The Role of Oil− Water Microinterface in Immobilized Phase-Transfer

Ind. Eng. Chem. Res. 2003, 42, 5983-5987

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The Role of Oil-Water Microinterface in Immobilized Phase-Transfer Catalysis Noritaka Ohtani,* Tomoaki Ohta, Kohji Watanabe, Kiyoto Endo, and Jun Mukudai Department of Materials-process Engineering and Applied Chemistry for Environments, Faculty of Engineering and Resources Science, Akita University, Akita 010-8502, Japan

Under two-liquid conditions, the third phase formed by linear-polystyrene-based cationic ionomers and the corresponding single-network ionomer gels absorbed the soluble quaternary salts either from the oil phase or from the aqueous solution. Polystyrene-based IPN gels, in which one network contained quaternary salts and another benzyl chlorides, were compared in terms of the feasibility of esterification of the chloromethyl groups in addition to the triphase catalysis (TPC) activities. IPN gels afforded not only very poor TPC activities but also slow esterification. Single-network gels, on the other hand, showed high TPC activities and high esterification rates. When decane was used as the oil, the TPC activity was increased, while the esterification became slower. Introduction Quaternary ammonium or phosphonium salts have been used as catalysts for phase-transfer catalysis (PTC)1-5 or as the catalytic moieties of the corresponding polymer-supported phase-transfer catalysts.6-9 Recently, we reported on the phase behavior of the quaternary salts in nonpolar oils and in aqueous electrolyte solutions.10,11 The solubility curves in oils had specific features that were characterized by critical values corresponding to the Krafft point and critical micelle concentration. These values are strongly dependent on the oil molecular volume and on the counterions. This behavior is rather similar to the solubility of common ionic surfactants in water. The Krafft boundary and the lower consolute curve, on the other hand, featured the solubility behavior of quaternary salts in an aqueous solution. The shape of the liquidliquid immiscibility gap was assumed to be a closed loop at times, and the region of the gap was sharply dependent on the concentration of the coexisting electrolyte. This behavior is very similar to that of micelle-forming nonionic surfactants.12 We have also disclosed that quaternary salt/nonpolar oil/water/electrolyte four-component systems often form microemulsion phases, within which an oil phase and an aqueous phase can coexist.13,14 We showed how the parameters, such as temperature, alkyl chain length of quaternary cation, counteranion, and electrolyte concentration, influence the formation of microemulsion phases. Benzyltributylphosphonium salts and benzylalkyldimethylammonium salts are the quaternary salts that have been used most often for attachment to insoluble polymer matrix in the preparation of immobilized phasetransfer catalysts.6-9,15-18 The solution behavior and reactivity of the corresponding linear polymers containing the quaternary salts (cationic ionomers) indicate that the quaternary salts aggregate in nonpolar oils.19-21 Linear polystyrenes with low levels of benzyltribu* To whom correspondence should be addressed. Tel.: 8118-889-2744. Fax: 81-18-837-0404. E-mail: [email protected].

