsification and polymerization. At a tempera- 
ture of 10 the process is complete in about twenty-four hours always 
in less than forty-eight hours. The reaction is always characterized by 
an induction period, which at 10 usually lasts forty to sixty minutes. It 
is evident that the speed of polymerization of chloroprene is much greater 
(apparently at least 20-fold) in emulsion than otherwise. The particular 
nature of the interface appears to be of great importance in determining 
the rate of polymerization. The rate is much more rapid with sodium 
oleate than with egg albumen although both of these emulsifying agents 
produce very small particles. 

A small amount of free acid is developed during the polymerization 
of the emulsions, and this gradually brings about coagulation during stor- 
age. However, if a little ammonia (e. g., 5 g. of NH 3 per liter) is added 
to the latex after completion of the polymerization this tendency is avoided. 
Latex stabilized in this way can be stored indefinitely without change. 
In addition to the ammonia it is ordinarily desirable to add an antioxidant 
such as phenyl-|3-naphthylainine since this greatly prolongs the life of 
articles prepared with the latex. 

The particle size of latex prepared according to the above example is 
very small and remarkably uniform. Figure 4 gives the results of some 
measurements made by Dr. J. B. Nichols with the ultracentrifuge. When 
sodium oleate is used as the emulsifying agent the mean radius of the 
particles is about 0.063/x and more than (>0% of the particles lie between 
0.05 and O.OTju- It appears that the ultracentrifugal method has not 
been applied to natural latex, but according to Hauser (23) the latex from 
mature Hevea trees contains particles ranging in diameter from 0.5 to 
3/x as well as a considerable proportion of smaller particles. The particle 
size of the synthetic latex can be controlled to a certain extent by suitably 
modifying the nature and the amount of the emulsifying agent. The 
use of lithium oleate in the ratio of two grams to one hundred grams of 
chloroprene gives particles having a mean radius of about 0.087/x. 

In the preparation of the synthetic latex the ratio of chloroprene to 
water can be varied over a wide range. As long as the concentration lies 
below 55% by weight of poly chloroprene, the latices are very fluid. Above 
this concentration there is a sharp increase in viscosity, and a 60% latex 
is quite thick. 

The synthetic latex is rapidly coagulated by acids, alcohol, acetone and 
many salts. The polychloroprene separates as a coherent mass, which is 
at first quite soft and plastic. However, as soon as the water is squeezed 
out, this plasticity is lost. The mass then has the properties already 



indicated for /z-polychloroprene it is analogous to a soft, vulcanized 
natural rubber. 

Applications of the Latex. The chloroprene latex can be directly 
applied to many uses after the manner of vulcanized natural latex. In 
this connection it has some special advantages owing to its peculiar prop- 
erties. Thus on account of its small particle size it penetrates porous 

articles such as leather and .__________________,____ 

wood in a manner that can 
hardly be approached with 
natural latex. 

Shaped articles are readily 
prepared by dipping forms of 
glass, metal or porcelain into 
the latex and coagulating the 
resulting film or allowing it to 
dry. By repeated dipping, 
articles of any desired thick- 
ness can be built up. 

The latex can be mixed with 
dyes, fillers and modifying and 
protective agents of various 
kinds to adapt it to specific 

Polymerization of Chloro- 
prene in Porous Materials. 
Attempts to impregnate such 
porous materials as leather, 
wood and tile with natural 
rubber are unsuccessful, 
whether the rubber is used in 
the form of latex or dissolved 
in a solvent such as benzene; 
but a very intimate impreg- 
nation of these materials with 

chloroprene rubber can be accomplished by soaking them in chloroprene 
and then allowing the rubber to be formed in place. Chamois or kid leather 
saturated with its own weight of chloroprene and sealed to prevent evapo- 
ration until polymerization is complete becomes translucent and assumes 
a rubber-like flexibility without extensibility. Impregnated spruce be- 
* Fraction of total weight (ck) corresponding to a certain radius (r) . 

100 200 300 

Radius in millimicrons. 

Fig. 4. Weight distribution curve for chloro- 
prene lattices. The units are such that the 
area under the curves for any radius interval 
equals the fraction of the total weight of parti- 
cles whose radii lie in that interval. Curve A, 
latex made with sodium oleate. Curve B, latex 
made with lithium oleate. 


comes water resistant but its appearance is unchanged. This process 
may be used with any porous or bibulous material that does not con- 
tain inhibitors for the spontaneous polymerization of chloroprene. 

Conclusion. If space permitted it would be possible to review the 
literature on synthetic rubber from diene hydrocarbons and to show that 
almost every recorded peculiarity and complication in this field finds 
some analogy in the behavior of chloroprene. Chloroprene is therefore 
capable of serving adequately as a representative diene in studying as 
a scientific problem the synthesis of rubber-like materials, and for this 
purpose it has the great advantage of its very high speed of polymeriza- 
tion as compared with dienes previously available. 

On the economic side the greatly diminished costs of producing natural 
rubber have obviated the need for an artificial material having the same 
properties. There remains, however, the need for a synthetic rubber that 
is free from some of the inherent defects of the natural product. The 
differences between polychloroprene and natural rubber are sufficient to 
suggest considerable potentialities for the new synthetic product. 


Chloro-2-butadiene-l,3 (chloroprene) is described arid its structure 
established through reactions leading to its conversion into butane- a, 
/3,7,5-tetracarboxylic acid, and into /3-chloroanthraquinone. 

Within ten days under ordinary conditions in a closed vessel containing 
a little air, chloroprene spontaneously changes into a transparent, resilient, 
strong, non-plastic, elastic mass resembling vulcanized rubber. This 
product is called ^-poly chloroprene. By interrupting the polymerization 
before it has proceeded to completion one obtains a soft plastic product 
(a-polymer) that resembles unvulcanized rubber. Under the action of 
heat the a-polymer rapidly changes to the ^-polymer. Other polymers 
of chloroprene described are volatile (0-) polymer, granular (co-) polymer, 
and balata-like polymer. The structures of the polymers are discussed 
as well as the effect of conditions on the formation of each type. 

Unlike any previously described synthetic rubbers, ^-polychloroprene 
resembles natural rubber in the fact that when it is stretched its x-ray 
diffraction pattern shows a point diagram. 

The transformation of chloroprene into /i-polychloroprene occurs very 
rapidly in aqueous emulsion. The resulting product constitutes a syn- 
thetic (vulcanized) latex. It has a much smaller particle size than natural 
latex and it penetrates porous materials more readily. 


Chloroprene can also be polymerized in the pores of porous or bibulous 
materials. The materials thus become intimately impregnated with syn- 
thetic rubber. 

Compared with natural rubber the new synthetic rubber is more dense, 
more resistant to absorption or penetration by water, less strongly swelled 
by petroleum hydrocarbons and less permeable to many gases. It is much 
more resistant to attack by oxygen, ozone, hydrogen chloride, hydrogen 
fluoride and many other chemicals. 

Bibliography and Remarks 

(1) Nieuwland, Calcott, Downing and Carter, Journ. Am. Chem Sac , 53, 4197 (1931). 

(2) data are based on vapor pressure measurements made with an isoteniscope by Dr. 
H. W Stark weather 

(3) Diels and Alder, Ber , 62, 2337 (1929). 

(4) Auwers and Jacob, tbid., 27, 1114 (1894) 

(5) Farmer and Warren, /. Chem. Soc , 897 (1929). 

(6) The same behavior has already been observed in rubber by Hock, Gummi-Ztg., 39, 1740 
(192. r >) 

(7) Katz, Chem -Zlg , 49, 353 (1925); Meyer and Mark, Ber , 61, 1939 (1928). 

(8) "Die Rontgentechnik in der Materialprufung," Egbert and vSchiebold, Akademische Verlags- 
gesellvschaft, Leipzig, 1930* p. 142 

(9) Meyer and Mark, "Der Aufbau der hochpolymeren organischen NaturstofFe," Akademische 
Verlagsgesellschaft, Leipzig, 1930 

(10) Hauser and v Susich, Kantschuk, 7, 145 (1931). 

(11) Harries, Ber., 37, 2708 (1904); 38, 3985 (1905); Pummerer, Kbermayer and Gerlach, ibid , 
64, 809 (1931). 

(12) Harries, " Untersuchungen," Julius v Springer, 1910, p 222 

(13) Midgley and Henne, Journ. Am. Chem Soc , 52, 2077 (1930). 

(14) Cf. Pummerer and Koch, in Memmler's "Handbuch der Kautschukwissenschaft," S Hirzel, 
Leipzig, 1930, p. 270. 

(15) Staudinger and Bondy, Ann , 488, 127 (1931) 

(16) vSome of the most suggestive are Staudinger, Kautschuk, 5, 911, 1261 (1929), Meyer and 
Mark, "Der Aufbau der hochpolymeren organischen Naturstoffe," Akademische Verlagsgesellschaft, 
Leipzig, 1930; Fikentscher and Mark, Kautschuk, 6, 2 (1930). 

(17) Pummerer and Koch, loc. cil , p 284 

(18) Hofmann and Tank, Z aneew Chem , 25, 1405 (1912). 

(19) Van Romburgh and Van Romburgh, Proc Roy. Acad Amsterdam, 34, 224 (1931) 

(20) Couant and Peterson, private communication 

(21) Kondakow, / prakl. Chem., [2J 64, 109 (1901). 

(22) Pummerer and Koch, loc at , p 268. 

(23) Hauser, "Latex," Theodor vSteinkopff, Leipzig, 1927, p. 56. 


III. The Addition of Hydrogen Chloride to Vinylacetylene* 

Vinylacetylene (I) constitutes the simplest possible example of a con- 
jugated enine system. Recorded information concerning the addition 
reactions of such systems is exceedingly meager. It has already been re- 
ported (1) that chloroprene (chloro-2-butadiene-l,3, III) can be obtained by 
the addition of (aqueous) hydrogen chloride to Vinylacetylene, and the 
present paper is concerned with a further description of the mechanism 
of this reaction and the nature of the products to which it leads. 

It is shown that the initial step consists in 1,4 addition, and the primary 
product thus formed is chloro-4-butadiene-l,2 (II) (b. p. <SS), a new com- 
pound of rather unusual structure and curious properties. Under certain 
conditions this chloro-4-butadiene-l,2 can be isolated as the major reaction 
product, but it readily undergoes an isomerization involving migration of 
the chlorine atom and a shift of a double bondf. Chloroprene (b. p. 59.4) 
is formed thus, and the transformation occurs with such facility in the 
presence of hydrogen chloride that chloroprene always constitutes a part 
of the reaction product. Certain salts reinforce the catalytic effect of 
hydrogen chloride on this transformation, and when cuprous chloride is 
present no chloro-4-butadiene-l,2 is found in the reaction product. When 
sufficient amounts of hydrogen chloride are present the reaction proceeds 
further with the formation of dichloro-2,4-butene-2 (IV). 


CHC CH=CH 2 > CH,=C==CH CH,,C1 > 


CH 2 =C CH=-CH 2 > CH 3 C=CH CH 2 C1 

I I 

Cl Cl 


Influence of Conditions on the Reaction. The reaction between 
Vinylacetylene and hydrogen chloride is conveniently carried out in the 
following manner. Fifty grams of Vinylacetylene and 175 cc. of concen- 
trated hydrochloric acid (about 2.2 moles of hydrochloric acid) are placed in 
a pressure bottle, and the bottle is shaken to promote contact between the 
aqueous and the hydrocarbon layers. After the completion of the reaction 
the oily layer is separated, dried, mixed with a small amount of an atiti- 

* W. H. Carothers, G. J. Berchet and A. M. Collins, Journ. Am. Chem. Soc., 54, 
4066-70 (1932); Contribution No. 96 from the Experimental Station of E. I. du Pont 
de Nemours and Co. 

Received June 9, 1932. Published October 5, 1932. 

f For a further study of the chemical behavior of chloro-4-butadiene-l,2 and for 
a discussion of its rearrangement to chloroprene, see this volume, page 335. 


oxidant such as catechol or pyrogallol, and distilled in vacuo through an 
efficient column. Data from a very large number of experiments of this 
type are available. Owing to the fact that in most cases neither the tem- 
perature nor the speed of shaking was precisely controlled these data 
cannot be used as a basis for a quantitative description of reaction veloci- 
ties; nevertheless, they give a clear idea of relative rates. 

In a typical experiment of the kind described above, about 43% of the 
vinylacetylene was utilized in seven hours, and analysis of the reaction 
product yielded chloro-4-butadiene-l,2 and chloroprene in the ratio 2.2:1. 

In general this ratio is diminished by increase in the temperature, con- 
centration of hydrogen chloride, or time of contact; and the proportion of 
the chloro-4-butadiene-l,2 in the product is less the more completely the 
vinylacetylene is utilized. This fact demonstrates that part at least of 
the chloroprene is formed by the isomerization of the chloro-4-butadiene-l,2 
during the course of the reaction, and it seems reasonable to conclude that 
all of it is formed by this demonstrated mechanism. 

The reaction is considerably accelerated by the presence of certain salts. 
Thus when 23 g. of calcium chloride is present in the reaction mixture the 
time required to obtain 40% conversion is decreased by about one-half, 
but the ratio of the two isomeric chlorobutadienes present in the reaction 
product at a given percentage conversion remains practically unaffected. 
Cuprous chloride is a much more powerful catalyst. When 23 g. of this 
salt is present in the reaction mixture, about 90% of the vinylacetylene 
reacts in four hours at 20. In this case the product consists for the most 
part of chloroprene, and no chloro-4-butadiene-l,2 is present. It is, how- 
ever, not necessary to assume that the cuprous chloride directs the* reaction 
so that addition occurs at the acetylenic linkage. Separate experiments 
show that cuprous chloride reinforces the catalytic effect of hydrogen 
chloride on the isomerization of chloro-4-butadiene-l,2 and this effect seems 
adequate to explain the absence of chloro-4-butadiene-l,2 in the reaction 

Chloroprene reacts further with hydrogen chloride to produce dichloro- 
2,4-butene-2. This reaction is also catalyzed by cuprous chloride, and the 
relative velocities are such that any conditions of concentration, tempera- 
ture, and time of contact that suffice to convert all of a given sample of 
vinylacetylene result in the formation of a certain amount of the dichloro- 
butene. The following experiment is illustrative. Fifty grams of vinyl- 
acetylene, 173 cc. of concentrated hydrochloric acid, 23 g. of cuprous chlo- 
ride, and 10 g. of ammonium chloride were placed in each of forty bottles. 
The bottles were gently shaken in a bath at 20 for four hours and then 


allowed to stand for twelve hours at 0. The contents of all the bottles 
were combined and the mixture was steam distilled in vacua into a receiver 
placed at the bottom of a 2-meter jacketed, carborundum-packed column 
provided with a dephlegmator and a second receiver each cooled to 80. 
The pressure was kept at about 150 mm. until all of the unchanged vinyl- 
acetylene had collected in the second receiver, and the pressure was then 
gradually reduced to 10 mm. until all of the chloroprene had collected in the 
second receiver. The dichlorobutene remained in the first receiver. The 
yields were: unchanged vinylacetylene, 115 g.; chloroprene, 2802 g.; di- 
chlorobutene, 117 g. In mole percentages these values are 5.7, 84.2 
and 2.5, respectively. The deficit amounting to 7.6 mole per cent, was 
mostly comprised in intermediate fractions. When this deficit is dis- 
tributed proportionately among the three major fractions, the percentage 
yields become: unreacted vinylacetylene, 6.1%; chloroprene, 91.2%; 
dichlorobutene, 2.7%. The corresponding figures for the calculated yields 
based upon unrecovered vinylacetylene are chloroprene, 97%, and di- 
chlorobutene, 3%. Further experiments indicate that the conditions of 
this experiment lie very close to the optimum for the conversion of vinyl- 
acetylene to chloroprene in a batch process : a higher ratio of chloroprene to 
dichlorobutene can be obtained only by utilizing a smaller proportion of the 
applied vinylacetylene, and a more complete utilization of the vinyl- 
acetylene results in a larger proportion of dichlorobutene. 

The Properties of Chloro-4-butadiene-l,2 and the Proof of Its Struc- 
ture. Chloro-4-butadiene-l,2 is a colorless liquid boiling at 87.7 to 
88.1. It is only slightly soluble in water but miscible with most of the 
common organic solvents. It has a peculiar, sharp odor. Some other 
properties are : n 1.4775; df, 0.9891; M tt calcd., 24.61; A/ R found, 25.30. 
Anal. Calcd. for C 4 H 5 C1: C, 54.23; H, 5.64; Cl, 40.11. Found: C, 55.04, 55.11 ; 
H, 5.70, 5.90; Cl, 39.75, 40 03. 

Its chlorine atom is exceedingly reactive. When mixed with alcoholic 
silver nitrate it rapidly yields a copious precipitate of silver chloride. 
This fact in itself indicates that the compound is not a 1,3-diene, since in 
such a structure the chlorine atom would, of necessity, be attached to a 
doubly bonded carbon. Furthermore, the compound does not react with 
maleic anhydride or with naphthoquinone. It is not a true acetylenic 
compound either since it does not yield any derivative with ammoniacal 
cuprous chloride. When treated with ozone it yields formaldehyde and 
(after oxidation with potassium permanganate) chloroacetic acid. The 
compound has also been directly oxidized with potassium permanganate. 
The only product obtained was chloroacetic acid. Acetic acid and oxalic 


acid were absent. This behavior demonstrates the presence of the groups 
CH 2 = and =CH CH 2 C1, and the compound must therefore have the 
structure chloro-4-butadiene-l,2. This structure is further confirmed by 
the fact that under the action of cold concentrated sulfuric acid, chloro-4- 
butadiene-1,2 is readily converted into chloro-4-butanone-2 (2). 

The transformation of chloro-4-butadiene-l,2 into chloroprene exempli- 
fies a type of reaction that is common to many substituted allyl halides. 
Such halides (e. g. t CH 3 CHXCH=CH 2 or CH 3 CH=CHCH 2 X) arise 
quite generally by the addition of halogens or hydrogen halides to 1,3- 
diencs, and in these cases the possibility of isomerization frequently makes 
it difficult or impossible to determine whether the primary product is the 
result of 1 ,2 or 1,4 addition. Chloro-4-butadiene-l, 2, however, differs from 
other allyl halides: the a, 7 shift brings the adjacent double bonds into the 
more stable conjugated configuration and the chlorine atom becomes at- 
tached to a doubly bonded carbon where its mobility is lost. A reversal of 
the isomerization is therefore impossible, and chloro-4-butadiene-l,2 can- 
not be other than a primary product of the addition of hydrogen chloride to 

The transformation of chloro-4~butadiene-l,2 into chloroprene has been 
observed under a variety of conditions : by the action of powdered potas- 
sium hydroxide, by the action of hot quinoline (140-150), by the action of 
heat (290) in the presence of silica gel, and by the action of hot dilute 
hydrochloric acid (3) . However, the isomerization occurred most smoothly 
and rapidly in the presence of hydrochloric acid containing cuprous chlo- 
ride. Fifty grams of chloro-4-butadiene-l ,2 was refluxed for three and one- 
half hours with 20 g. of cuprous chloride in 100 cc. of 18% hydrochloric 
acid. The oily layer was decanted, dried and distilled. The entire speci- 
men except for a small amount of undistillable residue came over between 
o9 and ()3 and the distillate was pure chloroprene. When the chloro-4- 
butadiene-1,2 was similarly treated with aqueous cuprous chloride alone it 
was recovered unchanged. 

Chloro-4-butadiene-l, 2 unlike its isomer chloroprene shows no tendency 
to polymerize. It can be distilled unchanged at ordinary pressure, and 
specimens stored under the ordinary laboratory conditions for many months 
remain unaltered. It undergoes no change even when submitted to a 
pressure of 6000 atmospheres for forty- five hours at 50 (4). 

Experimental Part 

Oxidation of Chloro-4-butadiene-l, 2. To a mixture of 30 g. of chloro-4-butadiene- 
1,2 with 250 cc. of water containing a little sodium carbonate was added in small por- 


tions with constant stirring 214 g. of potassium permanganate. The mixture was fil- 
tered, the filtrate acidified and continuously extracted with ether for several hours. 
The ether solution on distillation gave a liquid boiling at 185 which solidified on cooling. 
This was chloroacetic acid, identified by its melting point, neutralization equivalent 
(found, 96; calcd., 94.5) and transformation into chloroacetamide, m. p. 119. No 
acetic or oxalic acid was found. 

Ozonization of Chloro-4-butadiene-l,2. A solution of 20 g. of chloro-4-buta- 
diene-1,2 in 20 cc. of chloroform was treated with ozone for twelve hours at 0. The 
solvent and the unchanged material were evaporated in vacua and the remaining ozonide. 
was decomposed with water. Formaldehyde was detected in the aqueous solution by its 
strong odor and the formation of methylene-di-/3-naphthol; white needles melting at 
204 (corn). 

Chloroacetaldehyde was not detected directly. The aqueous solution was treated 
gradually with 50 g. of potassium permanganate, the excess permanganate destroyed 
with sulfur dioxide, and the filtrate extracted with ether. Distillation of the ether so- 
lution left a residue which crystallized on cooling. This product was chloroacetic acid, 
identified by its melting point and neutralization equivalent. 

Hydration of Chloro-4-butadiene-l,2.- Into 250 cc. of concentrated sulfuric acid 
88.5 g. of chloro-4-butadienc-l,2 was added dropwise with stirring, the temperature be- 
ing maintained at 5 to 3. The dark reaction product was poured onto cracked ice, 
partly neutralized with sodium carbonate and extracted with ether. The ethereal 
solution was washed, dried and distilled, yielding 58 g. of crude chloro-4-butanone-2, 
b. p. 110-123. On redistillation it boiled at 120 to 122 at 760 mm. 

Anal. Calcd. for C 4 H 7 OC1: C, 45.07; H, 6.57; Cl, 33.33. Found: C, 45.42, 
45.42; H, 6.81, 6.47; Cl, 32.26, 32.42. 

When treated with pheiiylhydrazine it gave a derivative having the correct melting 
point (77) and analysis for phenylmethylpyrazoline (5). 

Anal. Calcd. for CioHi 2 N 2 : C, 75.00; H, 7.50; N, 17.50. Found: C, 74.32; 
H, 7.60; N, 17.10. 

Dichloro-2,4-butene-2. This material is obtained as a by-product in the prepara- 
tion of chloroprene and it is readily prepared in quantity by shaking vinylacetylene with 
an excess (4 moles) of hydrochloric acid containing cuprous chloride. It is a colorless 
liquid having a characteristic odor; other properties are: boiling point 127-129 at 
756 mm., 61-63 at 70 mm., 53 to 54 at 50 mm., d* 1.1591, 2 D 1.47239, w 2 c ? 1.46988, 
w 2 F 1.48187, A/R calcd. 29.94, M R found 30.27. 

Anal. Calcd. for C 4 H 6 C1>: C, 38.40; H, 4.80; Cl, 56.80. Found: C, 38.53; 
H, 4.87; Cl, 56.90. 

The proof of the structure of this compound will be presented in a future paper deal- 
ing with its reactions. 

