Catalytic chain transfer (CCT) is a process that can be incorporated into radical polymerization to obtain greater control over the resulting products. Introduction Radical polymerization of vinyl monomers, like methyl (metha)acrylate of vinyl acetate is a common (industrial) method to prepare polymeric materials. One of the problems associated with this method is, however, that the radical polymerisation reaction rate is so high that even at short reaction times the polymeric chains are exceedingly long. This has several practical disadvantages, especially for polymer processing (e.g. melt-processing). A solution to this problem is catalytic chain transfer, which is a way to make shorter polymer chains in radical polymerisation processes. The method involves adding a catalytic chain transfer agent to the reaction mixture of the monomer and the radical initiator. Historical background Boris Smirnov and Alexander Marchenko (USSR) discovered in 1975 that cobalt porphyrins are able to reduce the molecular weight of PMMA formed during radical polymerization of methacrylates. Later investigations showed that the cobalt dimethylglyoxime complexes were as effective as the porphyrin catalysts and also less oxygen sensitive. Due to their lower oxygen sensitivity these catalysts have been investigated much more thoroughly than the porphyrin catalysts and are the catalysts actually used commercially. Process In general, reactions of organic free radicals (•C(CH3)(X)R) with metal-centered radicals (M•) either produce an organometallic complex (reaction 1) or a metal hydride (M-H) and an olefin (CH2=C(X)R) by the metallo radical M• abstracting a β-hydrogen from the organic radical •C(CH3)(X)R (reaction 2). These organo-radical reactions with metal complexes provides several mechanisms to control radical polymerization of monomers CH2=CH(X). A wide range of metal-centered radicals and organo-metal complexes manifest at least a portion of these reactions. Various transition metal species, including complexes of Cr(I), Mo(III), Fe(I), V(0), Ti(III), and Co(II) have been demonstrated to control molecular weights in radical polymerization of olefins. The olefin generating reaction 2 can become catalytic, and such catalytic chain transfer reactions are generally used to reduce the polymer molecular weight during the radical polymerization process. Mechanistically, catalytic chain transfer involves hydrogen atom transfer from the organic growing polymeryl radical to cobalt(II), thus leaving a polymer vinyl-end group and a cobalt-hydride species. The Co(por)(H) species has no cis-vacant site for direct insertion of a new olefinic monomer into the Co-H bond to finalize the chain-transfer process, and hence the required olefin insertion also proceeds via a radical pathway. The best recognized chain transfer catalysts