contemporary hiztory zeriez
LEONARD editedFINE by: Columbia University New Ywk, NY I0027
ERIC S. PROSKAUER
The Development of Polymer Chemistry in America-The Early Days C. S. Marvel University of Arizona. Tucson, AZ 85721 Polvmer chemistrv in America started first in industrial laboratories where plastin were prepared from natural prndurts such as h d a n d horn. At that stage it was not a truly scientific field of work but pure technology in converting the natural ~ r o d u c t into s materials like combs. esoeciallv decorative combs for ladies' hair. Chemistry may have &en involved in the transformations that were effective, hut certainlv no reactions could be written l o descrihe the changes. Soon after this stage, the development nf practical polymers from cellulose hecame an important industry. Cellulose esters were ~ r e ~ a r so e dthat they could he dispersed in variousmedia give cellulnne again. and ihen subsequently llydrcrlyzed Xanthates made from reaction of sodium hydroxide and carbon disulfide on cellulose were capable of being converted into useful polymers. Xanthatesareextruded into an acid bath to regenerate the original cellulose. This gives a clear film of cellulose (later called cellophane). Nitrate esters were prepared. A completely nitrated cellulose called gun cotton was used in making smokeless powders. Lesc completely nitrated cellulose was called nvruxvlin and was soluble in ether and acetone. The lower &atescould he spun from these solutions to give fibers and films which were useful but were very flamable. Pyroxylin blended with camphor gave a plastic, celluloid. which had extensive use in molded obiectc such as collars, buttons, and similar things, but it, too,was excessively flammable and many had fires resulted from its use. The cellulose derivatives were not truly synthetic polymers, but thev were modified natural oroducts. he synthetic polymer inhustry had its start when Bakelal~d.a Beleium-horn chemist. develo~eda method to control the reaction of phenol with formaldehyde to give a powder
which cnuld be comnressed and molded under heatine..to eive . a useful material (H'akeiitel for electrical insulation. I t cwld he made into sheets which could he used in thr conatrurtitm of cabinets such as radio cabinets and other similar objects. If not too hi~hlhlvwlvmerized, it muld he used to make varnishes which ctiuid he later further set hy heat. This plastic, the first to he made, is still an extremelv useful material as a building hlock in industry. It was many years after its devels opment hefore the reactlnnc which t w k place in t h ~ formatim were well understood. Soon after the development of Bakelite, American Cyanamid developed their urea-formaldehyde resins. They were especially valuable because they were colorless and could he c o n v e r d into decnrative materials such as ccmnetic cases and similar smaller articles. It was ohvious that the formaldehyde and urea had reacted to give monomethylol urea and dimethylol urea, but the remaining steps in the conversion of the prnduct to a useful plastic were not determined for many years. The third complex polymer was developed by General Electric and American Cyanamid from glycernl and phthalic anhydride, which received the name of glyptal resin. If the glycerol and phthalic anhydride were cooked together for some time. the mass became verv intractible. hut i f it were not heated fnr t w long a period, the material could still he d ~ s ~ e r s e dand made into varnishes. The addition o f a m e mo. nohasic acid to balance the active carhnxyl groups in phthalic acid with the hydroxyl groups ofglycerol would lead tn more tractihle products. This was develnped into varnish materials. Another early plastic was thiacol, an aliphatic polysulfide,
C d S. Mm.1 w8s tcm in Waynesvllls. lllimls in 1894 and r-ived his PM) hornme University 01 lllinolr In 1920.Far 41 years he wason me lawby ol me uniiersily of llfimis: his work was originally in aganic rynthes~s01 chemletry bul he became interested in lhe slruclure polymersand alter 1933 devoted most ol his research to lhal area. He lnvestl08ted %~llurdioxide addition oolvmers. men turned to the studv
and
suited in a large p u p ol mtw polymers over the nen 10 years. In 1950's he turned his anentim tothe syn(hesk0tmaterials able tototerate the high temperatures involved in space nights and made many ~nIrlbYtiOnS,the most signillcant 01 which was me preparation ol a polymer of benzimldizole which had outslandlog high-lemwatwe prwrtiar. In 1961. Marvel moved to lhe University of Arizona *mere he rtiil continues his work wlm e large research goup. t!e ha5 received many awards. among them me American Chemical Sociely's Nichols Award (1944). Gibbs Medal (1950).and Riestley Medal f 1956).