Acknowledgment. The writers are indebted to Mr. O. R. Kreimeier 
for assistance in the experiments on the addition of hydrogen chloride to 


The results of experiments on the action of aqueous hydrogen chloride 
on vinylacetylene are described. The initial step consists in 1,4 addition 


and the primary product is chloro-4-butadiene-l,2. This readily under- 
goes isomerization, yielding chloroprene, which always constitutes a part of 
the reaction product. When cuprous chloride is present in the reaction 
mixture the isomerization proceeds more rapidly and no chloro-4-buta- 
diene-1,2 is found in the reaction product. When sufficient hydrogen 
chloride is present the reaction proceeds further, yielding dichloro-2,4- 

Bibliography and Remarks 

(1) Carothers, Williams, Collins and Kirby, Journ. Am. Chem. Soc., 53, 4203 (1931); this volume, 
page 281. 

(2) Cf. Gustavson and Demjanoff, J. prakt. Chem., [2] 38, 201 (1888); Bouis, Ann. chim., [10] 9, 
402 (1928). 

(3) We are indebted to Dr. D. D. Coffman for some of these observations. 

(4) We are indebted to Dr. H. W. Starkweather for this observation. 

(5) Maire, Bull. soc. chim., [4] 3, 272 (1908). 

IV. The Addition of Hydrogen Bromide to Vinylacetylene, 
Bromoprene and Dibromobutene* 

Observations on the combination of vinylacetylene with hydrogen chlo- 
ride (1) have been extended to the analogous case of hydrogen bromide. 

The two reactants are closely similar in their behavior, but hydrogen 
bromide appears to act somewhat more slowly than hydrogen chloride. 
Concentrated aqueous hydrobromic acid containing cuprous bromide, 
when shaken with vinylacetylene at the ordinary temperature, yields the 
two products, bromo-2-butadiene-l,3 (bromoprene, II) and dibromo-2,4- 
butcne-2 (III). It seems likely, in view of the results already described 
for the hydrogen chloride reaction (Ib) that the primary product of reaction 
between hydrogen bromide and vinylacetylene is bromo-4-butadiene-l,2 
(I), which then rearranges to yield bromoprene, but no decisive direct 
evidence for the formation of this primary product is yet available. Its 
presence among the reaction products has not been established even when 
no cuprous bromide (catalyst) was used. The structure of the dihydro- 
bromide has not yet been directly established either, but formula III, 
in view of the results with hydrogen chloride, is not open to serious doubt. 

* W. H. Carothers, A. M. Collins and J. E. Kirby, Journ. Am. Chem. Soc., 55, 
786-8 (1933); Contribution No. 106 from the Experimental Station of the E. I. du 
Pont de Nemours and Co. 

Received August 11, 1932. Published February 9, 1933. 





2 =C CH CH 2 Br 



CH 8 CBr= CH CH 2 Br 

The third step in this series of reactions has a relatively high velocity, 
and appreciable amounts of the dihydrobromide (III) are present in the 
reaction mixture even at an early stage when the major part of the vinyl- 
acetylene applied remains unchanged. In this respect also the behavior 
of hydrogen bromide appreciably differs from that of hydrogen chloride. 
Bromoprene is an oil having a faint greenish-yellow color and an odor 
rather closely resembling that of butyl bromide. It boils at 42 to 43 at 
165 mm. The proof of its structure follows the course already indicated 
for chloroprene. It reacts with maleic anhydride to yield, after hydrolysis, 
a product whose composition agrees with that required for bromo-6- 
cyclohexene-5-dicarboxylic-2,3 acid (IV). Bromoprene furthermore reacts 
readily with naphthoquinone. The primary product is a crystalline solid, 
which probably has the formula bromo-2-tetrahydro-l,4,4a,9a-anthra- 
quinone-9,10 (V). It is rapidly oxidized by air in alkaline solution to 0- 
bromoanthraquinone (VI). 






CH 2 






Experimental Part 

Preparation of Bromoprene. Fifty grams of cold vinylacetylcne was placed in a 
pressure bottle containing 225 cc. of a thoroughly chilled solution prepared by dissolving 
75 g. of moist, freshly prepared cuprous bromide in 200 g. of concentrated hydrobromic 
acid (sp. gr. 1.55). The bottle was allowed to warm up to room temperature and was 
then mechanically shaken for seven hours. After standing overnight and shaking for 


another two hours the unused vinylacetylene was allowed to evaporate. The residual 
oil was separated, washed with water, dried over calcium chloride and distilled from 
hydroquinone under reduced pressure. Dry, oxygen-free nitrogen was led through the 
capillary tube during the distillation. The yield of bromoprcnc boiling at 42-43 at 
165 mm. was 30 g. (24%); df , 1.397; w 2 D 1.4988; Af R calcd , 27.50; MR found, 27 94. 

Anal. Calcd. for C 4 H 5 Br: C, 30.11; H, 3.78; Br, 60.15. Found: C, 36.32, 36.47; 
H, 4.13, 4.21; Br, 60.91, 61.43. 

Higher yields were obtained by the use of more concentrated hydrobromic acid. 
For example, by using 2 moles of hydrobromic acid (sp. gr. 1 .66) for each mole of vinyl- 
acetylene a yield of 44.8% of bromoprenc was obtained. 

Dibromo-2,4-butene-2. After the distillation of the bromopretie a considerable 
amount of higher-boiling residue remained. From this residue there was obtained 12 g. 
(6%) of a strongly lachrymatory, light yellow oil corresponding in analysis to a dibromo- 
butene. It had the following physical constants: b. p. (760 mm ) 168 to 169 with loss 
of HBr; b. p. (23 mm.) 73; c/J 1.8768, n 1.5485; M R calcd., 35.74; MH found, 

Anal Calcd for C 4 H 6 Br,: C, 2243, H, 2.80; Br, 7477. Found: C, 2298, 
23.06; H, 3.05, 2.95; Br, 74 94, 75 34 

Reaction of Bromoprene with Maleic Anhydride. Preparation of Bromo-6-cyclo- 
hexene-5-dicarboxylic-2,3 Acid. To 5 g. (0037 mole) of bromoprenc was added 3 g 
(0.030 mole) of maleic anhydride. After the mixture had stood at room temperature 
for about one hour a spontaneous reaction set in and after three or four hours the mass 
solidified. The reaction product was then dissolved in benzene and shaken for twenty 
minutes with an excess of 10% sodium hydroxide. The aqueous layer was separated 
and acidified. The crude acid which separated was purified by recrystallization from 
water. It separated from water in the form of a mixture of thick plates and blunt needles 
melting sharply at 186.5 to 187. 

Anal Calcd. for C 8 H 9 O 4 Br: C, 38.57; H, 364, neutral equivalent, 124.5. 
Found- C, 3837, 38.65; H, 3.75, 3.65; neutral equivalent, 1254 

Action of a-Naphthoquinone on Bromoprene. Conversion to 0-Bromoanthraqui- 
none. To a benzene solution of 15.4 g. (0 12 mole) of brornoprene was added 10 g 
(0.06) mole of a-naphthoquinone. After standing at room temperature for two days 
the solution was gently refluxed for one hour The benzene was then distilled off under 
reduced pressure. The residual sticky, dark red solid was pressed on a porous tile and 
then extracted with warm alcohol The alcohol was decanted from an insoluble tar and, 
on cooling, 4.9 g. (49%) of unchanged naphthoquinonc crystallized out. When the al- 
cohol mother liquor was diluted with water, 2.6 g of a nearly black solid separated. 
After recrystallization from alcohol it was obtained in the form of small, soft, nearly 
white crystals. When slowly heated on a copper block it began to turn blue at about 
115 and slowly deepened in color as the temperature was raised. At 138 it melted 
sharply; as the temperature was raised further it volatilized completely. 

The analyses corresponded with the values calculated for bromo-2-tetrahydro- 
1 ,4,4a,8a-anthraquinone-9, 1 0. 

Anal. Calcd. for CnHuOaBr: C, 57.74, 11,3.99. Found: C, 57.06, 57.65, H, 
3.89, 4.04. 

About 0.1 g. of the substance was dissolved in alcohol and a few drops of 10% so- 


dium hydroxide solution added. The solution was dark red in color. As air was bub- 
bled through the solution, the red color gave way to green, which in turn disappeared 
leaving a yellow solid. After recrystallization from amyl alcohol the oxidation product 
melted at 205-207. /3-Bromoanthraquinone melts at 207 (2). 


Vinylacetylene reacts with aqueous hydrobromic acid to form bromo-2- 
butadiene-1,3 (bromoprene) and dibromo-2,4-butene-2. Bromoprene re- 
acts with maleic anhydride yielding bromo-6-cyclohexene-o-dicarboxylic- 
2,3 acid, and with naphthoquinone yielding a bromotetrahydroanthra- 
quinone which is readily oxidized to /3-bromoanthaquinone. 

Bibliography and Remarks 

(1) (a) Carothers, Williams, Collins and Kirby, Journ. Am. Chem. Soc., 53, 4203 (1931); this 
volume, page 281; (b) Carothers, Berchet and Collins, Journ. Am Chem. Soc., 54, 4066 (1932); this 
volume, page 306 

(2) Heller, Ber., 45, 672 (1912). 

V. The Polymerization of Bromoprene (Third Paper on New 
Synthetic Rubbers*)! 

Chloroprene (I, X = Cl) polymerizes spontaneously to yield a rubber-like 
product (1), and the speed of this transformation is roughly 700 times 
greater than the analogous transformation of isoprene. The present paper 
deals with the behavior of bromoprene (2) (I, X = Br), and the results may 
be summarized in the statement that it shows no significant qualitative 
differences from chloroprene, although its speed of polymerization under 
most conditions appears to be somewhat greater. 

CH 2 C CH==CH 2 CH=C H 

I I 

X X 


In connection with the behavior of these materials the following analo- 
gies are of interest. Vinyl chloride and vinyl bromide (II, X = Cl and 
Br) polymerize spontaneously yielding products of very high molecular 
weight (3), but this behavior is not shown by propylene, which polymerizes 
only in the presence of special catalysts or under drastic conditions and 

* For the Second Paper in this sub-series, see this volume, page 384 reference to 
the reprinted paper by Carothers and Coffman, Journ. Am. Chem. Soc., 54,4071(1932) 

f W. H. Carothers, J. E. Kirby and A. M. Collins, ibid., 55, 789-95(1933); Con- 
tribution No. 107 from the Experimental Station of E. I du Pont de Nemours and Co. 

Received August 11, 1932. Published February 9, 1933. 


then yields products having only moderately high molecular weights. The 
haloprenes (I) bear the same structural relationship to isoprene that the 
vinyl halides bear to propylene. Thus the very powerful activating effect 
of a single halogen atom on ethylene is also manifested in butadiene 
(when the halogen atom is on the j3-carbon). The rate of polymerization 
is of great importance in studying the behavior of dienes since a high rate 
not only makes it possible to obtain experimental results in a reasonable 
length of time, but it permits one to extend the observations over a wide 
range of conditions. It becomes possible then to recognize the different 
types of reactions involved in the spontaneous polymerization and to obtain 
data on the way in which these different types of reactions are affected by 
changes in the conditions. Data of this type on chloroprene have already 
been presented, and it will be useful to review them briefly (with some 
extensions) and to make comparisons with those now available for bromo- 
prene and for other dienes. 

The polymerization of chloroprene leads to the four well defined and 
qualitatively distinct types of polymers shown in the chart. 

Volatile ((3) polymer -< - monoprene - > plastic (a) polymer 

/ / 

granular (co) polymer non-plastic GU) polymer 

The influence of various conditions on each of these reactions is indicated 
in Table I. 


Monoprene to Monoprene to a-Polyprene to Monoprene to 

Condition 0-polymer a-polyprene /i-polyprene w-polyprene 

Temperature + + + + + Autocatalytic, ini- 

Pressure + -f- + + ? tiated by strong 

Light ++ 0? ultraviolet light 

Oxygen + + + + + ? and by metal 

Antioxidants --- a surfaces 

Certain substances commonly classified as antioxidants (e. g., phenyl-0-naphthyl- 
amine) act as inhibitors; others (e. g. t benzidine) act as accelerators: + accelerates, 
no effect, inhibits. 

Of these different types of polymers the most important are the a- and 
the /x-polyprenes since the former corresponds to unvulcanized rubber 
and the latter corresponds to vulcanized rubber. 

The ju-polyprene is the final product of the spontaneous polymerization of 
chloroprene; the a-polyprene is an intermediate step in the formation of 
the ^-product. Isolation of the a-polymer free of At-polymer is possible 



only when the reaction is conducted under certain conditions : the tempera- 
ture must not be too high, since elevated temperature accelerates the 
transformation of a-polymer to /z-polymer more than it accelerates the 
formation of a-polymer from monomer; the reaction must not be too slow, 
since most inhibiting influences have a greater decelerating effect on the 
formation of the a- than on the /i-product; moreover, very slow reaction 
frequently leads to the formation of the co-polymer. Light and pressure 

both appear to have a greater ac- 
celerating effect on the formation of 
the a-polyprene than on the trans- 
formation of this into the ju-poly- 

The /3-polymer, a terpene-like 
material, is an undesirable by-prod- 
uct in the rubber synthesis. In the 
chloroprene polymerization the pro- 
duction of 0-polymer becomes ap- 
preciable only at temperatures higher 
than are necessary to bring about 
a i rapid formation of a-polyprene. 
Butadiene, isoprene and dimethyl- 
butadiene polymerize very slowly; 
to get the transformation to proceed 
at a reasonable rate, elevated tem- 
peratures are generally used, and this 
results in the formation of relatively 
large amounts of /3-polymer. 

The co-polychloroprene is also a 
useless product; it is made up of 
discrete rubber-like particles (ir- 
regular globules) which are non-plastic and not even swelled by rubber 
solvents. The formation of this polymer is autocatalytic. When a speck 
of the co-polymer appears (or is introduced) in a specimen of incompletely 
polymerized chloroprene, the entire specimen is soon more or less com- 
pletely converted into the co-polymer. The formation of nuclei of the 
co-polyprene is favored by strong ultraviolet light and by metal surfaces 
(e. g. y sodium, potassium, mercury, iron, copper and aluminum). The op- 
portunities for the formation of such nuclei are also increased by a long 
reaction time under any particular set of conditions. Similar polymers, fre- 
quently described as cauliflower-like masses, have been obtained from 

Fig. 1. x-Ray diffraction pattern 
of /z-polybromoprene (from latex) 
stretched 500%. 


isoprcne, butadiene and dime thy Ibutadiene, and one suspects that they 
may be the forms in which the polymers of these dienes are most frequently 
obtained. The very slow rate of the polymerization of these dienes would 
be especially favorable to the formation of co-polymer. 

The term rubber-like is vaguely used to cover a multitude of the most 
diverse properties, and the literature of synthetic rubber is exceedingly 
obvscure. Most of the agencies that are available to hasten the very slow 
polymerization of isoprene and butadiene are such as have been found 
in the case of chloroprene to affect more strongly the conversion of the 
a- into the jii-polymer than the formation of the a-polymer. One may 
expect therefore that the isolation of a true a-polyprene (4) from the iso- 
prene and butadiene products will present especial difficulties. So far as we 
are aware no clear disclosure has ever been made of an a-polyprene from iso- 
prene or from butadiene. However, when isoprene is subjected to a pres- 
sure of 12,000 atmospheres until 30% of the isoprene has polymerized, 
the polymer is at least 90% soluble in ether (5). If the reaction is allowed 
to proceed further until 80% of the isoprene has polymerized, the product is 
completely insoluble (5, 0) . Thus the formation of a completely vulcanized 
rubber-like product without the aid of sulfur is by no means peculiar to 
chloroprene. In fact the differences between the behavior of chloroprene 
and the behavior of other dienes appear to be differences of degree rather 
than differences of kind. 

The Spontaneous Polymerization of Bromoprene. A sample of bromo- 
prene when allowed to stand at the ordinary laboratory conditions in a 
stoppered flask containing a small amount of air becomes noticeably more 
viscous after twelve to fifteen hours. As the reaction proceeds, the vis- 
cosity increases; after about five days the sample sets to a stiff, elastic jelly 
containing a considerable amount of unchanged bromoprene. Usually 
after eight to ten days all of the bromoprene has reacted, but the time 
required varies considerably in different experiments. The product has a 
density of about 1.74, and this is 24% greater than the density of the 
bromoprene. (The increase in density in the formation of /x-polychloro- 
prene is about 28%.) 

This product, ju-polybromoprene, corresponds in its properties with the 
//-polychloroprene already described. It is tough, resilient and elastic 
but harder than the analogous product from chloroprene. On standing it 
gradually undergoes further change: it becomes still harder and less ex- 
tensible and the resemblance to soft vulcanized rubber pretty largely dis- 
appears although it still remains very tough and retains considerable 
elasticity and resiliency. These changes in the nature of the product are 


due, at least in part, to a progressive action of air and they can be retarded 
by the application of antioxidants to the surface of the sample. ju-Poly- 
bromoprene is similar to ju-polychloroprene in its behavior toward solvents. 
It is greatly swelled by chloroform, carbon tetrachloride and aromatic 
hydrocarbons but remains practically unchanged after prolonged immer- 
sion in alcohol, ether, or aliphatic hydrocarbons such as gasoline. 

a-Polybromoprene. The preparation of an a-polybromoprene (plastic 
polymer) presents no difficulties. A sample of bromoprene was exposed to- 
light from a Cooper-Hewitt lamp at 25. After twenty-four hours 50% 
of the material had been converted into polymer. (This rate is about 
40% greater than that usually obtained with chloroprene under the same 
conditions.) The product was an exceedingly viscous, yellow sirup. 
When it was mixed with a large volume of alcohol the a-polybromoprene 
was precipitated as a soft, plastic mass. This product showed no tendency 
to resist permanent deformation and sheeted out very readily on cold rolls. 
Two per cent, of phenyl-jS-naphthylamine was worked into the plastic 
mass to prevent spontaneous conversion into an elastic polymer (7). The 
sample was compounded with about 5% of its weight of zinc oxide and 
heated in a mold at 120 to 125 for twenty minutes. The product was 
non-plastic, strong, resilient and extensible (500 to 700%). However, 
compared with a similar product from chloroprene it was somewhat lacking 
in snap, and its permanent set was rather high. It also showed a greater 
tendency to "freeze/' After about two hours at ordinary temperatures it 
became very stiff, but the original pliability was restored when it was 
heated to 80 for a few minutes. 

The Polymerization of Bromoprene in Aqueous Emulsion. Like 
chloroprene, bromoprene is readily dispersed in water and the resulting 
smooth emulsion polymerizes with great rapidity to form a stable latex. 
The preparation and polymerization of such an emulsion is illustrated in 
the following example. 

Twenty-five cubic centimeters of 2% aqueous sodium oleate in a wide-mouthed 
bottle was surrounded by a bath of ice and water. Two drops of triethanolamine was 
added to the solution, and then, with vigorous stirring, 25 g. of bromoprene. A smooth, 
milk-like emulsion resulted. After the mixture had stood in the ice-bath for five hours 
an aliquot portion was removed and poured into a large volume of alcohol. The weight 
of the precipitate thus obtained indicated that 78% of the bromoprene had polymerized. 
Under the same conditions chloroprene is only about 20-30% polymerized. The bromo- 
prene emulsion described above was transferred to a refrigerator. After seventeen hours, 
more than 95% of the bromoprene had polymerized. Five cubic centimeters of 3% 
ammonium hydroxide was added to stabilize the emulsion and a small amount of an 


aqueous dispersion of phenyl-/3-naphthylamine (2% on the rubber content) added to 
function as an antioxidant. The resulting latex was very stable. 

As in the case of polychloroprene, the dispersed particles in the synthetic 
latex derived from bromoprene correspond more closely to the p.- than to 
the a-polymer. Nevertheless, when the latex is coagulated by the addi- 
tion of acids, the particles coalesce and cohere very firmly. The coagulum 
is at first soft and plastic, but it quickly becomes tough, elastic and non- 
plastic. Homogeneous, coherent films are obtained by allowing the water 
to evaporate from a thin layer of the fluid latex on a plate of porous por- 
celain. The films are readily stripped from the plate and the removal 
of the water can be completed by drying them for a few hours in an oven 
at 80. Such films are exceedingly tough and more resistant to tearing 
than analogous films prepared from chloroprene latices. A typical speci- 
men had a breaking strength of 160 kg./sq. cm. and an elongation at break 
of 740%. (The elongation of similar polychloroprene films is usually 
about 800%.) The films exhibited the high permanent set and the tend- 
ency to freeze or stiffen already referred to in connection with the vul- 
canized a-poly bromoprene. Like the latter, they were non-plastic and 
did not dissolve but merely swelled in chloroform and benzene. 

/3-Polybromoprene.^The conversion of bromoprene into a volatile 
liquid polymer (0-polybromopreiie) occurs under conditions similar to 
those already described for chloroprene (8) . A sample of bromoprene con- 
taining about 5% of thiodiphenylamine, a substance which powerfully 
inhibits the conversion of the haloprenes to rubber-like polymers, was 
heated in a sealed tube at 80 for five days. When the resulting black 
oil was poured into alcohol a small amount of a black tar separated. From 
the alcohol there was obtained by distillation a small amount (about 15%) 
of a yellow oil boiling at 104 to 110 at 11 mm. It had a fragrant, ter- 
pene-like odor very similar to that of /3-polychloroprene and was a mild 
lachrymator. It showed no tendency to polymerize further. 

co-Polybromoprene. The formation of co-polybromoprene, like the 
formation of co-polychloroprene, occurs under conditions which result in 
very slow polymerization. For example, samples of bromoprene con- 
taining 0.2% of phenyl-/3-naphthylamine or 0.5% of tetramethylthiuram 
disulfide and 0.3% of sulfur slowly became more viscous and, after two to 
six weeks, a white deposit having a crystalline appearance began to form. 
After formation of this substance had started, conversion of the whole 
mass was complete in a few days. The product turned dark brown in 
color on standing in air. co-Polybromoprene is soft, opaque and non- 


coherent, while a>-polychloroprene is a mass of glistening, hard, rubbery 
granules. Like co-polychloroprene, it is not swelled by benzene. 

The Structure of Polybromoprene. Evidence has already been pre- 
sented for concluding that a-polychloropreiie is precisely analogous to 
natural rubber in its chemical structure. The analogies between poly- 
chloroprene and polybromoprene are sufficiently close to justify the as- 
sumption of a similar structure (III) for the latter compound. The x-ray 
evidence is especially interesting. The fact that ju-polychloroprene when 
stretched furnishes a fiber diffraction pattern has already been disclosed (1). 
Polybromoprene shows a similar behavior, but it furnishes an even sharper 
pattern (Fig. 1) (9). So far as we are aware, it has not been possible to ob- 
tain fiber diagrams from any other synthetic rubbers, and this fact per- 
haps justifies the conclusion that polychloroprene and polybromoprene 
are more regular in their molecular structure than any other known syn- 
thetic rubbers. 

(Til) . . . CH 2 CH=C CH 2 - -CH-r CH=C CH., CHa CH=C CH, 

I I I 

Br Br Br 


The polymerization of bromoprene is closely analogous to that of chloro- 
prene, but somewhat more rapid. Spontaneous polymerization yields as 
the final product ju-polybromoprene, which resembles vulcanized rubber 
but is more dense than rubber or /x-polychloroprene. A plastic (a) poly- 
bromoprene is readily isolated from partially polymerized bromoprene, 
and it is converted to the //-product by the action of heat. At elevated 
temperatures in the presence of inhibitors a volatile liquid (|8) polymer is 
formed. w-Polybromoprene, a granular, insoluble, rubber-like mass, is 
produced under conditions that lead to very slow polymerization. 

