Polymer Latexes - American Chemical Society

Chapter 26

Polymer Colloids as Catalyst Supports 1

3

Downloaded by UNIV MASSACHUSETTS AMHERST on August 6, 2012 | http://pubs.acs.org Publication Date: June 3, 1992 | doi: 10.1021/bk-1992-0492.ch026

2

Warren T. Ford, Rickey D. Badley, Rama S. Chandran, S. Hari Babu, M. Hassanein, Sanjay Srinivasan, Hayrettin Turk , Hui Yu, and Weiming Zhu 4

5

Department of Chemistry, Oklahoma State University, Stillwater, OK 74078

We have designed and used functional colloidal particles as catalyst supports. The colloidal catalysts are prepared by emulsion copolymerization as cationic or anionic latexes. Subsequent reactions of the latexes can produce particles-with up to 80 mol percent of charged repeat units. Anionic transition metal complex catalysts and anionic reagents bind to cationic particles, which serve as sites of locally high concentration of reactant and catalyst in aqueous dispersions. Cationic catalysts bind similarly to anionic particles. Surface modification of colloidal silica with silane coupling agents also can produce colloidal catalysts. The colloidal supports increase the activity of most catalysts tested for oxidations and hydrolyses of organic compounds. Heterogeneous catalysis relies on high surface area supports to expose the active catalysts toreactantsin the surrounding fluid. In the high activity limit the rates of heterogeneous reactions depend only on the rates of mass transfer of reactants to and products away from the active sites on the surface (1). High surface area is normally achieved by use of macroscopic particles of porous materials such as silica, alumina, and active carbon as supports. High porosity may lead to catalytic reactions limited by rates pore diffusion. The active catalysts are often colloidal particles or clusters of metals, oxides, or sulfides that are not inherently very active, but capable of fast chemical reactions at high temperature whenfinelydivided and adsorbed on a high surface area support Under high temperature fast reaction conditions, mass transport and pore diffusion are often the rate-limiting steps. The oxide supports and the catalysts themselves usually are chemically stable for use at temperatures as high as synthetic transformations of organic compounds can be performed. Polymers too can be obtained in porous, high surface area forms, but most polymers lack the high temperature stability of oxide materials. Over the last two decades polymer supports have been investigated for catalysis of a variety of chemical reactions, mostly in 20-100 °C temperature range in organic solvents (2-4). The ^ r r e n t address: Phillips Petroleum Company, Bartlesville, OK 74004 Current address: National Starch and Chemical Company, Bridgewater, NJ 08807 Current address: Tanta University, Tanta, Egypt Current address: Ethyl Corporation, St Louis, MO 63104 Current address: Anadolu University, Eskisehir, Turkey

2 3

4 5

0097-6156/92A)492-0422$06.00A) © 1992 American Chemical Society In Polymer Latexes; Daniels, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.


