Polymers Are Everywhere Raymond B. Seymour Department of Polymer Science, University of Southern Mississippi, Haniesburg, MS 39406
Pre-Renalssance Chemlstry The first humans were quick to observe their environment, which consisted of a mixture of gases in the atmosphere and fresh and salt water in the lakes and oceans. If they looked at their reflections in apoolof water or looked a t the plant and wildlife, they would have observed, but not understood, polymers, such as a cellulose and starch in the plants and proteins and nucleic acids in the animals and in the humans. The ancients ate animal proteins, which consisted of giant molecules, or polymers based on many amino acids (RCH(NH2)COOH)joined together through covalent amide bonds (-CONH-). They alsoate starch, a polymer consisting of many molecules of glucose (C6H1206)joined together through alpha acetal (-CH-0-CH-) bonds and used wood for shelter and fuel. The carbohydrate (CH20,) polymer in wood is cellulose, which is similar to starch except that its glucose molecules are joined together by beta acetal linkages, which have a different arrangement in space than the alpha linkages. It is known that Neolithic Swiss dwellers cultivated flax, which was a source of cellulose fibers. The ancients also obtained proteinaceous wool fibers from the fleece of domesticated sheep and collected silk, which was spun by the silkworm (Bombyx mori). The major difference in these various proteins and the edible meat was the sequence in which the amino acids were arranged in long polypeptide chains (-CONH-),. The rocks and sand that were also observed by our ancestors were inorganic polymers consisting of multiple groups of silicates (-SiOa) and silica (SiOz). Hence, it is no wonder than one of man's early expressions was "Polymers are everywhere." Of course, his everywhere had limited boundaries, and the word polymer was not coined until Berzelius used this term in 1827. The terms protein and nucleic acid were coined by Mulder in the 1830's and Miescher in 1869,respectively. Actually, Miescher used the term nuclein, which was changed to nucleic acid in the 1890's Organic polymers were essential for food, shelter, and clothing. Moreover, the ancient artisans learned how to tan hides to produce leather about 5000 years ago. This industry, which is based on the crosslinking of proteins by gallic acid (C~HZ(OH)~COOH) is the oldest industry in continuous production. Embalming, which also involves the cross-linking of proteins is based on the condensation of formaldehyde (H2CO) with proteins. The ancient Egyptians and Greeks (3000-1500 B.C.) preferred to concentrate on the inorganic chemistry of metals and salts. The Greek philosophers, such as Empedocles (490-430 B.C.) described four elements, viz., earth, air, fire, and water. They overlooked the polymers that were everywhere hut did state that flesh and blood were combinations of their four elements. Pliny, the elder, was killed when he made too close an observation of the Vesuvius volcano in 79 A.D. Later, in the Middle Ages, the alchemists were concerned primarily with metallurgy and transmutation. Dispersed pigments had been used for decorative purposes for many centuries, but the first polymeric coatings
(paints) were made from linseed oil (a polyunsaturated oil) in the 14th centurv. White lead t2l'bCO,.l'biOH1,1 was oroduced in 400 from vinegar and leadplatis &d usedaas a white pigment. It is believed that this lead compound formed a salt with some of the free linoleic acid present in the oil and served as a catalyst for the reaction of oxygen and the polyunsaturated oil (drying). This cross-linking (curing) reaction was dependent on the formation of a peroxy free radical (-0-O.), that is, an electron-deficient group that initiated the chain polymerization (addition reaction). Oleoresinous paint was the major decorative coating material until the 1930's when it was displaced, to a large extent, by solvent-based and water-dispersed polymeric coatings. Renaissance Chemlstry In the 16th century, I'aracelsus emphasized rhe &hemists'searrh for gold and recognized that life was essentially a chemical process in which metallic elements and their salts must be in balance. In the 17th rentury, Hoyle and Kunkel emphasized investigation of gases and blow-pipe analysis, respectively, bur much progress in chemistry was retarded by Stahl's phlogiston theory. l'riestley, who is acrredited with the discovery of oxygen in 1774, missed an opportunity to makes conrribution to oolvmerscience when. in 1 7 0 . he coined the work rubber when he discovered that ( ~ e u e a brasiliensis) could he used to erase (rubout) ~encilmarks.In spite of its lack of commercial significance, t i e rubher eraser was the first successful application of natural ruhber by Europeans. The next application of this elastomer was a waterproof laminate produced by Macintosh in 1823.
