I
C. E. SCHILDKNECHT, Stevens Institute of Technology, Hoboken, N. J.
Advances in Ionic Polymerization of Vinyl-Type Monomers 1
Useful tailored polymers and copolymers may be developed by such techniques. Synthesis of cis- 1,4 polymers from isoprene and butadiene may lead to revolutionary industrial developments.
R E C E N T ADVANCES in the preparation of homopolymers of high molecular weight are of outstanding industrial interest, especially for plastics, adhesives, and rubbers made by ionic polymerization of unsaturated compounds. The report is based on recent European and American literature and on work in this laboratory: Characteristics of ionic polymerization reactions (in contrast to free radical polymerizations), and relations between monomer structure and response to different catalyst types, and between catalyst systems and polymer structure. These aspects have special interest at this time in connection with the development of so-called oriented (707), stereoregular, stereospecific (78), or eutactic (27) polymerizations, as well as other new industrial applications of ionic polymerizations. Although the ionic polymerization art is even older than the use of peroxide catalysts, most early examples, such as the thickening of turpentine by mineral acids, did not form polymers of high molecular weight in good yields. Particularly in ionic polymerization, much research and development of a high order separate the first laboratory experiments from the plant process for preparing a reproducible plastic or rubber of high molecular weight. There are several other types of ionic polymerizations besides cationic and anionic vinyl addition polymerizations. One of the most interesting, which often occurs with alkaline catalysts, involves migration of an active hydrogen atom to the double bond of another molecule, for which the names “hydrogen migration polymerization” or “H-polymerization” are hereby suggested.
With ethylenic monomers bearing electron-attracting or negative substituents such as acrylic monomers, Hpolymerization normally occurs, in a reverse Markownikoff way. Each step is similar to a Michael reaction, These polymerizations often occur best under anionic conditions and with retarders of free radical polymerization ( 8 ) .
There are known many ionic polymerizations of monomers by ring opening. Some are unsaturated compounds; here again copolymers may occur from two types of chain structures resulting from two mechanisms of polymerization-e.g., ionic vinyl polymerization and ionic ring opening. For example, the polymerization of 3,4-di-
- -CHzCH?CHzS- polymer
hydrofuran catalyzed by boron fluoride etherate was found to occur largely by a vinyl addition reaction (3). The polymerizations by which polyamides, polyesters, epoxy resins, and formaldehyde condensates are prepared
CHa=CHCH2SH
-+
O r two or more compounds may react; a divinyl compound can give linear polymers (55, 62) : CHz=CHCOOCHzOOCH-CHz
f HNH + bH2CH20H --CHzCHzCOOCHzOOCH&HzN- -
I
CHICHQOH In other cases H-polymerization may occur with acid catalysts or under free radical conditions. Numerous examples of such polymerizations, a t least to polymers of low molecular weight, have been discussed, but no adequate review of the subject has been published. Vinyl compounds apparently can sometimes form copolymers containing blocks from both normal vinyl polymerization and hydrogen-migration polymerization. Such mixed-type copolymers would be expected to show unusually wide solubility and compatibility, ‘and they may be present in polymers from acrylamide, described by Schildknecht and others (709) and by Breslow and others (8). They have received little attention as yet. Some ethylenic monomers bearing electron-donating groups such as divinyl ethers also may undergo H-polymerization-for example, with glycolsto give acetal polymers (reversed Michael addition), Discussion of the ionic mechanisms by which hydrogen is transferred lies beyond the scope of this article.
are believed to be ionic reactions, but are not discussed here. New ether polymers such as Penton of Hercules, Delrin of Du Pont, and crystallizable polypropylene oxides are among the interesting nonvinyl polymers prepared by ionic polymerization. By far the greatest industrial production of vinyl-type polymers is accomplished now by free-radical polymerizations initiated by peroxides or azo catalysts, usually with heating, but in some cases a t lower temperatures. However, the high polymeric productsButyl rubber, polyisobutylenes, poly(vinyl ethers), and some high-density polyethylenes-are prepared commercially by ionic polymerizations. Products of lower molecular weight obtained by cationic polymerization using acidic catalysts include coumarone-indene resins, terpene resins, and so-called petroleum resins. Russian butadiene rubber and the new cyanoacrylate adhesives are examples of commercial products employing basic catalysts (anionic polymerization). Development of new prodVOL. 50, NO. 1
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ucts by ionic polymerization, especially particular molecular structures by stereospecific or stereoregular polymerizations, is receiving much attention a t this time. Free radical methods have not been able to compete with ionic polymerizations in the synthesis of such new stereoregular polymers-for example high cis-1,4-p0lybutadiene rubbers. That ionic polymerizations with particular catalysts and usually in heterogeneous systems can be capable of controlred propagation to give different isomeric polymers was shown first in cationic polymerization by the writer and coworkers, who used alkyl vinyl ethers (770); in anionic polymerization it was demonstrated by Natta and others with I-olefins (84), and by a number of workers in this country with diolefins (60, 715, 778). Characteristics of Ionic Polymerizations