26. FORD ET AL.

Polymer Colloids as Catalyst Supports

423

near-ambient use temperatures allow less stable but much more active homogeneous molecular organic and organometallic catalysts to be employed. The solvent adds further important considerations to the design and understanding of catalytic processes. Catalysts that otherwise would be soluble must be fixed to the support by ionic or covalent bonds. Since solvent may dissolve polymers, most polymer supports are cross-linked but still swellable by solvent and used in gel form. Swelling means that active sites can be located in the gel phase as well as on the surface, and that reactant intraparticle diffusion may be still another physical process limiting the rates of chemical reactions. Supports for conventional heterogeneous catalysts are macroscopic particles used in batch, packed bed, or fluidized bed reactors. TTie catalytic species itself may be colloidal particles of a metal or oxide bound to the macroscopic support. Molecular, homogeneous catalysts bind to colloidal as well as to macroscopic polymer particles. Colloidal particles provide high surface area and can be prepared in a variety of sizes and compositions. Functional monomers can create specific binding sites for catalysts in the polymer. We review in this paper progress in design and use of polymer latexes and colloidal sol-gel silica as catalyst supports. In 1984 we discovered naively that cationic latexes used as phase transfer catalysts coagulated in the presence of concentrated aqueous sodium cyanide (5). We resumed the study of colloidal particle supports about two years later with emphasis on oxidation reactions that do not require high concentrations of electrolytes. In addition to our research, colloidal polymeric sulfonic acids (6) and imidazoles (7-9) have been used as hydrolysis catalysts, poly(sodium acrylate) and poly(sodium styrenesulfonate) latexes were used as catalysts for reaction of CoCNHs^Br " with hydroxide ion (10), and enzymes have been immobilized as catalysts on latexes (11). Stable colloidal particles are either charge stabilized or steric stabilized. To retain high surface area a colloidal catalyst must not coagulate and precipitate during use. Electrolytes which screen charged particle-particle repulsions, and organic components which cause collapse of the polymer loops and tails of a steric stabilizer on the surface may cause coagulation. Most of our investigations of colloidal catalyst supports have used charge stabilized particles dispersed in water. The latexes produced by emulsion polymerization have been cross-linked to prevent dissolution in water or any organic solvent. Ionic active catalysts bind electrostatically to the charged particles. These colloidal catalysts in some ways resemble other types of catalysts, such as micelles, polyelectrolytes, and ion exchange resins. Like micelles they may have charged surfaces and nonpolar interiors. A colloidal catalyst typically has a diameter of 100 nm and is polymeric inside, whereas a micelle has a diameter of 5 nm and consists of aliphatic hydrocarbon inside. Like polyelectrolytes, colloidal catalysts may have highly expanded charged polymer chains in water, but those chains are anchored to the cross-linked polymer support. Ion exchange latexes with charged sites both on the surface and inside the particles may have the same primary structures and compositions as ion exchange resins. However, typical ion exchange resins have diameters of 10 to 10 nm, whereas a typical latex diameter is 10 nm. Frequently with ion exchange resin catalysts intraparticle diffusion of reactants as well as intrinsic chemical reactivity limit the reaction rates. Since surface area per unit mass of catalyst is inversely related to particle diameter and to intraparticle diffusiontimes,colloidal particles offer an average diffusion pathlength at least 100timesshorter forreactantsto reach active sites, eliminating the intraparticle diffusional limitation toreactions.Thus high surface area does not necessarily mean thatreactionsoccur only on the surface of the particles. The goals of our colloidal catalysis research are to understand where and how chemical reactions proceed in the colloidal environment and to create highly active catalysts. The fundamental understanding should aid the practical design of catalysts. 4

4

6

2

In Polymer Latexes; Daniels, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.