Bibliography and Remarks 

(1) Carothers, Williams, Collins and Kirby, Journ. Am. Chem. Soc., 53, 4203 (1931; this volume, 
page 281). 

(2) Carothers, Collins and Kirby, Journ Am Chem Soc., 65, 786 (1933); this volume, page 311. 

(3) Staudinger, Brunner and Feisst, Helv Chim. Acta., 13, 805 (1930). 

(4) With the aid of swelling agents, softener and lubricants it is possible to confer a certain amount 
of plasticity on /^-polychloroprene It is also possible to obtain from chloroprene plastic polymers 
that, on being heated, losa their plastic properties very incompletely or not at all. The material that 
we refer to as a-polychloroprene is an inherently plastic, polymerizable polymer; its plastic properties 
are completely lost and its elastic properties become fully developed when it stands or is heated. 

(5) Conant and Tongberg, Journ. Am. Chem. Soc., 52, 1667 (1930) 

(6) According to observations made by Dr. H. W. Starkweather in this laboratory, isoprene 
polymers prepared in this manner are also completely non-plastic 

(7) Ref 1, page 4219, this volume, page 298. 

(8) Ref. 1, page 4211; this volume, page 290. 

(9) The x-ray data will be discussed in more detail in a future paper by Dr A. W. Kenney [Such 
a paper has not apparently been published hitherto. Kus.] 


VI. Vinylethinylmagnesium Bromide and Some of Its 


As might be expected from its structure, vinylacetylene (I) reacts rapidly 
with ethylmagnesium bromide. The reaction proceeds smoothly and 
apparently involves only the acetyleriic hydrogen; the behavior of the 
product indicates that it is Vinylethinylmagnesium bromide (II). It re- 
acts in the typical manner with a variety of reagents. 

Acetone yields vinylethinyldimethylcarbinol (III), a colorless liquid 
whose structure is established by its hydrogenation to n-butyldimethyl- 
carbinol. On standing, it polymerizes to a colorless, transparent resin. 

The action of carbon dioxide on Vinylethinylmagnesium bromide ap- 
parently gives vinylpropiolic acid (IV), but it was not found possible to 
isolate this substance in a state of high purity. Above 110 it polymerizes 
explosively, and even at lower temperatures it is rapidly converted into a 
tough, insoluble, rather elastic mass. 

The products (V and VI) obtained from a-naphthyl isocyanate and from 
triphenylchloromethane are stable crystalline solids. 

CH2=CH C=iC MgBr CH, CTI C=C COH(CH 3 ) 2 


CH2=CH C--C C(C 6 H 5 ) S 

Experimental Part 

Preparation of Vinylethinylmagnesium Bromide (II). The reaction vessel was 
provided with a refrigerated return condenser (ice salt) and dropping funnel. The 
vinylacetylene (10 to 20% excess) dissolved in ether was added in portions with con- 
tinuous stirring to the ethylmagiiesium bromide. The reaction proceeded smoothly 
with sufficient evolution of heat to keep the ether refluxing gently. The reaction prod- 
uct remained dissolved in the ether. 

Vinylethmyldimethylcarbmol (III). Forty grams of purified acetone was added 
slowly to a 15% excess of Vinylethinylmagnesium bromide. The product of the reaction 
distilled without residue at 5059 (15 mm.). On redistillation, 40 g. of a colorless 
liquid boiling at 5961 (17 mm.) was collected. The yield in pure vinylethinyldi- 
methylcarbinol was 53%. B. p. 67 (24 mm.); 2 T ? 1.4778; <# 0.8872; MR calcd., 
33.32; found, 35.07. Exaltation, 1.75. 

Anal. Calcd. for C 7 H 10 O: C, 76.36i ; H, 9.09; mol. wt., 110. Found: C, 76.18; 
H, 8.88; mol. wt. (cryoscopic in benzene), 119. 

* W. H. Carothers and G. J. Berchet, Journ. Am. Chern. Soc. t 55, 1094-6(1933) ; Con- 
tribution No. 110 from the Experimental Station of E. I. du Pont de Nemours and Co. 
Received August II, 1932. Published March 7, 1933. 


The carbinol became increasingly viscous on standing. After two weeks it was a 
hard, tough, transparent mass insoluble in the common organic solvents. This trans- 
formation was accompanied by the absorption of oxygen; the analytical values for car- 
bon and hydrogen became progressively lower. The values for the completely poly- 
merized product were C, 70.7, and H, 8.7. The polymerization was greatly retarded by 
the presence of hydroquinone. 

Hydrogenation of Vmylethmyldimethylcarbinol. Twenty grams of the carbinol 
was dissolved in 75 cc. of alcohol. Four-tenths of a gram of platinum oxide was added 
and the mixture was shaken in a reduction apparatus. It absorbed 0.506 mole of hy- 
drogen in forty-five minutes, or about 93% of the theoretical amount, calculated for 
three moles of hydrogen absorbed per mole of carbinol. After evaporation of the alco- 
hol the residue distilled at 71-72 at 48 mm. It was a colorless liquid with a pleasant 
camphor-like odor. Its physical constants agreed closely with those given in the litera- 
ture for dimethylbutylcarbinol. 

Found Given in the literature 

B. p. 139.5-141 (7B1 mm.) B. p. 141-142 (755 mm.) 

w 2 D 3 1.4189 n D 1.41592 

dll 0.817 d 0.8155 

a Henry and Dewaei, Bull. Acad. Roy. Belg., 957 (1908); Chem. Z., I, 1854 (1909). 

Anal. Calcd. for C 7 H 16 O: C, 72.41; H, 13.79. Found: C, 72.31, 72.37; H, 
13.68, 13.64. 

Vinylpropiolic Acid.- One mole of vinylethinylmagnesium bromide was treated with 
dry carbon dioxide at until a color test showed the absence of any RMgBr. The 
product was then decomposed with water and the aqueous solution was submitted 
to continuous extraction with ether for eight hours. The solvent was evaporated in 
vacua. The residue weighed 50 g. (calcd., 96 g.). Attempts to distil the product at 
this stage by the usual methods always resulted in explosions. A partially successful 
distillation was effected at low pressure (about 0.05 mm.) in an all-glass apparatus. 
This consisted of a flask sealed to a receiver cooled in liquid air. The flask was heated to 
about 60. Evaporation occurred at a moderately rapid rate, and the distillate froze 
to a crystalline solid in the receiver. It became liquid below room temperature. On 
redistillation in a stream of carbon dioxide most of the volatile product came over be- 
tween 64 and 71 at 2 mm. It was a colorless, water-soluble liquid which turned yellow 
on standing. Its molecular weight, determined by titration with TV/10 sodium hydrox- 
ide, was 102, instead of the calculated value 96. The product was evidently not quite 
pure. It reduced permanganate instantly in acetone solution. 

A tube containing a sample of the acid was evacuated with a water-pump and sealed 
off. The tube was heated for eighty-five minutes at 75, at the end of which time a rub- 
ber-like yellow substance had formed. This was insoluble in water, alcohol, ether, ben- 
zene and acetic acid at the boiling points of these solvents. It was partly soluble in 
hot sodium hydroxide, imparting a yellow color to the solution, from which an amor- 
phous solid separated on neutralization. 

tt-Naphthylamide of Vinylpropiolic Acid (V). A solution of 9 g. of a-naphthyl 
isocyanate in anhydrous ether was added slowly to an excess of vinylethinylmagnesium 
bromide. The reaction proceeded smoothly. The mixture was refluxed for thirty 
minutes, then poured onto crushed ice. On extraction of the aqueous solution with 


ether, 1.5 g. of dinaphthylurea was left undissolved. On evaporation of the ether solu- 
tion, a yellowish solid separated. After two crystallizations from 50% alcohol it was 
obtained in the form of small yellowish needles melting at 125 to 126 (copper block); 
yield, 9 g. It was readily soluble in ether, benzene, methanol and ethanol. It reduced 
permanganate in acetone solution very rapidly. In chloroform solution it absorbed 
bromine slowly with the evolution of hydrogen bromide. 

Anal. Calcd. for Ci 6 H n ON: C, 81.44; H, 4.97; mol. wt., 221. Found: C, 
81.50, 81.27; H, 4.92, 4.85; mol. wt. (in boiling ethylene chloride), 221, 219. 

Vinylethinyltriphenylmethane (VI). A slight excess of vinylethinylmagnesium 
bromide was treated with a solution of 10 g. of triphenylchloromethane in anhydrous 
ether. After completion of the reaction, the mixture was worked up as usual. The 
ether solution left on evaporation 7.5 g. of a yellowish crystalline solid. After recrys- 
tallization from hot alcohol, it melted at 134-135 (copper block). It reduced perman- 
ganate in acetone solution and absorbed bromine, though slowly, in chloroform solution. 

Anal. Calcd. for C 23 H 18 : C, 93.87; H, 6.12; mol. wt., 294. Found: C, 94.17, 
93.52; H, 6.14, 6.24; mol. wt. (in boiling benzene), 320, 315. 


Vinylacetylene reacts with ethylmagnesium bromide yielding vinyl- 
ethinylmagnesium bromide. This behaves in the typical manner toward 
acetone, carbon dioxide, a-naphthyl isocyanate and triphenylchlorometh- 
ane. The derivatives thus produced are described. 

VII. Sodium Vinylacetylide and Vinylethinylcarbinols* 

Vinylacetylene, like other true acetylenic compounds, reacts with Grig- 
nard reagents to form the corresponding organo magnesium halide, and 
this may be used to introduce the vinylethinyl group into compounds of 
various types (1). The present paper is concerned with the formation of 
sodium vinylacetylide and with its use in a similar manner, especially in the 
synthesis of Vinylethinylcarbinols. 

Sodium vinylacetylide is readily obtained by the action of Vinylacetylene 
on metallic sodium. The metal may be applied directly to the liquid 
hydrocarbon, or the latter may be diluted with an inert solvent such as 
ether, toluene or liquid ammonia. The reaction occurs with great ra- 
pidity if the sodium is dissolved in liquid ammonia. These methods, 
however, have the disadvantage that the acetylide produced is frequently 
contaminated with appreciable amounts of polymeric material. More 

* W. H. Carothers and R. A. Jacobson, Journ. Am. Chew. Sac., 55, 1097-1101 
(1933); Contribution No. Ill from the Experimental Station of E. I. du Pont de 
Nemours and Co 

Received August 11, 1932. Published March 7, 1933. 


uniformly satisfactory results are obtained by the action of powdered 
sodamide on the hydrocarbon. The latter is preferably dissolved in ether 
or liquid ammonia. 

The sodium vinylacetylide obtained in this manner is a dusty, white 
powder, and vinylacetylene is regenerated in high yield when it is cau- 
tiously treated with water in the presence of a diluent. Its stability is 
sufficient to permit its storage for two or three days in a stoppered bottle, 
and with some care it can be handled in the presence of air. However, 
if the air is moist the acetylide sometimes ignites spontaneously. It 
attacks many organic reagents with explosive violence, but cooling and 
dilution permit sufficient control to obtain smooth reaction in most cases. 
Isolation of the sodium compound is not necessary for its application as a 
reagent; the intended reactant is preferably added directly to the mixture 
resulting from the action of sodamide on vinylacetylene. 

In dealing with aldehydes and ketones a still simpler and more satis- 
factory procedure consists in adding powdered sodamide in portions to a 
mixture of the carbonyl compound with vinylacetylene. In most cases it is 
advantageous to have present a diluent such as ether. With simple 
aliphatic and alicyclic ketones this method is very satisfactory. The 
yields are good and large scale operations are much simpler than with the 
vinylethinylmagnesitim halides. However, sodium vinylacetylide has a 
more limited range of applicability in the synthesis of carbinols than has 
vinylethmylmagnesium bromide. Aliphatic aldehydes are partly resinified 
by the sodium compound, and un saturated aldehydes and ketones arc 
usually resinned completely. The magnesium compound, however, even 
with very sensitive aldehydes and ketones, generally yields the expected 
monomeric carbinols (2) . 

Vinylethmyldimethylcarbinol has already been described (1) and the 
other carbinols listed in Table II resemble it in their properties. They react 
readily with bromine and decolorize permanganate; in the presence of 
platinum they are smoothly hydrogenated to the corresponding w-butyl- 
carbinols. They can be distilled in vacuo without decomposition, but if the 
temperature of distillation is too high the tertiary carbinols tend to lose 
water with the production of the corresponding substituted divinylacety- 
lenes. Vinylethmyldi-w-propylcarbinol, for example, thus leads to the 
compound C^HB CH=C (C 3 H 7 )C=C CH=CH 2 . Divinylacetylene 
itself was obtained by heating vinylethinylmethylcarbinol with p-toluene- 
sulfonic acid. 

The tertiary carbinols all polymerize spontaneously on standing (3). 
The transformation sets in rather quickly and progresses to the stage of a 


thick sirup during the course of a few days. The mixture then sets to a 
tough rather elastic mass, but the final product, a hard, transparent glass- 
like mass, is obtained only after several weeks or months. The trans- 
formation is greatly accelerated by certain catalysts, e. g., benzoyl peroxide, 
especially in the presence of light. The final glass-like products adhere 
very tenaciously to glass. They are insoluble in the common organic 
solvents. The polymerization of the carbinols also proceeds more rapidly 
at elevated temperature, e. #., 100, but the final product obtained under 
these conditions is fusible and completely soluble in the common organic 
solvents. The spontaneous polymerization of the carbinols is strongly 
inhibited by the presence of a small amount of hydroquinone. The 
secondary carbinols polymerize very much more slowly than the tertiary 

Experimental Part 

Preparation of Sodium Vinylacetylide. Powdered sodamide (19.5 g, 0.5 mole) 
was slowly added to a solution of 75 g. of vinylacetylene in 250 cc. of liquid ammonia. 
The mixture was stirred for six hours and the ammonia evaporated in a stream of nitro- 
gen, finally at (30 . The residue was a white powder (38 g., ealcd. 37 g.) which showed a 
tendency to ignite spontaneously when exposed to the air. It was covered with toluene, 
and water was slowly added with constant stirring at 75. The acetylide finally dis- 
solved without appreciable residue in the aqueous layer. Vinylaeetylene was distilled 
from the mixture and collected in a cold receiver. The yield was 21.3 g. or 82%. 

Preparation of the Carbinols. -The general procedure used for the preparation of 
the tertiary carbinols is illustrated by the following example. The reaction mixture 
consisted of 555 g. (7.7 moles) of methyl ethyl ketone (Eastman Kodak pract.), 551 g. 
(10.6 moles) of vinylaeetylene, and 500 cc. of dry ether. The solution was contained in 
a 3-necked flask provided with a mercury-sealed stirrer and a coil condenser, which in 
turn was connected to a trap. The reaction flask and trap were surrounded with carbon 
dioxide snow and the coil condenser was kept cold in the same manner. To the cold 
solution was slowly added 300 g. (7.7 moles) of powdered sodamide and stirring was con- 
tinued for a total of six hours. The reaction mixture was made acid to litmus by means 
of 10% sulfuric acid, the ether layer separated and dried with sodium sulfate, and then 
distilled in a vacuum; 677 g. of pure vinylethinylmethylethylcarbinol was obtained; 
yield 71%. The carbinol was stabilized with 0.1% hydroquinone in order to prevent 
spontaneous polymerization. 

The same procedure was applied to the preparation of the secondary carbinols but 
the results were less satisfactory. The preparation of vinylethinylmethylcarbinol is 
used as an illustration. To a solution of 44 g. (I mole) of freshly distilled acetaldehyde 
and 75 g. of vinylaeetylene in 75 cc. of dry ether at -10 was slowly added 39 g. of 
powdered sodamide. After a few grams of sodamide had been added, the mixture be- 
came so gummy that additional ether was added. Finally, the mixture formed a cake 
and stirring was discontinued. After four hours the cake was broken up with a stirring 
rod, more ether added, and the suspension stirred at room temperature for three hours. 
The reaction mixture was allowed to stand overnight and then decomposed with water 


and dilute sulfuric acid, The mixture was extracted several times with ether, and the 
latter dried with sodium sulfatc and distilled. A considerable quantity of ether-insoluble 
resin remained in the reaction mixture. After removal of the ether and a small amount 
of low boiling liquid, 25 g. (26%) of vinylethinylmethylcarbinol was collected. It was 
a colorless liquid with an alcoholic odor slightly resembling that of butyl alcohol. It 
reacted with dinitrobenzoyl chloride to form the 3,5-dinitrobenzoate of vinylethinyl- 
methylcarbinol, white needles from dilute alcohol; m. p. 106-106.2. 

Anal. Calcd. for C,3H 10 N 2 O 6 : C, 54.27; H, 3.41. Found: C, 54.34; H, 3.47. 

Catalytic Reduction of the Vinylethinylcarbinols. The carbinols derived from 
acetone and from methyl ethyl ketotie absorbed four atoms of hydrogen very rapidly 
when dissolved in alcohol and shaken with hydrogen in the presence of Adams' platinum 
oxide catalyst. The resulting w-butylcarbinols corresponded in their properties with 
those already reported in the literature, and the yields were almost quantitative. Hy- 
drogenation of the carbinols derived from methyl octyl ketone and from acetophenone 
yielded the new saturated carbinols described below. 

w-Butylmethyloctylcarbinol. -Colorless liquid, b. p. (3 mm ) 94; n 1.4418; 
rf? 0.8318; MR calcd., 68.38; MR found, 68.03. 

Anal. Calcd. for C u H 2 oO: C, 78.50; H, 14.02. Found: C, 77.20, 77.30; H, 
14.18, 14.25. 

tt-Butylmethylphenylcarbinol. Colorless liquid, b. p. (6 mm ) 107 to 109; w 2 ? 
1.5118; df 0.9616; MR calcd., 55.52; M R found, 55.52. 

Anal. Calcd. for C 12 H, 8 O: C, 80.90; H, 10.11. Found: C, 81.02, 81.16; H, 
10.38, 10.24. 

n-Propyl-5-octadiene-l,5-ine-3. When the attempt was made to distil vinyl- 
ethinyldi-w-propylcarbinol in vacuo in a flask provided with a long column, dehydration 
occurred and the non-aqueous distillate was the hydrocarbon n-propyl-5-octadiene- 
l,5-ine-3. On redistillation it was obtained as a pale yellow liquid having a characteris- 
tic odor; b. p. (6 mm ) 57 to 58; </? 0.8047; w 2 D 1 4949; MR calcd., 50.06; MR found, 
53.62. The sample was perhaps not quite pure. 

Anal. Calcd. for C U H 16 : C, 89.19; H, 10.81. Found: C, 87.47, 90.27, 87.73; 
H, 11.11, 9.94, 10.61. 

Preparation of Divinylacetylene by the Dehydration of Vinylethinylmethylcarbi- 
nol. To 25 g. of -toluenesulfonic acid in a 500-cc. flask provided with a stirrer, separa- 
tory funnel, and condenser was slowly added 20.8 g. of vinylethinylmethylcarbinol. 
The flask was warmed on a water-bath to start the reaction, after which it proceeded 
vigorously. After two hours, the condenser was replaced by a distilling column and the 
contents of the flask distilled in vacuo. Divinylacetylene distilled from the mixture and 
collected in a trap surrounded by solid carbon dioxide and acetone. The divinyl- 
acetylene was not further purified, but was converted into the hexabromide. The 
melting point of this hexabromide was identical with that of the hexabromide from a 
known sample of divinylacetylene. The melting points were as follows 

Hexabromide from above synthesis m. p. 105-106 

Hexabromide from known sample of divinylacetylene 104-106 

Mixed m. p. 105-106 



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Polymerization of the Carbinols. The behavior of vinylethinylmethylethylcarbinol 
is typical of the tertiary carbinols. On standing in a stoppered bottle its viscosity in- 
creased very rapidly during the first few days, and at the end of a week the product was a 
thick, colorless, transparent sirup. After three weeks it had set to a tough, elastic, 
rather rubber-like mass which still contained a considerable proportion of unchanged 
carbinol. After six weeks it had changed to a very hard, translucent mass. 

Fifty grams of vinylethinylmethylethylcarbinol containing 0.5 g. of benzoyl perox- 
ide was exposed to a Cooper-Hewitt light (mercury arc in glass). The product obtained 
after seventy- two hours was a hard, transparent, pale yellow, glass-like resin. It was in- 
soluble in the common organic solvents. When heated it softened somewhat at 125 
to 150, but it did not liquefy completely even at much higher temperatures. 

Twenty-five grams of the carbinol containing 0.25 g. of benzoyl peroxide was ex- 
posed to a 150-watt Mazda light. In four days a product similar to that described in the 
preceding example was obtained. 

Twenty-five grams of the carbinol containing 0.25 g. of uranyl nitrate was exposed 
to a Cooper-Hewitt light. In seventy-two hours, a hard, transparent, amber-colored 
resin was obtained. When no catalyst was present under the same conditions, the 
transformation to a hard resin required about one week. 

A sample of vinylethinylmethylethylcarbinol was heated for several hours at 100 
while a slow stream of air was bubbled through it. The product was a brown sirup. At 
room temperature it solidified to a brittle resin which dissolved readily in the common 
organic solvents. 


Sodium vinylacetylide obtained by the action of sodium or of sodamidc 
on vinylacetylene is a very reactive white powder which shows a tendency 
to ignite spontaneously in the air. Under properly controlled conditions 
it is a convenient reagent for introducing the vinylethinyl group into 
reactive organic compounds. Vinylethinylcarbinols are readily obtained 
by treating a mixture of vinylacetylene and a ketone with sodamide. 
The same method can also be applied to aldehydes but with less favorable 
results. Nine new carbinols prepared by this method are described. 

Bibliography and Remarks 

(1) Carothers and Berchet, Journ. Am Chan. Soc. t 55, 1094 (1933), this volume, page 321. 

(2) Unpublished results. 

(3) The behavior of the vinylethinylcyclopentauol was exceptional. It had not polymerized after 
standing for one year at the ordinary conditions. 


VIII. a-Alkyl-0-Vinylacetylenes * 

Vinylacetylene is a prolific source of new and interesting compounds. 
The present paper is concerned with homologs of vinylacetylene, which are 
readily accessible through the mediacy of sodium vinylacetylide (1). 
Vinylethinylmagnesium bromide reacts with very active alkyl halides such 
as triphenylchloromethane (2), but not with simple alkyl halides. Sodium 
vinylacetylide on the other hand reacts almost explosively with simple 
alkyl halides. The reaction can be moderated, however, by employing 
low temperatures, and when the halide is cautiously added to the acetylide 
in liquid ammonia it proceeds smoothly and furnishes good yields of the a- 
alkyl-/3-vinylacetylenes. Alkyl sulfates or sulfonates can be used with 
similar results. The properties of alkyl vinylacetylenes obtained by these 
methods are indicated in Table I. The compounds are colorless liquids 
with characteristic odors. On standing they slowly polymerize, yielding 
viscous, yellow sirups. 


a ^ B. p , C. </4 2 i? M R calcd - M n found Exaltation 

CH 3 59 2 at 700 mm. . 7401 1 . 4490 22 . 82 23 . 94 1.12 

C 2 H { , 84 . 5-85 . 3 at 758 mm . . 7492 1 . 4522 27 . 44 28 . 82 1 . 38 

H-C 4 H 9 02-03 at 01 mm. . 7830 1 . 4592 30 . 08 37 .71 1 . 03 

?7-C 7 H,, 74. 5 at 9 mm. .7902 14000 50.53 51.05 1.12 

Experimental Part 

l-Methyl-2-vinylacetylene. A one-liter, 3-necked flask was fitted with a mercury- 
scaled stirrer, a dropping funnel, and an exit tube. The exit tube was connected to a 
vertical condenser which in turn led to a gas-washing train consisting of an empty bottle, 
a second bottle containing water, and a third containing 10% sulfuric acid. The exit 
tube from the latter was connected to a calcium chloride drying tower and this in turn 
led to a receiver immersed in a Dewar flask maintained at 78. 

