Catalysts, general mutual activation

Organic reactions have been performed through the combination of incompatible basic and acid catalysts by two general methodologies, namely by using a mixture of two supported catalysts with different active centres on separate supports or multifunctional single catalysts where incompatible reagents are rendered mutually inactive by confinement effect. 

No support can be regarded as inert with respect to the active centres. By its universally positive effect on the activity of centres, MgCl2 is superior to any other support. In spite of the great technical importance of Mg in active centres, generally not much is known of their structure in third-generation catalysts (or perhaps because of its positive effects all the important producers have published hundreds of patents, but the crucial factors may still be kept secret). It is suspected that the separation (dilution) of transition metal atoms by a barrier of Mg atoms enables the majority of transition metals to become part of the active centres on these centres, the polymer grows more rapidly than on centres without Mg. Mutual contact of the centres is hindered, bimolecular termination of centres (transition metal reduction to a less active oxidation state) is limited, and the centres live longer. 

The process of polymerization consists in general of three steps initiation, propagation, and termination. In radical polymerization, a catalyst is usually employed as a source of free radicals, the primary radicals. A fraction of these initiate a rapid sequence of reactions with monomer molecules, the primary radical thus growing into a polymer radical. Radical activity is destroyed by reaction of two radicals to form one or two molecules. This termination reaction is called mutual recombination, if only one molecule is formed. Termination by disproportionation results in two molecules. For many common monomers, recombination is the normal mode of termination and the kinetic treatment here is based on this termination reaction. Only slight modifications are required for polymerizations in which termination occurs by disproportionation. If both termination processes occur, another variable must be introduced to describe the kinetics of the system fully. 

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