In general, ionic polymerizations of vinyl compounds using acidic or basic catalysts exhibit a wider range and variety of phenomena than free radical polymerizations. Below is a list of characteristics, some of which require further substantiation by experimental work. Others do not occur universally. Types of Catalysts. The best known catalysts are strong anhydrous Lewis acids such as aluminum chloride and organometallic bases such as lithium butyl, but, as shown below, certain monomers bearing very strong electronwithdrawing or donating groups can respond even to weakly basic and weakly acidic catalysts, respectively. The ionic agents added are more correctly called catalysts than initiators, as they often exert their influence throughout the propagation reactions in contrast to the free radical initiators in vinyl polymerization. The fact that ionizing radiations have produced few definite examples of ionic polymerizations to give linear products of high molecular weight suggests that isolated ions, protons, or electrons do not readily act as catalysts, but that two ions of opposite charge are necessary. Complex ion catalysts growing in industrial interest, in many cases are particularly effective and give novel polymer structures. Monomer Structure. As shown in Table I, the polarity at the monomer double bond seems to be directly related tQ the best catalyst type and perhaps also to the base or acid strength most useful in forming high polymers. Monomers can be so grouped that the response of new monomers may be predicted. Inhibitors a n d Retarding Agents. Molecular oxygen and phenols which normally retard free radical polymerization have little influence on many ionic polymerizations. Agents that neutral-
108
ize the catalysts prevent polvmerization; many aromatic and heterocyclic compounds retard or terminate polymerization. It is especially characteristic that the presence as impurities of many olefinic and acetylenic compounds, and their dimers or other low polymers, may prevent formation of high polymers readily. Temperatures a n d Activation Energies. These are generally lower than in free radical polymerization. Diluents are used almost universally, to prevent rapid exothermic and uncontrolled reactions giving discolored products of low molecular weight. Only a few ionic processes at high temperatures have given linear high polymers-for example, the Phillips process. Rates of Polymerization. I n contrast to free radical polymerization, high polymers in some cases may form almost instantly, especially when acidic catalysts are used, as in the polymerization of isobutylene and certain vinyl ethers. In other cases both polymerization and propagation reactions may be carried out slowly, if monomer is pure and care is taken in design of the system. Rates increase with monomer and catalyst concentrations, but simple equations are not always followed. Copolymerizations. Monomer reactivity ratios depend to a greater extent upon polarity considerations than in free radical copolymerization. The numerical values are different in cationic copolymerization than in anionic copolymerization (or free radical copolymerization). I n many cases with a given catalyst only one of two monomers in a mixture homopolymerizes, leaving the second monomer substantially unreacted. Few cases of ionic copolymerizations give alternating copolymers. Stereospecific o r Stereoregulated Polymerizations, Beginning with the vinyl alkyl ethers, a number of cases of ionic polymerizations have given specially ordered or eutactic polymers, while free radical reactions have not been shown to give such oriented polymerizations at rates of industrial interest. This seems to be associated in part with control throughout the propagation reaction by the catalyst, or complex of catalyst with polymer, especially when the reaction occurs at surfaces or proliferously. I n the latter case polymerization occurs in the presence of preformed polymer and monomer must diffuse through a polymer medium of high viscosity before adding to growing chains of long life. Mechanisms. Few precise mechanisms for the formation of high polymers by ionic polymerization have been well established as yet. As ionic polymerizations can be carried out under many different conditions to give polymers of varied structure, the mechanisms are expected to differ at least in detail.
INDUSTRIAL AND ENGINEERING CHEMISTRY
The discussion of mechanisms is beyond the scope of this. For orientation, propagation reactions by which isotactic polymer molecules are formed in some cases may be represented as follows: CATIONIC POLYMERIZATIOX
CHz /’
CHZ
CHe
\ / \ / \
X-C
Y \
X-C
Y \
x-Cf
+ CHZ=CXY
Y \
ANIONICPOLYMERIZATION
A represents a complex anion associated with the positively ionized end of the growing polymer molecule, which not only facilitates addition of monomer molecules but also favors a particular steric arrangement-e.g., ddd or isotactic. M + represents a cation, usually a metal or metal complex. The complex ions may include solvation by solvent, by monomer, or by special activators, as well as polar links to surfaces. X and Y represent substituents-e,g., H and OR in isotactic cationic polymerization of vinyl ethers, and H and CHI in anionic polymerization of propylene.