tylphosphonium chlorides barely dissolve in either phase of coexisting toluene/water two phases; rather, they emulsify one or both of the layers, or they are liberate as an opaque third phase at the boundary between the two layers. Previously, we reported that, under liquid/liquid/solid (L/L/S) triphase catalytic (TPC) conditions, the interior of quaternary salt/immobilized polystyrene resins consisted of two microseparated phases, oil and water, at the interface of which reactions took place.15,16,22,23 An understanding of the resin microstructure has progressed to the point of being able to predict the mechanism of triphase catalytic reaction systems. In this paper, we demonstrate some additional evidence for the presence of a microinterface and discuss its role in triphase catalysis. We show that soluble quaternary salts form a mixed microinterface with the polymerattached quaternary salts within the third phases or within the cross-linked ionomers. After preparing two types of polystyrene ionomer gel, we evaluate the continuity of the oil or water microphase and the mobility of the styrene moiety or reagents in the cross-linked ionomer. One is a singlenetwork gel that contains both benzyltributylphosphonium chlorides and benzyl chlorides. The other is an IPN gel in which one network contains only benzyltributylphosphonium chlorides and another network contains only benzyl chlorides. In addition to the triphase catalytic activities of these gels, the feasibility of forming acetate from the chloromethyl groups was measured as a function of oil structure or aqueous NaOAc concentration. Experimental Section Materials and Equipment. 1H NMR spectra were recorded on a Varian Gemini 300H, 300-MHz spectrometer. Decyl methanesulfonate was prepared by the reaction of methanesulfonyl chloride with 1-decanol in pyridine. Hexadecyltributylphophonium bromide (HTBPBr) was purchased from Tokyo-Kasei and was used without further purification. Benzyltributylphosphonium bromide (BTBPBr) was prepared by the quaternization of tributylphosphine with benzyl bromide in

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5984 Ind. Eng. Chem. Res., Vol. 42, No. 24, 2003

toluene at 90 °C for 72 h. All of the phosphonium salts had purities above 98%. Linear ionomers were prepared and characterized by the method reported previously.19 The degree of polymerization was 340 unless otherwise stated. Singlenetwork polystyrene-attached quaternary salts were prepared by the reaction of tributylphosphine or alkyldimethylamines with a cross-linked polystyrene containing a prescribed amount of chloromethyl groups (1% divinylbenzene, -200 + 325 or -100 + 200 mesh).16 The quaternization was fully carried out for the preparation of PxBuX or AxRX resins, where P and A stand for phosphonium and ammonium, respectively; x is the ionexchange capacity of the resin in millimoles per gram; Bu and R represent the alkyl group of the quaternary salt; and X is the counterion of the quaternary salt. The chloromethylated groups were partially reacted to prepare P(a - b)BuCl resins, where a indicates the ionexchange capacity of resin in millimoles per gram and b is the content of chloromethyl groups in millimoles per gram. The content of quaternary salts was determined by GLC by analyzing the amount of 1-chlorodecane that was formed through the reaction of the ionomers with excess decyl methanesulfonate in toluene at 90 °C. The content of chloromethyl groups was quantitatively analyzed in the same way after the groups were converted to the corresponding quaternary salts through reaction with an excess amount of pyridine. Preparation of IPN Polymers. A fully quaternized single-network resin, AxRCl or PxBuCl, was soaked in a dichloromethane solution containing prescribed amounts of styrene, chloromethylstyrene, benzoyl peroxide, and divinylbenzene. After the dichloromethane had been evaporated, a second suspension polymerization was carried out, using poly(vinyl alcohol) as a stabilizer. The resulting interpenetrating-network resins, IPN(a - b), were washed successively with water, toluene, methanol, and THF; extracted (Soxhlet) with THF under nitrogen; and dried under reduced pressure. Noncatalytic Reactions between AxRCl or PxBuCl and Decyl Methanesulfonate. The reactions were carried out under pseudo-first-order reaction conditions where decyl methanesulfonate was present in great excess over gel-attached onium salts. An oil bath used for kinetic experiments was temperaturecontrolled using a Tabai contact thermometer. Reaction mixtures were stirred with a Teflon-coated stirring bar at a speed of over 1000 rpm. Reaction mixtures were analyzed by GLC on a Hitachi model 163 FID instrument with a 1-m column of SE-30 or PEG-20M at 170 °C. In a typical kinetic run, a 30-mL culture tube with a Teflon-lined screw cap was charged with 0.05 mmol of ionomer-bound onium chlorides and 4 mL of toluene containing pentadecane as an internal standard. The tube was placed in an oil bath, and the mixture was stirred for 30 min with a Teflon-coated magnetic stirring bar, after which 0.82 mmol of decyl methanesulfonate was injected via micropipet. Aliquots of the toluene layer were withdrawn periodically with a microsyringe to determine the concentrations of the substrate and 1-chlorodecane by GLC. Sorption of Quaternary Salts. (1) GC Method. The general procedure is as follows. A 30-mL culture tube with a Teflon-lined screw cap was charged with prescribed amounts of ionomer with 0.05 mmol of phosphonium salts, sodium halide plus prescribed