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which is a semi-ruhhery plastic. It had an early use as a caulking agent on the flight decks of airplane carriers. Recently, it has become important as the best hinder to use with rocket fuels which carry our spaceships into arhit. The basic chemistry of none of these processes was well understood and the ideas concerning the true nature of pnlymeric materials had not been clearly set forth a t that time. Staudinger did this in about 1920 when he began to call polymers macromolecules. Many chemists did not accept his views entirelv. Thev still helieved that polvmers were large . . molecules which weie held together by some force other than ordinary valence forces. They did not realize that polymers were mixtures of molecules of varied molecular weight, and thev did not realize that macromolecules could have some poperties which were determined by the size and shape of these big molecules. My first introduction to a polymer came as a result of a reaction which one of my students, Martin Friedrich, was carrying out between lithium ethyl and tetraethyl arsonium chloride in an attempt to prepare a pentaalkyl arsenic compound similar to the one which Cahours had thought he had made a t an earlier date. No pentaalkyl arsenic compound was obtained, hut the arsenic compound was converted to triethyl arsine and some gaseous products were produced. Analysis of gaseous oroducts showed that thev were about 95% ethane and 5% ethylene. It would have been an easy thing to account for an equal mixture of ethylene and ethane, or for the gas to have been butane, hut there was no reasonable way to account for essentially pure ethane as a product. As a result, I postulated that in our reaction equal amounts of ethane and ethvlene had been oroduced and as our reaction stood, the excess ;rf the lithium ethyl had caused most ofthe ethylene to polymerize and thus to disappear in the reaction mixture. To show that this could happen, we prepared a Nujol solution of lithium ethvl and passed ethvlene into it at atmospheric pressure. ~ t h y l e n was e ieadily ahiorbed and in a day & two there was a solid white polymer separated from the hydrocarbon solution. This waiundoubtedly linear polyethyl&e, although that was unknown a t that time and polyethylene was not considered to he very important then. So no further work was done on this discovery a t that time. This was written up and puhlished in 1928 in the Journal of the American Chemical Society. Many years later, the British discovered how to polymerize ethylene under high pressure with a peroxide catalyst. This gives a low density polyethylene, which has developed into one of our maior . nlastics . My next experirnre u,ith polymer-. rnmt: with the synthesis of l,rumoalkvl dialk\.l amines. \r,h~rhI had vlanned I I I use in &her synthetic wori. All of these materials self-reacted and oroduced oolvmeric auaternarv ammonium salts and cvclic . . quaternary ammonium salts. The polymeric materials contained both ionic and nonionic bromine and by comparing the amounts, it was possible to tell how many monomer units had combined to form the polymeric product. This led to a study of various hromoalkyl units with various carbon chain lengths between the hromine and the nitrogen to determine the amount of polymeric and amount of cyclic product which would be oroduced. This problem occupied our efforts from about 1929 to 1933. While I was interested in these polymers, I became a consultant for the DuPont Company at the Experimental Station in Wilmington, Delaware. On an early trip there, I talked with Dr. Elmer Bolton, who called my attention to a patent by Matthews and Elder issued in England in 1914, which claimed that sulfur dioxide would react with simple olefins such as ethylene, propylene, hutylene, etc., to give polymeric products. Rolton asked me if I thought such a reaction would occur. I was very skeptical because it did not seem to fit orthodox organic chemistry. He pointed out the fact that such a polymer would he exceedingly cheap if it could he obtained. I suggested that 1 try out the reaction in my laboratory a t Urhana and see if it really did occur.
