Non-hydrocarbon Heterocyclic and

Coordination catalysts, which are usually applied for the polymerisation and copolymerisation of heterocyclic and heterounsaturated monomers, involve a wide range of metal derivatives characterised by a moderate nucleophilicity and relatively high Lewis acidity. Compounds of group 2 and 3 metals, such as zinc, cadmium and aluminium, as well as transition metals, such as iron, are representative coordination catalysts. The appropriate Lewis acidity of the metal and the appropriate nucleophilicity of the metal substituent in the catalyst make the monomer coordination favourable prior to the nucleophilic attack of the metal substituent on the monomer not yet coordinated. 

The differentiation between the coordination polymerisation of heterocyclic and heterounsaturated monomers and their nucleophilically initiated anionic polymerisation lies in the covalent nature of the metal-heteroatom bond (but being polarised as in the Mt 5+— Xs bond) in the coordination catalyst, which activates the monomer by its coordination, enhancing the nucleophilicity of the metal substituent simultaneously, and the ionic character of the metal-heteroatom bond in the nucleophilic initiators. 

Catalysts for the polymerisation of heterocyclic and heterounsaturated monomers contain a heteroatom, such as, for example, Cl (i.e. with an Mt-Cl active bond), or the group bound to the metal atom via the heteroatom (Mt X for example, X = 0, S, N) as the initiating substituent. However, there are also catalysts that possess an alkyl initiating substituent at the metal atom, especially when they are used for the polymerisation of heterounsaturated monomers. 

The metal-carbon bond in these catalysts becomes the metal-heteroatom bond after the monomer enchainment. 

Note that the mechanism of incorporation of the coordinating epoxide into the Mt-X active bond actually occurs in a multicentred transition state but not in a four-centred transition state. This is implied by the participation of another metal atom of the same or different catalyst molecule in this transition state. As a consequence, an inversion of the configuration at the nucleophilically attacked carbon atom of the coordinated epoxide takes place [63,68,71,116 121]. 

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Polymerisation of Non-hydrocarbon (Heterocyclic and Heterounsatu rated) Monomers... 

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