POLYMERIZATION A Greet
Field of Chemistry G
S
Ahead
WHITBY
University af A k r o n , 302 East Buchtel A v e . , A k r o n 4 , O h i o
T H E synthetic production, by poly-· • merization, of materials of very high molecular \\ r eight is a v a s t field of organic chemistry i n t o the exploration and exploitation of which chemists have entered only during recent years. Of the untold millions of carbon compounds capable of existence, chemists have isolated from natural sources or have synthesized perhaps half a million. B u t until recently almost ail t h e carbon compounds studied were compounds of relatively low molecular weight. T h e 1910 edition of Richter's Lexikon of carbon compounds lists about 150,000 compounds which had been put on t h e record up t o November 1909. Of these only a b o u t 0.4 per cent (644 entries) are compounds with more t h a n 50 carbon atoms, and only a b o u t 3 per cent compounds with more t h a n 30 carbon atoms. I n the current edition of Beilstein's H a n d book of Organic Chemistry, covering the chemical literature t o 1919, approximately 180,000 compounds are recorded. Of these only about 0.2 per cent (388 entries) are compounds above C5o a n d only a b o u t 3 per cent compounds above C3o· These statistics indicate clearly t h a t prior t o t h e last quarter of a century synthetic chemists h a d concerned themselves little 1 Address delivered before tiie Canadian Chemical Conference, Toronto, Canada, J u n e 5 to 7, 1944.
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with carbon compounds of very high molecular weight such as fall in the new field of polymer chemistry. Further, the compounds of relatively low molecular weight with which formerly organic chemists were almost exclusively concerned a r e of interest chiefly because of their chemical properties, whereas macromolecular organic products are of interest chiefly b e cause of their mechanical properties. I n order t o obtain useful mechanical strength in organic products, it is essential t h a t they shall have very high molecular weights—a minimum of, say, 20,000 in the most favorable types, and 10 times as high in other types. T h e number of high molecular types occurring in n a t u r e is small. Cellulose, in t h e .forms of wood, cotton, flax, etc.; proteins, in t h e forms of wool, hair, horn, etc.; rubber: these are t h e most important naturally occurring macromolecular organic m a t e rials. A much wider variety of macromolecular types can b e made synthetically by polymerization. Chemists in important numbers undertook t h e systematic study of high polymer chemistry only within the past two decades. Already considerable insight has been secured into t h e principles of polymerization, and already a not inconsiderable n u m b e r of synthetic polymeric products with useful practical applications
CHEMICAL
M o l t e n nylon polymer being extruded onto a water-sprayed casting wheel. The strip thus Formed is later chopped into flake, subsequently melted and extruded as filaments.
have been developed. I t m a y confidently be expected t h a t , w i t h fuller exploration of this vast new field of organic chemistry, the n u m b e r and variety of valuable polymeric materials will be very greatly increased. I t is a sign of t h e times t h a t t h a t sterling old journal of organic chemistry, the Journal fur praktisdie Chemie, founded 110 years ago, in 1834, has lately, under t h e editorship of H . Staudinger, become, in effect, a specialized journal of high polymer chemistry. T h e building up of very large organic molecules may be accomplished either by (1) addition polymerization or (2) condensation polymerization.
Addition Polymerization Addition polymerization is the self' addition of small molecules t o form large ones. I t is dependent on the presence of unsaturated centers in t h e small (monomer) molecules. Simple aliphatic olefinic hydrocarbons are in general difficult t o polymerize to a high level. By means, however, of ultra-high pressures (about 1,000 atmospheres) even the simple olefin ethylene can (in t h e presence of a little
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ENGINEERING
NEWS
oxygen as catalyst) be converted to a high polymer, - ( C H 2 . C H 2 ) 3 - . The product is a tough, flexible solid, which, being substantially a paraffin of very high molecular weight (say, 20,000) has outstanding electrical properties and is finding practical applications because of these properties. Of the simple olefins, the unsymmetrical compound isobutylene seems to have the greatest capacity for polymerization. Staudinger showed that activated clay (Florida earth) was capable of converting it to relatively low polymers. It has since been found that halide catalysts, such as boron fluoride, applied at very low temperatures, will convert isobutylene (b.p. —6° C.) almost instantaneously into solid, elastic high polymers. On the basis. of this observation, Butyl rubber has been developed. This synthetic rubber is made by polymerizing isobutylene in admixture with a small proportion of isoprene. The polymers of straight isobutylene are saturated and unvulcanizable. The introduction of a small proportion of isoprene confers unsaturation on the polymers and makes them vulcanizable. Although the simple olefins are not in general readily convertible to high polymers, it has become clear that the double bond can be activated and polymerizability greatly enhanced by (1) substitution which confers polarity on it, (2) its participation in a semibenzenoid conjugated system or in a "potentially conjugated" aliphatic system. Examples of the first are the readiness of vinyl chloride, CH 2 :CHC1, and vinylidene chloride, CH 2 :CC1 2 , t o yield high polymers, thanks t o the high polarity conferred on the double bond by the substituent chlorine atoms. B y contrast, the symmetrically substituted dichloroethylene, C1CH:CHC1, has no similar proneness to polymerize. Another example of polar activation is vinyl acetate, CH 2 :OH(OCOCH 3 ), which readily yields high polymers. Examples of the influence of semibenzenoid conjugation in activating a double bond are seen in styrene and indene.