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dioxide wiih k h e r olefins such as cyclohexene, hutene, propylene, etc. The reaction was catalyzed hy peroxides to produce nice white polymeric products. 1 continued working on this problem for about ten years. We had to determine the limits of the reactions, what olefins would react, and what the structures of the products were. By hydrolysis we established that they were polysulfones with head-to-tail structures. All the polymers were one to one olefin SO2polymers except that prepared from vinyl chloride, which always came out two molecules of vinyl chloride for one of S02. Later it was shown that sometimes the temperature affected the . valvmer cum. hinations and some could he made with different ratios and SO? olefin. All 1-olefins would eive ~ o l v m e r sand some 2bonds conkgated with carhonyl groups would also not react in this polvmerization reaction. Otherwise, functional groups would "oGnterfere with the formation of polymers. AII of the volvmers proved to he slightly unstable a t their melt in^ . points, and no good molding resins could ever he made. It was particularly difficult to do anything with the sulfur dioxide kthylene p&ner because it was i&duhle and intractable. The hutylene polymers gave nice poker chips in the molding test hut there &asalwa& at least one star in each showing that the polymer had decomposed to gaseous products. As far as I know no one has ever found a stabilizer that would prevent the decomposition of these polymers a t the melting point. It was about this time when Carothers was employed by the DuPont Company a t the Experimental Station and began carrying on his classical studies on polymerization, especially condensation polymerization. He demonstrated beyond any doubt that polymers were really macromolecules such as Staudinger had postulated. He wrote a review article for Chemical Reuiews in which he -gave definitions to such important terms as recurring unit, end group, copolymer, condensation ~olvmer. and addition . volvmer. After that article, . . . the mystery of polymer chemistry was pretty well cleared up, and it was possible for less talented people to make good contributions in the field. Carothers pointed out the extreme imvortance of using pure monomers in polvmerizations if good products were to beexpected. His work ied to the first&thetic fiber, hexamethylene adipamide (nylon 6.6) and in this work the discovery was made that polymeric materials could he oriented by cold drawing to give greatly improved tensile streneths. This was one of the critical discoveries that makes ~ ~-~ synthetic fibers useful. Julian Hill was one of the men working with him who helped in this discovery. Paul Flory was also working with Carothers's group and contrihuted much to the theoretical side of early condensation polymer chemistry. It is interesting to note that the discovery of cold drawing fibers was more or less accidental. Yet it was a most important part of synthetic fiber development. Nylon had been made and seemed not to have any especially useful properties and put aside on the shelf without patenting. Work was continued on the polyester series which gave more soluble products, easier to handle, and thus simpler to work with in the laboratory. It was while working with one of these softer materials that Julian Hill noted that if he gathered a small hall of such a polymer on the end of a stirring rod and drew it out of the mess, that it was extended and became very silky in appearance. This attracted his attention and that of the others working with him, and it is reported that one day while Carothers was downtown, Hill and his cohorts tried to see how far they could stretch one of these samples and took a little hall on a stirring rod and ran down the hall and stretched them out into a string. It was in doing this that they noticed the very silky appearance of the extended molecules and they realized that they were orienting the polymer molecules and increasing the strength of the product. While polyesters were too low melting fur use in textile products, they went hack to study ~
