EMULSION POLYWERIZATION1 W. B. REYNOLDS Phillips Petroleum Company, Bartlesville, Oklahoma
Tms paper attempts to summarize in a very ~eneral form solubiizin~ .. micelles. Consequently, - . the general ..
\my the prrtinent feature? (rf emuIsi(m p o ~ j ~ e r i m t i o npivtl~reprevailing i n most eruu~sioipolymerizations is reactions. Unfortunately, there has bem ronsidcmble that given in Iligl~re1. Different rmulviun polymerizncontroversy concerning certain theoretical aspects of tion systems vary chiefly in the nature of the chain emulsion polymerizations, most notably, perhaps, those initiating and chain terminating factors. The oil and involving a consideration of the loci in which polymeri- water solubilities of initiator and terminator ingredients zation occurs. Such conflicts in theory have arisen are important in determining polymerization loci and largely because of unwarranted generalizations to all the molecular weight of the polymer. emulsion polymerizations of data obtained from a study of a particular system. The author claims little CHAIN INITIATION originality in the ideas presented. He has merely sumIt is now well established thai emulsion polymerizamarized current viewsof emulsion polymerization theory tions proceed by a free radical mechanism ( 2 ) . Initiaand interjected a certain amount of "editorial" com- tor systems are those which generate free radicals at a ment. controlled rate. The radicals thus generated initiate About the only thing common to all emulsion poly- polymer chains primarily through addition to a monomerization systems is a multiplicity of phases. Usually mer molecule (R . M -+RM .). However, a secondthe coutiuuous phase is aqueous and usually a water- ary method of initiation is believed to involve dehydrosoluble emulsifying agent is present. On the other ena at ion of a monomer molecule by a radical (R f M hand, methods for chain initiation and chain termiua- ~ R HMI.). tion vary widely in different systems, and each particu"Catalysts" for mass free radical polymerizations are lar system must be examined in great detail before con- oil soluble peroxides, diazoamino compounds, diazo clusions can be drawn relative to reaction mechanisms thio ethers and the like. Such materials are also used and loci. Considerations of paramount importance in emulsion polymerizations. However, emulsion sysare: (a) the nature of the emulsifying agent used, i. e., tems in addition permit the use of water soluble "catawhether or not solubilizing micelles are present; (b) lysts" and combinations of water soluble and oil soluble the nature of the system used for generating initiating initiator ingredients. free radicals; and ( c ) the nature of the chain-transfer A few typical reactions by means of which free radiagent if such is employed. Depending upon factors ( a ) cals are generated in emulsion polymerizations are sumand (b), the predominant locus of the polymerization marized in the following outline: may be in the micelles (early stage of GR-S polymeriza1. Thermal Decompositions tion), in true water solution (hydrogen peroxide ferrous n n (1 ion catalyzed polymerization of acrylonitrile), or in the li monomer phase (benzoyl peroxide catalyzed polymeri(a) R-A-O-O-C-R-2R zation of methyl methacrylate in the absence of solu(b) ROOH RO. + OH. R. + R ' S Nz bilizing micelles). ( c ) R-N=N-S-R' The loci of most emulsion polymerizations are not as .obvious as those cited. Usually several loci are of importance. In addition to the micelles, the true water solution, and the oil phase mentioned above, the polymer particles themselves containing dissolved monomer I1 R-C-O+ Fe+*+ become important loci for polymerizations initiating in ( b ) H202+ FeC'- OH. + OHFef++ micelles or aqueous solution. Although the oil water (c) R-N=N-SR' + Fe++-R. RS-+ FeC++ Na interface may be a locus for generating initiating free radicals, as Harkins (1) has pointed out, it can scarcely 3. Sin~le Electron Oxidation-Reduction be considered a locus for chain growth. (a) K.S20s + RSH RS. + SO4-. KHSOI K f OH. + SO4= SO4-. + OHMost emulsifiers used in emulsion polymerization re( b ) KzS,Oa + KAO8 K&04 + SO,-. 4- S20r-. 2K+ actions (e. g., fatty acid soaps, fatty alcohol sulfates, SO,- + OH. SO1-. + OHaliphatic sulfona,tes, and alkylated aromatic sulfonates) 2BOa-. S20a-
+
.
+
-
-
- ---
Presented before the Division of Chemical Education a t the 112th Meeting of the American Chemiral Society, New York City, September, 1947.
+
+ + +
+
+ +
Most of the above reactions have been studied sufficiently t o indicate that the courses shown are a t least 135
JOURNAL OF CHEMICAL EDUCATION
136
07
'? P
0
MONOMER OIL SOLUBLE INGREDIENTS.