CÏÏ===CiÎ2
Two double carbon bonds disposed in a conjugated arrangement, as in (to take the simplest case) butadiene, have long been recognized as an arrangement favoring polymerization. A triple conjugated system (—CH:CH.CrI:CrI. CH:CH—) in Chinawood oil accounts f o r the ready gelation of that glyceride. In a conjugated system of carbon linkages, as in a simple olefinic system, polar substituents tend to enhance polymerizability. Thus 2-chlorobutadiene (chloroprene) CH 2 :CH.CCl:CrI 2 polymerizes far more rapidly than does butadiene. And what is perhaps more important, the polarity of the monomer has a directing influence on the growth of t h e polymer chains, keeping them linear, so that the polymerizate has better elastic properties than that from butadiene, i n the polymerization of which branching of the molecular chains is likely to occur because the monomer molecules unite in both 1,4and 1,2-positions. The introduction of a second polar substituent in an internal position in the butadienoid system still further enhances polymerizability. 2,3-Dichlorobutadiene, CHo : CC1.CC1 : CH 2 , polymerizes more quickly than 2-chlorobutadiene. Chlorine substituted in a terminal position has no such effect. 1-Chlorobutadiene, CH 2 :CH.CHrCHCl, does not polymerize much more quickly than butadiene* Cyano substitution in the terminal position, as in 1-cyanobutadiene, C ΡΙ2 : CII.CH : CH.C ; Ν raises the rate of polymerization of buta diene. But here, as in the case of acrylo nitrile (supra) the cyano group may be considered as introducing extra (poten
tial) conjugation. Polarity and enhanced polymerizability is conferred to the buta dienoid system by an acetoxy group, as in CH 2 :CH.C(OCOCH 3 ):CH 2 (cf. vinyl ace tate, supra). From the examples just given, it is clear that certain generalizations as to the relation between chemical structure and ability to polymerize have already made themselves apparent. Such gener alizations are of assistance in the search for new polymerizable substances. Factors in Polymerization Reactions Again, with regard to the actual conduct of addition polymerization, useful generali zations have been established concerning the factors which influence the rate at which polymerization takes place and the level to which it proceeds. The molecular weight of the polymeric product in general rises, as the percentage of the monomer which has undergone polymerization in creases. Increase in the temperature at which polymerization is conducted and in the concentration of catalyst added gener ally speeds up the reaction but reduces the ultimate molecular weight attained. Dilu tion of the reactants with an indifferent solvent in general leads to a lower molecu lar weight in the polymer. B y maintaining the monomer in sus pension in water, the speed of polymeriza tion is increased, and it is raised still more by actual emulsification of the monomer. For example; if 2-chlorobutadiene is al lowed to stand in bulk at room tempera ture, it undergoes polymerization in about 10 days; if it is merely emulsified in a soap solution, polymerization is com plete in 2 to 8 hours at room temperature.
CH 2 "CH=CH
Styrene
Indene
Examples of potential conjugation in activating a double bond are to be seen in the polymerizability of acrolein, C H 2 : C H . C : 0 ; acrylic esters, H CH2:CH.C:0;
itaconic
ester,
OR RO CH 2 : C.C: O; acrylonitrile, CH 2 : CH.C: N ; CH 2 COOR methyl vinyl ketone, CH 2 :CH.C:0. CH 3 V O L U M E
2 2,
N O .
Condensation polymerization. Du Pont plant, Seaford, D e l . Autoclaves and control panel for nylon polymer production by condensation of hexamethylene ciamine *vt4 adipiic acid.
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2 5,
1 9 4 4
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Copolymcrization W h e n a m i x t u r e of two monomers is subjected to conditions under which each alone would polymerize, it is usually foxind t h a t , instead of each polymerizing •separately t o yield a mechanical mixture of the products which the components would give if polymerized separately, t h e compounds undergo co- or interpolymerization, forming molecules into wliich both enter. T h a t is to say, xA -fr H gives, not Ax -f- Rr, h u t (AH),. Fxirther, mixtures composed, not merely of c