Response to Different Catalyst Types
Price (99) discussed the effects of different substituents upon the electron density at the double bonds o€ vinyl compounds and pointed out that monomers responding best to cationic polymerization contain groups promoting the release of electrons to the double bond. The writer and coworkers ( 7 7 7 ) listed a number of monomers according to response to cationic, free radical, or both methods of polymerization and proposed a classification of monomers with regard to cationic, free radical, and anionic polymerization ( 705). A more complete summary of the best catalyst types in relation to monomer structure is given in Table 1. Only most recent references are included, as others are available in books (105, 706). In general, the response to different catalyst types corrrlates fairly well with the electron displacement by the substituents attached to the ethylene group, with orientation effects in benzene substitution, and with sigma values of Hammett. Among the monomers having greatest electron density at the double bond (most basic or nucleophilic) are isopropenyl alkyl ethers and vinylidene ethers, These compounds can be poly merized by relatively weak Lewis acids. For example, isopropenyl isopropyl ether can be polymerized readily by boron fluoride-methanol ( 7 14). Strong
IONIC P O L Y M E R I Z A T I O N Lewis acids such as aluminum chloride and boron fluoride are best for polymerization of isobutylene a t low temperatures (706), and for some other compounds of the type C H p C C H , . R These catalysts, as well as weaker ones such as boron fluoride etherates, are suitable for preparing high polymers from alkyl vinyl ethers, certain alphamethylstyrene derivatives, and betapinene. The growing list of 1,2-disubstituted monomers which can give polymers of fairly high molecular weight in appropriate cationic systems includes some vinyl ether-type compounds such as 1,2-dimethoxyethylene, cumarone, dihydrofuran, and 2,5-dihydropyran, as well as hydrocarbons such as indene. Not all have fivemembered rings, as was formerly thought necessary for such polymerizations, By free radical methods it has not been possible in general to obtain vinyltype homopolymers of high molecular weight except from monomers containing =CH2 or =CF2. The monomers which have responded only to free radical conditions to give homopolymers of high molecular weight can be best characterized as haloethylenes and vinyl esters of organic acids (Table I, column 11). Here the substituents are only moderately electronattracting. Halogen and acetate groups orient ortho and para in benzene. Cationic and anionic polymerization systems have not yet given linear polymers of high molecular weight from these monomers. Low molecular weight products have been obtained from chloroprene, for example (73). The chloroand fluoroethylene compounds have been most studied, but the bromine and iodine compounds present no exceptions known to the writer. When the halogen is not directly attached to the ethylene group, ionic polymerizations have more success, as in the cases of haloalkyl vinyl ethers and nuclear-halostyrenes. Monomers bearing one or two highly negative or electron-withdrawing groups (Table I) are not homopolymerized readily by free radical means nor by acidic. catalysts. Our knowledge has been increased in recent years by Goodrich workers, who studied vinylidene cyanide (45) and by Eastman men who studied cyanoacrylic esters (75, 67, 68). Some of the latter also may be polymerized by radical methods. The older work on nitroethylene polymerizations is now seen in a new light. A number of these acidic monomers can be polymerized by addition of any of thousands of inorganic and organic compounds which are even mildly basic, including water, sodium bicarbonate, ammonia, amines, amides, alcohols, and ethers. Even basic mon-
omers such as vinyl ethers act as catalysts for polymerization of vinylidene cyanide and cyanoacrylate esters. One hesitates to use the word “catalyst,” because any compound capable of supplying electrons causes polymerization of the group of monomers, whereas only acids such as phosphorus pentoxide and sulfur dioxide act as stabilizers against polymerization. The cyanosorbate esters also polymerize readily under the action of aqueous alkali. These monomers are acidic in the Lewis sense and can be homopolymerized best to high polymers under controlled conditions by adding bases (57, 52). Besides the vinyl-type compounds which respond even to aqueous alkalies to give high polymers, other unsaturated compounds polymerize best with anhydrous highly nucleophilic catalyst systems. Among these are the dihydronaphthalenes, which Scott polymerized by using sodium naphthalene and ether solvents (772, 776). Recently the polymerization of beta-nitrostyrene by sodium methylate has been restudied (73). These and similar monomers have given only polymers of low or moderate molecular weights. Polymerizations by free radical methods have been used commercially largely for the acrylic and methacrylic monomers, but there has been increasing progress toward commercial polymers of high molecular weight by use of anionic methods-for example, in liquid ammonia (4). Recently polymerization of methyl methacrylate using lithium amide or potassium amide in liquid ammonia has given polymers of high molecular weight and of favorable narrow molecular weight range for molding (46). Rohm & Haas found that methyl methacrylate polymerizes by catalysis with sodium methylate. Still another group of monomers gives high polymers by cationic and by free radical methods, but they are stabilized against polymerization by adding alkali with antioxidants. These include a number of vinyl-nitrogen ring compounds such as N-vinylcarbazole and N-vin ylpyrrolidone. Table I shows that free radical homopolymerization to give polymers of high molecular weight is favored by conjugation in the monomer, as well as by mod-. erate electron withdrawal from the ethylenic double bond. If the polarity a t the double bond is considerable, as in vinylidene cyanide or alkyl vinyl ethers, repulsive forces (50) in radical polymerization may oppose the addition of successive monomer units, but the formation of alternating copolymers with a comonomer of opposite polarity will be favored. A growing list of monomers can be homopolymerized by all three methods:
ethylene, and a number of compounds bearing conjugated double bonds, especially styrene, 1,3-butadiene, and isoprene. Methyl vinyl ketone belongs in this group (707), and apparently alpha-methylstyrene does also. Cationic homopolymerization has been most used with alpha-methylstyrene, but polymerization using sodium was developed by Jones (77, 78, 65, 66). The monomer gave polymers of moderate molecular weight by heating under high pressure (704, and solid polymers can be obtained by slow photopolymerization at room temperature (707). The newer ionic polymerizations of olefins and diolefins are discussed below. Publications on the free radical homopolymerization of propylene seem to be inconclusive at this time. I n reviewing the great number of monomers which have been polymerized it is noticed that homopolymers of high molecular weight have not been reported with certainty from several vinyl-type monomers such as isopropenyl chloride, isopropenyl acetate, phenyl vinyl ether, and methyl vinyl ether. I t is evident that the substituents of the first three make the ethylene group comparatively neutral electronically.