-- 5z4
WATER PLUS WATER SOLUBLE, INGREDIENTS
--0-
droplets for emulsions of varying average particle diameters. Harkins suggests that the area of the solubilized oil layer in the micelles ( 2 ) is given by the following equation :
g, +
Sitvation Prevailing in EmuMon Polymerirations Whsn, the Emulsifier Forms Solubilizing M i d l a ('1 (after HsrkIm1)
Figure 1.
probable (3). Certain of the reactions occur in a singlr phase while others involve ingredients substantially in differentphases. Thus, the locus of free radical generation may be (a) the oil phase, (b) the true aqueous phase, (c) the oil-water interface, or ( d ) the micelles. When the free radicals are generated by thermal decomposition of an oil soluble component such as benzoyl peroxide or a diazo thio ether, the locus may be either the oiI phase or the micelles since such oil soluble components are distributed between the oil phase and the micelles. Free radicals may be generated in true water solution either through decomposition of a water soluble ingredient or single electron oxidation-reduction involving two water soluble ingredients. When such an oxidation-reduction involves an oil soluble and a water soluble component as in the mercaptan persulfate system, the locus may be the oil-water interface or the micelles. For reasons set forth below, the more important locus is believed to be the micelles. In the absence of solubilizing micelles, the reaction between dodecyl mercaptan and potassium persulfate is very slow. In the presence of rnicelles the reaction is rapid. This demonstrates the powerful influence of micelles on the important initiating reaction involved in the production of GR-S synthetic rubber. However, it does not prove that the micelles are actually the reaction locus since they may merely influence favorably an oil-water ihterfacial reaction. In this connection, it is revealing to consider the relative areas of the oil-water interface and the total area of the monomer solubilized in the micelles. Considerations offered by Harkins (1) are useful in making these calculations. Consider, for example, a typical emulsion polymerization system comprising 100 parts monomer of specific gravity 0.7 and 180 parts water which is 0.1 N in potassium oleate. The low critical concentration of potassium oleate makes it possible to neglect the amount of soap in isotropic solution. If the assumptions are made that the oil droplets are covered by a monolayer of adsorbed soap molecules, each molecule occupying 24 sq. A of the interfacial area, and the remainder of the soap is in the form of micelles, it becomes a simple matter to calculate the relative areas of solubilized oil and oil
Z =
28 X N / 2 sq. A.
where 28 represents the area produced by each double soap molecule and N is the total number of soap molecules in the micelles. Since the solubilized oil layer in the micelles has two sides, the total available area of the solubilized oil is 28 N. Consequently, the ratio of the monomer interfacial area in the micelles to the monomer interfacial area in the droplets is given by z = R = .(area, single droplet,) (number droplets) Nd = 28 N (=dZ)(140X l O ' V / ( d / 6 ) 3 X 10"
-
where d is the average particle diameter expressid in Angstrom units. N is equal to the total number of soap molecules charged (0.018 X 6.06 X lot3)less the number required to form a monolayer on the oil droplets (3.5 X 10a5)/d. Thus, N = (10.9 X lo2' d-3.5 X lOZ5)/dwhere d is still expressed in Angstrom units. There is then obtained for the ratio of the areas:
Expressing d in microns gives R = 3.64d-1.17. Calculating the value of the ratio for varying droplet average diameters gives the following results: Average droplet diameter Rntio of area of a'l in micelles (microns) to area of oil droplets
n
A stable emulsion of a hydrocarbon in 0.1 N potassium oleate solution has an average particle diameter of one to two microns. The presence of electrolyte decreases the stability of this type of emulsion witha consequent increase in particle diameters. The polymeri-' zation system described above does not form a stable emulsion even when the Agitation is fairly violent. Layering of the oil occurs within a few minutes after agitation is stopped. The average particle diameter is thus probably larger than two microns. In any event, the available area of oil in the micelles is certainly several fold greater than the area a t the droplet interface and, in all probability, it is many fold greater. In a polymerization of this type, the concentration of mercaptau in the solubilized monomer in the micelles is roughly the same as its concentration in the oil droplets. Thus, it is evident that the great majority of free radicals in this system is generated in or on the micelles.
