Coordination polymerization of heterocyclic and heterounsaturated monomers

Polymerizations and copolymerizations of non-hydrocarbon monomers, such as heterocyclic and heterounsaturated monomers, in the presence of coordination catalysts constitute a distinct group of coordination polymerization processes which differ essentially from the coordination polymerization of unsaturated hydrocarbon monomers in terms of the kinds of catalyst applicable and the polymerization mechanism. While the coordination polymerization and copolymerization of numerous hydrocarbon monomers with catalysts based on transition metals have become the backbone of several significant branches of the plastics and rubber industries, the coordination polymerization and copolymerization of heterocyclic and heterounsaturated monomers (carried out mainly in the presence of catalysts based on main group metals) have been restricted in industry within a much smaller volume. However, the latter polymerizations are of great significance for extensive mechanistic studies and exploratory investigations to obtain more efficient catalysts, as well as for the discovery of novel synthetic routes for polymers, new polymers or for improving existing polymers. In this review, coordination polymerization and copolymerization of heterocyclic and heterounsaturated monomers are presented in terms of the type of monomer, type of catalyst, structure of the active sites and polymerization mechanism. Attention has been paid to the coordination homopolymerization of oxacyclic, thiacyclic, azacyclic, and phosphacyclic monomers, the coordination copolymerization of oxacyclic monomers with other heterocyclic monomers or heterounsaturated monomers, and the coordination homopolymerization of heterounsaturated monomers. Coordination polymerizability of heterocyclic monomers such as epoxides, episulfides: oxetanes, lactones, alkylene carbonates, alkylene thiocarbonates, dicarboxylic acid anhydrides and a-aminoacid carboxyanhydrides, morpholinedione, oxyphosphonoyloxytrimethylene, as well as heterounsaturated monomers like carbon dioxide, carbon disulfide, sulfur dioxide, isocyanates, aldehydes, ketones and ketenes, has been taken into consideration. The mechanisms of propagation operating in the coordination polymerization of the heteroatom-containing cyclic and acyclic monomers mentioned above have all been shown to be analogous, irrespective of the type of catalyst (containing mononuclear or multinuclear active species), and a general scheme concentrating on similarities of these mechanisms is proposed and discussed.