Polymers In the 19th Century As mentioned previously, Berzelius coined the term polymer in 1827. He was known as the great organizer of chemistry, hut his progress was hampered by his development of the dualistic system and belief in the "vital force" concept. The latter was shown to be nonexistent when Wohler oroduced the organic compoound urea (H2NCONH3 by heating the inorganic salt, ammonium cyanate (NH&NO), in 1828.
Wohler's rearranaement catalyzed considerable interest in organic chemistryand revived interest in inorganic chemistry that had been fostered by the alchemists. However. most of the devehpments in polymer science were by inventors, who were attempting to solve application problems. Thus. in 1839. Charles and Nelson Goodvear heated natural ruhher with small and large amounts ofsulfur to produce soft and hard vulcanized cross-linked ruhber. The elastic soft ruhher, which is still in use today in automobile tires, is said to have a low cross-linked density and the nonelastic hard ruhher (ehonite) is said to have a high cross-linked density. This cross-linking of linear (continuous chain) polymen is comparable t o the tanning of leather or embalming of corpses. The linear chains made up of a few hundred covalently joined isoprene molecules (HzC=C(CH3))CH=CH2 may he represented by a wavy line, and the crosslinked polymer may be depicted as a network structure. Volume 65 Number 4
April 1988
327
In 1833 Braconnot, reacted cellulose with nitric acid to produce the nitrate ester and this product was improved in 1847 bv Schonbein. who used a mixture of nitric acid and sulfuric acid. If onedisregards the tanning of leather, the air curing of unsaturated oil paints, and the vulcanization of rubber, the production of cellulose nitrate (erroneously called nitrocellulose) was the first manmade derivative of a --" -----
Most of the advances in applications of polymers in the 19th century, like the produ~iionof silk and paper in China in 2640 B.C. and I00 A.D. were based on physical rather than on chemical changes. Fur example. Menard made collodion by dissolving cellulose nitrate in an equimolar mixture of ethanol and ethvl ether. the Hvatt brothers made celluloid by the addition-of camphor to~cellulosenitrate, and Swan obtained the first manmade fiber bv forcine collodion through small holes (spinnerets) and &p~rat&g the solvent from the extruded filaments. Nevertheless, the utility of these explosive fibers was enhanced by Chardonnet who denitrated the filaments to produce regenerated cellulose (rayon). An improved technique for the production of rayon was developed by Cross, Bevan, and Beadle who produced watersoluble cellulose xanthate (Cell-SCS-, Na+) by the condensation of alkali cellulose with carbon disulfide (CSz). The xanthate was decomposed to produce regenerated cellulose fibers when the extruded fibers were immersed in an acid bath. The other reactions, such as the production of polystyrene by the free radical chain polymerization of styrene (HzC=CH(CsHs) by Simon in 1839, the polymerization of vinylidene chloride and vinyl chloride hy Regnault and Baumann in 1838 and 1872, respectively, the cationic polymerization of isohutylene (CHFC(CH~)Z) by Goryainov and Butlerov in 1870, and the condensation of phenol with formaldehyde by Bayer in 1872 were of interest from a chemical viewpoint, but the application of these polymers was delayed for many decades. In spite of the lack of commercialization, the chemical reactions devised by these pioneer scientists did demonstrate many techniques that are still in use today. The conversion of a small molecule. such as stvrene. to a piant molecule consisting of many repeating units of styrenemonomers is a chain reaction that. unknowinelv at that time. was initiated by free radicals (R.). The c&version of other small molecules, such as isobutylene, to larger molecules (oligomers) consisting of several repeating units of isohutylene was also a chain reaction that was initiated by the protons (H+) from the sulfuric acid (HzS04). Lourenco produced polyesters by t h e condensation of ethylene glycol (HO(CHz)z(OH)) and succinic acid (HOOC(CHz)z(COOH)) in 1863. This reaction and the condensation of phenol (C6HsOH) with formaldehyde (HCHO) were step-reaction polymerizations that could be catalyzed by either an acid or an alkali. I t is important to note that the cross-linking (vulcanization) of linear polyisoprene by heating it with sulfur; the flexibilization of cellulose nitrate by the addition of camphor as a plasticizer, the extrusion of a polymeric solution, such as cellulose xanthate, to produce rayon; and the curing (drying) of unsaturated oils in the wresence of oxvgen and a catalvst (drier) continue to he usedto produce ruhi)er forautomohilr tires. Flexible polyvinyl chloride (I'VC iilms) for packaging, thr deromposition product of soluble CPIIIIIOSP xanthafe tila328