amounts of water, and 2 mL of an organic solvent. The tube was placed in a temperature-controlled oil bath, and the mixture was stirred for 30 min with a Tefloncoated magnetic stirring bar. Then, 2 mL of an organic solvent containing prescribed amount of soluble phosphonium salts was injected. After a prescribed time, an aliquot of the organic layer was withdrawn with a syringe and added to a toluene solution containing an excess amount of decyl methanesulfonate. The concentrations of the quaternary salts were determined by GLC by analyzing the amounts of 1-halodecanes formed through the reaction of the quaternary salts with the excess decyl methanesulfonate at 90 °C. (2) NMR Method. 1H NMR spectra were recorded on a Varian Gemini 300H, 300-MHz spectrometer. The difference between the NMR method and the GC method was only the use of benzene-d6 or D2O for the solvent of the phase whose composition was determined. After a mixture had been equilibrated at a constant temperature, 0.7 mL of the phase with a deuterated solvent was withdrawn. The methylene peaks of the quaternary salts were integrated to obtain the concentration. Intraresin Reaction. A 30-mL culture tube with a Teflon-lined screw cap was charged with prescribed amounts of resin with 0.1 mmol of phosphonium salts, 2 mL of an organic solvent, and 0.1 mL of water. The tube was placed in a temperature-controlled oil bath, and the mixture was stirred for 30 min with a Tefloncoated magnetic stirring bar, after which 2 mL of NaOAc aqueous solution was injected. After a prescribed time, the mixture was cooled to 0 °C and filtered. The resin was reacted with an excess amount of methyl methanesulfonate and then reacted with an excess amount of pyridine. The yield of chloride ions was quantitatively analyzed by the above-mentioned method. Results and Discussion Sorption of Soluble Quaternary Salts onto Intraresin Microinterfaces. Benzyltributylphosphonium bromide (BTBPBr) is unlimitedly soluble both in benzene and water. The equilibrium state of the BTBPBr/benzene/water three-component system, however, depends on the composition of the constituents. The system affords a liquid (O)/liquid (M)/liquid (W) threeliquid state when the weight fraction of BTBPBr is above a certain value. The M phase is a BTBPBr-rich microemulsion phase, and BTBPBr is assumed to aggregate in the M phase.12 If the weight fraction of BTBPBr is low, BTBPBr predominantly resides in the aqueous layer. Benzyltributylphosphonium chloride also gives an O/M/W three-liquid state when the aqueous phase contains a certain amount of NaCl. In fact, a linear polystyrene with a low level of benzyltributylphosphonium chlorides (LP1.0BuCl) hardly dissolves in either phase of a coexisting toluene/water two-layer system; rather, it is liberated as an opaque third phase at the boundary of the two layers. The third phase contains considerable amounts of water and toluene. This solubility of the linear ionomer also suggests the presence of the oil-water microinterface within the network of cross-linked ionomers. On the other hand, hexadecyltributylphoshonium bromide (HTBPBr) is predominantly present in the toluene layer of a toluene/water two-phase system. As shown in Figure 1, HTBPBr was absorbed by the third phase that a benzyltributylphosphonium ionomer,

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Figure 1. Sorption of hexadecyltributylphosphonium bromide (HTBPBr) from the toluene layer to the third phase formed by the polystyrene-based linear ionomer containing benzyltributylphosphonium bromide (LP1.0BuCl) and to the cross-linked phosphonium ionomer (P1.0BuCl) under toluene/water two-liquid conditions. LP1.0BuCl or P1.0BuCl, 0.04 mmol; water, 1.0 mL; toluene, 4.0 mL; HTBPBr, 0.04 mmol; 30 °C; GC method. Q+Brand Q+Cl- represent HTBPBr and HTBPCl, respectively. The solid and broken lines represent the data for P1.0BuCl and LP1.0BuCl, respectively.