MARCH, 1949
Thus far, attention has been given to free radical generation. If the free radicals are hydrophobic (e. g., dodecyl mercapto), chain initiation occurs either in the oil phase.or in the micelles, the relative importance of the two loci depending largely upon the locus of generation of the radicals. However, it should be borne in mind that the initially generated free radical may not he the actual chain intiator. For example, a hydrophobic radical (R.) might react with water a t an interface to produce the hydrophilic hydroxyl radical. When hydrophilic radicals are produced in the presence of water soluble monomers, chain initiation occurs predominantly in true water solution. In the absence of water soluble monomers, hydrophilic radicals may initiate chains a t the droplet or micelle interfaces. However, the great "activating" effect of water soluble monomers in systems in which hydrophilic radicals are generated indicates the comparative inefficiency of such interfacial reactions. CHAIN GROWTH
Chain growth occurs substantially in the locus of chain initiation. As pointed out above, this may involve the oil phase alone as in the henzoyl peroxide catalyzed polymerization of styrene in the absence of solubilizing micelles; it may involve the true water solution as in the hydrogen peroxideferrous ion polymerization of acrylonitrile; it may involve the micelles as in the early stages of the G R S polymerization; or it may involve the polymer-monomer particles as in the later stages of the GR-S polymerization, and also in the later stages of the above-mentioned acrylonitrile polymerization. It is obvious that the chain growth locus depends upon many factors, and conclusions should be drawn concerning loci only after careful analysis of all important factors. Certain criteria are particularly useful in det,erminingchain growth locus. One of the most forceful methods for differentiating between polymerization in the oil phase and in the aqueousphase is that of visual obsemation under a microscope. This technique was introduced by Vinograd, Fong, and Sawyer (4). When a droplet of oil is placed in a small amount of soap solution and viewed under a microscope, the diameter of the droplet can be observed to decrease linearly with time as the oil diffuses into the soap micelles. When a monomer containing a small amount of mercaptan is used as the oil droplet and the soap solution contains dissolved persulfate, turbidity appears in the aqueous phase as polymer forms and themonomer droplet eventually disappears (Harkins (1)). This is powerful visual evidence against the oil phase as polymerization locus in a mercaptau-persulfate system in the presence of micelles. On the other hand, however, the same technique proves the oil phase to be the polymerization locus for the bensoy1 peroxide catalyzed polymerization of styrene in the absence of micelles. In the latter example, the monomer droplet is converted into a polymer particle. Evidence in favor of the aqueous phase (micelles) as the important locus for merciptan-persulfate initiated
systems is obtained by examination of the oil phase at conversions up to 50 per cent. When agitation is stopped the unreacted monomer (except for that dissolved in the polymer particles in the aqueous phase) forms a layer a t the top of the reaction mixture. This layering occurs within a few minutes and the monomer separating contains no dissolved polymer. If polymerization were occurring in the oil phase, such separation of pure monomer could not occur.' A third line of evidence bearing upon chain growth locus is the particle size of the latex produced. When an oil phase polymerization occurs in the absence of micelles, large polymer particles approaching the average diameter of the original oil droplets are obtained. On the contrary, the latex average particle diameters from mercaptan persulfate initiated systems in the presence of micelles are quite small (40&1200 A.). The x-ray data of Hughes, Sawyer, and Vinograd (7) and of Harkins, Mattoon, and Corrin (8) prove quite conclusively that micellcs can serve as chain growth loci. However, this evidence is less diagnostic than that cited above in proving the most important loci under actual emulsion polymerization conditions. An additional important experimental approach to establishing chain growth loci is the effect of varying types of chain transfer agents on the molecular weight of the resulting polymer. For example, if equivalent amounts of normal dodecyl mercaptan and normal hexadecyl mercaptan are compared as modifiers in a G R S recipe, the polymer resulting from the dodecyl mercaptan recipe is of very much lower molecular weight than that from the hexadecyl mercaptan recipe. If polymerization were occurring in the monomer phase, the molecular weights should be equal since the two mercaptans have been demonstrated to have equivalent inherent chain transfer efficacies (9). The answer lies in the extremely slow rate of diffusion of the C16 mercaptau from the monomer phase to the chain growth locus in the micelles or in the polymer-monomer particles (9). As mentioned previously, it is scarcely pertinent to consider the oil-water interface as the chain growth locus. Finally, it might be mentioned that copolymer composition as a function of monomer charge ratio has been used as a criterion for chain growth locus. If one monomer is more water soluble than another, the monomer ratios in the oil phase and in true aquesus solution would be quite different. Fordyce and Chapin (10) carried out emulsion polymerizations of varying mixtures of styrene and acrylonitrile and found the compositions of the copolymers produced in the early stages of conversion to be simila~to those of comparable mass polymerizations. The same results were obtained when the catalyst was either benzoyl peroxide or potassium 1 The initial suggestion that chain growth of certain emulsion polymerizations miy occur in the aqueous phase was due to Fikentscher (6). Subsequently, many invegtigators have adduced evidence for aqueous phase polymerizations. Special mention should bemade of the work of Fryling and Harrington ( 8 ) m d of Harkins (1).