424

POLYMER LATEXES

Colloidal polymer supports are likely to provide more active catalysts than larger polymer particles for two reasons: High surface area can overcome mass transport and diffusional limitations to reaction rates, and solubility of the organic reactants in the polymer concentrates reactants in the same phase with the active catalytic sites. These reasons to expect high activity do not consider intrinsicreactivityat the active sites, which might be higher or lower than in larger polymeric catalysts. Colloidal catalyticreactionsmay occur at the particle surface, which must be charged and highly hydrated to stabilize the colloid, or inside the particles in environments that could vary widely, from that of a lipophilic hydrocarbon polymer to that of a hydrated ion exchange resin. The interior of the particle may be tailored to suit the hydrophile/lipophile balance needed for high intrinsic chemical reactivity. The groups on the particle surface can be varied, but they must be charged to stabilize the colloid. Colloidal Supports Anionic oxidizing agents, anionic catalysts, or both have been employed in oxidation and hydrolysis reactions with colloidal catalysts, so we will emphasize positively charged supports. Emulsion polymerization of nonpolar monomers such as styrene promoted by cationic surfactants such as hexadecyltrimethylammonium chloride (CTAC1) produces cationic latexes having all of the positive sites derived from the surfactant. If persulfate is used as the initiator, the polymer surface will also have a small number of negatively charged sulfate sites which will serve as counterions to the surfactant. Dissociation of part of the chloride counterions provides the net positive charge on the particle. Much larger numbers of cationic sites can be created in two steps by copolymerization with a monomer such as chloromethylstyrene followed by reaction with an amine to form ammonium ions. One can synthesize a colloidal particle having a quaternary ammonium ion in almost every repeat unit by use of only chloromethylstyrene and a small amount of a cross-linking monomer, which prevents the highly ionic quaternized product from dissolving in water. Since cationic polyelectrolytes also can be highly active catalysts for reactions of anions (12), we have studied only cross-linked latexes. Cationic latexes can even be produced using an anionic surfactant and chloromethylstyrene followed by amine treatment The CMS latex made with sodium dodecyl sulfate early during treatment with trimethylamine coagulates at its composition of zero charge and then quickly redisperses as positively charged particles after further quaternization (13). Since our goal is to understand latexes as catalysts, and cationic micelles can also be active catalysts, we have prepared some latexes with polymerizable, micelleforming surfactants such as monomers 1-4 (Scheme 1) (14,15). Similar 2-tailed surfactants have been used to stabilize bilayer vesicles by polymerization (16-20). A surfactant covalently bound to the particles cannot act independently as a micellar catalyst, but it might be possible for excess surfactant such as CTACI to dissociate and act as an aqueous phase catalyst. After copolymerizations of 2 mol percent of the monomers 1-3 with 1% DVB (divinylbenzene) and 97% styrene, we proved that the charged monomers were covalently bound by ultrafiltration of the latexes to remove soluble electrolytes and analysis of both the filtrate and the particles for bromide ion. The particles contained 95-97% of the amount of bromide ion originally charged as monomer. Ultrafiltration was used to purify most of the latex catalysts by removal of soluble by-products of the synthesis. After ultrafiltration and extensive washing of the particles with water the conductivity of the filtrate is a factor of 5 or 10 less than the conductivity of the initialfiltratefrom the reaction mixture. However, even thefinalfiltrateusually has conductivity at least ten times that of the wash water. The latexes from monomers 1-3 were slightly polydisperse 60 nm diameter spheres (14).


26. FORD ET AL.

425


C H j ^ - C O O C H i C H j N T C ^ ^ C H ^ n ^ Br

1

+

0(CH ) N (CH ) Br 2

12

3

3

4 > CH CI


15

2

O C C H ^ n ^ C H ^ Br"

3

£

+

/\—CH N (CH ) 2

3

3

cr

5

3 Scheme 1. Quaternary ammonium ion monomers The monomer 4,firstreported by Tsaur and Fitch (21), was used to explore the effects of monomer and divinylbenzene levels on the particle sizes and stabilities of latexes. The particle size decreased and the polydispersity increased as the amount of surfactant 4 increased from 0.5 to 5.0 mol percent and as the amount of DVB in the monomer mixture increasedfrom1 to 5 mol percent Within experimental error all of surfactant 4 was incorporated into particles. Transmission electron microscopy showed 22-95 nm diameter particles with nonspherical shapes from the various polymerizations. Particle aggregation increased with the amount of DVB. The morphology suggested that the final particles were formed by aggregation of smaller primary particles. Since all of the latexes produced from polymerizable monomers 1-4 were prone to coagulation in the presence of the organic substrates in catalysis experiments, we turned to more highly ionic latexes prepared with vinylbenzyltrimethylammonium chloride (5), the non-micelle-forming salt formed by quaternization of chloromethylstyrene with trimethylamine (22). Early during emulsion polymerization copolymer containing the quaternary ammonium ions serves as the charge stabilizing surface of the growing colloidal particles. Up to 5 mol percent of sodium styrenesulfonate was incorporated into cross-linked polystyrene latexes by the shotgrowth technique of Kim, El-Aasser, and Vanderhoff (23). Their technique is similar to but not identical with seed growth emulsion polymerization. In our syntheses of cationic latexes an initial batch of monomers containing 1 mol percent of 5 was polymerized to 90-95% conversion, and then a second shot of monomers containing a larger amount of 5 was added, and the polymerization was completed. In the presence of 90-95% polymerized particles, all of the second shot of monomer 5 was incorporated into existing particles instead of forming new ones. The products of shot growth polymerization are highly monodisperse, as in seed growth polymerizations. Use of the shot growth technique with monomer mixtures containing 1% DVB and varied amounts of styrene and chloromethylstyrene followed by quaternization of the chloromethyl groups with trimethylamine produced the family of latexes in Table I which containfrom0.6 to 60 mol percent of quaternary ammonium ion repeat units. The particle number in shot growth emulsion polymerization is established during the first stage, and by use of 1.05 weight percent of charged monomer 5 in every polymerization along with varied mixtures of styrene and chloromethylstyrene, approximately the same size of particles was produced over the entire range of copolymer composition. In Polymer Latexes; Daniels, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