LP1.0BuCl, formed in the presence of a toluene/water two-liquid system. This suggests that the soluble quaternary salt forms a mixed microinterface with the polymer-attached quaternary salts within the third phase. It is generally observed that quaternary salts are absorbed by the third phase that another quaternary salt forms under oil/aqueous solution (liquid/liquid) conditions. The corresponding cross-linked ionomer (P1.0BuCl) similarly absorbed the soluble quaternary salt HTBPBr (Figure 1), as did the ammonium-type ionomer (Figure 2). It was found that absorption equilibrium was attained within several tens of minutes. The absorbed amount was much larger than the value anticipated for a simple partition of the soluble quaternary salts between toluene and resinous toluene phases. The soluble quaternary salts might form a mixed microinterface within the beads of the cross-linked ionomer. Benzyltributylphosphonium bromide (BTBPBr) was almost present in the aqueous phase of an O/W twophase system if the content of BTBPBr was lower than 5% and no electrolyte had been added. However, BTBPBr was almost present in the benzene phase of an O/W two-phase system if the content of NaBr was higher than 10% in the aqueous solutions. The amount of BTBPBr in the other phase could be neglected under these conditions. Interestingly, the cross-linked ionomer (P0.95BuCl) can absorb BTBPBr either from the benzene layer or from the aqueous layer, irrespective of the location at which BTBPBr resides in the systems (Figure 3). The quantity of absorbed BTBPBr increased with increasing amount of P0.95BuCl. If the aqueous layer was removed from the system, the amount of sorption was reduced; 62% of BTBPBr remained in the benzene layer at the equilibrium state. BTBPBr was

Figure 2. Sorption of hexadecyltributylphosphonium bromide (HTBPBr) from the toluene layer to the cross-linked ammonium ionomer (A1.0BuCl) under toluene/water two-liquid conditions. A1.0BuCl, 0.04 mmol; water, 1.0 mL; toluene, 4.0 mL; HTBPBr, 0.04 mmol; 30 °C; GC method. Q+Br- and Q+Cl- represent HTBPBr and HTBPCl, respectively.

Figure 3. Equilibrium sorption of benzyltributylphosphonium bromide (BTBPBr) from the benzene layer (solid lines) or from the water layer (broken line) to the cross-linked ionomers under benzene/aqueous two-liquid conditions as a function of ionomer amount. From benzene (water): BTBPBr, 0.04 mmol; water (water-d2), 1.0 mL; NaBr, 1.08 mmol (0 mmol); benzene-d6 (benzene), 1.0 mL; 60 °C; NMR method.

also absorbed from the benzene layer by ammonium ionomers A0.87C8Cl and A0.93C16Cl. Intraresin Reactions. Two types of polystyrene ionomer gels were prepared. One ionomer is a singlenetwork gel that contains both benzyltributylphosphonium chlorides and benzyl chlorides in a polymer chain. The other is an IPN gel in which one network contains only benzyltributylphosphonium chlorides and another network contains only benzyl chlorides. Before we examined the rate of the reaction between these immobilized quaternary salts and chloromethyl groups, the reaction of benzyl chloride with aqueous sodium acetate under several reaction conditions was first performed. All reactions examined obeyed pseudo-

5986 Ind. Eng. Chem. Res., Vol. 42, No. 24, 2003 Table 1. Pseudo-First-Order Rate Constants [105 kobsd (s-1)] of Nucleophilic Substitution Reactions between Benzyl Chloride and Sodium Acetate under Two-Liquid Conditionsa catalyst solvent decane toluene

none rpm)b

3.60 (100 6.37 (880 rpm)b 1.34 (880 rpm)b

BTBPCl

P(1.1-1.0)BuCl

7.35 2.45

11.4 3.43

a Catalyst, 0.05 mmol if present; benzyl chloride, 70 µL; reaction temperature, 90 °C; 2.5 mol/L sodium acetate aqueous solution, 2 mL: organic solvent, 2 mL. b Stirring speed with magnetic stirring bar.