polyamides and found nut that although they were a little harder they still cnuld be cold drawn, and applying this technique to polyhexamethylene adipamide led to the production of Nylon. Shortly after the introduction of nylnn as a synthetic fiber, the British Calico Workers Association discovered that ethylene terephthalate made a gnod fiber and gave this t.he name of terylene. It is interesting to note that Carnthers had made mnst of the polyesters and all that he had made were too low melting nr too easily hydrolyzed to show promise as textile fibers. But he had nnt used terephthalic acid, and this was the one acid which gave useful fihers. Emmett Izzard a t the DuPont film department in Buffalo found that ethylene terephthalate made an excellent film when it was stretched hiaxially. This discovery was made a t about the same time the British discovered the use of this ester as a fiber. The film was introduced to the market as Mylar and sonn afterward DuPont developed the fiber which they called Dacron. It was not long after the Dacron development that DnPont produced the first vinyl-type polymer as a fiher, palyacrylonitrile, which they called Orlon, and which Monsanto soon intrnduced as Acrylan. These fibers were nnt as versatile as either Nylon or Dacron, hut they have found an extensive use, particularly as sweater yarns. The work of Carothers and his group on condensation polymers clarified that phase of the polymer problem rather completely. They did some work on vinyl polymers. It resulted in the discovery of the first useful synthetic ruhber, hut they did very little on the mechanism of vinyl polymerization. In reporting the discnvery of this new synthetic rubber it is worth reporting some of the background of that discovery. Dr. Calcott of Jackson Laboratory of DuPont had noted Father Nieuwland's work a t Notre Dame on the dimerization and trimerization of acetylene to give monu and divinyl acetylene. These unsaturated hydrocarbons intrigued Calcott and he thought they should he starting materials for a synthetic ruhher. Accordingly, he acquired the patent rights to the twv materials from Father Nieuwland and started research toward converting them into higher polymers. He was not having very much success, so he came over and asked Carothers a t the Experimental Station if he would look into the matter. Carothers was interested and asked Arnold Collins, one of his helpers, to purify a sample hy distilling the crude mixtures of dimer and trimer. In doing this, he obtained a small sample of material which boiled above the boiling point of monovinyl acetylene and below the boiling point of divinyl acetylene. Instead of tossing it away, he set it aside and over the weekend it solidified. He noted that solid seemed ruhhery and dropped it on the bench and found that it bounced. Examinatinn showed that it did, in fact, have good ruhber properties and was the hydrochloric acid addition product of monovinyl acetylene. This observation of Collins's led to further work which produced Neoprene, the first useful synthetic ruhher and one that is still a very important stable, strong, oil-resistant rubber. Vinyl polymerization was being studied during this time in universities and industry. One great advance was the recognition that it preceded in the three distinct steps-initiation, propagation, and termination, as do most chain reactions. Once this step-wise polymerization was recognized, rapid advances in the field came about. The wnrk of Mayo and Walling on reactivity ratios was a great step forward in the study of copolymers. Before the many advances in theory were made, a considerable amount of work was devoted to determine the structure of vinyl polymers. The question was whether they wonld build up regularly in a head-to-tail fashion as noted by Staudinger, or would they build up in a headto-head, tail-to-tail fashion, and the third possibility was a random arrangement of recurring units. Chemically, it was established that most of the prnducts were head-to-tail arrangements of recurring units, and this, of course, became the expected structure, once the mechanism of polymerization was
well understnnd. There have been a few cases discovered in which some randnm strurtures occurred. In the case o f .~ o.l v vinyl acetate and the pnlyvinyl alcohd obtained frvm it. Flory found evidence that there were a few head-to-head, tail-to-tail units and he demnnstrated this hy shnwing that the polyvinyl alcohol was cleaved by periodic acid oxidation indicating 1,2-glycol units in the polymer. Apparently, the stahilization of the radical intermediate is such that either structure can result. In the case of polyvinyl fluoride there has alsn heen evidence ornduced to show that there are fluorines on adiacent carhon atoms, which ind~catessome random strurture in this oolvmer. large radicals, or the chain transfer reaction with the polymer or an added agent. An external transfer agent is often used to regulate the molecular weight of the polymer, and thus telomerize the growing chain and introduce the desired functional groups at the end of the chain. In addition tn free radical polymerization, reactive cvmpnunds may he polymerized by cationic or anionic initiation. These are sometimes effective where the free radical initiation is nut. An example is the polymerization of isobntylene, which goes hest by cationic initiation. Monomers like acrylonitrile, acrvlic esters. and stvrene can