To a solution of 104 g. of vinylacetylene in GOO cc. of liquid ammonia was slowly 
added 58.5 g. (1 5 moles) of powdered sodamidc. The mixture was stirred for three 
hours and then concentrated to approximately 300 cc. by evaporating the ammonia 
in a current of nitrogen; 189 g. (1.5 moles) of dimethyl sulfate was added slowly through 
the separatory funnel. The reaction was very vigorous and about four hours were re- 
quired for the addition. The ammonia was allowed to evaporate and the reaction flask 
was finally heated on a water-bath. Part of the l-methyl-2-vinylacetylene collected in 
the first bottle and part in the second. The portions were combined, dried over calcium 

* R. A. Jacobson and W. H. Carothers, Journ. Am. Chem. Soc., 55, 1022-4(1933); 
Contribution No. 112 from the Experimental Station of E. I. du Pont de Nemours and 

Received September 21, 1932. Published April 0, 1933. 


chloride, and distilled. Some low-boiling material came over first and then 37.6 g. 
(38%) of l-methyl-2-vinylacetylene was collected. It was a colorless, volatile liquid 
possessing a powerful hydrocarbon odor somewhat similar to that of vinylacetylene. 

Anal. Calcd. for C 6 H 6 : C, 90.91 ; H, 9.09. Found: C, 90.97; H, 8.63. Mol.Wt.: 
calcd., 66; found, 66.1, 66.3 (cryoscopic, benzene). 

l-Ethyl-2 -vinylacetylene. A solution of 104 g. (1.5 moles) of vinylacetylene in 
500 cc. of liquid ammonia was treated with 58.5 g. (1.5 moles) of powdered sodamide as in 
the preceding experiment. After three hours, 231 g. of diethyl sulfate was added slowly 
through the dropping funnel. The mixture was allowed to stand overnight while the 
ammonia evaporated. Water was added to the reaction flask and the upper layer, 
weighing 51 g., was separated. After drying with calcium chloride, the liquid was dis- 
tilled. A small amount of low-boiling liquid came over first, after which 37 g. of 1-ethyl- 
2-vinylacetylene distilled. The product was a colorless liquid with an odor similar to 
that of l-methyl-2-vinylacetylene. 

Anal. Calcd. for C 6 H 8 : C, 90; H, 10. Found: C, 89.47, 89.73; H, 10.20, 9.44. 
Mol. Wt. : calcd., 80; found, 78.4, 79.5 (cryoscopic, benzene). 

l-Ethyl-2- vinylacetylene was also prepared by treating sodium vinylacetylidc 
with ethyl ^-toluenesulfonate according to the method recently employed by Truchet 
(3): 58.5 g. (1.5 moles) of sodamide was slowly added to a solution of 104 g. of vinyl- 
acetylene in 200 cc. of butyl ether at 10 and the mixture was stirred for three hours. 
A solution of 300 g. (1.5 moles) of ethyl ^-toluenesulfonate in 200 cc. of butyl ether was 
then added drop by drop during several hours. The thick mixture was heated in a water- 
bath at 80 for three hours and allowed to stand overnight. Water was added but such 
a troublesome emulsion formed that the mixture was set aside for twenty-four hours in a 
separatory funnel. The ether layer was separated, dried with calcium chloride, and dis- 
tilled. A considerable quantity of low-boiling material first distilled, and then a fraction 
weighing 45 g. and boiling at 78-88 was collected. Upon redistillation 28.5 g. (23.7%) 
of 1 -ethyl-2-vinylacetylene boiling at 8485 was obtained. Of the two methods of pre- 
paring this compound, the first was the better. 

l-Butyl-2-vinylacetylene. A solution of 100 g. of vinylacetylene in 400 cc. of 
liquid ammonia was treated with 39 g. (1 mole) of powdered sodamide. After three 
hours, 137 g. (1 mole) of butyl bromide was slowly dropped into the solution during 
about four hours. The mixture was allowed to stand overnight, water was added, and 
the upper layer separated. The liquid was dried with calcium chloride and distilled. 
The product was a colorless liquid with a characteristic hydrocarbon-like odor. The 
liquid polymerized during the course of three months to a yellow viscous sirup. 

Anal. Calcd. for C S H I2 : C, 88.88; H, 11.12. Found: C, 88.65; H, 10.70. 
Mol. Wt. : calcd., 108; found, 103, 104 (cryoscopic, benzene). 

l-Heptyl-2-vinylacetylene. A solution of 75 g. of vinylacetylene in 400 cc. of 
liquid ammonia was treated with 39 g. (1 mole) of powdered sodamide. After three 
hours, 150 g. (0.84 mole) of heptyl bromide was slowly added during four hours. The 
mixture was allowed to stand overnight, water was added, and the upper layer (128 g.) 
separated. After drying with calcium chloride, the liquid was distilled; 101 g. of 1- 
heptyl-2-vinylacetylene was collected. Based on the heptyl bromide used, the yield 
was 80% of the theoretical. On standing for two months the liquid polymerized to a 
yellow viscous sirup. 


Anal. Calcd. for d,H 18 : C, 88.00; H, 12.00. Found: C, 87.65, 87.51; H, 
11.55, 11.81. Mol. Wt. : ealcd., 150; found, 143, 145 (cryoscopic, benzene). 


Sodium vinylacetylide reacts with alkyl halides, sulfates or sulfonates, 
yielding a-alkyl-/3-vinylacetylenes. Compounds of the formula CH^-^ 
CH C^C R are described in which R is methyl, ethyl, n-butyl and 

Bibliography and Remarks 

(1) Carothers and Jacobson, Journ. Am. Chem. Soc. t 55, 1097 (1933), this volume, page 323. 

(2) Carothers and Berchet, Journ. Am. Chem. Soc., 55, 1094 (1933), this volume, page 321. 

(3) Truchet, Compt. rend., 191, 854 (1930). 

IX. l-Alkyl-2-chloro-l,3-butadienes and their Polymers 
(Fourth Paper on New Synthetic Rubbers)* 

Chloroprene (I) polymerizes very rapidly to form a rubber-like product of 
excellent quality (1). 

CH 2 -=C CH CH 2 (I) 


Replacement of the hydrogen at the 3-position by methyl does not appreci- 
ably affect the rate of spontaneous polymerization, but the rubber-like 
product is somewhat deficient in extensibility (2). On the other hand, the 
introduction of methyl at both the 3- and the 4-positions greatly diminishes 
the tendency to polymerize, and the product, although highly extensible, 
is lacking in resilience (2). 

The present paper is concerned with chloroprenes in which a hydrogen 
in the 1 -position has been replaced by alkyl. These compounds are readily 
obtained by the action of hydrogen chloride on the corresponding a- sub- 
stituted vinylacetylenes. The latter have already been described (3). 
Doubtless because of their lesser solubility in water they react more slowly 
with aqueous hydrochloric acid than does the parent hydrocarbon, which 
reacts practically completely when shaken for five hours at room tempera- 
ture with two moles of concentrated hydrochloric acid containing cuprous 
chloride (4). Under the same conditions a-methylvinylacetylene is less 

* R. A. Jacobson and W. H. Carothers, Journ. Am. Chem. Soc., 55, 1624-7 (1933); 
Contribution No. 113 from the Experimental Station of E. I. du Pont de Nemours and Co. 
Received September 28, 1932. Published April 6, 1933, 


than 40% utilized. Reaction of the higher homologs is still slower, so that 
elevated temperature was needed for the butyl compound, and the heptyl 
compound required the addition of alcohol to function as a solvent. 

The physical properties of the new homologs of chloroprene are indicated 
in Table I. They are colorless liquids with characteristic odors. Like 
chloroprene, they react with a-naphthoquinone to form addition products, 
which are readily oxidized to a-alkyl-/3-chloroanthraquinoiies (II), and 
their identity is established by this reaction. 

No additions to systems of the type alkyl C^C -CH^- CH 2 have been 
recorded hitherto. It is therefore of interest to observe that the result 
of adding hydrogen chloride under the conditions described is to place H 
at (1) and Cl at (2), but in view of the mechanism already established for 
vinylacetylene (4) it seems probable that the first product is alkyl CH -- 
C -CH CH 2 C1, which then rearranges to the substituted chloroprene. In 
any event, current theories, either empirical or electronic, do not appear to 
account for this result. 


Nature of 

B. p , C. 

in mm 








H 1 








CH 3 



1 . 4785 





C 2 H 6 





33 . 84 


1 21 

w-C 4 H 9 

04 65 



. 9360 

43 . 04 

43 77 

o 7;j 

w-C 7 H 15 



1 4785 


56 9.3 

57 79 


Polymerization of the Substituted Chloroprenes. Chloroprene polymerizes prac- 
tically completely to an elastic, rubber-like mass in forty-eight to eighty hours when di- 
rectly illuminated by a 150-watt Mazda lamp at 30 to 35. The substituted chloro- 
prenes listed in Table I all polymerize much more slowly. The methyl compound re- 
quires about six or seven weeks, and the higher members of the series require still longer 
times. The products, with the possible exception of that derived from the heptyl com- 
pound, are definitely rubber-like but much inferior in quality to poly chloroprene. The 
product from the methyl compound is the best. Polymerization under direct light from 
a 150-watt Mazda lamp during one month at the ordinary temperature gave a soft mass 
containing a considerable proportion of unchanged monomer. The polymer precipi- 


tated by the addition of alcohol was a soft, plastic mass resembling milled smoked sheets. 
It was compounded with 10% of its weight of zinc oxide and then heated at 120 for 
twenty minutes. The plastic properties were quite largely suppressed by this treat- 
ment, but the product appeared to be incompletely vulcanized. It was strong and 
tough and had a high extensibility, but it recovered from stretch rather slowly, and was 
deficient in resilience. Vulcanizates obtained from higher members of the series were 
still vSofter and more deficient in resilience. 

Preparation of l-Alkyl-2-chloro-l,3-butadienes. The general procedure was simi- 
lar to that already described for chloroprene (4). One mole of the hydrocarbon was 
shaken with approximately 2.2 moles of concentrated aqueous hydrochloric acid con- 
taining about 0.25 mole of cuprous chloride and 0.2 mole of ammonium chloride. The 
nonaqueous layer was separated, stabilized with hydroquinone and distilled; or, in some 
cases, the reaction mixture was distilled with steam after the addition of hydroquinone. 
The reaction time varied from five to sixteen hours and the temperature from 23 to 


Nature of 
alkyl C 




Mol wt 
in benzene) 







CH 3 58.56 












C 2 H 6 61.82 












tf-C 4 H 9 66.45 












>/-C 7 Hs & 70.79 10.19 19.02 186.5 70.39 10.04 19.26 189 193 

45. The product in each case appeared to consist entirely of the substituted chloro- 
prcnc and the unchanged hydrocarbon. Yields were good, but conversions were not 
complete. No appreciable addition could be obtained with the heptyl compound in 
aqueous solution, but a fair conversion was obtained when the hydrocarbon was shaken 
with 3.4 moles of hydrochloric acid in ethyl alcohol with 0.35 mole of cuprous chloride 
and 0.44 mole of ammonium chloride for five hours at 70 to 80. 

Condensation of Naphthoquinone with the l-Alkyl-2-chloro-l,3-butadienes. 
The substituted chloroprenes were each heated with a-naphthoquinone in the ratio of 
about 1 g. to 0.5 g. at 100 for about two hours. Alcohol containing sodium hydroxide 
was added, and air was bubbled through the resulting suspension. The solids were 
crystallized as indicated below and were thus obtained in the form of yellow crystals 
(generally needles). 




R = 

Cryt>t. from 

M. p , C. 





CH 3 

Acetic acid 




70 16 


C 2 H h 







-C 4 H 9 







tt-C 7 Hi5 a 







a In this case the intermediate tetrahydro compound was isolated, probably 1- 
heptyl-2-chloro-4,4a,9,9a-tetrahydro-9, 10-anthraquinone ; white crystals from methanol, 
m. p. 96-98. 

Anal. Calcd. for C 2 iH 2 6O,.Cl: C, 73.16; H, 7.26. Found: C, 72.48; H, 7.44. 


Polymerization of the l-Alkyl-2-chloro-l,3-butadienes 

l-Methyl-2-chloro-l,3-butadiene.-~ A sample of l-methyl-2-chloro-l,3-butadiene 
was exposed at room temperature to an ordinary incandescent light (150-watt Mazda) 
for one month. During this period the liquid increased in viscosity slowly at first, 
but more rapidly later until a soft, transparent, elastic, rubber-like solid was obtained. 
The rubber-like solid was macerated with alcohol to remove monomer and polymers of 
low molecular weight. The residual polymer was a tough, rubbery, plastic material. 
Ten per cent, by weight of zinc oxide was incorporated by means of steel rolls and the 
plastic mass then heated at 120 for twenty minutes. The product was strong, tough 
and elastic, but recovery from stretch was rather slow. 

A sample of l-methyl-2-chloro-l,3-butadiene was exposed to a 150-watt Mazda 
light at 30-35 for six and one-half weeks. The liquid progressively increased in vis- 
cosity until finally a pale yellow rubbery solid was obtained. This product was more 
completely polymerized than the product of the preceding experiment. The polymer 
was highly elastic and resembled cured natural rubber. 

l-Ethyl-2-chloro-l,3-butadiene. A sample of l-ethyl-2-chloro-l,3-butadiene was 
exposed to the light of a 150-watt Mazda lamp at 30 to 35 for about four weeks. The 
product was a pale yellow, transparent, viscous sirup. This was macerated with a large 
volume of alcohol to remove unchanged monomer. The soft, sticky, coherent mass that 
remained undissolved by the alcohol was mixed with 10% of zinc oxide and heated in a 
mold for twenty minutes at 120. The product was a tough, elastic material resembling 
the vulcanized product obtained from l-methyl-2-chloro-l,3-butadiene, but it was 
softer and less resilient. 

l-Butyl-2-chloro-l,3-butadiene. A sample of l-butyl-2-chloro-l,3-butadiene, 
when allowed to stand at the ordinary conditions in a stoppered bottle, after nine and 
one-half months had changed to a thick, sticky, brown sirup. Precipitation with alco- 
hol then gave a 60% yield of rather soft, rubber-like material. The reaction was accel- 
erated by light. A sample directly exposed to a 150-watt Mazda lamp at 30 to 35 for 
six weeks had changed to a yellow, viscous, sirup. After five weeks more it was consider- 
ably thicker. It was washed with alcohol arid the residual sticky solid was mixed with 
zinc oxide. It vulcanized very incompletely on being heated. 

A sample of l-butyl-2-chloro-l,3-butadiene was subjected to a pressure of 6000 
atmospheres at 38. At the end of ninety-six hours it had polymerized to a transparent 
soft, sticky solid; 90% of this solid was now insoluble in alcohol. The portion insoluble 
in alcohol was mixed with 10% of its weight of zinc oxide, 2% of stearic acid, and 1% of 
benzidine and then heated at 120. The physical properties of the product indicated 
that it was very incompletely vulcanized. It was elastic but rather weak and somewhat 

l-Heptyl-2-chloro-l,3-butadiene. A sample of this dierie showed no apparent 
change in color or viscosity when allowed to stand at the ordinary conditions for nine 
months. Polymerization occurred slowly when a sample was exposed to light from a 
Mazda lamp at 30 to 35. After three and one-half weeks the sample had changed to a 
colorless, transparent sirup. After six more weeks it had become thick and very vis- 
cous. It was finally washed with alcohol and attempts were made to vulcanize the in- 
soluble polymer in the presence of zinc oxide. The product was soft and sticky. 

A sample of l-hcptyl-2-chloro-l,3-butadiene was subjected to a pressure of 6000 
atmospheres at 38. At the end of ninety-six hours it had polymerized to a transparent, 


sticky, elastic mass. Only 4% of the material was soluble in alcohol. The alcohol- 
insoluble polymer was mixed with 10% of zinc oxide, 2% of stearic acid and 1% of 
benzidine and heated at 120. The product was a sticky solid possessing very slight 

Acknowledgment. We are indebted to Dr. H. W. Starkweather for 
the experiments at high pressure. 


Substituted chloroprenes of the formula CH 2 CH CC1=CH R in 
which R is methyl, ethyl, w-butyl and w-heptyl are described. These 
compounds all polymerize much more slowly than chloroprene and the 
polymers, though rubber-like, are inferior in quality to poly chloroprene. 
The methyl compound polymerizes most rapidly and yields the best poly- 
mer, but compared with polychloroprene the polymer is lacking in re- 

Substituted anthraquinones derived from the substituted chloroprenes 
are described. 

Bibliography and Remarks 

(1) Carothers, Williams, Collins and Kirby, Journ Am Chem. Soe , 53, 4203 (1931), this volume, 
page 281 

(2) Carothers and Coffman, Jonrn Am. Chem Soc., 54, 4071 (1932), this volume, page 384. 

(3) Jacobson and Carothers, Journ Am Chem Soc., 55, 1622 (1933), this volume, page 329. 

(4) Carothers, Berchet and Collins, Journ Am Chem Soc , 54, 4000 (1932); this volume, page 3()<>. 

X. The Chlorination of the Hydrochlorides of Vinylacetylene* 

The action of hydrogen chloride on vinylacetylene gives rise to the two 
monohydrochlorides, chloro-4-butadiene-l,2 (I) and chloroprene (II), and 
the dihydrochloride, dichloro-2,4-butene-2 (III) (1). 

CHE=C- CH CH, > CH 2 ==C==CH CH,C1 > 


CHa=-= CC1 CH=CH 2 > CH 3 CC1=CH CH 2 C1 
(II) ' (HI) 

All of these compounds react rapidly with chlorine and we now record some 
observations on the products to which they lead. 

* W. H. Carothers and G. J. Berchet, Journ. Am. Chem. Soc., 55, 1628-31(1933); 
Contribution No. 144 from the Experimental Station of E. I. du Pont de Nemours and 

Received September 28, 1932. Published April 0, 1933. 


Chloro-4-butadiene-l,2 in a series of experiments was chlorinated under 
various conditions and the combined distillable product was redistilled 
through an efficient column. Most of the material segregated into two 
fractions, one boiling at 40 to 41 at 10 mm. and the other at 64 to 65 at 
10 mm. Each had the composition C 4 H 5 Cl3. The lower boiling com- 
pound on oxidation with permanganate yielded a,/3-dichloropropionic acid 
as the only recognizable product, and was thus identified as trichloro-1,2,3- 
butene-3 (IV). The higher boiling compound when similarly oxidized 
yielded only chloroacetic acid. The compound was therefore trichloro- 
l,3,4-butene-2 (V). The two compounds evidently arise by the addition 
of chlorine at the interior and the terminal members of the pair of con- 
tiguous double bonds. Other allenes have been observed to behave in a 
similar manner (2). 

, CHr-CCl CHC1 CH 2 C1 v 

CHo C^CH CHoCl ( (IV) 2 CH 2 C1CC1 2 CHC1 CH 2 C1 

N CH,C1 CC1=CH CH 2 C1 ' (VII) 


Further experiments showed that although the two trichlorobutenes were 
the principal products when one mole of chlorine was added to chloro-4- 
butadiene-1,2, the ratio in which they were formed varied considerably with 
the conditions used. At 40 to 50 the 1,3,4-compound predominated, 
at 60 to 70 the 1,2,3-compound. Saturation of chloro-4-butadiene- 
1,2 with chlorine led to a compound having the composition C^sCU. 
In view of its origin it may be assigned the formula pentachloro- 1,2, 3,3,4- 
butane (VII). 

In all of the chlormations of chloro-4-butadiene-l,2 a considerable frac- 
tion (up to about 20% of the total product) consisted of undistillable ma- 
terial. In the chlorination of chloroprene (1 mole : 1 mole) the proportion 
of undistillable product was still greater (up to about 50%). The distil- 
lable product formed either at 40 to 50 or at 60 to 70 was a difficulty 
separable mixture, but the principal fraction (25 to 30% of the total 
product) was closely similar in its physical properties to the compound al- 
ready identified as trichloro-l,3,4-butene-2. This obviously might arise 
from chloroprene by a process of 1,4 addition. 

The chlorination of dichloro-2,4-butene-2 leads to the formation of 
trichloro-l,2,3-butene-3 (IV), tetrachloro-l,2,3,3-butane (VI) and penta- 
chloro-l,2,3,3,4-butane (VII). This series of products might be accounted 
for as follows 


+ Cl a -HC1 
CH 3 CC1=CH CH 2 C1 > CH 3 CC1 2 -~CHC1--CH 2 C1 > 


+ C1, 

CH 2 CC1 CHC1 CH 2 C1 > C1CH 3 CC1 2 CHC1 CH 2 C1 

(IV) (VII) 

The isolated tetrachlorobutane however is a stable compound, that is, 
it shows no tendency to lose hydrogen chloride spontaneously. More- 
over, a very curious feature of the reaction lies in the fact that at elevated 
temperature (e. g., 40 to 60) very little hydrogen chloride is evolved 
and the predominating product is the tetrachlorobutane; at low tempera- 
tures (e. g., 60 to 70), on the other hand, hydrogen chloride is formed 
in copious amounts, and the predominating product is the trichlorobutene 
or the pentachlorobutane, depending upon the amount of chlorine applied. 
It appears therefore that the trichlorobutene does not originate from the 
tetrachlorobutane as such, but that both of these compounds arise from a 
common prior intermediate. Such an intermediate might be VIII, which 
would be formed by the addition of the chlorine molecule at the deficient 
carbon in the active form of the double bond (3). Rearrangement of this 
intermediate would lead directly to tetrachlorobutane; loss of hydrogen 
chloride followed by rearrangement would lead to trichlorobutene. A 
sufficient difference in the temperature coefficients of the primary processes 
would account for the observed facts. 