Figure 5. Influence of network structure on the intraresin reaction. Polymer-attached phosphonium salts, 0.05 mmol; toluene, 1.0 mL; 2.5 M NaOAc aqueous solution 1.0 mL; 60 °C.

Figure 4. Effects of coexisting quaternary groups and solvent on the esterification of polymer-attached chloromethyl groups. The active chloride of CMPS is 1.0 mmol/g of resin. Amount of chloromethyl groups, 0.1 mmol; toluene, 2.0 mL; 2.5 M NaOAc aqueous solution 2.0 mL; BTBPCl, 0.1 mmol if present; 90 °C. All constituents of the reaction with P(0.6-0.2)BuCl were used in onehalf the amounts used for the reactions with the other gels. The solid and broken lines represent the data for toluene and decane, respectively.

first-order kinetics. The data are reported in Table 1. The reaction proceeded even in the absence of catalyst, indicating that the reaction took place at the bulk liquid/ liquid interface. It is noted that the P(1.1-1.0)BuClcatalyzed reaction was faster than the BTBPCl-catalyzed reaction and that this tendency was more pronounced when decane was used as the oil. Next, a single-network ionomer gel was contacted with toluene and 2.5 M NaOAc aqueous solution. The rate of esterification of immobilized chloromethyl groups was investigated. Two steps are necessary for this reaction to proceed. The first step is the ion exchange of gel-attached quaternary salts from chloride to acetate ion form by way of the migration of both anions through the continuous aqueous microphase within the gel. The second step is the reaction at the oil-water microinterface. The gel-attached chloromethyl groups must move to the microinterface where the ion pairs with the acetate form exist; we use the term “site-site interaction” to represent the contact of these two species. As shown in Figure 4, the chloromethyl groups of the ionomer gel, P(0.6-0.2)BuCl or P(1.1-1.0)BuCl, were readily transformed to an acetate form. The reaction rate, however, did not follow pseudo-first-order kinetics, and the reaction tended to become slower with increasing

reaction time, indicating that the apparent reactivity of the immobilized chloromethyl groups was dispersed. Some chloromethyl groups seemed to remain unchanged even after prolonged reaction time. In other words, they appeared to be isolated from site-site interactions. Figure 4 also shows that the presence of immobilized phosphonium groups is necessary for the facile esterification of chloromethyl groups. A single-network gel with only chloromethyl groups (CMPS) did not react at all. The reaction was very slow even in the presence of a soluble phosphonium salt (BTBPCl) catalyst while the reaction tended to be nearly of pseudo-first-order kinetics. Although decane was a more efficient solvent than toluene for phase-transfer catalysis (Table 1), the intraresin reaction hardly proceeded in decane except in the initial stage. This poor reactivity might be closely related to the amounts of solvents absorbed by the ionomer gel. When P(1.1-1.0)BuCl gel was equilibrated with an oil and 2.5 M NaOAc aqueous solution, 1 g of the gel absorbed 0.93 g of toluene but only 0.032 g of decane, whereas the gel absorbed ca. 0.4 g of water irrespective of the coexisting oil. This means that the mobility of the polystyrene chain attached by the chloromethyl groups was highly restricted in decane. The substrate used for triphase catalysis, that is, benzyl chloride, can readily approach the reaction sites (intraresin oil-water microinterface) as it was not immobilized.15 However, the polymer-attached benzyl chloride had great difficulty approaching the microinterface. The chloromethyl groups of an IPN gel scarcely reacted even in toluene (Figure 5). The presence of phosphonium salts on another network does not function catalytically at all. This is due to the low ionexchange ability of IPN gels. For example, the degree of ion exchange of phosphonium salts from chloride to acetate form was only 8.0% after 3 h. Acetate ions were not introduced into this IPN gel. Therefore, the triphase catalytic activity of the IPN gel was quite poor. However, the nucleophilic reactivity of phosphonium or ammonium chlorides attached to an IPN gel was not always low compared with that of the quaternary salts of single-network ionomers. The reactivity of chloride ions toward decyl methanesulfonate was examined as a function of the loading of ammonium chlorides. As