Journal of Chemical Education
men& (rayon), and the application of oleoresinous paint are all in use today. That so much progress occurred in the absence of little sound theoretical foundation is surprising. In spite of the word pol> mer having been coined hy Rerzelius, the existence of giant molecules (macn~moleculesjwas not recognized and the development of the colloidal theory by Graham in 1861 served to confuse the issue by supplying an erroneous alternate aggregate concept instead of a macromolecular conrept. Hlasiwetz and Habermann, in 1871, and \lusculus and Meytr in 1881, recognized that proteins, dextrin, starch, and cellulose were polymers. In 1888, Brown and Morris used Rauult'scry~~sropic technique (freezing-pointdepression) to obtain a molecular weight value uf 30,000 for amylodextrin, but these conrepw that are acceptahle today were nut accepted by the proponents of the association theurv. Most of the leading scientists of the late 19th century preferred to consider polymers as aggregates of smaller molecules, held together by physical, rather than by covalent forces. Unfortunately, for the progress of polymer science, the proponents of these erroneous concepts continued to discredit macromolecular concepts during the first few decades of the 20th century. Synthetic Elastomers (Rubbers)
Manmade paper, rayon, celluloid, and coatings (paint and glyptal resins) were all commercially available a t the heginning of the 20th century. However while Heoea rubber had been modified by vulcanization with sulfur, it was still a natural product, and, in spite of ronsiderahle experimentation, there were nomanmade counterparts. In 1826. Faraday made an elemental analvsisof Hroea ruhber and in 1860. the isoprene building block(^^^^) was obtained by ~ i l l i & b y the pyrolysis of natural rubber. Since isoorene had undergone radical chain polymerization in ~ i i l i a m ' sand other laboratories, i t is surprising to note that the first synthetic rubber, called methyl ruhber, was produced in 1900 by Kondakov by the anionic polymerization of 2,3-dimethylbutadiene (H2C=C(CH3)C(CH3)=CH2). Unreinforced methyl rubber was used for truck tires by the German army in World War I. These tires failed hut probably would have been successful if they had been reinforced by carbon black. Because of the failure of methyl ruhber tires during World War I and the depressed price of plantation rubber in the post-World War I era, there was little economic incentive for the development of other types of synthetic elastomers. The first American synthetic ruhber was a polysulfide elastomer that was produced in 1927 by Patrick through the condensation of ethylene dichloride (CI(CH2)zCl) and sodium tetrasulfide (NazSa) in an aqueous suspension. This solvent-resistant elastomer, which was called Thiokol. after the Greek word for sulfur, had an obnoxious odor, andhence its production was not acceptable to those livine near the Tbiokol plant. Nevertheless, enough Thiokol A was produced to demonstrate its usefulness as a cable coating and as a component of gasoline hoses. The most widely used Thiokol product is an oligomeric liquid (LP-2) which is produced by the reduction of Thiokol A. Thiokol LP-2 is readily converted to a solid elastomer at room temperature hy t h i addition of lead dioxide (Ph02):l'his product is widely used as R raulking mareria1 and as a hinder for solid rocket nronellants. Neoprene, another ~merican-madesynthetic elastomer, was produced in the 1930's bv Collins and Carothers bv the emulsion polymerization of c h l o r o p r e n e (H;c-CCICH=CH2j. It is interestina tonote that the orecurs#~r for chloroprene was the explosive-vinyl-acetylene ~H~C=CHC--CH), which was discovered by Nieulands, who was a hotanist-priest a t Notre Dame. Neoprene is asolvent and heat-resistant elastomer that continues to be produced for use in hostile environments. Over 75,000 tons ojneoprene is produced annually in the United States.
