Matrices polystyrene, insoluble

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 immobilization of phase transfer catalysts on solid substrates allows a clean reaction with no contamination of the products by the catalyst. Insoluble polystyrene matrices have been used as a solid support. The polymer matrix does not affect the velocity of the reaction, apart from steric hindrance with respect to the reagents. In the case of immobilization on modified silica the active centre is linked to the support by an alkyl chain of variable length. This length strictly determines the adsorption capacity of the polar support, which then controls the rate of reaction. A three-phase catalytic system is set up. Two distinct phases, containing reagents, come into close... 

Supported Organotin Reagents on Insoluble Polystyrene Matrix... 

However, the shortcomings of the chemistry, notably low stepwise yields (40-80%), have limited the applicability of the solid-phase techniques compared with solution-phase syntheses. One of the unavoidable problems was the striking disparity in the properties between the non-polar polystyrene matrix and the polar ionic phosphodiester oligonucleotide chain, which led to incomplete couplings due to functional group inaccessibility. Overall, despite all the efforts of many researchers and some local successes like the use of uncross-linked insoluble isotactic polystyrene [151], phosphodiester syntheses on polystyrene resins were practically limited to the level of tri-and tetranucleotides. 

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