426

POLYMER LATEXES Table I. Ion Exchange Latex Particles Using Monomer 5

radii in nm by


sample

1N+ 2N+ 5N+ 10N+ 25N+ 50N+ 75N+

.DLS 2,6,2\6'CoPcTS tetra-terf-butyl-1,4-diphenoquinone Mnni(a)TPPCl TS styrene + NaOCl —> styrene oxide Mn (a)TPPCl TS alkenes + NaOCl —> epoxides and diols alkenes + KHSO5 —> epoxides and diols Mn (a)TPPClgTS cyclooctene + — > cyclooctene oxide (NH4) Mo7024-4H 0 tetrahydronaphthalene (tetralin) + 0 ---> 2

2

2

2

ref.

2

8

nl

8

ni

6

2

14

IEL, silica

c

IEL IEL IEL IEL IEL

30 31 d d 32

ASL, ACL

24

2

n

Co (pyr)

a-tetrol + a-tetralone a

6

in

CoPcTs = cobalt phthalocyaninetetrasulfonate. Mn (Cl)TPPCl8TS = chloromanganese(III) meso-tetrakis(2,6-dichlorophenyl)porphyrintetrasulfonate. (NH4)6Mo70 4-4H 0 forms unidentified oxomolybdate anions in hydrogen peroxide. Co (pyr)6 = Co in the presence of at least 6 molar equivalents of pyridine. SCL = cationic surface charged latex. IEL = cationic ion exchange latex. ASL = anionic sulfonate latex. ACL = anionic carboxylate latex. Hari Babu, S., Ford, W. T., / Polym Sci. Part A: Polym. Chem., in press. Zhu, W., Ford, W. T., / . Polym. Sci. Part A: Polym. Chem., in press. 2

n

2

11

b

c

d

Table HI. Hydrolyses with Colloidal Catalysts reaction (CeHsOkP^OQ^s-p-NCb - -> (C6H50) P0 " +" OC6H5-p-N0 diazinon —> diethyl thiophosphoric acid (C H )3CF—>(C6H )3COH 2

6

5

2

5

2

catalyst

support

ref.