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experimental evidence might be required to definitively reach this conclusion. Literature Cited

Figure 6. Nucleophilic activity of IPN resins toward decyl methanesulfonate. IPN or AxCnCl, 50 mg; decyl methanesulfonate, 0.82 mmol; toluene, 4.0 mL; 90 °C.

shown in Figure 6, the reactivity of the ammonium chlorides was slightly lower than that of single-network ionomers. Role of Intraresin Interface in Triphase Catalysis. All of the experimental results obtained in this study run counter to the homogeneous model being proposed for the microstructure of triphase catalyst gels. The interfacial reaction mechanism proposed here gives a clearer explanation for all of these results. The microstructure that single-network ionomer gels form under liquid/solid/liquid triphase catalysis conditions is highly heterogeneous. Quaternary salts strongly interact each other under the conditions where oil and water are both present. Quaternary salts are not isolated from each other but, rather, are aggregated in the ionomer gels. Thus, the soluble low-molecular-weight quaternary salts are sorbed by the insoluble ionomer gels in which an oil-water microinterface is formed. IPN gels, which cannot form any oil-water microinterface within the gels, afforded not only very poor TPC activities but also very low rates of esterification. Chloride ions were not ion-exchanged into an acetate form under the TPC conditions because of the absence of water droplets in the gels. Single-network gels, on the other hand, showed high TPC activities and high esterification rates because of the heterogeneous character of the gel interior. When decane was used as the oil instead of toluene, the TPC activity was increased because of the increase in the local concentration of substrate at the microinterface. This indicates that the mobility of styrene moieties is not necessarily necessary for the TPC activity provided that the oil and water microphases are both continuous. The esterification of polymer-attached chloromethyl groups, however, became slow because the restricted mobility of the chloromethyl groups prevents them from approaching the microinterface. The results of this work also suggest that the low TPC activity of macroporous catalysts cited by many studies is possibly due to the advantage of the microinterface, at which a substrate and polymer-attached active quaternary salts collide with each other within the resins, although more

