Heterogeneous polymeric catalysts

It is necessary to note the limitation of the approach to the study of the polymerization mechanism, based on a formal comparison of the catalytic activity with the average oxidation degree of transition metal ions in the catalyst. The change of the activity induced by some factor (the catalyst composition, the method of catalyst treatment, etc.) was often assumed to be determined only by the change of the number of active centers. Meanwhile, the activity (A) of the heterogeneous polymerization catalyst depends not only on the surface concentration of the propagation centers (N), but also on the specific activity of one center (propagation rate constant, Kp) and on the effective catalyst surface (Sen) as well ... 

IV. Heterogeneous Polymerization Catalysts Derived from Transition... 

A closer similarity exists between the C2-symmetric octahedral isospecific model sites, which have been proposed for the heterogeneous polymerization catalysts,13 15 and some slightly distorted octahedral metal complexes, including bidentate or tetradentate ligands, which have recently been described as active in isospecific olefin polymerization in the presence of MAO.128-130 In fact, all these catalytic systems can be described in terms of racemic mixtures of active species with A or A chiralities. 

Esterolytic Reactions of Active Esters Using Heterogeneous Polymeric Catalysts Containing Imidazole Groups... 

Transition metal complexes, zeolites, biomimetic catelysts have been widely used for various oxidation reactions of industrial and environmental importance [1-3]. However, few heterogenized polymeric catalysts have also been applied for such purpose. Mild condition oxidation catalyzed by polymer anchored complexes is attractive because of reusability and selectivity of such catalysts. Earlier we have reported synthesis of cobalt and ruthenium-glycine complex catalysts and their application in olefin hydrogenation [4-5]. In present study, we report synthesis of the palladium-glycine complex on the surface of the styrene-divinylbenzene copolymer by sequential attachment of glycine and metal ions and investigation of oxidation of toluene to benzaldehyde which has been widely used as fine chemicals as well as an intermidiate in dyes and drugs. 

Gfoup 4 bis(arene) complexes (arene = mesitylene) have also been used as precursors for heterogeneous polymerization catalysts. Their reaction with alumina gives 1 equiv. of mesitylene/grafted M, and it has been proposed that this reaction yields [(AlsO)M(C6H3Me3) (H)] (Scheme... 

Historically, heterogeneous polymerization catalysts have been the workhorse of the polymer industry. Although these catalysts offer many important advantages over their homogeneous counterparts in commercial production, they also have a significant number of drawbacks. For example, hetereogeneous catalysts typically have multiple active sites, each of which has its own rate constants for monomer enchainment, stereoselectivity, comonomer incorporation, and chain transfer. Therefore a substantial amount of empirical optimization of these catalysts is necessary before polymers of relatively uniform molecular weights, composition, and stereochemistry can be produced. 

Table IV reports the main asymmetric reactions which have been attempted using synthetic optically active polymers as soluble catalysts. Note that polyimines are the most studied polymers, together with poly-L-a-aminoacids often complexed with metals. Whereas with heterogeneous polymeric catalysts (Table II) only reduction reactions were studied, a larger chemical versatility is displayed by homogeneous systems addition, oxidation, hydrogenation and hydrolysis reactions have been carried out. Optical yields are good (up to 90%) in certain addition reactions (Table V) and rather low in the other cases.

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