be initiated hv aninnic svstems. Once the polymers are produced there does not seem to he much difference between those initiated bv one method or the other. During this perind all of these systems were actively investigated for commercial production. Most of the relatively available monomers had been thornuzhlv studied and the products they produced were marketed by the end of the 1950's. The plasticization of polymers hy an outside agent was developed and is very effective in making such things as nolvvinvl chloride more useful. Semon a t B. F. Goodrich Co. found that esters like tricresyl phosphate and dioctyl ohthalate would olasticize oolyvinvl . . . chloride and make a much mare useful product from it, and that is largely resoonsible for the immense -growth in the production of this particular polymer. Major advances in polymerization practice and theory came during the government ruhher program, which started in 1941. With the take-over of the Pacific area by the Japanese a t the start of World War 11, this country was suddenly aware that it moved on ruhher and the supply of natural ruhher was nc longer available. A government ruhber office was established and a ruhher czar was appointed to see that a program was set up, funded, staffed adequately, and given every opportunity to develop a new product to satisfy the need. This government office built the needed plants, financed the research necessary, manufactured a synthetic ruhher that could he used, marketed it. and when the war was over. sold the nlants back to industrv. he results of the program, "nder thdgeneral supervision of Bradlev Dewev. .. who was the first ruhher czar in the research phase, was phenomenal. Dewey collected in his group chemists from the various rubber comoanies and universities such as Massachusetts Institute of ~kchnology,Cornell, Case Western, Chicago, Minnesota, and Illinois. In addition, companies like Bell Telephone Laboratories, Esso (now called Exxon), DuPont. and other contrihuted manpower. These chemists worked together in harmony, shared all research advances with each other, shared the problems that were most acute, and cooperated in every way to develop a synthetic rubber which wonld meet our needs. Within a year a satisfactory product had been produced. By the end of the war, a really good synthetic was available for tire rubber. The synthetic was not sa&sfactory for large truck tires and large bomher tires because the heat buildup in operat.ion was too great, but it was still very useful for autnmnbile tires and is used in competition with natural rubber now. At the end of hostilities with Germany, the Office of Rubber Reserve sent a team of five chemists to Germany to see what Volume 58 Number 7
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progress had been made there in synthetic ruhher during the war years. It was a t that time that the importance of redox polymerization was stressed hy the German chemists, who were trving LO develop a recipe for continuous polymerizatim. oi'butadiene and styrene. The Americans adapted this recipe llrr the production of ruhber in a low-temperature process which gave us so-called cold ruhber. It was possihle to polymerize the styrene-hutadiene mixture by the redox process at 5'C as rapidly as the previous process used a t 7O0C. T h e new cold ruhher had a narrower molecular weight distribution and a s a result it performed much better a s a tire rubber and is still the standard ruhher for passenger car tires. Durine these vears the nolvmer industrv crew to he one of . .. the maj& indusiries in ~meri'ca.The whole chemical industry was really the top industrial operation in the country. The inlimnat ion that was collected became so voluminous that new juurnals were started to take care of the matter. In 194fi. Pmfessor Mark a t Hrwklyn Polytechnic started the Journal 111l'r~lvmrr Srirnrf,. In 195%the Journal of A ~ o l i r d Pdvmcw 6hrmistr? began publication. In l9fifithe ~ ; , h r n a lof ~ a c mmolrcular ('h~mislrycame into heing. and in 1968, the American Chemical Society started the Macmmolrrular Journal. All ol' these publications did much 111 stimulate further work in pr~lymerscience. A new system of pnlymerization, the Ziegler-Natta system. was developed a t ahnut this time. This system used a comhination of aluminum alkyls and titanium chloride to initiate polymerization. This new process wuuld palymeri7e ethylene a t r w m temperature and atmospheric pressure. It was found that isoprene polymerieed by this system gave a product which was all head-to-tail and all cis which was essentially identical with natural rubber. This development came just as the war ended and natural rubber became availahle again. For a while. svnthetic rubber comoeted well with natural rubber. but the