(1) Starks, C. M.; Liotta, C. Phase Transfer Catalysis Principles and Techniques; Academic Press: New York, 1978. (2) Weber, W. P.; Gokel, G. W. Phase Transfer Catalysis in Organic Synthesis; Springer-Verlag: Berlin, 1977. (3) Dehmlow, E. V.; Dehmlow S. S. Phase Transfer Catalysis, 2nd ed.; Verlag Chemie: Weinheim, Germany, 1983. (4) Makosza, M. Phase-transfer catalysis. A general green methodology in organic synthesis. Pure Appl. Chem. 2000, 72, 1399. (5) Brandstrom, A. Principles of phase transfer catalysis by quaternary ammonium salts. Adv. Phys. Org. Chem. 1977, 15, 267. (6) Regen, S. L. Triphase Catalysis. Angew. Chem., Int. Ed. Engl. 1979, 91, 464. (7) Montanari, F.; Landini, D.; Rolla, F. Phase-Transfer Catalyzed Reactions. Top. Curr. Chem. 1982, 101, 147. (8) Hodge, P.; Sherrington, D. C. Polymer-Supported Reactions in Organic Synthesis; John Wiley & Sons: New York, 1980. (9) Ford, W. T.; Tomoi, M. Polymer-Supported Phase Transfer Catalysts: Reaction Mechanisms. Adv. Polym. Sci. 1984, 55, 49. (10) Ohtani, N.; Hosoda, Y. Phase Behavior of Tetrabutylammonium Salt in Aromatic Hydrocarbons or Aqueous Solutions. Bull. Chem. Soc. Jpn. 2000, 74, 2263. (11) Ohtani, N. Solubility behavior of benzylhexadecyldimethylammonium salts in oils. Stud. Surf. Sci. Catal. 2001, 132, 189. (12) Ohtani, N.; Tsuchimoto, D. Phase Behavior of Benzyltributylphosphonium Salts in Aromatic Hydrocarbons or Aqueous Solutions. Bull. Chem. Soc. Jpn. 2001, 74, 1225. (13) Ohtani, N.; Yamashita, T.; Hosoda, Y. Phase Behavior of Tetrabutylammonium Salt/Oil/Water/Inorganic Salt Four-Component Systems. Bull. Chem. Soc. Jpn. 2000, 74, 2269. (14) Ohtani, N.; Morimoto, Y.; Naitou, M.; Kasuga Y.; Tsuchimoto, D. Phenomenological Properties and Phase Behavior of Benzylalkyldimethylammonium Salts in the Presence of Benzene and Electrolyte Solutions. Langmuir 2001, 17, 3829. (15) Ohtani, N.; Inoue, Y.; Mukudai, J.; Yamashita, T. Polystyrene-Supported Onium Salts as Phase-Transfer Catalysts. In ACS Symposium Series; Halpern, M. E., Ed.; American Chemical Society: Washington, DC, 1996; Vol. 659, p 248. (16) Ohtani, N.; Nakaya, M.; Shirahata, K.; Yamashita, T. Reactivity and Catalytic Activity of Cationic Ionomers for the Nucleophilic Substitution Reactions. J. Polym. Sci. A 1994, 32, 2667. (17) Ohtani, N.; Inoue, Y.; Nomoto, A.; Ohta, S. Polystyrenesupported ammonium fluoride as a catalyst for several basecatalyzed reactions. React. Polym. 1994, 24, 73. (18) Ohtani, N.; Inoue, Y.; Shinoki, N.; Nakayama, K. PhaseTransfer Nucleophilic Reactions Using Water-Insoluble Alcohols as Organic Solvents. Bull. Chem. Soc. Jpn. 1995, 68, 2417. (19) Ohtani, N.; Inoue, Y.; Mizuoka, H.; Itoh, K. Solution Viscosity Behavior of Polystyrene-Based Cationic Ionomers. Effect of Ion Content and Solvent. J. Polym. Sci. A 1994, 32, 2589. (20) Ohtani, N.; Inoue, Y.; Kaneko, Y.; Okumura, S. Solution Viscosity Behavior of Polystyrene-Based Cationic Ionomers: Effects of Quaternary-Group Structure and Counter Ion. J. Polym. Sci. A 1995, 33, 2449. (21) Ohtani, N.; Inoue, Y.; Kaneko, Y.; Sakakida, A.; Takeishi, I.; Furutani, H. Solution Viscosity Behavior and Swelling Behavior of Polystyrene-Based Cationic Ionomers. Effects of Added Salts and Counterion. Polym. J. 1996, 28, 11. (22) Ohtani N.; Wilkie, C. A.; Nigam, A.; Regen, S. L. Triphase Catalysis. Influence of Percent Ring Substitution On Active-Site Mobility, Macroenvironment, Microenvironment, and Efficiency. Macromolecules 1981, 14, 516. (23) Ohtani, N.; Regen, S. L. Influence of Aqueous Salt on Triphase Catalytic Activity. Macromolecules 1981, 14, 1594.

Received for review February 18, 2003 Revised manuscript received May 30, 2003 Accepted July 27, 2003 IE030147+