Recent Catalysts for Living Polymerization

Nb and Ta halides and their mixtures with suitable organometallic co-catalysts (similar to those for Mo and W halides) work as excellent catalysts for the polymerization of various disubstituted acetylenes, while they cyclo-trimerize monosub-stituted acetylenes. This is probably because the steric hindrance of disubstituted monomers can hamper the cyclo-trimerization to selectively give polymers. A problem with Nb and Ta catalysts is that they are deactivated by polar groups in the monomer and solvents. 

It is noteworthy that the Grubbs-Hoveyda catalyst can polymerize monosub-stituted and diphenylacetylenes [67]. The Grubbs-Hoveyda catalyst and a series of Ru carbene complexes catalyze the polymerization of o-substituted PhAs, as represented by (o-isopropoxy)phenylacetylene [68]. The substituents at the ortho position of monomers are assumed to serve as supportive ligands that maintain and prolong the life of unstable propagating carbene species. 

Recently, many examples of catalysts for living polymerization of substituted acetylenes have been reported. These catalysts are useful for the elucidation of the polymerization mechanism as well as precision synthesis of various polymer architectures such as block copolymers and star polymers. Table 15.2. 

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