High-Density Polyethylenes The formation of linear polyethylene or polymethylene was observed before this century, when solutions of diazomethane in solvents such as diethyl ether were found to form a white flocculent polymer on storage (94, 706). Recent studies have shown this polymerization to be an ionic reaction especially catalyzed by boron compounds (74) and by finely divided metals such as copper, silver, and nickel (77). Ether seems essential for the copper-catalyzed polymerization of diazomethane, but in benzene solution polymethylenes of high molecular weight can be obtained by use of trimethyl borate, boron fluoride, or triphenyl borate (70, 707). Polymethylenes of particularly high molecular weight can be obtained by adding boron fluoride etherate to soluto tions of diazomethane a t -15’ 5’ C. (38, 90, 97). Because of the hazards of working with diazomethane, only small quantities of these polymethylenes have been prepared, and few chemical and physical properties of the polymers have been published. Melting points have been reported ranging from 128’ to 137’ C., the highest temperature being favored. Modifications of the Fischer-Tropsch synthesis using carbon monoxide and hydrogen have given waxlike hydrocarbons of high molecular weight. High melting polyethylenes of high density, have been obtained from carbon VOL. 50, NO. 1
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has been polymerized experimentally a t low temperatures by using gamma radiation from cobalt-60 for initiation ( 7 7 ) . Polyethylenes of relatively high density (0.93 to 0.94 gram per cc.) are being produced by Spencer Chemical Co., Imperial Chemical Industries (48), and others using free radical methods. Liquid polymers of low molecular weight have been obtained by many workers from ethylene, using FriedelCrafts or Lewis acid catalysts (cationic polymerization). Fischer prepared high molecular polyethylene in what seems to be a cationic system (40). The application of diborane (54)as a catalyst for preparation of ethylene polymers having remarkably high softening temperatures seems also to be a case of cationic polymerization. The solid, heterogeneous catalyst systems of Phillips Petroleum Co. also may be classified with cationic polymerization, although the mechanism is uncertain. In a patent application of 1945 a nickel oxide-silica-alumina catalyst activated by heating in air at 400' to 700' C. had been used to obtain a viscous liquid polymer mixture, from which a fraction of waxlike solid polyethylene was separated by cooling (2). The preparation of such nickel oxideToble I Summary of Responses of Different Types of Ethylenic Monomers to Homopolymerlzation aluminosilicate catalysts has been disunder Cationic, Free Radicol, and Anionic Conditions cussed (58). However, the chromic Cationic (Acidic. Electrooxide-silica-alumina catalysts developed n/ philic Catdpats) CHs=C by Phillips led to the first large pro\ duction of high density polyethylene in I I11 ~ _ _ _ America (Marlex 50) (74, 98). CH*=C(CH*r. CH.=CHBr CH.=C!CN:r CH?=CHC! CH?=CCN Although basic catalysts such as lithium CH! CH?=CCH, CH*=CCI! 1 (07, 6 8 ) 11 CF?=CCI: COOR butyl in hydrocarbon solution had been I COR R CH?=CCN CF?=CF: (113) used to catalyze polymerization of ethylCF?=CFCI CHJ CH?=CHOR S0.R ene (&), no unique properties had been CH.=CHF CH?=CF? CH?:=CCOOR CH: C H : = C ~ O R observed in the products. However, 1 CH?==CFCI (11% COR 1 SOrR Roedel, in a patent application of 1944. CHJ CH?=CCOOR reported that polymerization of ethylene I CI from benzene solution a t 150' C., and 0 0 CH=CH 1000 atm. for 10 hours in the presence CH!=CCH=CH: CH.=CHNO1 ' \ 1 CH? 'I OCHa OCHa of ethyl magnesium chloride (formed in CI CH.=CN09 , situ), gave white, fibrous polymers of CH:=CHOOCCHa CHa unusually high tensile strength and elonCH:=CHOOCR CH.=CNOz gation (703). Later the use of preI CH9=CHOOCCH.CI c1 formed Grignard reagents was proCH=CHCH=CCN posed (726). Polyethylene of 20,000 I , (Tf,22) CHz COOR molecular weight has been reported by use of Alfin catalyst (sodium isoproCH.=CHCOOR CHs=CHCONR* poxide and allyl sodium), but the polymer CHs-CH C H -CCOOR CHt=CCONRi , I, '- I was not claimed to have high crystalCHg C H CHJ CHJ linity ( 6 ) . '0' CH1=CHCN Ziegler and coworkers found that CH,=C CH9=CCN addition of transition metal halides I 'COOR CH, such as titanium tetrachloride to aluminum alkyls in hydrocarbon dispersion promoted the polymerization of ethylene even a t atmospheric or moderate pressures to give polymers having a wide range of molecular weights and CH9=CHa CHg=CHCH$ properties (720, 725, 728). In some ? cases polyethylenes of comparatively C H ? = C o CH.=CHC-CHa high density and high average molecular I' 0 CHa weight have been obtained. Wide CH2=C-CH=CH2 CHt=CHCH=CHz ranges of nucleophilic agents and of I CHI transition halides have been disclosed