CH 3 C CH CH 2 C1 


The above statements in regard to the influence of temperature on the 
chlorination of dichloro-2,4-butene-2 are illustrated by the following ex- 

One mole (125 g.) of the dichlorobutene in a flask cooled with a slush of 
solid carbon dioxide and acetone was treated with chlorine gas until 1.33 
moles of chlorine had been absorbed. The mixture was allowed to warm 
up while a slow stream of air was passed through it to remove dissolved 
gases. During this operation there was a sudden and very copious evolu- 
tion of hydrogen chloride. The loss in weight corresponded with the 
evolution of 0,9 mole of hydrogen chloride. The hydrogen chloride 


collected and titrated was only 0.73 mole, but some was lost owing to 
the suddenness of the evolution. Distillation of the mixture gave 

Trichloro-l,2,3-butene-3 . 440 mole 

Tetrachloro-1 ,2,3,3-butane . 089 mole 

Pentackloro-l,2,3,3,4-butane 0. 198 mole 

The rest (0.273 mole) was contained in intermediate fractions and residue; 
the latter comprised 10.3% of the total product. If the losses are dis- 
tributed proportionately among the three chief fractions, the percentage 
yields of these are 

Trichlorobutene 60.6% Tetrachlorobutane 12.2% Pentachlorobutane 27.2% 
Since in the formation of the pentachloro compound the trichloro com- 
pound is a necessary intermediate, 88% of the dichlorobutene was con- 
verted to the trichlorobutene, and only 12.2% to the tetrachlorobutane. 

One mole (250 g.) of the dichlorobutene was chlorinated at ordinary 
temperature, the rate being controlled so that the temperature of the mix- 
ture was between 45 and 60. The increase in weight corresponded with 
the absorption of 0.01 mole of chlorine, and at the same time 0.36 mole of 
hydrogen chloride was liberated. Distillation of the mixture gave 

Trichloro-1 ,2,3-butene-3 . 207 mole 

Tetrachloro-l,2,3,3-butane . 343 mole 

Pentachloro-l,2,3,3,4-butane . 100 mole 

Chlorination in this case was incomplete and there was considerable loss in 
residue and intermediate fractions, but the data show that the yield of 
tetrachlorobutane was greater than the combined yields of the trichloro 
and the pentachloro compounds. Further experiments showed that by 
careful adjustment of the amount of chlorine applied at low temperatures 
the isolated yield of trichloro-l,2,3-butene-3 could be raised to 80%, while 
at high temperatures the yield did not exceed 25%. 

Oxidation of Trichloro-1, 2, 3-butene-3. Twenty grams of the com- 
pound was stirred with 200 cc, of water while 70 g. of potassium perman- 
ganate was added in portions. The alkaline solution was filtered, treated 
with sulfur dioxide, filtered, acidified and continuously extracted with 
ether for nine hours. Evaporation of the ethereal solution gave 8.5 g. of 
acidic oil which distilled at 125 at 25 mm. It was identified as a,/3-di- 
chloropropionic acid by its melting point (49-50) and its neutralization 
equivalent (calcd., 143; found, 141.1). 

Oxidation of Trichloro-1, 3, 4-butene-2. Oxidation of this compound 
under the same conditions as those described above gave chloroacetic acid 




B p, 






Trichloro- 1 




1.3430 1.4944 




Trichloro- 1 



64 -( 



1 3843 1.5175 









1 4204 1.4958 









1.5543 1.5157 

Anal. - - 

45 01 



Empirical formula 







C 4 H 6 C1 3 





29 . 95 




C 4 H fi Cl 3 






3 . 01 



C 4 H 6 C1 3 





C 4 H B C1 5 


2. If) 













as the only product. It was identified by its melting point (o'3), mixed 
melting point and neutralization equivalent (calcd., 94.5; found, 95.1). 


Chlorination of chloro-4-butadiene-l,2 gives mixtures of trichloro- 1,2, 3- 
butene-3 and trichloro-l,3,4-butene-2 which react further to produce 
pentachloro-l,2,3,3,4-butane. The chlorination of chloroprene (chloro-2- 
butadiene-1,3) gives considerable amounts of trichloro- l,3,4-butene-2. 
The chlorination of dichloro-2,4-buteiie-2 at 40 to 60 proceeds with little 
loss of hydrogen chloride, and the product formed in largest amount is 
tetrachloro-l,2,3,3-butane. At low temperatures (-60 to -70) large 
amounts of hydrogen chloride are liberated during the chlorination, and the 
principal products are trichloro- 1, 2, 3-butene-3 and pentachloro- 1, 2,3,3,4- 

Bibliography and Remarks 

(1) Carothers, Berchct and Collins, Journ Am Chcm Soc , 54, 40(iG ( 1932) , this volume, page 300. 

(2) Bouis, Ann. clnm , [10] 9, 451 (1928) 

(3) Carothers, Journ Am Chem. Soc , 46, 2227 (1924). Evidence that chlorinalions involve 
primarily attack by the chlorine molecule has been presented by Sopcr and vSmitli, J. Chem. Soc. t 1582 


XL Dichloro-2,3-butadiene-l,3 and 
Trichloro-1 ,2 ,3-butadiene-l ,3 * 

Chloroprene (I) under ordinary conditions polymerizes to a rubber-like 
product about 700 times as rapidly as isoprene (or butadiene) (1). Hence 
in the diene polymerization a chlorine atom at the /^-position has a powerful 
activating influence. A bromine atom in the same position has an even 
greater positive effect (2). On the other hand a-chlorobutadiene poly- 
merizes not much more rapidly than isoprene, and the product, though 
elastic, has very little strength (3). The corresponding bromine compound 
also polymerizes spontaneously but no rubber-like properties have been 
ascribed to it (4) . Hence the activation produced by the halogen atom is 
very sensitively related to its position, and multiple substitution does not 
modify this conclusion, for tetrachloro-l,2,3,4-butadiene-l,3 has been de- 
scribed without any indication that it polymerizes at all (5). The effect 
of a single ^-halogen atom on diene behavior is in fact unique so far as 
recorded facts go. No other type of substitution has yielded a compound 
that greatly exceeds butadiene in the speed of its spontaneous polymeriza- 
tion and at the same time leads to a rubber-like product. In the methyl 
series isoprene polymerizes somewhat more rapidly than butadiene, and 
/3,7-dimethylbutadiene perhaps yet faster ((5); and the product from the 
latter, though somewhat inferior in snap and extensibility, is still rubber- 
like. On the other hand, a-substitution by methyl reduces the tendency 
to polymerize and unfavorably affects the quality of the product (7). It 
appears that both the terminal methylene groups of butadiene must be 
free : if either of them is substituted even by an activating group the speed 
of polymerization is diminished and/or the properties of the product are 
adversely affected (8). 

These conclusions stimulated interest in dichloro-2,3-butadiene-l,3 (II) 
which has at once the two unsubstituted terminal methylene groups and two 
chlorines properly located to produce a doubly activating effect. To provide 
further comparisons trichloro- 1,2, 3 -butadiene- 1,3 (III) was also prepared. 

Dibromo-2,3-butadiene-l,3 has already been described, but not very 
fully: "In a sealed tube it remains limpid for several hours, then becomes 
turbid, and is gradually transformed into a white polymer, but the trans- 
formation is complete only at the end of several days." (9). Otherwise 
information concerning its behavior is lacking. 

* G. J. Berchet and W. H. Carothers, Joum. Am. Chem. Soc., 55, 2004-8 (1933); 
Contribution No. 115 from the Experimental Station of E. I. du Pont de Nemours and 

Received October 3, 1932. Published May 6, 1933. 


Preparation of Dichloro-2,3-butadiene-l,3 and Trichloro-l,2,3-buta- 
diene-1,3. Starting materials for the preparation of the two dienes were 
found in the series of products obtained by chlorinating the hydrogen chlo- 
ride addition products of vinylacetylene (10). The dichlorobutadiene was 
obtained from trichloro-l,2,3-butene-3, and the trichlorobutadiene from 
pentachloro-l,2,3,3,4-butane. The reactions starting from vinylacetylene 

HC1 HC1 
CH-=C CH=CH 2 > CHjp=CCl CH^= CH, > 

CH 3 CC1----CH CH 2 C1 > CHa=CCl CHC1 CH,C1 

CH 2 C1 CC1 2 CHC1 CH 2 C1 - 



CH 2 = CC1 CC1=CH 2 
(III) (IT) 

The elimination of hydrogen chloride from the trichlorobutene proceeds 
very rapidly under a variety of conditions. When the trichloro compound 
is mixed with a slight excess of approximately N methyl alcoholic potas- 
sium hydroxide, the theoretical amount of potassium chloride precipitates 
out within fifteen minutes. Dilution with water then precipitates the di- 
chlorobutadiene as a heavy oil, which, after being stabilized with hydro- 
quinone, is obtained in 86% yield by vacuum distillation. The losses are 
almost entirely due to polymerization. There is no evidence of the forma- 
tion of any by-products. Dichloro-2,3-butadiene-l,3 is also obtained by 
the action of alcoholic potash on butadiene tetrachloride (tetrachloro- 
1,2,3,4-butane) but the yields are less favorable. 

The dichlorobutadiene could not be induced to react with naphthoqui- 
none or with maleic anhydride, and hence no direct and decisive proof of 
its structure was possible. The structure dichloro-l,3-butadiene-l,3 is not 
excluded by the methods of synthesis used, but this structure seems very 
unlikely in view of the rapidity with which the compound polymerizes. 

Pentachloro-l,2,3,3,4-butane also reacts rapidly with alcoholic potash. 
Besides the trichlorobutadiene the product was found to contain some tetra- 
chloro-l,2,2,3-butene-3 whose identity was established by its oxidation to 
a,o:,j3-trichloropropionic acid (11). This compound is doubtless an inter- 
mediate in the formation of the trichlorobutadiene, and the latter should 
therefore have the structure (III). 


Polymerization of Dichloro- and Trichlorobutadiene. -Under ordinary 
conditions dichlorobutadicne polymerized completely in about twenty- 
four hours, at 85 to 90 in about forty minutes. The corresponding times 
for chloroprene are about ten days, and about twelve hours (estimated). 
Thus the dichloro compound reacts about ten times as rapidly as chloro- 
prene and about seven thousand times as rapidly as isoprene. The poly- 
merization is inhibited by hydroquinone and accelerated by air or by ben- 
zoyl peroxide. The polymer is almost entirely devoid of rubber-like 
properties. It is a white, opaque, tough, hard mass, non-plastic and lack- 
ing in extensibility. It is somewhat (or partly) soluble in chloroform, but 
very slightly soluble in other common organic solvents including dichloro- 
butadiene, and under most conditions it separates as an opaque precipi- 
tate from the dichlorobutadiene as it is formed. At elevated temperatures 
(e. g., 85 to 90), however, the polymer remains dissolved until the poly- 
merization is practically complete. The product is at first a soft, colorless, 
transparent, sticky, elastic mass, but it becomes opaque, hard, and tough 
in a very short time. The absence of rubber-like properties in the polymer 
is consistent with other observations on the influence of substitutents : the 
spontaneous polymers from CH 2 ^CCl--CMe==CH 2 (12) and from CH2- 
CMe CMe=CH 2 are both deficient in extensibility and snap as com- 
pared with those from the monosubstituted dienes CH2~CC1 CH=CH2 
and CH 2 =CMe~^CH=CH 2 . 

The trichlorobutadiene polymerized much more slowly than the dichloro 
compound. At the ordinary conditions it changed to a jelly-like mass con- 
taining about 50% of unchanged diene in ten to twelve days. After one 
month polymerization was apparently complete. The product was a dark 
colored, rather soft and friable mass. 

Conclusions. The chlorinated 1,3-butadienes now known may be ar- 
ranged in the following order so far as the speed of their spontaneous trans- 
formation into high polymers is concerned 

j8, 7 , 0, 7 > a, 0, 7, 5 

and only the second member of the series yields a polymer that is definitely 
rubber-like. The generalizations outlined in the introduction thus receive 
further confirmation. 

Preparation of Dichloro-2,3-butadiene-l,3 (a) from Trichloro-l,2,3-butene-3. 
253 g. (15% excess) of potassium hydroxide dissolved in 760 cc. of methanol was placed 
in a flask provided with a stirrer and an efficient reflux condenser, and 6,34 g. of trichloro- 
l,2,3-butene-3 was added with stirring at such a rate that the temperature of the mixture 
remained between 10 and 15, the flask being cooled in a bath of ice and water. After 
all the trichlorobutene had been added the mixture was stirred for two hours, and the 


potassium chloride was filtered off; yield, 293 g. or 99.4% of the theoretical. The filtrate 
was poured into a large volume of water and the heavy oil was separated, dried with 
calcium chloride and distilled in the presence of hydroquinone. The product boiled 
at 39 to 41 at 80 mm. ; yield, 304 g. or 86% of the theoretical. Other properties deter- 
mined on a purified specimen are : b. p. 41 to 43 at 85 mm . , 98 at 7(30 mm. ; n 2 ,? 1 .4890 ; 
dl 1.1829; MR calcd., 29.47; found, 30.21. 

Anal. Calcd. for C 4 H 4 C1 2 : C, 39.02; H, 3.25; Cl, 57.72. Found: C, 38.76; 
H, 3.78; Cl, 57.09. 

(b) From Tetrachloro-l,2,3,4-butane. -The tetrachlorobutaiie was prepared by 
chlorinating butadiene. The sample used was a mixture which contained the solid and 
liquid isomers in the ratio 1 :1.12 (13). In a flask provided with a stirrer and a reflux con- 
denser was placed a solution of 150 g. of potassium hydroxide in 500 cc. of mcthanol. 
To this solution 234 g. of the tetrachlorobutane mixed with 100 cc. of methanol was 
slowly added. The temperature of the mixture was kept between 10 and 18. Po- 
tassium chloride separated immediately. After the addition was complete, the mixture 
was stirred for two hours at 25 and then filtered. The yield of potassium chloride was 
156 g. (88%). The filtrate was poured into a large volume of water and the heavy oil 
which separated was dried and distilled over hydroquinone. The distillate segre- 
gated into two fractions (1) b. p. 39 to 45 at 80 mm., 57 g., and (2) b. p. 45 to 110 at 
80 mm., 41 g. The first fraction was /3,7-dichlorobutadiene; on redistillation it boiled 
39 to 40 at 80 mm. and it showed the correct refractive index and chlorine content. 
The second fraction on redistillation boiled chiefly at 84 to 86 at 27 mm. Its chlorine 
content (found, 57.67, 57.75) agreed with that required for a dichlorobutadiene, and it 
was perhaps an impure isomeric dichlorobutadiene: d% 1.287; n 2 1.4999. 

Preparation of Trichloro-l,2,3-butadiene-l,3 and Tetrachloro-l,2,2,3-butene-3. 
One-half mole (115 g.) of pentachloro-l,2,3,3,4-butane was slowly added with stirring 
to a solution of one mole of potassium hydroxide in 270 cc. of methanol. The mixture 
was stirred at room temperature for about two hours. The precipitated potassium 
chloride was filtered off; yield, 61 g. or 82%. The filtrate was poured into a large volume 
of water, and the precipitated oil was dried and separated by distillation into two frac- 
tions (a) crude trichlorobutadiene boiled at 56 to 65 at 26 mm., 44 g.; and (b) crude 
tetrachlorobutene boiling at 66 to 75 at 26 mm., 24.5 g. They were further purified 
by distillation. 

Trichloro-l,2,3-butadiene-l,3. B. p., 33 to 34 at 7 mm ; n 1.5262; d 1.1060; 
MR calcd., 34.34; MR found, 34.39. 

Anal. Calcd. for C 4 H 3 C1 3 : C, 30.47; H, 1.90; Cl, 67.61. Found: C, 29.50, 
29.43; H, 2.33, 2.19; Cl, 68.42, 68.50. 

Tetrachloro-l,2,2,3-butene-3.- B. p. 41 to 42 at 7 mm.; >/, 2 D 1.5133; rfj 1.4602; 
Af R calcd. 39.67; MR found, 39.91 . 

Anal. Calcd. for C 4 H 4 C1 4 : C, 24.74; H, 2.06; Cl, 73.19. Found: C, 25.49, 
25.16; H, 2.23, 2.24; Cl, 72.66, 72.16. 

Oxidation. A sample (23 g.) of the tetrachlorobutene was oxidized with excess 
aqueous potassium permanganate. After filtration and treatment with sulfur dioxide 
the solution was extracted continuously with ether. Evaporation of the ether gave a 
liquid residue which distilled at 120 to 125 at 22 mm. It crystallized on cooling and 
after being washed with petroleum ether melted at 48 to 50 . Neutral equivalent found, 
176.4; calcd. for trichloropropionic acid, 177.5. 



Dichloro-2,3-butadiene-l,3 and trichloro-l,2,3-butadiene-l,3 have been 
prepared and their properties are described. The dichloro compound 
polymerizes more rapidly than chloroprene, the trichloro compound more 
slowly than chloroprene. The polymers are not rubber-like in either case. 
The chlorobutadienes now known may be arranged in the following order 
so far as their speed of spontaneous polymerization is concerned: 0,y ^> 
^> a, 0,7 > a ^> a,#,7,5; and only the second member of the series 
(chloroprene) yields a definitely rubber-like polymer. 

Bibliography and Remarks 

(1) Carothers, Williams, Collins and Kirby, Journ. Am Chcm Soc , 53, 4203 (1931), this volume, 
page 281. 

(2) Carothers, Collins and Kirby, Journ Am Chem. Soc , 55, 789 (1933), this volume, page 314 

(3) Cf. Ind Eng Chcm 26, 30 (193O). 

(4) Willstatter and Bruce, Ber., 40, 3979 (1907). 

(5) Muller and Huther, ibid., 64, 589 (1931). Cf. also pentachloro and hcxachlorobutadienc- 
1,3, Beilstein, 4th ed., Vol. 1, page 250. 

(6) Whitby and Crozier, Canadian J. Research, 6, 203 (1932); Whitby and Kat/, tbtd , 6, 398 

(7) Macallum and Whitby, Trans. Roy. Soc. Can., 22, 39 (1928); Fisher and Chittendcn, Ind 
Eng. Chcm , 22, 869 (1930); Whitby and Gallay, Can. J. Res , 6, 280 (1932). 

(8) This statement is also supported by observations on the following compounds in which the 
speed of polymerization is in the indicated order: CH 2 =CC1 CH=CH 2 , CIl2=CCl CMe=CH 2 ^>> 
MeCH=CCl CH=CH 2 . CH 2 =CC1 CMe=CHMe; Jacobson and Carothers, Journ. Am Chem. Soc , 
65, 1624 (1933); this volume, page 331; Carothers and Coffman, Journ Am Chem. Soc., 54, 4071 (1932). 

(9) Lespieau and Prevost, Cnmpt rend., 180, 675 (1925). 

(10) Carothers and Berchet, Journ. Am. Chem. Soc., 55, 1628 (1933), this volume, page 335 

(11) The structure CHC1=CH CC1 2 CH 2 C1 is not absolutely excluded but it seems very uii 
likely in view of the ease with which hydrogen chloride is lost from the group CH 2 C1 CHCla . 

(12) Carothers and Coffman, Journ Am. Chem. Soc., 54, 4071 (1932); this volume, page 384 

(13) Cf. Muskat and Northrup, Journ Am Chem. Soc , 52, 4054 (1930). 

XII. The Addition of Thio-p-cresol to Divinylacetylene* 

The reaction of acetylene with itself to form an open-chain trimer was 
described in the first paper of this series (1). This compound, which has the 
molecular formula C 6 H 6 , contains no true acetylenic hydrogen; on hydro- 
genation it yields w-hexane; and it differs from the already known di- 
methyldiacetylene (I). It was therefore assigned the structure divinyl- 
acetylene (V) (2). The formulas II, III and IV, which are equally con- 
sistent with the above indicated properties, may perhaps be rejected as 
inherently unlikely; but in connection with a detailed study of the acety- 

* W. H. Carothers, Journ. Am. Chem. Soc., 55, 2008-12 (1933); Communication 
No. 86 from the Experimental Station of E. I. du Pont de Nemours and Co. 
Received September 29, 1932. Published May 6, 1933. 


lene trimer it seemed desirable to obtain direct and decisive experimental 
proof of the structure represented by formula V. 

CH 3 C^C C=C CH 3 


CH 3f -=C=C==C=CII CU CH2=-C==C=CH CH=-CH 2 

(II) (III) 


(IV) (V) 

Efforts in this direction at first met with difficulties : the compound has a 
great tendency to combine with itself in the presence of polar reagents, 
and it does not readily yield well-defined derivatives capable of easy identifi- 
cation, but a solution of the problem was found in the reaction with thio-p- 

It has been shown by Posner (3) that vinyl compounds readily add thio- 
phenols. In the case of styrene the reaction proceeds in accordance with 
the equation 

CeHsCH CH 2 + RvSH > C6H 5 CH 2 CH 2 - vS R 

Since the direction of addition alleged by Posner is exclusively the reverse 
of that required by well-known empirical generalizations and by currently 
popular electronic theories, the reaction was lately re-examined by Ash- 
worth and Burkhardt with the result that Posner's conclusion was com- 
pletely verified (4). Thus for the purpose in view the thiophenols as re- 
agents have the advantage that their mode of addition is thoroughly estab- 
lished. Moreover, neither heat nor catalysts are required to induce their 
reaction with reactive carbon double bonds. 

Acetylene trimer (one mole) readily dissolved thio-/?-cresol (two moles). 
When the fluid mixture was allowed to stand at the laboratory temperature 
during ten clays it gradually set to a magma of thin, transparent, leaf- 
like crystals, and the odor of the thiocrcsol almost entirely disappeared. 
The reaction was powerfully accelerated by light, and when the mixture 
(40 g. of thiocresol and 12.5 g. of acetylene trimer) in a reagent bottle of 
soda glass was directly illuminated by a mercury arc, reaction was com- 
plete in about five hours. The yield of crystalline product was about 80% 
of the theoretical, and some unidentified oily material was formed. 

After crystallization from alcohol and from acetic acid the reaction prod- 
uct melted at 74.5-75.5. Its composition agreed with that required 
for di-(-tolylthio)-l,()-hexine-3 (VI). The verification of this structure 
through reactions described below decisively establishes the structure of 
the acetylene trimer as divinylacetylene (V). In its reaction with thio-p- 


cresol the double bonds of divinylacetylene function independently of the 
triple bond. As will be shown in future papers, however, this behavior is 
not typical: other reactions indicate conjugation between the ethylenic 
and acetylenic linkages. 

Di-(-tolylthio)-l,6-hexine-3 readily ad4s two atoms of bromine to 
form the dibromide X. The action of potassium permanganate leads to a 
series of oxidation products. When the di-(/?-tolylthio)-l,6-hexine-3 is 
dissolved in chloroform and shaken with cold dilute sulfuric acid to which 
permanganate is added in portions, the acetylenic disulfone, IX, is obtained. 
When carbon tetrachloride is used as the solvent in this process oxidation 
proceeds further and one obtains the dikctone, VIII. In acetone solution, 
alkaline permanganate causes rupture of the carbon chain with the forma- 
tion of the already known /3--tolylsulfonepropionic acid (VII). The 
acetylenic disulfone, IX, readily adds bromine to form the dibromide XI, 
and sulfuric acid converts it into the ketone XII. 