-
Butyl rubber, which was another American-made synthetic elastomer, was produced by Sparks and Thomas in the 1930's. In contrast to Thiokol, which was made by stepreaction polymerization, and neoprene, which war made by radical chain polymerization, butyl rubher, like Gorvainov's polyisobutylene (LR~UJ, was produced hy catimic chain polymerization using a Lewis arid tAlC1.l~as the initiator. In contrast to ool\~isobutvlene.which. like unvulcanized natural rubber hadlimited utility, butil rubber could be cross-linked (cured) usina natural rubber com~oundineformulatrons. Huts1 r u b k r tvas an unsaturated polyolefin produced by the copolymer~zationof isobntslenr with small amountsof isoprene-(10%).In contrast to the neoprene macromolecule, which is a homopolymer consisting of repeating units of chloroprene, butyl rubber is a copolymer with repeating units of both isobutylene and isoprene. Thus, a homo~olvmermav be revresented as A. and a comlvmer mav be iepresentedas (AB),. Butyl rubber and chforinated anh brominated butvl rubber have excellent resistance to easeous permeation and are used in automobile tires to prevent leakina of the pressurized air. he-development of methyl rubber was encourged by Kaiser Wilhelm prior to World War I. Likewise, the development of ~ u u aelastomers, - ~ which were patented by ~ s c h u n kur and Boch in 1933, was encouraged by Hitler prior to World War 11. It is of interest to note that elastomers, like methyl rubber, could be made by the polymerization of butadiene (Bu) in the presence of sodium (Na) and merited the acronym of B u n a . T h e copolymers of b u t a d i e n c e (H2C=CHCH=CH2) and styrene (HzC=CH(CsH5)), Buna S, made by Tschunkur and Boch, and butadiene and acrylonitrile (Buna N), made hv Konrad and Tschunkur, were made by radical chain polymerization of the monomers in an aqueous emulsion that used potassium persulfate (K2SyOs) and not sodium (Na) as the initiator. I. (;. Farhen Industrie made the Buna patents available to Standard Oil of New Jersey, as part of a previous agreement on the hydrogenation of coal and oil. Hence, much of the know-how was available to American rubher firms who nroduced the butadiene (75%) styr&(25%) &ast&eric copolvmer under the trade name of GRS durine World War 11. his elastomer, which is now called SBR, isthe world's most widely used elastomer. Over 8 million tons of synthetic rubber (SR) is produced annually worldwide. The American consumption of SBR and natural rubber (NR) is 1.78 million tons and 735 thousand tons, respectively. About 60 thousand tons of the acrylonitrile-butadiene copolymer (NBR) is produced annually in the United States. NBR is used for the production of flexible objects used in the presence of oil and other solvents. A cis polymer of isoprene, which has been produced by nature f i r millenia, was produced in 1954 in m&h the same manner as that used for the production of methyl rubber in the early years of the 20th century. The preferred initiator for this stereoregular elastomer is butyllithium (CpHsLi). The relative amounts of the elastomeric cis stereoisomer
-
~
~~~
.
can be regulated by choice of initiator. This polymer, which is similar to NR, has been replaced, to a large extent, by the less expensive cis-polybutadiene.
cis-Polybutadiene (PB) is being produced in the United States at an annual rate of 360 thousand tons by the chain polymerization of butadiene in the presence of initiators, such as butyllithium and titanium tetraiodide (TiIa). The copolymer of ethylene (HzC=CHe) and propylene (HzC=CH(CHa)) (EPM) is produced at an annual rate of about 500 thousand tons, in the United States, by the copolymerization of these olefinic monomers in the presence of Ziegler-type initiators, such as diethylaluminum chloride ((C2Hs)AlCl) and vanadium chloride (VCI*). Most of the commercial products (EPDM) have a moderate amount of uusaturation, which is the result of the addition of a small amount of a third monomer, such as dicyclopentadiene. In addition to these widely used synthetic elastomers, there are also several commercial specialty elastomers, such as acrylic elastomers (-CH2-CH(COOCzH+), chlorosulfonated polyethylene (Hypalon), epichlorohydrin polymers
silicones
-Si-0-
Si-
I
I
CHB
CHB
fluorocarbon polymers, phosphazenes
and blends of various elastomers. Thermoplastic Elastomers As polymer scientists learned more about structure-propertv relationships of polvmers, they recoanized that it was possible to syn