monoxide and hydrogen upon special catalysts such as ruthenium oxide (20) and molybdenum oxide (22). In one example 1 mole of carbon monoxide and 2 moles of hydrogen reacted in tetrahydronaphthalene solution in the presence of a nickel molybdite catalyst a t 175' C. at 900 atm. After 14 hours a low conversion was obtained io orientable, highly crystal polyethylene melting a t 135' C. The polymer contained some carbonyl groups. Highdensity, crystalline polyethylenes which gave tough films or high tenacity fibers were also obtained by other modifications of Fischer-Tropsch synthesis (36. 49, 69). One patent describes the deposition of polyethylene in the FischerTropsch synthesis as a difficulty to be overcome (70). Commercial manufacture of polyethylenes from carbon monoxide and hydrogen is not regarded as promising a t this time. The processes for polymerization of ethylene to form conventional branched polyethylenes (density about 0.92 gram per cc.) by free radical initiation under high pressure, which have developed from the discovery of Fawcett, Gibson,
and Perrin (37), permit variation in polymer branching (77) depending upon such factors as conversion, temperature, and initiator concentration, More linear polyethylenes of higher density have been developed by using special conditions of free radical polymerization-e.g., lower temperatures and conversions. Ethylene can be polymerized at comparatively low temperatures by using azo catalysts (23, 63, 64). Polymerization of ethylene at lower temperatures with ultrahigh pressures gave substantially linear polyethylenes (79, 25). In one example ethylene in benzene solution was polymerized a t 45' C. and 7000 atm., using an azo catalyst. The product had less than one branch per 200 carbon atoms of the main chain. I n Du Pont laboratories liquid ethylene has been polymerized slowly below its critical temperature of 9.6' C., by use of free radical initiators to give high density polyethylenes (27, 26, 93). Polymerization of ethylene a t 7500 atm. and 70' C. upon a solid azo catalyst gave a solid mass of polyethylene of density 0.95 gram per cc., showing high crystallinity (56). Ethylene also
~
qi:
CH.=Fo
1 10
'
INDUSTRIAL AND ENGINEERING CHEMISTRY
~
IONIC P O L Y M E R I Z A T I O N in patents, many of which give little information about the polymerization reactions or the characteristics of the polymers formed (727, 722, 724, 727, 729). The polyethylene molecular weight can be controlled to some extent by adjusting the molar ratio of metal organic compound to that of the heavy metal halide (32, 723). Much research effort is being directed toward overcoming numerous difficulties with catalyst systems of the general Ziegler type, which discourage plant operation. These include catalyst hazards, poor reproducibility of the catalyst (black precipitates of uncertain composition obtained from the reaction of an aluminum alkyl or aluminum alkyl chloride with transition metal halides), unfavorable molecular weight range, undesirable cross linking, and catalyst residues causing corrosion of molding equipment. In addition to the usual acid-alcohol quench, alkaline treatments of the polymer are said to remove catalyst residues which cause corrosion of metal mold surfaces (33, 34). Addition of certain metal salt (37) and epoxy (35) stabilizers against polymer discoloration on heating is another example of technological development in Germany. The Koppers Co. has disclosed methods of continuous addition and removal of components using compounds of the type RR'AlX, with heavy metal halides as catalysts (59). Efforts are under way to prepare definite preformed complexes as complete catalysts for anionic olefin polymerization, which are soluble or a t least more compatible with the monomer and diluent, as many industrialists believe that polymerizations a t solid surfaces present formidable technological difficulties. Natta and coworkers have polymerized ethylene a t low pressures, using bicyclopentadienyl titanium compounds such as Ti(CsH6)&12 with Al(CzH5)3 and Ti(Cd&)g(C&&)2 with Al(C&)s. (C~H5)20 (89). The same workers have polymerized ethylene in heptane solution a t 40 atm. and 95' C., using a soluble crystallizable complex believed to be (C5H6)2TiCl2Al(C2H5)2 (88). Such a soluble blue complex containing titanium and aluminum has been found to be a highly active polymerization catalyst, if the ethylene contains traces of oxygen (7, 9). This and other evidence suggest that high catalytic activity in these ethylene polymerization catalysts requires the presence of a t least some quadrivalent titanium. In Germany the catalytic activity of the complex precipitates, derived from heavy metal halides reacting with organic compounds of alkali metals, alkaline earths, aluminum, or zinc, has been enhanced by oxygen treatment (29); a specified proportion of molecular oxygen
also may be maintained with ethylene in the reaction mixture, depending upon the particular organometallic catalyst (30). The complex catalysts obtained by reaction of transition metal halides with aluminum alkyls or other highly nucleophilic organometallic compounds are unusual in the wide range of different monomers which can be polymerized, and because under certain conditions stereospecific or stereoregulated polymerization of unsymmetrically substituted ethylenic monomers may give unusually regular chain configurations (eutactic polymers). The Standard Oil Co. of Indiana has patented a wide range of heterogeneous inorganic catalysts for polymerization of ethylene, especially molybdenum oxide compositions (57, 95, 96), nickel or cobalt on charcoal (28, 97), and these agents along with alkali metals, or hydrides of alkali or alkali earth metals (39). As these catalysts contain reducing agents or are activated by heating in a reducing atmosphere, anionic polymerizations may take place, but insufficient evidence has been published.