(X) C 7 H 7 SCH 2 CH,CBr=-CBrCH>CH,SC 7 H 7 ^ 


(VI) C 7 H 7 vSCH 2 CH 2 CE-CCH 2 CH 2 SC 7 H 7 

KMnO 4 

(VII) C 7 H 7 SO,CH 2 CH 2 COOH < 

(VIII) C 7 H 7 S0 2 CH 2 CH 2 COCOCH 2 CH 2 S0 2 C 7 H 7 

(IX) C 7 H 7 SO 2 CH 2 CH 2 C^CCH 2 CH 2 vSO 2 C 7 H 7 


C 7 H 7 vSO 2 ClI 2 CH 2 CBr CBrCH.>CH 2 SO 2 C 7 H 7 C 7 H 7 vSO 2 CH 2 CH 2 COCH 2 CH 2 CH 2 vSO 2 C 7 H 7 
(XI) (XII) 

Di-(/>-tolylthio)-l,6-hexine-3 (VI). -M. p. 74.5-75.5. Anal. Calcd. for C 2 oH 2 >S, 
C, 73.60; H, 6.79; S, 19.65; mol. wt., 326.3. Found: C, 73.50; H, 7.18; S, 20.1 
niol. wt. (in freezing benzene), 291, 299, 302, 308. 

Oxidation of Di-(p-tolylthio)-l,6-hexine-3 (VI) to -Tolylsulfonepropionic Acid 
(VII). Ten grams of di-(-tolylthio)-l,6-hexine-3 suspended in 100 cc. of pure acetone 
in a bottle was constantly stirred and maintained below 32 while 41 g. of powdered 
potassium permanganate was added during two and a half hours. The permanganate 
was rapidly reduced. The manganese dioxide was removed by filtration and washed 
with acetone and water; the nitrates were evaporated, dissolved in water, treated with 
decolorizing carbon, filtered and acidified. The semi-crystalline precipitate was dissolved 
in a mixture of ether and chloroform, and the filtered solution was extracted with aqueous 
sodium bicarbonate solution. The aqueous solution was acidified. The precipitated 
solid (4 g.) melted at 102-113. After crystallization from alcohol and then from water 
it was obtained in the form of needles melting at 110-1 13 (5). 


Anal. Calcd. for CioHi 2 O4vS: neutral equivalent, 228. Found: 234. 

Partial Oxidation of Di-(/>-tolylthio)-l,6-hexine-3 (VI). Formation of Di-(/>- 
tolylsulfone)-l,6-hexandione-3,4 (VIII) -To 3.2 g. of di-(/>-tolylthio)-l,6-hexine-3 in 
30 cc. of carbon letrachloride, 100 cc. of 3 N sulfuric acid was added and then ice and 
powdered potassium permanganate in small portions with constant shaking. About 
0.5 g. of permanganate was consumed. The mixture was decolorized with aqueous 
sodium bisulfite. A white solid, which was suspended in the mixture, was filtered off, 
dissolved in chloroform, and precipitated with petroleum ether. After crystallization 
from ethylene chloride, glacial acetic acid, and butyl acetate, it was obtained in the form 
of fine, yellowish needles which softened at 197-200 and melted at 200-201. 

Anal. Calcd. for C2oH 22 O 6 S 2 : C, 56.85; H, 5.22. Found: C, 50.37, 50.74; H, 
5.23, 5.37. 

This material was insoluble in ether, petroleum ether, ligroin, benzene, and carbon 
tctrachloride; readily soluble in warm chloroform and ethylene chloride. It reacted 
with phenylhydrazitie to form a crystalline derivative melting above 215. 

Partial Oxidation of Di-(p-tolylthio)-l,6-hexine-3 (VI). Formation of Di-(/>- 
tolylsulfone)-l,6-hexine-3 (IX) -Three grams of di-(p-tolylthio)-l,G-hexine-3 in -40 
cc. of chloroform together with 100 cc. of 3 N sulfuric acid was vigorously shaken in a 
stoppered bottle while ice and powdered potassium permanganate were added in small 
portions until a purple color persisted in the mixture. A total of 0.7 g. of potassium 
permanganate was applied. The mixture was treated with sulfur dioxide to dissolve the 
precipitated manganese dioxide. The aqueous layer was discarded and the chloroform 
layer was washed with dilute alkali and water, dried, filtered and evaporated. Crys- 
tallization of the solid residue from dilute acetic acid gave 2.4 g. of white crystals melting 
at 150-158; recrystallized from butyl acetate; needles, m. p. 157-158; insoluble in 
boiling carbon tetrachloride. 

Anal. Calcd. for C 20 H 2 2O 4 S 2 : C, 01.50; H, 5.08; S, 10.43. Found: C, 01.00 
01.48; H, 5.85, 0.17; 8,16.54,16.68. 

Action of Bromine on Di-(-tolylsulfone)-l,6-hexine-3 (IX). Formation of Di- 
(/?-tolylsulfone)-l,6-dibromo-3,4-hexene-3 (XI). In 10 cc. of 13 M bromine in chloro- 
form, 0.38 g. of di-(-tolylsulfoiie)-l,6-hexine-3 was dissolved. The mixture was illu- 
minated by a 40-watt Mazda lamp for fifteen minutes. About 40 cc. of petroleum ether 
was added and the crystalline precipitate was recrystallized from butyl acetate; m. p. 

Anal. Calcd. for C 20 H 22 O 4 S 2 Br 2 : Br, 29.00. Found: 28.87,28.92. 

Action of Sulfuric Acid on Di-(-tolylsulfone)-l,6-hexine-3 (IX). Formation 
of Di-(-tolylsulfone)-l,6-hexanone-3 (XII) - One gram of di-(/>-tolylsulfone)-l,0- 
hcxine-3 was dissolved in 10 cc. of cold coned, sulfuric acid. The colorless solution was 
allowed to stand for fifteen minutes and was then poured into 50 cc. of water. The re- 
sulting white precipitate was crystallized twice from alcohol; m. p. 134-135. 

Anal Calcd. for CaoHuOfiS*: 8,15.77. Found: S, 10.41. 

Action of Bromine on Di-(p-tolylthio)-l,6-hexine-3 (VI). Formation of Di-(- 
tolylthio)-l,6-dibromo-3,4-hexene-3 (X). Di-(-tolylthio)-l.G-hexme-3 in dilute chloro- 
form solution was allowed to stand for a few minutes with an excess of standard bromine, 
and the excess bromine was then titrated with standard thiosulfate. Approximately 
1.05 moles of bromine was consumed by one mole of di-(-tolylthio)-l,G-hexine-3. 


To 1.6 g. of di-(-tolylthio)-l,6-hexine-3 in 15 cc. of chloroform, 20 cc. of N bromine 
in chloroform was added. The solution was allowed to stand for five minutes. It was 
then shaken with 2 N thiosulfate solution, washed with water, dried with sodium sulfate, 
filtered and evaporated. From the residual oil, colorless crystals gradually separated. 
These were isolated and recrystallized from a mixture of alcohol and ether as trans- 
parent columns, m. p. 46-47.50. 

Anal. Calcd. for CaoHazSsB^: Br, 32.85. Found: Br, 32.98, 33.07. 


The divinylacetylene described in a previous paper reacts with thio-p- 
cresol to form a crystalline derivative whose structure as di-(^-tolylthio)- 
1 ,0-hexine-3 is demonstrated by its oxidation to the known /3-p-tolyl- 
sulfonepropionic acid. The following transformation products of the new 
derivative are described : di-(/?-tolylthio)-l,6-dibromo-3,4-hexene-3, di- 
(/?-tolylsulfone)-l ,0-hexaiidione-3,4, di-(/>-tolylsulfone)-l ,6-hexine~3, di-(p- 
tolylsulfone)-! ,(i-dibromo-3,4-hexene-3, di-(/>-tolyl-sulfoiie)-l,G-hexanone-3. 

Bibliography and Remarks 

(1) Nieuwlanci, Calcotl, Downing and Carter, Journ. Am. Chem. Soc. t 53, 4197 (1931). 

(2) This compound has perhaps been obtained in small amounts, though not completely char- 
acterized, by Farmer, Laroia, Switz and Thorpe, /. Chem. Soc., 2948 (1927). See also Migaouac and 
cie vSaint-Aunay, Compt. rend., 188, 959 (1929) 

Lespieau and Guillemonat in a recent publication [Compt. rend., 195, 245 (1932)] have described 
divinylacetylene under the title "A New Isomer of Benzene." Apparently they have entirely over- 
looked the paper of Nieuwland, Calcott, Downing and Carter referred to above. They also state 
that they have obtained about 12 cc. of (impure) vinylacetylene, of which hitherto only 1.4 g. has been 
prepared (referring to Willstatter and Wirth), It may, therefore, be of interest to state that in the 
laboratories and works of the du Pont Company many hundreds of kilograms of vinylacetylene and di 
vinylacetylene has been prepared by the process of Nieuwland, Calcott, Downing and Carter. 

(3) Posner, Ber , 38, 646 (1905). 

(4) Ashworth and Burkhardt, /. Chem. Soc., 1791 (1928) 

(5) Kohler and Reimer [Am Chem. J , 31, 175 (1904)] describe /3-/>-tolykulfonepropionic acid as 
needles melting at 1 10-113. It is partially decomposed by repeated crystallization from boiling water. 

XIII. The Action of Chlorine on Divinylacetylene* 

Scarcely any information has been recorded concerning the behavior of 
multiply conjugated enyne systems in reactions of addition. A system of 
this kind is found in divinylacetylene, which has been made available 
through discoveries described in the first paper of this series (1). In its 
reaction with thio--cresol only the ethylenic bonds of divinylacetylene are 
involved (2). 

* D. D. Coffman and W. H. Carothers, Journ. Am. Chem. Soc., 55, 2040-7 (1933); 
Contribution No. 116 from the Experimental Station of K. I. du Pont de Nemours and 

Received October 22, 1932. Published May G, 1933. 


Chlorine behaves in a different manner. Reaction proceeds very 
rapidly, and a point of apparent saturation is reached when six atoms of 
chlorine have been absorbed. The product then consists of a crystalline, 
volatile hexachloride (CeHeCle), and a pale yellow, very viscous sirup 
which also has the composition CeHeCle. The sirup is not volatile and it 
has a higher molecular weight than that required by the simple formula. 
The crystalline hexachloride resists chlorination, ozonization and oxida- 
tion by hot nitric acid; and the action of alkaline permanganate leads to 
total destruction. Nevertheless, there can be no doubt that its structure 
is correctly represented by the formula hexachloro- 1,2, 3,4,5, (>hexene-3 
(IV), and a study of the di- and tetrachlorides throws some light on the 
mechanism by which it is formed. 

The di- and the tetrachlorides are both liquids, and they are not easily 
obtained in good yields since the application of as little as one mole of 
chlorine converts part of the reacting divinylacetylene to the hexachlo rides. 
It appears on the other hand that only one dichloride and one tetrachloride 
are produced: the same tetrachloride is obtained by the chlorination of 
either divinylacetylene or its dichloride; and chlorination of the tetra- 
chloride yields the hexachloride (IV) already referred to. 

The dichloride still contains an open straight chain of six carbon atoms, 
since it can be hydrogenated to w-hexane. Oxidation with permanganate 
yields as the only identifiable product, chloroacetic acid; hence one of the 
chlorines is contained in the grouping C1CH 2 C= and there is no terminal 
CH.i group. The other chlorine is attached to a doubly bonded carbon, 
since the action of boiling aqueous sodium carbonate liberates only one 
chloride ion per molecule of the compound. The only reasonable structure 
for the dichloride consistent with these facts is dichloro-1 ,4-hexatriene-2,3,5 


I . CH2=CH C=C CH==CH 2 


III. C1CH 2 CH=CC1 CC1'-=CH CH 2 C1 


Ha. C1CH 2 CHC1 C^C--CH=CH 2 

lib. C1CH 2 CH=C=C=CH CH 2 C1 

Ilia. CICHa CH=0=CC1 CHC1 CH 2 C1 

Illb. C1CH 2 CH=CC1 CC1 2 CH=CH 2 

IVa. C1CH 2 CHC1--CC1 2 CC1= CH CH 2 C1 

V. CH 2 ==CH -CC1==CC1 CH=-CH 2 

VI . CH 2 = CH CCl=O==C=CHs 



IX. CH 2 =C=CC1 CCl=C=CH a 


The tetrachloride of divinylacetylene when oxidized with potassium 
permanganate also yields as the only identifiable product chloroacetic 
acid, and hydrolysis with aqueous sodium carbonate shows that two of 
the chlorine atoms are reactive and the other two inactive. This behav- 
ior indicates that it has the structure tetrachloro-l,3,4,6-hexadiene-2,4 


Formally, the relation between the three chlorides and the parent 
hydrocarbon is that each member of the series is derived from the pre- 
ceding one by a process of 1,4 addition. It is impossible to demonstrate 
conclusively that they actually originate by this mechanism. Addition 
either 1,2 or 1,6 followed by an a,y shift would lead to the same result. 
The dichloride (II) might arise thus from Ila or lib, the tetrachloride from 
Ilia or II Ib and the hexachloride from IVa. 

However, this dual mechanism appears highly improbable since it pro- 
vides no visible reason for the fact that the observed products are without 
exception compounds that would result from their precursors by a process 
of 1,4 addition. There are moreover other independent facts to indicate 
that 1,4 addition may be a favorite mode of reaction for enyne compounds. 
Thus the action of hydrogen chloride on vinylacetylene yields chloro-4- 
butadiene-1,2, CH 2 ^C^CH CH 2 C1 (3), and this must be a primary 
product since in the rearranged halicle, CH2=CC1 CH=CH 2 , the chlorine 
is attached to a doubly bonded carbon where its mobility is lost. It is 
perhaps significant in this connection also that the dichloride (II) of 
divinylacetylene shows no tendency to rearrange into the completely 
conjugated triene (V) under experimental conditions that might be ex- 
pected to favor especially such a transformation. 

When the dichloride (II) is treated with alcoholic potash it rapidly loses 
one molecule of hydrogen chloride. The terminal chlorine and its ad- 
jacent hydrogen are involved in this reaction, and the product has the 
structure chloro-3-hexatetraene-l,3,4,5 (VI). When hydrogenated it is 
converted into w-hexane, and oxidation yields oxalic acid. 

In the presence of cuprous chloride the dichloride (II) absorbs one 
molecule of hydrogen chloride from aqueous hydrochloric acid. In the 
resulting trichloro compound all three of the chlorines are reactive toward 
aqueous sodium carbonate. It is therefore the central double bond bear- 
ing the inactive chlorine atom of the dichloride which disappears, and the 
trichloro compound must have the structure trichloro-l,4,4-hexadiene-2,5 
(VII). When oxidized it yields chloroacetic acid. 

The tetrachloride (III) of divinylacetylene also loses hydrogen chloride 
when treated with alcoholic potash and yields a trichloro and a dichloro 


compound. In this case only two positions are available for the loss of 
hydrogen chloride, namely, the 1,2 and 5,6. The trichloro compound 
must therefore be trichloro- l,3,4-hexatriene-2,4,5 (VIII) and the dichloro 
compound must be dichloro-3,4-hexatetraene- 1,2,4,5 (IX). 

The chloro compounds described above furnish several novel and unusual 
examples of triad systems potentially capable of allylic rearrangement. 
One rather striking anomaly that appears in this connection has already 
been referred to. In the dichloride, II, the terminal chlorine atom shows 
no tendency to undergo a shift to the 7 position, in spite of the fact that 
such a shift would result in a completely conjugated triene. It is true 
that the 7 carbon is at the center of a pair of twinned double bonds, but 
this in itself cannot be responsible for the absence of a tendency to re- 
arrange since precisely the same configuration is found in chloro-4-buta- 
diene-1,2, CH 2 =C=CH CH 2 C1, which undergoes the 01,7 transposition 
with great facility. In the trichloro hexadiene, VII, also, a triad shift 
of one of the central chlorines would bring the double bonds into the 
conjugated configuration. No special attempts have been made to bring 
about rearrangement in this case, but it has not been observed to occur 
spontaneously. The absence of spontaneous rearrangements in the 
compounds III and VIII is not surprising, since a shift of the mobile 
chlorine atom in these compounds would destroy the conjugation of the 
double bonds. 

Four of the chloro compounds described above (II, VI, VIII and IX) 
exhibit a structural feature that has, so far as we are aware, not been 
exemplified in any compounds described hitherto. They contain a pair 
of conjugated double bonds and at least one of the pair is the first mem- 
ber of a series of two or more contiguous double bonds, thus, 
- C=C=C C=C 

I II- The question arises, can the carbon atom bearing 


the twinned double bonds function as one of the ends of the conjugated 
system? The behavior of the dichloride II toward chlorine provides an 
affirmative answer to this question. The chlorine here, almost certainly 
adds 1,4 to produce the tetrachloride, III. On the other hand, the addition 
of hydrogen chloride to the dichloride apparently involves only the 3 and 
4 positions. The trichloro compound obtained (VII) is the compound 
that would be produced by 3,4 addition, and it is not likely that it arises 
by 1,4 addition followed by an allylic transposition, because the product, 
C1CH 2 CH=CH CC1 CH CH 2 C1, which would be formed by 1,4 
addition, is precisely analogous to dichloro- l,3-butene-2, CHa -CC1 


CH CH 2 C1, and deliberate attempts to rearrange this compound have 
given negative results (4). 

With two exceptions (IV and VII) all of the above described chloro 
compounds contain at least one pair of conjugated double bonds. Never- 
theless, none of them reacts either with naphthoquinone or with maleic 
anhydride at 100. This failure cannot be ascribed to the fact that in 
several cases the conjugated system terminates in a series of contiguous 
double bonds, since, as has already been pointed out above, this arrange- 
ment does not preclude 1,4 addition; moreover, this arrangement is 
not present in the tetrachloride III which also fails to react. Similar 
failures have been reported before. Thus, X, XI and XII all react very 

RCH=C CH=CH CH..=C C=--CHo CH*===C---C===CH CH 3 

I I I " " I I 

Cl Cl CHa Cl CH 3 

(R = H, CH 8 . C 2 H 6 , C 4 H 9 , or C 7 H, 6 ) (XI) (XII) 


smoothly with naphthoquinone (5) but XIII and XIV do not (6). It 
C1CH=-CH CH-=CH, CH 2 =C C=CH 2 


is evident that the diene reaction can be regarded as diagnostic only if it 
leads to positive results; comparatively trivial substitutions sometimes 
cause it to fail. 

Substitution similarly has a profound influence on the tendency of 
dienes to polymerize. Of the above described chloro compounds only 
the dichlorotetraene (IX) shows any great tendency to polymerize spon- 
taneously. In the course of twenty-four hours at the ordinary conditions 
it was converted into a dark brown, hard, brittle resin. The dichloride 
II polymerized very slowly oti standing, and after three months it was 
a viscous sirup. The polymerization was greatly accelerated by increased 
pressure. At 6000 atmospheres and 25 it was changed in twenty- three 
hours to a yellow, elastic, plastic mass. The chloro-3-hexatetraene- 
1,3,4,5 at 6000 atmospheres and 25 was changed in twenty hours to a 
hard coke-like mass. Pressure was not applied to the other compounds, 
but none of them showed any signs of polymerizing when allowed to stand 
for several weeks. We are indebted to Dr. H. W. Starkweather for the 
experiments at high pressure. 


Experimental Part 
Preparation and Proof of Structure of Dichloro-l,4-hexatriene-2,3,5 (II) 

Preparation of Dichloro-l,4-hexatriene-2,3,5 by Chlorination of Divinylacetylene. 

Chlorine was passed into a solution of 400 g. (5 moles) of divinylacetylene in 300 g. of 
dry carbon tetraehloride at 50. During five hours, 436 g. (6.1 moles) of chlorine 
was absorbed. Fractionation of the reaction product gave 191 g. (1.28 moles) of di- 
chloro-l,4-hexatriene-2,3,5 boiling at 45-50 (3 mm.) (25% of the theoretical amount). 
Continued distillation of the residue yielded 57 g. of higher boiling material (b. p. 50- 
115 (3 mm.)) which was a mixture of the tetrachloro and hexachloro derivatives. The 
remainder consisted of undistillable material. 

A chlorination similar to that described above was carried out by allowing only 332 
g. (4.67 moles) of chlorine to be absorbed during five hours at 50. There was ob- 
tained 145 g. (0.97 mole) of the dichloro derivative (19% of the theoretical amount) 
and 61 g. of tctrachloro-l,3,4,6-hexadieiie-2,4. Chlorination of divinylacetylene at 
(mole for mole) in carbon tetraehloride gave the dichloride in 20% yield. 

The properties of dichloro-l,4-hexatriene-2,3,5 are: colorless mobile liquid with a 
sharp characteristic odor; b. p. 38 at 1 mm., 45-46 at 3 mm.; d 1.1807; 2 D 1.5195; 
MR calcd. 38.24; MR found, 38.34. On standing it very slowly polymerized, and 
after three months it had changed to a viscous sirup. 

The same compound was obtained in very small yield by the action of aqueous 
hypochlorous acid on divinylacetylene. 

Anal. Calcd. for C 6 H 6 C1 2 : C, 48.35; H, 4.03; Cl, 47.62; mol. wt., 149. Found: 
C, 47.88; H, 4.00; Cl, 47.59; mol. wt. 155. 

Reduction of Dichloro-l,4-hexatriene-2,3,5- to w-Hexane. The dichloro compound 
(25 g., 0.168 mole) in 50 cc. of ethyl acetate (with 0.16 g. PtO 2 ) absorbed 0.484 mole of 
hydrogen during three hours (calcd. 0.672 mole). A large amount of hydrogen chlo- 
ride was formed during the reduction. The filtered ethyl acetate solution was frac- 
tionated, and the fraction boiling at 50-75 was collected and allowed to stand over 
aqueous alkali until saponification of the ester was complete. The water insoluble 
layer was then separated and further dried over solid alkali. There was obtained 
8 g. of w-hexane, b. p. 69-70; d 0.677; w 2 D 1.3812. There was also obtained 15 g. of 
higher boiling material (b. p. range 30-80 at 21 mm.) from which no constant boiling 
fraction was received. 

The Reactivity of the Halogens in Dichloro-l,4-hexatriene-2,3,5. Hydrolysis by 
Sodium Carbonate. The dichloro compound (50 g., 0.333 mole) was refluxed with 
agitation during eight hours with 300 cc. of water containing 72 g. (0.67 mole) of sodium 
carbonate. Analysis of the aqueous solution showed that 0.281 mole of sodium chloride 
was formed. Hence only one active halogen atom is present in C 6 H 6 C1 2 . The hydroly- 
sis product was a soft, sticky resin. 

Permanganate Oxidation of Dichloro-l,4-hexatriene-2,3,5 to Chloroacetic Acid. 
The dichloro compound 50 g. (0.33 mole) completely reduced 300 g. of potassium per- 
manganate added with agitation in small portions to the aqueous solution (200 cc.) dur- 
ing three hours at 35-10. Sulfur dioxide was passed into the reaction mixture; the 
manganese dioxide was filtered, and repeatedly washed. Continuous ether extraction 
of the strongly acidified aqueous solution gave 15 g. of acidic material, which was 
identified as chloroacetic acid, b. p. 61 to 63 at 2 mm. ; m. p. 58-59. 


To complete the identification the chloroacetic acid was converted, through its 
chloride, into phenylglycocoll anilide, which showed the correct melting point and 
mixed melting point. 