0-iodosobenzoate -SO3H -SO3H

IEL silica silica

22 26 26



428

POLYMER LATEXES

We have employed only readily available primary oxidants, such as dioxygen, hydrogen peroxide, potassium monoperoxysulfate, and sodium hypochlorite, with the aim of demonstrating processes that could be used on an industrial scale. Most of the oxidations have been carried out with water-immiscible substrates in the absence of added organic solvent in triphase dispersions of water, substrate, and colloidal particles. In every case one or more of the colloidal supports increased rates of reaction by two to thirtytimescompared with rates using the same catalyst in aqueous dispersions lacking the colloidal supports, but we have found no new spectacularly large rate increases. In kinetic studies the method of agitation of the triphase mixtures and the relative amounts of oxidant, substrate, catalyst particles, binding sites in the particles, and binding sites occupied by catalyst all affect the rates of oxidation, and most of these parameters are interdependent. The overall kinetics of most of the reactions depend on mass transport of the water-immiscible substrate as well as intrinsic chemical reactivity. Although none of the mechanisms of these heterogeneous oxidations is well understood at present, they have sufficient potential for large scale applications that further investigation to understand the kinetics is warranted. The autoxidation of mercaptans to disulfides catalyzed by cobalt phthalocyaninetetrasulfonate (CoPcTS, Scheme 2) is known as "sweetening" in the petroleum industry (27). Schuit, German, and their coworkers discovered very high co-catalytic activity of cationic polyelectrolytes in aqueous solutions for autoxidation of mercaptoethanol (28,29). In our oxidations of water-soluble mercaptoethanol (Table II) at 25 °C, surface modified silicas, surface charged latexes made from monomer 1, and ion exchange latexes with bound CoPcTS are slightly more active than CoPcTS in the absence of cationic colloids and CoPcTS bound to commercial anion exchange resins, but much less active than the cationic polyelectrolyte [3,6]ionene {[(CH2)3N (Me) (CH )6N (Me)2ln 2Br -} (Hari Babu, S., Ford, W. T., J. Polym. Sci. Part A: Polym Chem., in press.) 1-Decanethiol was autoxidized efficiently at 35 °C with CoPcTS bound to latexes prepared from monomers 1-3. The rate enhancements due to latexes were about the same as those due to CTABr micelles (14). 2,6-Di-terf-butylphenol is converted to only a quinone dimer by dioxygen and CoPcTS on ion exchange latexes at 70 °C, whereas the monomeric quinone is formed also in water alone (30). There must be a higher concentration of the active species that dimerizes in the latex phase than in a solution lacking latex. Although CoPcTS is reputed to be stable to oxidation, its visible spectrum changed and catalytic activity decreased during the oxidations. Epoxidation is one of the most important oxidation reactions in synthetic chemistry, both for industrial production of ethylene oxide and propylene oxide and for exploratory syntheses. The epoxidations in Table II show useful selectivities but not high overall activity for oxidations of alkenes. At 25 °C the hindered Mn porphyrin Mn (Cl)TPPClgTS (Scheme 2) bound to an ion exchange latex in strongly basic 0.38 M NaOH catalyzes the NaOCl Qaundry bleach) epoxidation of styrenes but +

+

2

2

111

in

not aliphatic alkenes (31). At 40 °C, cyclooctene, cyclohexene, a-methylstyrene, and P-methylstyrenes are oxidized to mixtures of epoxides and diols, and 1-alkenes fail to react (Zhu, W., Ford, W. T., / . Polym. Sci., Part A: Polym. Chem, in press) The relative reactivities depend upon both the nucleophilicity of the alkene toward the electrophilic oxidizing species, presumed to be a Mn =0 porphyrin, and the solubility of the alkene in the latex, which decreases in the order aromatic alkenes > cycloalkenes > acyclic alkenes. Although the Mn (Cl)TPPClgTS, fully substituted with chlorine atoms at the 2,6-positions of the phenylrings,is one of the more oxidatively stable Mn porphyrins known, it is deactivated by oxidation of the porphyrin ring during the NaOCl reactions. An oxomolybdate (Table II), generated from ammonium molybdate and hydrogen peroxide and bound to ion exchange latexes, also selectively catalyzes v