New Stereoregular Hydrocarbon Polymers
>
Whereas only amorphous, comparatively low polymers had been prepared from propylene (47), 'Natta and others (75,78,79,84), using heterogeneous catalysts, were able to prepare normally crystalline film-forming polypropylenes in which successive methyl groups are believed to have the same steric configuration-e.g., ddd-with three monomer units in a spiral or helical arrangement comprising the identity period of about 6.5 A. In one example the catalyst was prepared under an inert atmosphere by adding 0.04 mole of titanium trichloride to 0.10 mole of triethyl aluminum dispersed in 400 ml. of heptane (85). The vinyl polymers showing crystallinity by virtue of successive substituents on tertiary carbon atoms in the same steric position were called isotactic polymers. The isotactic polymerization of propylene has been studied in considerable detail by Natta and others (83). Crystallized purified titanium trichloride with the aluminum alkyl gave polypropylenes of more stereoregular structure than did the Ziegler catalyst employing titanium tetrachloride. The rate of polymerization was approximately a linear function of the concentration of titanium trichloride and of the partial pressure of propylene. The triethyl aluminum employed was discovered to contain about 7% of A10C2H6(C2H6)2. The over-all activation energy of this polymerization was 12 to 14 kcal. per
mole. Many other systems have been studied for preparation of isotactic polypropylenes-for example, titanium tetrachloride or vanadium tetrachloride in cyclohexane, to which was added lithium aluminum tetrahexyl, lithium aluminum tetradecyl, or phenylmagnesium bromide (24). Russian workers reported best preparation of isotactic polypropylene at 50' C. and 4 to 6 atm. using triethylaluminum with tita- * nium tetrachloride ( 7 77). As expected from Table I and the electron-donating properties of the methyl group, propylene polymerizes somewhat more slowly than ethylene by anionic methods. Isotactic polypropylenes are expected to be produced commercially in the near future. Molded Moplen polypropylene of Montecatini of specific gravity 0.90, as well as experimental polypropylenes examined by the writer, withstands temperatures u p to 150' C. Properties of pilot plant Italian polypropylene PR/56 have been discussed in detail ( 5 ) . Physical properties of isotactic polymers of other 1olefins have been observed; those from certain branched olefins such as isopropylethylene and isobutylethylene are especially notable for high softening temperatures (53, 86, 702). I n the isotactic polymers of isobutyl ethylene and isoamyl ethylene there are believed to be 3.5 monomer units per helix or 7 monomer units to an identity period of 2 helices. Natta, Pino, and Mazzanti copolymerized mixtures of 1-olefins a t 20' to 80' C. to obtain amorphous polymers (87). Among the advantages of isotactic polypropylenes in films and molded plastics compared to polyeihylenes are their transparency, heat resistance, freedom from environmental cracking, and satisfactory molding of even the highest molecular weight polymers. Crystalline and amorphous polypropylenes had nearly the same transition temperatures around -35' C. (52). Samples of American and German experimental isotactic polypropylenes were found in this laboratory to be completely soluble in boiling toluene and in boiling xylenes (707). O n cooling to room temperatures the solutions gave gels which underwent syneresis only very slowly. When stabilizede.g., with 0.1% Santowhite crystalsthe isotactic polypropylenes were stable to heating for 5 hours at 150' C. but on exposure for 3 weeks to ultraviolet light they developed pronounced infrared absorptions of carbonyl groups a t 5.8 microns and of double bonds at 6.1 microns, and became brittle. Crystallizable polystyrene having an isotactic structure was first prepared by Natta and others (76, 84, 92) using Ziegler-type catalyst, but in poor yields VOL. 50, NO. 1
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and in mixture with cross-linked polymer. Williams and others (779) were able to make completely soluble, crystallizable polystyrene employing Alfin-type catalysts developed by Morton. At Stevens Institute of Technology it has been found that alpha-methylstyrene polymers prepared using cationic and anionic as well as free radical polymerization have similar infrared absorption spectra, except for a region near 11 microns. For this reason, as well as the high softening points and considerations from molecular models. it is believed that these polymers may be largely dldl or syndiotactic in structure. This explanation of the high softening temperatures of alpha-methylstyrene polymers is different from that proposed recently (66). Natta's group has reported recently the preparation of high-softening isotactic polymers from alpha-vinylnaphthalene, o-methylstyrene, and 6-fluorostyrene. New Polymers from Dienes The syntheses of cis-1,4 polymers from isoprene and from butadiene are the most recent outstanding achievements in anionic polymerization and seem most likely to lead to revolutionary industrial developments. Relatively little has yet been published. Different ionic polymerization systems have been known for some years to permit greater variation in the structure of diene polymers than free radical initiation (778). Old references as well as recent work (72, 73, 98) indicate that resinous, brittle polymers often are obtained from dienes in cationic polymerizations under the action of acidic catalysts. These are believed to be rich in 1,2-diene structures, especially alternating, dldl, or syndiotactic (87) side vinyl groups along the chain molecules. Different alkali metal catalysts were known to give rubberlike polymers from butadiene and from isoprene containing different proportions of cis-I ,4-polymer segments; but until recently lithium was not known to be unique as a catalyst for obtaining the cis-1 ,4-isomer structure. The discoveries that lithium (775, 778) and lithium alkyls, as well as modified Ziegler-type catalysts (47, 60), can give isoprene polymers high in cis-I ,4 structure constitute a tremendous breakthrough in research. Lithium and lithium hydride have not given eutactic polymers readily from butadiene having high cis-1,4 content. Certain organoaluminum-titanium halide catalysts, as well as lithium alkyls, however, can be used with both isoprene and butadiene to give predominantly cis-I ,4- polymers having elastic recovery comparable to or in some cases better than natural rubber. Most of these anionic polymerizations of dienes are carried out in heterogeneous systems and the polymer forms in a proliferous solid or highly viscous phase. n-Butyl lithium in homo-
