Polymer-supported catalysts applications

Several industrial processes use phase-transfer techniques with soluble catalysts, mostly for fine chemical productions (42,43). It is easy to believe that this technology will find a greater application in the near future, perhaps with the use of polymer-supported catalysts. 

Second Generation Recyclable Boomerang Polymer Supported Catalysts for Olefin Metathesis Application of Ardu-engo Carbene Complexes, M. Ahmed, T. Arnauld, a. G.M. Barrett, et at, Synlett 2000, 1007-1009. 

Another interesting feature of polymer-supported catalysts containing quaternary ammonium salts involves the development of enantioselective catalysis using salts derived from cinchonia or ephedra alkaloids.11341 The first application of such chiral supported catalysts in the Michael reaction between methyl 1-oxoindan-2-carboxylate and methyl vinyl ketone revealed a high chemical yield in condensation product (60-100 %) although the enantioselectivities were only moderate (ee <27 %). 

Composite membrane catalysts can also be assembled with polymeric supports or intermediate layers [117-119]. These membranes were tested as membrane catalysts for selective hydrogenation of some dienic hydrocarbons and proved to be as selective as monolithic palladium alloy membranes [117]. The use of polyarilyde has been proposed in order to widen the temperature range of polymer-supported membrane application... 

Thus, a micro encapsulation technique has been shown to be quite effective for binding catalysts to polymers. Utilizing this technique, unprecedented polymer-supported, microencapsulated rare earth Lewis acids have been prepared. The catalysts thus prepared have been successfully used in many useful carbon-carbon bond-forming reactions. In all cases, the catalysts were recovered quantitatively by simple filtration and reused without loss of activity. This new technique for binding nonpolymer compounds to polymers will be applicable to the preparation of many other polymer-supported catalysts and reagents. 

Most recent research has been focused on the application of polymers as chiral auxiliaries in enantioselective Lewis-acid-catalyzed reactions. Studies of Itsuno and co-workers [44] culminated in the development of a polymer-supported catalyst containing a chiral oxazaborolidinone with oxyethylene crosslinkages which gave the Diels-Alder adduct of cyclopentadiene and methacrolein in 88 % isolated yield with an exotendo ratio of 96 4 and 95 % e. e. for the exo adduct. A variety of polymer-supported chiral Lewis acids was also investigated by Mayoral et al. [45]. Some supported catalysts were more active than their homogeneous analogs, but enantioselectivity was always lower. 

Yet in spite of these negative aspects, the cobalt catalysts are predominant in industrial applications. The reason is very simple the cost of rhodium. Given its cost, it is imperative that the catalyst be recovered completely. This consideration was partially responsible for the very early and intense interest in hydroformylation with polymer-supported catalysts. 

Our studies of chemically prepared catalyst powders in gas diffiision electrodes have demonstrated that conducting polymer supported catalysts can provide similar current densities to commercial carbon supported catalysts. They indicate that with further optimization, the ion conducting properties of certain polymer support catalysts may allow them to exceed the performance of carbon supported catalysts. However, it is clear that substantial improvements in the stability of the polymer support materials will have to be made before applications in fuel cells can be realized. 

The application of polymer-supported catalysts has now been extended to the synthesis of complexes between transition metal derivatives and structurally ordered macromolecular ligands to give catalytic systems exhibiting high activity and stereoselectivity. Polystyrene and polymethacrylate resin and polystyrene-divinylbenzene-polystyrene-polybutadiene block copolymers, as well as vinyl-functionalized polysiloxanes grafted onto silica, are very suitable polymers for heterogenization of mostly Pt and Rh complexes. Moreover, polyamides exhibit much higher thermal stability than conventional polystyrene supports (114). 

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