Polymer Supported Catalytic Groups

The polymer-supported catalysts are thus important conceptually in linking catalysis in solutions and catalysis on supports. The acid—base chemistry is fundamentally the same whether the catalytic groups are present in a solution or anchored to the support. The polymer-supported catalysts have replaced acid solutions in numerous processes because they minimise the corrosion, separation, and disposal problems posed by mineral acids. 

Poly(styrene—divinylbenzene) copolymers can be used as catalyst supports. Attachment of catalytic groups to the polymer supports can be achieved by... 

A more versatile method to use organic polymers in enantioselective catalysis is to employ these as catalytic supports for chiral ligands. This approach has been primarily applied in reactions as asymmetric hydrogenation of prochiral alkenes, asymmetric reduction of ketone and 1,2-additions to carbonyl groups. Later work has included additional studies dealing with Lewis acid-catalyzed Diels-Alder reactions, asymmetric epoxidation, and asymmetric dihydroxylation reactions. Enantioselective catalysis using polymer-supported catalysts is covered rather recently in a review by Bergbreiter [257],... 

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). 

The same hyperbranched polyglycerol modified with hydrophobic palmitoyl groups was used for a noncovalent encapsulation of hydrophilic platinum Pincer [77]. In a double Michael addition of ethyl cyanoacetate with methyl vinyl ketone, these polymer supports indicated high conversion (81 to 59%) at room temperature in dichloromethane as a solvent. The activity was stiU lower compared with the noncomplexed Pt catalyst. Product catalyst separation was performed by dialysis allowing the recovery of 97% of catalytic material. This is therefore an illustrative example for the possible apphcation of such a polymer/catalyst system in continuous membrane reactors. 

The affinity of the polymer-bound catalyst for water and for organic solvent also depends upon the structure of the polymer backbone. Polystyrene is nonpolar and attracts good organic solvents, but without ionic, polyether, or other polar sites, it is completely inactive for catalysis of nucleophilic reactions. The polar sites are necessary to attract reactive anions. If the polymer is hydrophilic, as a dextran, its surface must be made less polar by functionalization with lipophilic groups to permit catalytic activity for most nucleophilic displacement reactions. The % RS and the chemical nature of the polymer backbone affect the hydrophilic/lipophilic balance. The polymer must be able to attract both the reactive anion and the organic substrate into its matrix to catalyze reactions between the two mutually insoluble species. Most polymer-supported phase transfer catalysts are used under conditions where both intrinsic reactivity and intraparticle diffusion affect the observed rates of reaction. The structural variables in the catalyst which control the hydrophilic/lipophilic balance affect both activity and diffusion, and it is often not possible to distinguish clearly between these rate limiting phenomena by variation of active site structure, polymer backbone structure, or % RS. 

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). 

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