1 12
geneous hydrocarbon solution can polymerize isoprene at ordinary temperatures and pressures to give a high cis1,4-rubber (43). Characteristics of the polymerization of isoprene by lithium alkyls in hydrocarbons have been discussed by Gibbs and others (44). Both lithium dispersions and lithium alkyls have some disadvantages for industrial use. When n-butyllithium in solution in n-heptane reacts with isoprene, the polymer precipitate has about 90% cis-1,4- structure (67). When the reaction is carried out in ether solution, homogeneous polymerization occurs to a solution of isoprene polymer containing a mixed structure of 3,4 with 1,2 and 1,4 segments. In the anionic polymerizations using aluminum alkyls with titanium halides and related compounds, isomerism in the diene polymers has been controlled by variations in conditions. For example, adjustment of the relative proportions of the catalyst components as well as the temperature of polymerization can control the proportion of cis-1,4 to trans-l,4 configuration ( 7 ) . The polybutadiene rubbers very high in cis-1,4 structure are more rubbery than rubber in some respects, but they become brittle by crystallization at winter temperatures. However, the Phillips Petroleum Co. (700) has found 80% cis-polybutadiene rubbers to have excellent freeze resistance (Gehman freeze points as low as -100' (2.). There is considerable interest in copolymerizing dienes with readily available mono-olefins such as 1-butene for preparation of freezeresistant rubbers. A very stimulating objective of further research, which no doubt will also contribute to industrial processes, is determination of the relative importance in stereoregular or eutactic polymerizations of the specific catalyst, the specific monomer structure, the solvent, and the type of polymerization system with respect to phases. Until this time almost all such polymerizations of hydrocarbon monomers, as well as of the vinyl ethers discussed below, have occurred in heterogeneous systems, whether or not the locus of polymerization is at surfaces.
Isomeric Vinyl Ether Polymers Polymerization studies at Stevens Institute of Technology, as well as in a number of laboratories in Europe and America, have confirmed the preparations of crystallizable and permanently amorphous isomeric polymers from vinyl isobutyl ether and from methyl vinyl ether, first observed by the writer and coworkers ( 7 70, 7 7 7 ) . Both the crystalline and the substantially amorphous high polymers of isobutyl vinyl ether free of insoluble cross-linked fractions can be obtained in good yield by the proliferous and flash-type cationic poly-
INDUSTRIAL AND ENGINEERING CHEMISTRY
merizations, respectively. The structure of the isotactic polymers of isobutyl vinyl ether has been elucidated (72, 80) and compared with those of drawn isotactic polymers of I-olefins (80). The helical unit from three monomer molecules in ddd or 111 conformation was proposed by Natta and by Bunn for drawn isotactic isobutyl vinyl ether polymers. A typical high polymer of vinyl isobutyl ether prepared in this laboratory by the slow proliferous process with boron fluoride etherate catalyst at -78' C. in the presence of liquid propane and solid carbon dioxide was found to have about 30% crystallinity from x-ray patterns. This was observed directly from the polymer granules as formed (after heating at 50' C. in vacuum to removal volatiles). The degree of crystallinity of the polymer films containing isotactic segments increases with heat treatment at 50' C. The best amorphous high polymers show only slight crystallinity in spite of heat treatments and stretching. Evidence ivas presented that the slow proliferous cationic polymerization by which crystallizable polymers of isobutyl vinyl ether can be prepared may not be a surface reaction (707). Although the systems are heterogeneous, with liquid propane-monomer and catalyst-monomer-polymer phases, the polymerization may occur homogeneously within the growing. polymer phase where the preformed polymer maintains high viscosity and the monomer can penetrate by diffusion. Different vinyl ether monomers give characteristic high polymer growths-e.g.: isobutyl vinyl ether gives nodules attached by filaments. Further evidence of the relatively long life of the growing macroions in the proliferous cationic polymerizations has been obtained in addition to that reported with n-butyl vinyl ether. In the case of isobutyl vinyl ether, again about 10 to 15 minutes is required for the polymer viscosity of withdrawn samples to reach a maximum value (osp/C of about 7). Isopropyl vinyl ether and isobutyl vinyl ether are now found to be very different in their behavior in stereoregular polymerizations. Under similar conditions of proliferous polymerization, using carbon dioxide-propane diluent at -78' C. and boron fluoride-diethyl ether complex as catalyst, the isopropyl vinyl ether polymerizes about three times as fast as isobutyl vinyl ether and normally gives polymers of considerably higher viscosity as determined in toluene solution-e.%.,
*c
= 10.
