Polymer-supported quaternary onium

The direct quaternization of chloromethylated polystyrenes by tertiary amines or phosphines represents the easiest way to obtain polymer-supported quaternary onium salt (12,13). A lipophilic character of quaternary cation and a topology allowing sufficient cation-anion separation also play an important role (35,36). A linear spacer chain (of about 10 carbon atoms) between the catalytic site and the polymer backbone substantially increases the reaction rates. The loading of quaternary onium groups also affects catalytic efficiency, the influence being different for directly bonded and spaced groups, e.g. 10 and 11, respectively (37). 

Dispersed phase polymerisation of HIPEs has also been used to prepare polymer-supported quaternary onium phase transfer catalysts [162]. One strategy involved the polymerisation of a concentrated emulsion of vinyl benzyl chloride (VBC) in water and subsequent quaternisation of the polymer resin with tertiary amines and phosphines (Fig. 22). 

Polymer supported quaternary onium salts [68] and polymer supported catalysts containing phosphorus palladium complexes and quaternary onium... 

Marconi, P. F., and W. T. Ford, Catalytic Effectiveness Due to Mass Transfer Limitations in Triphase Catalysis by Polymer-Supported Quaternary Onium Salts, J. Catal., 83,160 (1983). 

PolyHIPE, in granular form, has recently been employed as a support for bicatalyst systems [136]. Styrene/DVB porous polymers were prepared with free vinyl groups (acryloyl, allyl and vinylbenzyl) on the surfaces of the cavities. Impregnation with solutions of quaternary onium monomers, with subsequent polymerisation, resulted in grafting onto the pendant double bonds, to give a surface-quaternised material (Fig. 19). 

The counter-ions of some of the quaternary onium groups were exchanged with an anionic phosphine compound, which was then used to complex palladium. Thus, a polymer material containing phase transfer catalyst and transition-metal catalyst groups was obtained (Fig. 20). The Heck-type vinyla-tion reaction [137] was used to examine the catalytic activity of the heterogeneous system. The polymer-supported catalyst was found to compare favourably with the homogeneous system (Fig. 21). 

T. Nishikubo, et al., Soluble polymer-supported catalysts containing pendant quaternary onium salt residues for regioselective addition reaction of epoxy compound with active ester. Macromolecules 1994, 27(25), 7240-7247. 

The majority of research on catalysis by anion-exchange resins is devoted to the use of resins in interfacial catalysis reactions [38-43]. In this connection, quaternary ammonium or phosphonium, onium salts have been commonly obtained [44]. Crown-ethers, cryptands and linear polyesters supported on polymers also catalyze similar reactions dealing with the interfacial transfer of reagents. In the simplest case, the mechanism of interfacial catalysis represents a substitution reaction of Sf 2 type typical for the interaction of the nucleophile, T , present in the aqueous solution with the alkyl halogenide, RX, in the organic phase ... 

Quaternary ammonium and phosphonium ions bound to insoluble polystyrene present an even more complicated mechanistic problem. Polystyrene beads lacking onium ions (or crown ethers, cryptands, or other polar functional groups) have no catalytic activity. The onium ions are distributed throughout the polymer matrix in most catalysts. The reactive anion must be transferred from the aqueous phase to the polymer, where it exists as the counter ion in an anion exchange resin, and the organic reactant must be transferred from the external organic phase into the polymer to meet the anion. In principle, catalysis could occur only at the surface of the polymer beads, but kinetic evidence supports catalysis within the beads for most nucleophilic displacement reactions and for alkylation of phenylacetonitrile. 

---