ra



26. FORDETAL.


429

m

Mn (Cl)TPPCl TS 8

Scheme 2. Water soluble phthalocyanine and porphyrin oxidation catalysts

epoxidation of cyclooctene but not 1-octene at 40 °C (32). The latex-bound oxomolybdate catalyst gradually lost half of its activity over five catalytic cycles. Potassium hydrogen monoperoxy sulfate, a commercial mixture of 2KHS05-KHS04-K2S04, is a powerful oxidant that reacts with alkenes in the absence of the Mn porphyrin catalyst and latexes (33). The presence of the Mn porphyrin and latex has little effect on KHSO5 activity. The autoxidation of tetralin to tetralone (Table II) is used in the production of methyl N-(l-naphthyl)carbamate, a widely used insecticide, and cobalt ion catalyzed autoxidation is used generally for large scale oxidation of alkyl aromatic hydrocarbons to carboxylic acids, such as the conversion of p-xylene to terephthalic acid for production of polyesters. The Co -pyridine complex in an acrylic acid/styrene copolymer latex is only twotimesmore active than in aqueous solution lacking latex (24). The oxidation proceeds via tetralin hydroperoxide at 50 °C, a low temperature for such processes. n


430

POLYMER LATEXES

0-Iodosobenzoate ions are highly active catalysts for the hydrolysis of phosphate esters (Table HI). Moss, Alwis, and Bizzigottifaf) discovered that CTAC1 micelles greatly increased the activity and achieved a half-life of hydrolysis of pnitrophenyl diphenyl phosphate of about 10 seconds at pH 8 and 25 °C. We found that 0.2 mg/mL of ion exchange latex particles are as active as CTAC1 micelles, and that the latexes 25N and 50N (25% and 50% of quaternary ammonium ion repeat units) have the highest activities of the latexes in Table I. The colloidal silica-sulfonic acid produced by surface modification of 55 nm diameter sol-gel silica has almost as high activity for hydrolysis of the insecticide diazinon (diethyl 2-isopropyl-6-methyl-4-pyrimidinyl phosphorothioate) as does aqueous HC1 at 62 °C, and much higher activity than polystyrene-based gel and macroporous sulfonic acid ion exchange resins (Table HI) (26).


+

+

Conclusions Latex and colloidal silica supports promote the oxidations and hydrolyses of organic compounds in aqueous dispersions at or near ambient temperature with readily available oxidants and catalysts. The new catalytic processes are attractive for their ability to work in the absence of organic solvent even with water-immiscible reactants. They might be used for the destruction of specific components of industrial wastewater or for chemical manufacturing processes. Ideally we would like to have a catalyst capable of autoxidizing all organic compounds in water to carbon dioxide, but there is not and may never be such a catalyst. Although the use of water-immiscible substrates in our research shows the utility of colloidal catalysts, it makes the heterogeneous reactions difficult to understand. It adds complications of solubilities and mass transfer to heterogeneous catalysis problems that are complex even with water-soluble reactants. Progress toward understanding these heterogeneous reactions will require water-soluble reactants and complete analysis of the distributions of every component of the reaction mixture between the aqueous and colloidal particle phases. Those distributions might be determined both by analysis of suitable kinetic data, as in the pseudo-phase model of micellar catalysis (35), and by extensive chemical analysis of the particle and aqueous phases. With known local concentrations the intrinsic rates of reaction in the particle phase can be determined quantitatively. This will enable analysis of the contributions of increased local concentrations in the particle phase and the intrinsic catalytic activity to the observed rates of reaction. Then systematic study of composition and structure of gel and surface-charged particles will reveal how the environments created via variations in colloidal particle synthesis contribute to the catalytic activity. Acknowledgment. We thank the U.S. Army Research Office for support of our colloidal catalyst research. Literature Cited

1. Satterfield, C. N. Mass Transfer in Heterogeneous Catalysis, MIT Press: Cambridge, MA, 1970. 2. Chauvin, Y.; Commereuc, D.; Dawans, F. Progr. Polym. Sci. 1977, 5, 95. 3. Pittman, C. U., Jr., In: Comprehensive Organometallic Chemistry; Wilkinson, G.; Stone, F. G. A.; Abel, E. W., Eds.; Pergamon: Oxford, 1983,vol. 8, pp 553-611. 4. Garrou, P. E.; Gates, B. C. In: Syntheses and Separations Using Functional Polymers, Sherrington, D. C.; Hodge, P., Eds.; Wiley: Chichester, 1988, pp 123-147.