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JOURNAL OF CHEMICAL EDUCATION
persulfate. The conclusion is made that emulsion polymerization occurs "within the oil droplet or a t its interface" when either an oil soluble or a water soluble catalyst is used. It is true that this work indicates that the polymerization did not occur in tme aqueous solution. However, this approach does not distinguish between oil phase polymerization and polymerization in the micelles (solubilizing micelles were present in the Fordyce and Chapin experiments). Since distribution equilibrium of monomers in the micelles is established rapidly, the monomer ratios in the micelles would be very close to their ratios in the oil phase. In a later publication (11) Fordyce recognized that the previous experiments did not differentiate between the oil droplets and the micelles as loci and redefined the term "oil phase" to include monomer dissoived in the micelles. This extension of the well-known connotation of the term "oil phase" in emulsion polymerizations is scarcely warranted. The Fordyce and Chapin polymerizations may well have occurred in the oil phase (monomer droplets), hut it remains to be established by other criteria. Also, similar experiments might be carried out with diene monomers since the vinyl monomers used are inherently more susceptible to oil phase polymerization.
the redox system and with each other.= Disproportionation is possibly also a chain terminating factor although experimental evidence hearing- upon - this auestion in inadequate. The locus for chain termination must, of course, he the locus of chain growth. In hulk polymerizations, modification is controlled simply by the concentration of the chain transfer agent. The same consideration applies to emulsion polymerizations occurring in the oil phase. However, modification in polymerizations such as GR-S in which the reaction occurs primarily in the micelles and the polymer-monomer particles is a considerably more complicated process and may depend upon the rate with which the chaintransferagent diffuses from the oil phase to the chain growth loci (18). Modification of GR-S is achieved satisfactorily through the use of dodecyl mercaptans or mercaptan mixtures averaging about Clz. Mercaptans of higher molecular weight may also be used under conditions encouraging more rapid diffusion from the oil to the aqueous phase, i. e., vigorous agitation, higher pH, and/ or higher mercaptan concentration. Low molecular weight mercaptans tend to produce over-modification because of too high concentrations in the chain growth loci. However, they can be used with certain very rapid recipes where the polymerization "keeps ahead" of the mercaptan, or in polymerizations carried CHAIN TERMINATION out a t low temperatures where the mercaptan diffusion Chains may he terminated by chain transfer, radical to the polymerization loci is slower. comhination, or disproportion according to the follow- LITERATURE CITED in@reactions:
-
- - +
(1) Chain Transfer M,.
+ RH
(2j W i c a l combination ZM.. or M,.
+ R.
M,H M."
+ R.
M,R
(3) Disproportionation HM,.
M,
R.
When free radicals are generated relatively slowly and an efficient chain transfer agent such as an aliphatic mercaptan is present, practically all chains are terminated by transfer. These conditions are met in GR-S polymerization. Snyder and coworkers (12) and Wall and coworkers (15) have demonstrated that there is approximately one atom of sulfur per molecule in mercaptan modified emulsion polymers produced in mercaptan-persulfate initiated systems. The mercaptan is consumed primarily by chain transfer since very little persulfate is consumed during the polymerization. The successful demonstration of the chain transfer mechanism in the GR-S system has led to a rather widespread belief that chain transfer is the sole mechanism by which modification in emulsion polymerizations is achieved. However, low molecular weight polymers are obtained with very low concentrations of chain transfer agents when free radicals are generated very rapidly in the polymerization mixture. The ratio of sulfur atoms to the number of molecules is much less than one in mercaptan modified "redox" recipes. It seems quite certain that in such recipes a large proportion of the chains are terminated by comhination of the growing chain radicals with free radicals produced in