These
isopropyl vinyl ether polymers do not crystallize at room temperature nor on heating at 50' C. as in the case of isotactic polymers of isobutyl vinyl ether. In an effort to prepare normally crys-
IONIC POLYMERIZATION talline isotactic polymers from isopropyl vinyl ether, different catalysts were tested and larger proportions of diluent were used to reduce the rate of reaction (Table 11). None of the polymers obtained from these typical proliferous polymerizations of isopropyl vinyl ether showed spontaneous crystallinity, but several showed x-ray fiber patterns on stretching, so that they are believed to contain stereoregular chain segments. The quenching of the polymerizations was carried out as described previously (770). Isotactic isopropyl vinyl ether polymer chains may crystallize less readily, because the space around the helical chain axis is less filled as shown in models than in the case of isotactic polymers of isobutyl vinyl ethers. However, the degree of isotaxy may be lower in the isopropyl vinyl ether polymers because of less favorable monomer configuration. Rotational isomerism has been suggested in alkyl vinyl ethers and the infrared absorptions of the vinyl groups have been found to be strikingly different in the two monomers (708). I n isopropyl vinyl ether the 6.1-micron component of the doublet is much stronger than the 6.2-micron absorption; the reverse is true in isobutyl vinyl ether. The high polymers of n-butyl vinyl ether prepared by similar conditions of proliferous cationic polymerization a t -78' C., like those from isopropyl vinyl ether, are tacky and They do not but give x-ray fiber Patterns while held a t high elongations as in the case of polyisobutylenes. In the further development of this field it must be remembered that not all stereoregular polymers may be normally crystalline as in the cases of isotactic isobutyl vinyl ethers and isotactic polypropylenes. The relative crystallinity of vinyl ether polymer films can be compared by the
Shore Type A Durometer or by cold drawing tests. Molded films of the isotactic isobutyl vinyl ether polymers, cut into strips such as 1 x 1/4 inch, can be cold drawn by hand. They show a sharp shoulder or line of demarcation between the drawn and undrawn zones and a definite limit to the drawing. The appearance of the drawn or "neckeddown" strips of isotactic vinyl isobutyl ether polymers is similar to that of drawn strips of isotactic polypropylenes, but the latter often show more opacity in the oriented zone. By these and infrared methods it has been proved that isotactic polymers of isobutyl vinyl ether can be prepared by using boron fluoride solution in methylene chloride instead'of a boron fluoride etherate, if the reaction is carried out in a proliferous manner in a diluent such as liquid propane below - 70' C. The stereoregular isotactic polymers of isobutyl vinyl ether show solubility properties typical of linear crystalline polymers. The isotactic polymers of isobutyl vinyl ether are completely soluble in isopropyl alcohol, tert-butyl alcohol, and dioxane a t their boiling temperatures, On cooling to room temperature the crystalline polymer precipitates directly out of these solvents without forming gels. Infrared studies (708) to be reported in more detail have shown the following: Films of isotactic and amorphous polymers of isobutyl vinyl ether give somewhat similar absorption maxima, but a n absorption a t 13.3 microns is associated with a high degree of crystallinity. The vinyl ether polymers, as quenched using m&hanol and aqueous ammonia at 500 show no and dried in terminal double bond% but after films have been heated for a week a t 50' C. in air, the normally crystalline polymerS especially begin to show the 5.8micron absorption of carbonyl groups
c*,
Table II.
Proliferous Polymerizations of Isopropyl Vinyl Ether (Monomer, liquid propane, 1 to 8 by weight; -78O.C.; 1 hour) Conversion % per 10-4 Catalyst % mol. cat. %P/C'" 2 Hz0 .BF3 0.05 0.02 CHzCHz
1
...
\
,OsBFa
CHzCHp (ClCHzCHz)20.BF3 (CBHhOH)z.BFa ( C H ~ C H Z C H Z C H BF3 ~)~~. above undistilled* [(CHa)zCHCH&HzI 20.BF3 AlCls etherate (CHsCHz)zO .BFa above undistilledb
0.24
0.10
0.19 3.6 14.8 le.? 16.0 25.6 32.0 38.1
0.12 3.9 14.8 18.7 18.4
...
...
1.0 1.8 11.2 3.1 6.1 1.0
24.6
7.1
20,6
io. 1
Solutions of 0.20 gram of polymer per 100 ml. of toluene at 25' C. using modified Ostwald capillary viscometer. UndistiUed catalysts contained some free BF3 and colored impurities.
and the 6.2-micron absorption of vinyl groups. In carbon tetrachloride solution isotactic and amorphous polymers Of ether give tion spectra.
Acknowledgment The author wishes to assistance Of *' wo 'lare
'Otter, J*
and J * E*
the
A* J' Buselli, Ranby' Herman Wexler, in preparing this review'
'
literature Cited (l)
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