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5. Bernard, M.; Ford, W. T.; Taylor, T. W. Macromolecules 1984,17,1812. 6. Fitch, R. M. In: Macromolecules; Benoit, H.; Rempp, P., Eds.; Pergamon: Oxford, 1982, p 39. 7. Hopkins, A.; Williams, A. J. Chem. Soc., Perkin Trans. II 1983, 891. 8. Kitano, H.; Sun, Z.-H.; Ise, N. Macromolecules 1983, 16, 1306. 9. Sun, Z.; Yan, C.; Kitano, H. Macromolecules 1986,19,984. 10. Ishiwatari, T.; Maruno, T; Okubo, M.; Okubo, T.; Ise, N. J. Phys.Chem. 1981, 85, 47. 11. Kitano, H.; Nakamura, K.; Ise, N. J. Appl. Biochem. 1982, 4, 34. 12. Ise, N. Acc. Chem. Res. 1982,15,171. 13. Campbell, G. A.; Upson, D. A. Macromol. Syn. 1990, 10, 1. 14. Hassanein, M.; Ford, W. T. Macromolecules 1988, 21, 525. Hassanein, M.; Ford, W. T. J. Org. Chem. 1989, 54, 3106. 15. Choubal, M.; Ford, W. T. J. Polym. Sci. Part A: Polym. Chem. 1989, 27, 1873. 16. Bader, H.; Dom, K. I.; Hupfer, B.; Ringsdorf, H. Adv. Polym. Sci. 1985, 64, 17. Fendler, J. H.; Tundo, P. Acc. Chem. Res. 1985, 14, 45. 18. Hayward, J. A.; Johnston, D. S.; Chapman, D. Ann. N. Y. Acad. Sci. 1985, 446, 267. 19. O'Brien, D. F.; Klingbiel, R. T.; Specht, D. P. Tyminski, P. N. Ann. N. Y. Acad. Sci. 1985, 446, 282. 20. Sadownik, A.; Stefely, J.; Regen, S. L. J. Am. Chem. Soc. 1986, 108, 7789. 21. Tsaur, S.-L.; Fitch, R. M. J. Colloid Interface Sci. 1987,115,450. 22. Ford, W. T.; Yu, H. Langmuir 1991, 7, 615. 23. Kim, J. H.; Chainey, M.; El-Aasser, M. S.; Vanderhoff, J. W. J. Polym Sci. Part A: Polym. Chem. 1989, 27, 3187. 24. Chandran, R. S.; Srinivasan, S.; Ford, W. T. Langmuir 1989, 5, 1061. 25. Badley, R. D.; Ford, W. T.; McEnroe, F. J.; Assink, R. Langmuir 1990, 6, 792. 26. Badley, R. D.; Ford, W. T. J. Org. Chem. 1989, 54, 5437. 27. Frame, R. R. U.S. Patent 4,298,463, 1981. 28. Zwart, J.; van der Weide, H. C.; Bröker, N.; Rummens, C.; Schuit, G. C. A.; German, A. L. J. Mol. Catal. 1977-78, 3, 151. 29. van Welzen, J.; van Herk, A. M.; Kramer, H.; German, A. L. J. Mol. Catal. 1990, 59, 291, 311. 30. Turk, H.; Ford, W. T. J. Org. Chem. 1988, 53, 460. 31. Turk, H.; Ford, W. T. J. Org. Chem. 1991, 56, 1253. 32. Srinivasan, S.; Ford, W. T. New J. Chem. 1991,15,693. 33. Zhu, W.; Ford, W. T. J. Org. Chem. 1991, 56, 7022. 34. Moss, R. A.; Alwis, K. W.; Bizzigotti, G. O. J. Am. Chem. Soc. 1983, 105, 681. 35. Bunton, C. A.; Savelli, G. Adv. Phys. Org. Chem. 1986, 22, 213. RECEIVED December 4, 1991


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