(1) HARKINS, W. D., J . Am. Chem. Soc., 69, 1428 (1947). (2) STAUDINGER, H., Ber., 53, 1081 (1920); TAYLOR, H. S., AND W. H. JONES, J. Am. Chem. Soc., 52, 1111 (1930); CAALMERS, W., ibid., 56, 912 (1934); ST~JDINGER, H., AND W. FROST., Ber., 68,2351 (1935); STAUDINGER, H., Trans. Faraday Soc., 32,97 (1936); EIXIRY, P. J., J. Am. . V.. d m E. HUSEChem. Soc.. 59.241 (1937) : S c a u ~ zG. Z. ihydik hem,, ~ 3 9 246 , (1938) { IRANY,E. P., MANN, J . Am. Chem. Soc., 62,2690 (1940); P a r c ~C. , C., R. W. J. Am. Chem. Soe., 64,1103 (1942). KELL,AND E. KREBS, Cf.: (a) HEY.D. H., AND W. A. WATERS, Chem. Rev., 21, 169 (1937); (b) K H ~ A S C M. H . S., S. S. KANE,AND H. C. BROWN, J. Am. Chem. Soc., 63. 526 (1941); (c) BAXENDALE, J. H., M. G. EVANS,AND G . S. PARK,Trans. parad. SOC.,42, 155 (1946); (d) REYNOL~S, W. B., AND
E. W. C O T ~ E N Paper , hefore High Polymer Section, A.A.A.S., Gibson Island, Maryland, July 4. 1946; (e) KOLTAOFF, I. M., A N D W. J. DALE,J . Pol. SCi., 3. 400 Wedox" recipes involve an initiating system comprising an oxidizing agent, a complex metallio catalyst, and usually a reducing agent. In such systems, free radicals are generated extremely rapidly through reaction of the oxidizing agent with the reduced form of the metal catalyst. It is not entirely clear as to the originators of redox polymerizations since several groups were working simultaneously and independent,lyin this field. It would appear that the earliest American investigators of this type of system wore Stewart (14) and Fryling (16). I. G. Farhenindustrie chemists in Germany had also developed redox polymerization systems, same of which were substantially faster than those described by Stewart and Fryling (16). Simultaneou~ly various British investigators, notably R. G . R. Bacon and L. A. Morgan were studying polymerisat,ions which they termed "reduetionactivation" (17). (Continued on page 146)
JOURNAL O F CHEMICAL EDUCATION EMULSlON POLYMERIZATION (Continued from page 198)
(4) (5) .. (6)
.
(7). (8) (9) 110) . . (11)
(1948); (f)BACON, R. G. R., Trans.Farad. Soe., 42, 140 L. R., Trans. Fnrod. Soe., 42, 169 (1946); (g) MORGAN, (1946). VINOGRAD, J. R.,L. L. FONG,G. S. RONAY,AN11 W. M. SAWYER.Papcr presented before the American Chemical Society, NOWY01.k~Sept~mber13, 1944. Chem.. 51.433 (1038). FIKENTSCHER. . R.. , Anoeur. " , . FRYLING, C. F.. A N D E. W. HARRINGTON, Ind. E f q . Chem., 36,114 (1944). AND J. R. VINOGRAD. J. HUGHES.E. W.. W. M. SAWYER. Chem.'~hys.,i3, 131 (1945). ' IIARKINS.W. D., R. W. MATTOON,A N D hI. L. CORRIN, J . Am. Chem. Soc., 68, 220 (1946); J. Colloid So:.,1, 105 (1946). REYNOLDS, W. B., AND P. J. CANTERINO.l'ap~rprtxenbed at symposium on "Mechanism of Polym~rizntion"at the Polytechnic Institute oi Brooklyn, Apl.i! 10, 1047. FORDYCE. R . G.. AND E. C. CHAPIN.J. Am. Chem'Soc. 69,581'(1947)1 eJ. FORDYCE, R. G., AND G. 5:. HAM,ibid.; 69,695 (1947). R. G.,ibid., 69,1903 (1947). FORDYCE,
. .
SNYDER, H. R., J. M. STEWART; R. E. ALLEN,AND R. J. DEARBORN, ibid., 68,1422 (1946). WALL,F. T., J. W. BANES,AND G. D. SANDS,ibid., 68, 1429 (1946). W. D., U. S. Patent 2,380,473 (to B. F. Goodrich STEWART, Co.). Application date, Feb. 19. 1941. FRYLINO, C. F.,U. S. Patent 2,379,431 (to B. F. Goodrich Co.). Application date, June 27, 1941. PBL 4670 I. G. Farbenindustrie A. G. (Wissensbaftliche Kommission fiir Ksutsehuk-February 11, 1943 by W. Kern). Cj,, Bibliography of Scientific and Industrial Reports, 5 (No. I), 31 (1947). CJ., also, HOEENSTEIN, TV. P., AND H. MARK,J. Pol. Sci., 1,549 (1946). British Patent 573,369 (to Imperial Chemical Industries). Application November 4, 1941. Cj., also, reference 3f and 3g. REYN~D W.~ B., , AND P. J. CANTERINO.Paper before High Polymer Section, A.A.A.S., Gibson Island, Maryland, July 4, 1946; SMITH,W. V., J . Am. Chem. Sac., 68, 2059, 2064, 2060 (1946); FRANK, R. L., P. V. SMITE, F. E. WOODWARD, W.B. REYNOLDS, AND P. J. CANTERINO, J.Pol.Sci.,3,39 (1948).