Chlorination of Dichloro-l,4-hexatriene-2,3,5 to Tetrachloro-l,3,4,6-hexadiene- 
2,4. Chlorine was passed into 100 g. (0.67 mole) of C 6 H 6 Cl2 in 54 g. of carbon tetra- 
chloride at 5 to 10 until 42 g. (1.18 moles) of chlorine was absorbed. Fractionation 
of the product gave 10 g. of tetrachloro-l,3,4,6-hexadiene-2,4; b. p. 85 to 92 at '3 rnm.; 
d\ Q 1.4902; n 2 1.5458. Part of the original dichloro compound (21 g.) was recovered. 
The remainder of the product consisted of higher boiling residue. The tetrachloro de- 
rivative is further described below. 

Preparation and Proof of Structure of Chloro-3-hexatetraene- 1,3,4,5 (VI) 

Chloro-3-hexatetraene-l, 3,4,5 Obtained by the Action of Alcoholic Alkali on Di- 
chloro-l,4-hexatriene-2,3,5. The dichloro compound (105 g., 0.67 mole) freshly dis- 
tilled was added during one and one-half hours to a solution of 43 g. of potassium hy- 
droxide (15% excess) in 200 cc. of absolute methanol with vigorous agitation at 10-15. 
Stirring was continued at 15 during two hours. The insoluble potassium chloride was 
filtered off, washed with methanol, and dried. The yield of potassium chloride was 
47 g. (94% of the theoretical amount). The methanol filtrate was poured into 500 cc. 
of water, and the water-insoluble material separated and dried. The yield of crude 
material was 74 g. 

The pure chloro-3-hexatetraene-l, 3,4,5 was obtained as a colorless liquid; b. p. 
127 at 760 mm. (with decomposition), 82 at 163 mm., 55 at 54 mm.; dj 0.9997; 
n 1.5280; MR calccl., 32.91 ; MR found, 34.64. 

The same compound was obtained by the action of sodium methylate (in absolute 
methanol) on dichloro-l,4-hexatriene-2,3,5. 

Anal. Calcd. forC 6 H 6 Cl: C, 64.02; H, 4.45; Cl, 31.53; mol. wt., 112.5. Found: 
C, 63.99; H, 4.61; Cl, 31.61; mol. wt. (cryoscopic in benzene), 110. 

Reduction of Chloro-3-hexatetraene-l, 3,4, 5 to n-Hexane. The chloro compound 
(13 g.) absorbed 87% (0.403 rnole) of the theoretical amount of hydrogen (catalyst 0.15 
g. of PtCW in 25 cc. of ethyl acetate during three hours. A large amount of hydrogen 
chloride was formed. The ethyl acetate solution was distilled and the distillate up to 
80 was collected and allowed to stand over aqueous alkali during four hours. The un- 
saponifiable material was separated and allowed to stand over solid potassium hydroxide 
during fifteen hours. There was obtained 2 g. of w-hexane, b. p. 64-70; d? 0.691; 
w 2 ,? 1.3766. A small amount of high boiling residue was obtained but not identified. 

Permanganate Oxidation of Chloro-3-hexatetraene-l, 3 ,4,5 to Oxalic Acid. 
The oxidation was made by adding in small portions 224 g. of potassium permanganate 
to a vigorously agitated, aqueous solution (200 cc.)of 80 g. of potassium carbonate and 
21 g. of chloro-3-hexatetraene-l, 3,4, 5 at 35-45. After ten hours the solution was de- 
colorized with sulfur dioxide and the manganese dioxide filtered and repeatedly washed. 
The combined filtrates were strongly acidified with sulfuric acid, while the reaction 
mixture was cooled in an ice-bath. Continuous ether extraction of the acidified aque- 
ous solution during fifteen hours gave 15 g. of oxalic acid dihydrate (calcd. 25 g.). The 
acid melted at 101 (copper block). 

Neutral equivalent. Subs., 0.2916 g. Required 41.10 cc. of 0.1117 N NaOH 
Calcd. for H 2 C 2 O4-2H2O; neutral equivalent, 63.0. Found: 63.5. 


Preparation and Proof of Structure of Trichloro- 1, 4, 4-hexadiene-2, 5 (VII) 

Addition of Hydrogen Chloride to Dichloro-l,4-hexatriene-2,3,5. Dichloro-1,4- 
hexatriene-2,3,5 (75 g., 0.5 mole) was shaken during twelve hours at 27 with 30 g. of 
cuprous chloride and 75 cc. of hydrochloric acid (sp. gr. 1.18). The water insoluble 
layer was separated, washed and dried. Fractionation gave 20 g. of the original di- 
chloro compound and 28 g. of trichloro-l,4,4-hexadiene-2,5, which boiled at 100-103 
at 4 mm.; d% 1.3036; w 2 D 1.5585; M n calcd. 43.58; M n found, 45.88. 

Anal. Calcd. for C 6 H 7 C1 3 : C, 38.83; H, 3.78; Cl, 57.39; mol. wt., 185.5. Found: 
C, 38.87; H, 3.83; Cl, 57.38; mol. wt. (cryoscopic in benzene), 185. 

Permanganate Oxidation of Trichloro-l,4,4-hexadiene-2,5 to Chloroacetic Acid.- - 
The trichloro compound (18 g.) reduced 84 g. of potassium permanganate (added in 
small portions with vigorous agitation) in alkaline solution during four hours at 30-40. 
The nitrates obtained after separating and washing the manganese dioxide were acidified 
and subjected to continuous ether extraction during fifteen hours. From the ether ex- 
tract there was obtained 10 g. of chloroacetic acid, which boiled at 65-68 at 4 mm. and 
melted at 58-59. It was further identified by conversion through its acid chloride into 
chloroacetanilide, which showed the correct melting point, 134-135. 

The Reactivity of the Halogens in Trichloro-l,4,4-hexadiene-2,5. Hydrolysis by 
Sodium Carbonate. -The trichloro compound (5 81 g , 0313 mole) was refluxed during 
seven hours with agitation in 110 cc. of water containing 8.6 g. of sodium carbonate. 
Analysis of the aqueous solution showed that 0.0763 mole of sodium chloride was formed. 
Therefore three active halogen atoms were present. 

Preparation and Proof of Structure of Tetrachloro-l,3,4,6-hexadiene-2,4 (III) 

Chlorination of Divinylacetylene to Obtain Tetrachloro-l,3,4,6-hexadiene-2,4. 
Chlorine was passed into divinylacetylene (234 g., 3 moles) at 40 to 50 with vigor- 
ous agitation. During five and one-half hours 340 g. (4.8 moles) of chlorine was ab- 
sorbed, corresponding to 80% chlorination to the tetrachloro derivative. Considerable 
loss of hydrogen chloride occurred. Distillation gave 119 g. of crude dichloro-1,4- 
hexatriene-2,3,5 and 96 g. of very crude tetrachloro derivative. Fractionation of 160 
g. of the crude tetrachloro derivative (b. p. 50-120 at 3 mm.) gave 50 g. of tctrachloro- 
1 ,3,4,6-hexadiene-2,4 boiling at 84-89 at 2 mm.; f/f 1.4013, 2 1.5465; Jl/ K calcd , 
48.43; 7lf R found, 49.71. 

Anal. Calcd. for CeHeCli: C, 32.76; H, 2.73; Cl, 04.51 ; mol. wt., 220. Found: 
C, 32.80; H, 2.49; Cl, 63.76, mol. wt. (cryoscopic in benzene), 222. 

Permanaganate Oxidation of Tetrachloro-l,3,4,6-hexadiene-2,4 to Chloroacetic 
Acid. The tetrachloroderivativc (58 g.) completely reduced 220 g. of potassium per- 
manganate (added in small portions with vigorous agitation) during five hours at 35-40. 
The filtrate obtained after separating and washing the manganese dioxide was strongly 
acidified and subjected to continuous ether extraction during fifteen hours. The ether 
extract gave 20 g.of chloroacetic acid which boiled at 64-66 at 3 mm. and melted at 
58-59 after recrystallization from petroleum ether. 

The Reactivity of the Halogens in Tetrachloro-l,3,4,6-hexadiene-2,4. Hydrolysis 
by Sodium Carbonate.- The tetrachloro compound (5 g., 0.0228 mole) was refluxed dur- 
ing eight hours with agitation in 100 cc. of water containing 7 g. (0.066 mole) of sodium 
carbonate. Analysis of the aqueous solution showed that 0.0414 mole of sodium chloride 
was formed. Therefore two active halogen atoms are present in the tetrachloridc. 


The Chlorination of Tetrachloro-l,3,4,6-hexadiene-2,4 to Hexachloro-1,2,3,4,5,6- 
hexene-3 (IV). Chlorine was passed into 27 g. (0.12 rnole) of tetrachloro- 1,3,4,6- 
hexadiene-2,4, during eight hours at 60-70. The product was distilled and from the 
portion boiling at 95-115 at 2 mm. was obtained 8 g. of pure hexachloro-1,2,3,4,5,6- 
hexene-3 which melted at 57 to 58 after two crystallizations from petroleum ether; 
yield, 23%. The remainder of the product was a viscous liquid not further investigated. 
The hexachloride is described in more detail below. 
The Behavior of Tetrachloro-l,3,4,6-hexadiene-2,4 with Alcoholic Alkali. Trichloro- 

l,3,4-hexatriene-2,4,5 (VIII) and Dichloro-3,4-hexatetraene-l,2,4,5 (IX) 

The tetrachloro compound (75 g., 0.34 mole) was added during two hours to a solu- 
tion of 44 g. of potassium hydroxide (15% excess) in 200 cc. of absolute methanol with 
vigorous agitation at 10-15. The reaction was allowed to proceed for one hour after 
which the potassium chloride was filtered off (KC1, 36 g. or 48%). The methanol 
filtrate was poured into water, and the water-insoluble layer was separated, dried and 
distilled. From the crude material (50 g.) two fractions were obtained; one was tri- 
chloro-l,3,4-hexatriene-2,4,5 (25 g.) which boiled at 50 at 1 mm.; d% 1.3132; 2 ,? 
1.5517; Mn calcd. 43.11; MR. found, 44.61. The other was dichloro-3,4-hexatetraene- 
1,2,4,5 (5 g.) which boiled at 38-40 at 8 mm.; 4 1.1819; w 2 D 1.5456; Mn calcd. 37.77; 
Mn found, 39.41. 

Trichloro-l,3,4-hexatriene-2,4,5. -Anal. Calcd for C 6 H 6 C1 3 : C, 3926; H, 2.73; 
Cl, 58.01; mol. wt., 183.4. Found: C, 39.70; H,2.75; Cl, 57.52; mol. wt. (cryoscopic 
in benzene), 190. 

Dichloro-3,4-hexatetraene-l,2,4,5.~ vlwa/. Calcd. for C 6 H 4 C1>: Cl, 48.27 
Found. Cl, 48.60, 48.32 (7). 

The Chlorination of Divinylacetylene at 60-70 to Obtain Hexachloro-1, 2,3,4,5,6- 
hexene-3 (IV). Divinylacetylene (100 g., 1.28 moles) in 160 g. of carbon tetrachloridc 
absorbed 197 g. (2.8 moles) of chlorine during twelve hours. Distillation of the product 
gave 30 g. (10% of the theoretical amount) of the hexachloride, which boiled at 110 to 
112 at 2 mm. After recrystallization from petroleum ether the white crystals melted 
at 58 to 59. The major part (150 g.) of the product from this reaction was an almost 
colorless, viscous sirup. This is described in more detail below. 

Hexachloro-l,2,3,4,5,6-hexene-3 is not attacked by ozone or by hot nitric acid, and 
the action of alkaline permanganate leads to total destruction. It fails to chlorinate 
further even at elevated temperatures in the presence of light. 

Anal. Calcd. for C 6 H 6 C1 6 : C, 24.76; H, 2.06; Cl, 73.16. Found: C, 25.14; H, 
2.18; Cl, 73.37. 

The viscous sirup that formed the major part of the product of the above-described 
reaction was obtained as a residue which remained after the removal of the hexachloro- 
l,2,3,4,5,6-hexene-3 by distillation. This material could not be distilled even at 185 at 
a pressure of 2 mm. although it darkened rapidly and showed a tendency to liberate 
hydrogen chloride under these conditions. Its chlorine content was generally 1-3% 
less than that of the pure hexachloride, and like the latter it showed no tendency to ab- 
sorb more chlorine even at elevated temperature. This sirup probably results from the 
coupling together of two or more of the six-carbon atom chains at some stage of the 

Molecular weight determinations were made on two specimens of the sirup both of 
which contained a considerable proportion of the dissolved hexachloride. The values 


obtained were (a) Cl, 68.90%; mol. wt. (cryoscopic in benzene), 360; (b) Cl, 72.90%; 
mol. wt., 370. 


Divinylacetylene reacts with chlorine to form a liquid dichloride and 
tetrachloride, a crystalline hexachloricle, and a sirupy product having 
approximately the composition of the hexachloride but a higher molecular 
weight. The three monomeric products are each formed from their pre- 
cursors by 1,4 addition and they have the formulas dichloro-l,4-hexa- 
triene-2,3,5; tetrachloro-l,3,4,()-hexadiene-2,4; and hexachloro-1, 2,3,4, - 
r),G-hexene-3. The following transformation products of these compounds 
are described: chloro-3-hexatetraene-l, 3,4,5; trichloro-l,4,4-hexadiene- 
2,5; trichloro-l,3,4-hexatriene-2,4,5; and dichloro-3,4-hexatetraene-l,2,4,5. 

Bibliography and Remarks 

(1) Nieuwland, Calcolt, Downing and Carter, Journ Am Chrm Soc., 53, 4197 (1931) 

(2) Carothers, ibid., 55, 2008 (1933); this volume, page 344. 

(3) Carothers, Berchet, and Collins, Journ Am Chem Soc , 54, 4000 (1932); this volume, page 300 

(4) Unpublished results. 

(f>) Carothers and Coffman, Journ Am Chem Soc , 54, 4071 (1932), this volume, page 384, 
Jacobson and Carothers, Jouni Am Chem Sor , 55, 1024 (1933); this volume, page 331 

(0) Carothers and Berchet, Journ Am Chem .Sor , 55, 2004 (1933); this volume, page 340. 

(7) The analyst found great difficulty in the proper combustion of this compound The carbon 
percentage was usually lower and the hydrogen percentage higher than the theoretical amounts. 

XIV. The Dihydrochloride of Divinylacetylene* 

The addition of thio-p-cresol (I) to divinylacetylene (2) involves only the 
terminal ethylenic linkage. Chlorine on the other hand reacts by 1,4 
addition to yield first a dichloride and then successively a tetra- and a 
hexachloride (3). Study of the action of hydrogen chloride on divinyl- 
acetylene has furnished further information concerning the behavior of 
this multiply conjugated enyne system. 

Divinylacetylene reacts fairly rapidly when shaken with aqueous hydro- 
chloric acid at room temperature. As in the case of vinylacetylene (4), 
the reaction is considerably accelerated by cuprous chloride. On the 
other hand, the same product is obtained whether cuprous chloride is 
present or not. Under ordinary conditions a point of apparent saturation 
is reached when two molecules of hydrogen chloride have been absorbed. 

* D. D. Coffman, J. A. Nieuwland and W. H. Carothers, Journ. Am. Chem. Soc., 
55, 2048-51 (1933); Contribution No. 117 from the Experimental Station of E. I dti 
Pont dc Nemours and Co. 

Received October 22, 1932. Published May 6, 1933 


The first step in the reaction undoubtedly involves the formation of a 
monohydrochloride, but apparently this reacts more readily than the parent 
hydrocarbon. Consequently the chief product is dihydrochloride even 
when a deficiency of hydrochloric acid is used. The monohydrochloride 
is produced in poor yields, and it is so difficult to separate from the un- 
changed hydrocarbon that it has not yet been obtained in a state of purity. 

Purification of the dihydrochloride to constant composition requires 
rather sharp fractionation, and the boiling range (80 to 82 at 17 mm.) 
is wide enough to suggest the presence of geometrical isomers. Only one 
chlorine atom of the dihydrochloride is reactive. When heated with 
naphthoquinone or maleic anhydride the only evidence of reaction is the 
formation of a small amount of dark gummy material. However, as has 
already been pointed out (3), negative results in the Diels- Alder reaction 
are of no value in the demonstration of structure, since many 1,3-dienes 
fail to react. The compound is stable. It shows no tendency to poly- 
merize spontaneously, but a sample that had been submitted to a pressure 
of 6000 atmospheres for ninety- three hours at 49 was changed to a dark, 
somewhat elastic and only slightly plastic mass. When heated with 
sodium acetate in acetic acid it yields a monoacetate, and the second 
chlorine atom remains unaffected. Similarly the action of methyl alcoholic 
potash results in a monomethyl ether. When oxidized with permanganate 
the dihydrochloride yields acetic acid and chloro acetic acid. 

These facts furnish a basis for a discussion of its structure. We assume 
that the hydrogen atoms of the hydrocarbon retain their original positions 
during reaction, and the results of oxidation then indicate that the di- 
hydrochloride contains the residues CH 3 CH C and C1CH 2 CH 2 C . 

Hence the chain has the formula CH 8 CH=C C=CH CH 2 C1. Only 

I I 
a hydrogen atom and a chlorine atom remain to be accounted for, and 

one of these must be attached to the third carbon atom and the other 
to the fourth carbon atom. No direct experimental method is available 
for a decision between these two alternatives, but a satisfactory con- 
clusion can be reached by a consideration of the probable mechanism 
of the reaction. In the following chart the two alternative formulas, 
dichloro-l,3-hexadiene-2,4 and dichloro-l,4-hexadiene-2,4 are designated 
as A and B, respectively; the positions of the hydrocarbon chain are 
marked with the numbers 1 to 6; and the H and Cl of the addendum are 
called a and b, respectively. Addition at the first stage may be 1,2,3,4, 
1,4 or 1,6 and it must bring the chlorine atom to position 1 or position 
3 (or 4). The formulas I to IV therefore include all the reasonable possi- 


bilities for the monohydrochloride. For further reaction the possibilities 



Mode of Monohydrochloride Mode of Dihydrochloride 

No. addition produced addition produced 

I la 4b CHi CH=C-CC1 CHCHi 3a, 6b CHt CH=CH CC1=CH CHjCl A 

II 4a' Ib CHjCl CH-C- CH CHCHt 6a, 3b ClCHr-CH=CCl CH=CH CH, A 

III 3a 4b CH,CH CHCCl CHCHt la, 6b CHr-CH-CH CCl-=CH CHiCl A 

6a, Ib CICHr CH=CH CC1=CH CHs B 

IV la 6b CHr CH=-CCCH CH 2 C1 3a, 4b CH CH=CH CC1CH CHjCl A 

4a, 3b CHj CH=CC1 CH=CH CH:C1 B 

I by 1,4 addition (3a, 6b) gives A III by 1,6 addition (6a, Ib) gives B 

II by 1,4 addition (6a, 3b) gives A IV by 1,2 addition (3a, 4b) gives A 

III by 1,6 addition (la, 6b) gives A IV by 1,2 addition (4a, 3b) gives B 

Thus of the six pairs of reactions that might lead to A or B, four lead to 
A and only two to B, and probability is two to one in favor of A as the 
formula of the dihydrochloride. Further than this, however, there are 
good reasons for rejecting both III and IV as possibilities. Evidence of 
1,6 addition in the reactions of divinylacetylene has never been observed, 
nor of 1,2 addition at the acetylenic linkage of either vinylacetylene or 
divinylacetylene. Moreover, 1,2 addition followed by 1,6 addition and 
1,6 addition followed by 1,2 addition both appear inherently improbable. 
The addition of chlorine to divinylacetylene proceeds 1,4: CH2=CH 
C C~CH CH 2 > CH 2 C1 CH=C=CC1 CH=CH 2 (3); and the 
primary step in the action of hydrogen chloride on vinylacetylene is 1,4 
addition: CH C CH=CH 2 -^ CH 2 =C=CHCH 2 C1 (4). It seems 
certain therefore that the dihydrochloride of divinylacetylene is dichloro- 
l,3-hexadiene-2,4 (A) and the mechanism by which it is formed is most 
probably represented by II. 

Experimental Part 

The Addition of Hydrogen Chloride to Divinylacetylene in the Presence of Cuprous 
Chloride.- Divinylacetylene (80 g ) was shaken al cither or room temperature during 
one hour with 1.5 liters of 12 N hydrochloric acid containing 300 g. of cuprous chloride. 
The reaction mixture was then subjected to steam distillation. By this process 80 g. 
of crude dihydrochloride was obtained (53% of the theoretical amount). Fractionation 
of 241 g. of material (b. p. 70-80 at 11 mm.) through an efficient column gave 170 g. 
of analytically pure dichloro-l,3-hexadiene-2,4 which boiled at 8082 at 17 mm.; 
t/f 1.1456; w 2 D 1.5271; M R calcd. 38.70; MR found, 40.42. 

Anal. (Carius). vSubs., 0.1201 g.; AgCl, 0.2305 g. Calcd. for C 6 H,C1 2 : Cl, 46.99. 
Found: Cl, 47.47. 

The Addition of Hydrogen Chloride to Divinylacetylene in the Presence of Calcium 
Chloride. Six pressure bottles each containing 150 cc. of hydrochloric acid (sp. gr. 
1.187), 30 g. of anhydrous calcium chloride and 80 g. of divinylacetylene were shaken at 


25 during one hundred and ten hours. The water-insoluble layer was separated, 
washed with aqueous sodium carbonate and dried. Distillation gave 242 g. of material 
boiling above 50 at 70 mm. 

Fractionation through an efficient column gave 94 g. of material boiling at 50-60 
at 52 mm., and 60 g. of dichloro-l,3-hexadiene-2,4 which boiled at 80-82 at 17 mm. 
The residue weighed 00 g. The yield of dihydrochloride was 6.8% of the theoretical 

Repeated fractionation of the low boiling material (b. p. 50-60 at 52 mm.) failed 
to give a product having the composition of the monohydrochloride of divinylacetylene 
(C 6 H 7 C1). The chlorine analysis of the samples collected was always less than the 
theoretical amount for the monohydrochloride, indicating that hydrocarbon material 
was present. 

The Permanganate Oxidation of Dichloro-l,3-hexadiene-2,4 to Obtain Acetic and 
Chloroacetic Acids. An analytically pure sample of the dihydrochloride (100 g.) was 
introduced into an aqueous solution (400 cc.) containing 46 g. of potassium carbonate. 
Potassium permanganate (486 g.) was added in small amounts during five hours with 
continuous agitation. The solution was decolorized with sulfur dioxide and the man- 
ganese dioxide filtered and washed. The combined filtrates were acidified with 100 cc. 
of concentrated sulfuric acid and the solution subjected to continuous ether extraction 
during fifteen hours. The ethereal extract was dried and the ether removed by evapora- 
tion. The acidic residue gave by fractionation 15 g. of acid boiling at 105-120 (Fract. 
C) and 10 g. of acid boiling at 62.5 at 3 mm. (Fract. A). 

The determination of the Duclaux values of Fract. C indicated that acetic acid 
was the material under examination. The Duclaux values observed are 7.3, 6.7, 7.4, 
compared to 6.8, 7.1, and 7.4 for acetic acid, ,3.95, 4.40, 4.55, for formic acid, and 11.9, 
11.7, 11.3, for propionic acid. The identification of Fract. C was completed by the 
preparation of />-acet-toluididc, which showed the correct melting point and mixed melt- 
ing point (149 to 150). Fraction A was similarly identified as chloroacetic acid by 
conversion to chloroacetanilide, which showed the correct melting point and mixed 
melting point (134 to 135). 

The Action of Alcoholic Potash on Dichloro-l,3-hexadiene-2,4. Preparation of 
Methoxy-l-chloro-3-hexadiene-2,4. Dichloro-l,3-hexadiene-2,4 (65 g.) in 200 cc. of 
absolute methanol containing 38 g. of potassium hydroxide was rcfluxed with vigorous 
agitation. Filtration of the reaction mixture gave 30 g. of dry potassium chloride 
(calcd. 34 g.). Aqueous dilution of the alcohol solution was followed by separation of 
the water-insoluble layer which was dried and fractionated. There was obtained 20 
g. of low boiling material and 21 g. of the methyl ether which boiled at 8892 at 30 
mm.; w 2 ,? 1.4928; d\ Q 1.0239; Jl/ u calcd. 40.10; Mn found, 41 .57. 

Ami. Calcd. for C 7 H,,C1O: C, 57.35; H, 7.51. Found: C, 50.91; H, 774. 

Action of Sodium Acetate on Dichloro-l,3-hexadiene-2,4. Preparation of Acetoxy- 
l-chloro-3-hexadiene-2,4(5). One hundred grams of the dichloro compound, 100 g. of 
fused sodium acetate, and 300 cc. of glacial acetic acid were refluxed together for four 
hours. The acetic acid was neutralized with 20% sodium hydroxide and the mixture 
was extracted with ether. The ether extract was dried with calcium chloride, and then 
distilled. Three fractional ions gave 55 g. of colorless liquid boiling at 84 to 85 at 3 
mm.; df 1.0915; n 2 ,? 1.4890; MR calcd., 44.61 ; M H found, 46.14. 

Anal. Calcd. for CsHnO.Cl: Cl, 20.31. Found: 20.58,20.78. 



The action of aqueous hydrochloric acid on divinylacetylene yields a 
dihydrochloride. The compound is oxidized by permanganate with the 
formation of acetic acid and chloroacetic acid, and this fact together with 
a consideration of the probable mechanism of its formation, indicates that 
it has the structure dichloro-l,3-hexadiene-2,4 and that it results from two 
successive acts of 1,4 addition. When treated with sodium acetate its re- 
active (terminal) chlorine is replaced by acetoxy, and the action of methyl 
alcoholic potash similarly leads to the formation of a methyl ether. 

Bibliography and Remarks 

(1) Carothers. Journ Am Chem. Soc , 55, 2008 (1933); this volume, page 344 

(2) Nieuwland, Calcott, Downing and Carter, Journ. Am. Chem Soc , 53, 4197 (1931). 

(3) Coffman and Carothers, Journ Am Chem Soc , 55, 2040 (1933); this volume, page 348. 

(4) Carothers, Berchet and Collins, Journ Am Chem Soc , 54, 4000 (1932) , this volume, page 300 

(5) We are indebted to Dr. W F Talhot for this experiment. 

XV. Halogen-4-butadienes-l,2. The Mechanism of 
1 ,4-Addition and of ^-Rearrangement* 

The reactions of vinylacetylene (I) and some of its derivatives exhibit 
peculiar features which provide a unique opportunity for testing certain 
assumptions concerning the mechanism of 1 ,4-addition and of a,7-rc- 
arrangement. We have already shown (1) that the primary product of 
adding hydrogen chloride to (I) is chloro-i-butacliene-1,2 (II) which is readily 
rearranged to chloroprene (III) (2). 


(I) (II) (III) 

We now present further analogous facts as a basis for discussion. 

The bromide II is obtained by the action of liquid hydrogen bromide on 

I at low temperature and by the action of sodium bromide on the chloride 

II at room temperature. The second method has also been applied to the 
preparation of the iodide II, and in both cases the product is free from the 
isomeric halide III. 

The chloride and the bromide II react readily with hot water (preferably 
in the presence of sodium carbonate) and yield the corresponding carbinol, 
a strongly lachrymatory liquid, which can then be converted back to the 

* W. H. Carothers and G. J. Berchet, Journ. Am. Chem. Soc., 55, 2807-13 (1933) ; 
Contribution No. 120 from the Experimental Station of E. I. du Pont de Nemours and 

Received December 20, 1932. Published July (, 1933. 


chloride or the bromide II by the action of phosphorus trihalide in the pres- 
ence of pyridine. The carbinol II is also converted to the chloride II by 
boiling dilute hydrogen chloride (18%). The action of sodium acetate on 
the chloride or bromide in boiling acetic acid solution yields the acetate II, 
The latter is also obtained from the carbinol by the action of acetic an- 
hydride plus sulfuric acid. These reactions all proceed normally, i. e., 
none of the isomeric compounds III has been detected in the products. 
In this respect they differ from those involving Grignard reagents de- 
scribed in the next paper (3). Phenylmagnesium bromide acts on the 
chloride II yielding a mixture of phenyl-4-butadiene-l,2 (normal) and 
phenyl-2-butadiene-l,3 (abnormal). Methyl- and heptylmagnesium 
halides gave only abnormal products, while benzylmagensium chloride gave 
only the normal product. The isomeric chloride III does not react with 
Grignard reagents. 

The bromide II, like the chloride (4), is rapidly converted to the isomeric 
compound III by dilute HX containing CuX. The iodide isomerizes still 
more readily: when heated alone to about 130 it suddenly evolves heat 
and the lower-boiling iodide III rapidly distils. It may be inferred that 
this transformation also occurs spontaneously at the ordinary temperature 
since, within one or two days, the iodide II is completely converted to a 
granular polymer almost certainly derived from III. Deliberate attempts 
to rearrange the acetate and the carbinol II have been unsuccessful. 

In r6sum6 the facts are : 

Normal Metathetical Reactions of II : RC1 > RBr (I); RCl(Br) >ROH(OAc); 

ROH > RCl(Br, OAc). 

Rearrangements: RCl(Br, I) II > III. 

Abnormal Reactions: RC1 (II) with Grignard reagents gives both normal and ab- 
normal products. 

The halides of series III polymerize rapidly: the speeds for chloroprenc 
and bromoprene (5) are roughly 700 and 1100 times that of isoprene, and 
iodoprene polymerizes so quickly as to interfere with the determination of 
its physical properties. None of the compounds of series II shows any 
tendency to polymerize spontaneously. (The apparent polymerization of 
the iodide II is accounted for by prior rearrangement.) 

Theories (6) concerning the mechanism of 1,4- addition and of a,7-rear- 
rangements are too numerous to review but, with some concession for 
differences in terminology, the following propositions (1-4) will be recog- 
nized as pertinent to various suggestions that have from time to time been 


1. That all apparent examples of 1 ,4-addition really involve (com- 
pleted) 1,2-addition followed by a,7-shift. This once attractive hypothesis 
appears to have been generally abandoned; the facts now show that it 
cannot possibly be true. The rearrangement of the halides II > III is 
irreversible, hence II, the 1,4-product, must be a primary product. 

2. That addition at conjugated systems is a two stage process: the 
cation of the addendum first attacks one end of the system yielding isomeric 
ions (or intermediate fragments) corresponding to the two isomeric prod- 
ucts. The ions arising from attack of (I) by hydrogen ion would be CH 2 = 
C+ CH=CH 2 (III) and CH 2 C=CHCH 2 + (II). Thus in the addition 
of HX to (I), the hydrogen ion attacks the acetylenic (positive?) end of the 
molecule; the resulting ions are those involved in the isomerizations dis- 
cussed under 4 below. 

3. That the probable nature of the predominating hypothetical inter- 
mediate or of the predominating product can be inferred from conclusions 
based on fact or on speculation as to which is the more stable. 

The facts show that this is not possible, since in the addition of hydrogen 
chloride or hydrogen bromide to (I) the unstable isomer (II) is a primary 
product, and is in all probability the only primary product. 

4. (a) That in triad systems it is the ion R+ not the molecule RX which 
rearranges, (b) That R + is a necessary intermediate in metathetical 
reactions of RX involving the formation of X". (c) That the ion R + , 
whether formed directly by dissociation of RX or as an intermediate in 
metathetical reactions of RX, will immediately assume the forms corre- 
sponding to the equilibrium products. 

The practical implication of (c) is that metathetical reactions of triad 
systems (involving X) must yield an equilibrium mixture of the isomers. 
This has been taken so much for granted that it forms the basis of a sup- 
posed demonstration (7) of the intimate mechanism in the hydrolysis of car- 
boxylic esters. It is now obviously directly opposed to the facts, since a 
whole series of metathetical transformations in compounds of type II has 
been carried out without any rearrangement to the corresponding isomers 
III. Not only does no isomerization occur under these conditions where, 
according to (b), ionization must be assumed to occur, but the iodide II 
rearranges under conditions that appear to be especially unfavorable to 
ionization. This throws considerable doubt on assumption (4a). 

Abnormal reactions of triad systems do, nevertheless, exist. The action 
of RMgX on the chloride II furnishes an unambiguous demonstration of 
this fact. The isomeric chloride III is (experimentally) incapable of 
reacting with RMgX; hence completed rearrangement cannot precede 


reaction. The isomeric hydrocarbons II and III cannot rearrange under 
the conditions used. The isomerization is therefore an integral part of the 
metathesis. Three explanatory assumptions appear to be possible: 

(a) The abnormal reaction consists in addition of RMgX at the double 
bond followed by elimination of MgX 2 . This has the disadvantage of being 
ad hoc since addition of RMgX at simple ethylenic linkages has never been 

(b) The initiating step results in the formation of a cation (or some other 
fragment) which adjusts itself before entrance of the anion of RMgX. 
This is the currently popular assumption (4a and b above). It presents 
this difficulty as shown in the comment under 4 : the facts now require that 
these intermediate fragments (if they exist at all) must be capable (con- 
trary to 4c) of resisting or escaping rearrangement under some conditions 
and not under others. In either case the conditions cannot be specified in 
advance and the assumption of intermediate ions contributes nothing to 
the explanation of the facts, but, at best, serves merely as a pictorial method 
of rationalizing the facts after they are known. 

(c) The initiating step is the formation of a complex (coordination 
compound). This assumption also, in the present state of knowledge 
concerning complexes, is too vague to furnish a basis for detailed predic- 
tions. It is, however, unquestionably closer to reality than the assumption 
of free ions or radicals as intermediates. The closest approach to a simple 
free alkyl anion is probably found in the alkali alkyls. These materials 
are extraordinarily reactive; the ethyl anion, for example, is capable of 
reducing the sodium ion to sodium hydride at temperatures below 100 
(8). Experimentally, simple alkyl cations are completely unknown; it is 
reasonable to suppose that they would be still less stable than alkyl anions. 
Metathetical reactions of organic compounds that can be formulated as 
proceeding through the intervention of ions generally involve media or 
catalysts capable of giving rise to coordination compounds (9). 

Johnson (10) has shown that coordination and chelation are probably in- 
volved in the mechanism of many types of organic reactions including ab- 
normal reactions and o:,7-rearrangemerits. Our application of his ideas to 
the present case is illustrated below. 

In a triad system the usually accepted valence angles are capable of 
placing the unshared electrons of X fairly close to the 7-carbon atom, 

R~CH==CH CH 2 -X- Polarization at ft 7 favored by the proximity 
of these unshared electrons permits still closer approach of X toward 
7 and consequent loosening at a; these tendencies finally bring the 


two forms into equilibrium. This mechanism is in accordance with the 
observed order I > Br > Cl ^> OAc, where the effective diameters of the 
atoms progressively diminish and the constraints on the electrons in- 
crease, while in OAc the Ac group may besides act as a steric obstacle. 
This may be supposed to apply to the rearrangement of the halides II 
themselves at elevated temperatures. In fact, for the chloride and the 
bromide II rearrangement at ordinary temperature appears to require the 
presence of salts having great coordinating power conditions merely 
favoring ionization, e. g. y water plus hydrogen chloride, do not suffice. 
_ CH=CH ^ n ^ e t ner hand, water plus hydrogen chloride plus 

x< / \CH cu P r us chloride, or dry ferric chloride alone, induces very 
-- X rapid rearrangement. An easily conceivable function of 

| the salt in this case is symbolized in the figure, where the 

^ etc * steric factors are analogous to those involved in chelation 

while the metal salt acts as a carrier of the entering group. In reactions 
of metathesis the chloride II yields abnormal products R ,OEt 

only with RMgX. Reaction with Grignard reagents ){Mg 

generally undoubtedly requires entrance of the reactant 
into the complex, RMgX(OEt) 2 . Coupling of a halide 
probably involves some intermediate step such as (A). Obviously, in 
view of the facts, such a purely "dative" electron shell as that which sur- 
rounds the magnesium is unusually vague and mobile. This mobility 
would provide the opportunity for a sufficient approach of R and R' in 
forms that are essentially cation and anion without ever requiring the inter- 
mediacy of free alkyl ions. If R' is a substituted allyl group the steric 
factors referred to above will permit the coupling to occur at the 7 as well 
as the a carbon. Reaction of the chloride II with sodium iodide to give 
the corresponding iodide might appear to require intervention of the 
cation II indicated in 2 above, but we reject this idea also, and prefer to 
assume that some kind of coordination precedes reaction and that the 
cation is never liberated as &free entity. On the other hand, the coordinat- 
ing power of the iodide ion is certainly relatively feeble, and there is nothing 
in its structure or known behavior to indicate that it could be very effective 
in the kind of coordination that is here suggested to account for rear- 
rangement or abnormal reaction. The observed absence of abnormality in 
other reactions of compounds II can be similarly explained. 

Experimental Part 

Bromo-4-butadiene-l,2 (II). Viiiylacelylene (otic mole) was passed into 3 moles of 
liquid hydrogen bromide during five to seven hours. The product, washed and dis- 


tilled, gave two fractions: crude bromo-4-butadiene-l,2, 53.5% of calcd., and dibromo- 
2,4-butene-2 (11), 36%. Careful redistillation yielded no other products. Under other 
conditions, bromoprene (11) was found in the reaction product. For example, liquid hy- 
drogen bromide (2.18 moles) added to vinylacetylene (4 moles) at ca. 50 after three 
hours gave the following products : bromoprene, 10%; bromo-4-butadicne-l,2, 19%; di- 
bromo-2,4-butene-2, 12%. The bromide II was also obtained from chloro-4-butadiene- 
1 ,2 and sodium bromide in acetone plus water. Conversion was far from complete, re- 
peated distillation was required to obtain a pure product, and the isolated yield was poor 
(25%); but there was no indication of any bromoprene in the product. Better yields 
of the bromide II (69%) were obtained from the carbinol II plus phosphorus tribromidc 
in the presence of a little pyridine at 10-20 (ca. one hour); a colorless, lachrymatory 
liquid; b. p. 64-66 (181 mm.), 109-111 (760 mm.); df 1.4255; w 2 D 1.5248. 

Anal. Calcd. for C 4 H 5 Br: C, 36.09; H, 3.76; Br, 60.15. Found: C, 36.61 ; H, 
3.91; Br, 60.13. 

Iodo-4-butadiene-l,2 (II). Chloro-4-butadicne-l,2 with one mole of sodium 
iodide was allowed to stand in 80% alcohol or in acetone at room temperature for three 
hours. The heavy yellow oil precipitated by dilution with water was distilled; yield, 
46%; b. p. 43-45 (38mm.); estimated b. p. at 760mm., 130; d% 1.7129; n 2 D 1.5709. 

Anal. Calcd. for C 4 H 5 I : C, 26.66; H, 2.77; 1,70.55; mol. wt., 180. Found: C, 
27.00; H, 2.97; 1,70.40; mol. wt. (in freezing benzene), 184. 

It reacted instantly with aqueous silver nitrate. When it was allowed to stand at 
ordinary temperature solid particles began to separate within twenty-four hours; within 
forty to fifty hours the transformation was usually complete and the product was a 
granular mass of amorphous, insoluble particles similar to the granular polymers ob- 
tained from chloroprene and bromoprene (5). The polymerization is doubtless preceded 
by isomerization. 

Anal. Calcd. for (C 4 H B I) n : I, 70.55. Found: I, 69.49, 69.44. 

lodoprene (III). When the iodo compound (II) was heated at 125 130 a short 
but lively reaction took place; the product then distilled at 111-1 13 (760 mm.), while a 
large amount of viscous residue remained in the flask. The change in physical proper- 
ties indicated that the distillate was the isomeric compound, iodo-2-butadienc-l,3; 
n 2 1.561. It polymerized completely within forty-eight hours. 

Hydroxy-4-butadiene-l,2 (II). Chloro-4-butadicnc-l,2 (6 moles) was stirred at 
60-90 with 6 moles of sodium carbonate in 1500 cc. of water for fifteen hours. Ether 
extraction and distillation gave the carbinol in 50% yield (losses were due to solubility 
in water and formation of higher-boiling products) ; colorless liquid miscible with water 
and most organic solvents; sharp pungent odor, lachrymatory, strongly vesicant; 
b. p. 68-70 (53 mm.), 126-128 (756 mm.); 4 0.9164; w 2 ,? 1.4759. The carbinol was 
also obtained in 50% yield by similarly hydrolyzing the bromide II. 

Anal. Calcd. for C 4 H 6 O: C, 68.57; H, 8.57. Found: C, 68.65; H, 8.68. 

Hydrogenation of the carbinol gave w-butyl alcohol identified through its 3-nitro- 
phthalic ester (mixed m. p. 145). 

The carbinol was recovered unchanged after being refluxed with sodium ethylate in 
ethanol, 25% aqueous sulfuric acid, or 2% aqueous hydrogen chloride. 

When the carbinol (0.5 mole) was refluxed with 100 cc. of 18% hydrogen chloride 
for one hour, a 9% yield of chloro-4-butadiene-l ,2 was obtained ; the rest was unchanged 


carbinol. By the action of phosphorus trichloride on the carbinol in the presence of a 
little pyridine a 62% yield of ehloro-4-butadiene-l,2 was obtained. No chloroprene 
was found in the product. 

Acetate of Hydroxy-4-butadiene-l,2. To hydroxy-4-butadiene-l,2 with excess 
acetic anhydride was added a drop of sulfuric acid. The lively reaction was moderated 
by cooling and the mixture was finally refluxed for one-half hour. Dilution with water, 
extraction and distillation gave a 75% yield of the acetate; b. p. 85-86 (125 mm.), 
140-140.5 (780 mm.); 4 0.9641; w 2 D 1.4504. The same compound was obtained by 
refluxing for seven hours one mole of chloro-4-butadiene-l,2 with 2 moles of sodium 
acetate dissolved in 200 cc. of glacial acetic acid; yield 59%. Similarly from bromo-4- 
butadiene-1,2 a 73% yield of pure acetate was obtained. There was no indication in 
either case of the presence of any isomeric compound. 

Anal. Calcd. for C 6 H 8 O,: C, 64.28; H, 7.14. Found: C, 64.30; H, 7.16. 

Hydrogenation of the acetate gave pure n-butyl acetate. vSaponification gave pure 

Rearrangement of Chloro-4-butadiene-l,2 (II). To the observations recorded 
previously (1) the following may be added. The chloride II (30 g.) with 50 cc. of 18% 
hydrogen chloride after two hours at 70-80 gave 2 g. of crude III. The rest was un- 
changed II. The chloride II (50 g.) with 150 cc. of 18% hydrogen chloride and 10 g. of 
cuprous chloride shaken at 20 for sixteen hours gave a 70% yield of III. The rest was 
polymer; no II was recovered. When 3% of dry ferric chloride was added to the 
chloride II heat was evolved and distillation set in at once. The distillate was a mixture 
of II and III. We are indebted to Dr. D. D. Coffman for the last two of these experi- 


Compounds of the formula CH 2 C~CHCH 2 X in which X is Cl, Br, 
I, OH and OAc undergo a series of metathetical reactions without yielding 
abnormal (rearranged) products. The chloride and the bromide are 
formed by the 1,4-addition of HX to vinylacetylene. They are easily and 
irreversibly rearranged to CH 2 =CX CH=CH 2 . The iodide rearranges 
spontaneously. The direct bearing of these facts on theories concerning 
the mechanism of 1,4-addition and of ^^-rearrangement is discussed. 

Bibliography and Remarks 

(1) Carothers, Berchet and Collins, Journ. Am. Cheni Soc , 54, 4000 (1932), this volume, page 306. 

(2) Carothers, Williams, Collins and Kirby, Jonrn Am Cheni Soc , 53, 4203 (1931) , this volume, 
page 281. 

(3) Carothers and Berchet, Journ. Am. Chem. Soc., 55, 2813 (1933); this volume page 308. 

(4) Carothers, Berchet and Collins, Joufn. Am Chem. Soc., 54, 4066 (1932) , this volume, page 300. 

(5) Carothers, Collins and Kirby, Journ. Am. Chem. Soc., 55, 789 (1933), this volume, page 314. 

(6) See for example, Gillet, Bull soc. chim. Belg., 31, 366 (1922); Prevost, Bull soc chim., 43, 
990 (1928); Burton, /. Chem. Soc., 1651 (1928); Burton and Ingold, ibid., 904 (1928); Whitmore, 
Journ Am. Chem. Soc., 54, 3274 (1932). 

(7) Ingold and Ingold, J. Chem. Soc., 756 (1932) 

(8) Carothers and Coffman, Journ. Am. Chem. Soc , 51, 588 (1929); 52, 1254 (1930). 

(9) Cf. Meerwein, Ann., 455, 227 (1927). 

(10) Johnson, Journ. Am. Chem. Soc , 55, 3029 (1933). 

(11) Carothers, Collins and Kirby, ibid , 55, 789 (1933), this volume, page 314. 


XVI. The Preparation of Orthoprenes by the Action of 
Grignard Reagents on Chloro-4-butadiene-l,2* 

The term orthoprene is here presented as a designation for derivatives of 
butadiene-1,3 having a single substituent and that in the 2-position (III). 
Isoprene is the historically important member of this class and (except 
for a single reference (1) to ethylbutadiene) it was the only one known until 
the discovery of chloroprene (2) and bromoprene (3). The extraordinarily 
superior properties of these compounds from the standpoint of rubber 
synthesis prompted the preparation and examination of other dienes (4). 
The results (5) suggested that similarly desirable properties are not likely 
to be found among any other types of dienes than the orthoprenes as such, 
and it became important to obtain further members of this class. The lack 
of any satisfactory general method for this purpose led to the development 
of the method described here, which is based on the observation that chloro- 
4-butadiene-l,2 (I) like other substituted allyl halides (6) reacts with 
Grignard reagents to produce abnormal (III) as well as normal (II) prod- 
ucts. The theoretical implications of this fact have been discussed in 
the preceding paper (7). 

CH.r=O=CHCH 2 Cl (I) CH 2 -=C==CHCH,R (II) CH2=CR CH===CH 2 (III) 

For the purpose in view the new method leaves much to be desired. 
Yields are rather low, and separation of the desired product from by- 
products is rather laborious. No doubt further study would lead to con- 
siderable improvement. In its present state, however, the method has 
sufficed for the isolation of two orthoprenes especially wanted : w-hepto- 
prene and phenoprene. As precursors of rubber neither of these com- 
pounds approaches chloroprene; they are in fact probably inferior to iso- 
prene. Heptoprene polymerizes rather more rapidly than isoprene but the 
product obtained under most conditions appears to be softer and weaker. 
Phenoprene polym