Monomers and Catalysts - Coordination

Coordination polymerisation was first proposed in 1956 for the unusual, at that time, low-pressure polymerisation of ethylene and polymerisation of propylene with the transition metal catalysts discovered by Ziegler in 1953 [1], and for the ferric chloride catalysed ring-opening polymerisation of propylene oxide to crystalline polymer reported by Pruitt et al. in a Dow patent [2]. 

Polymerisation carried out in the presence of a coordination catalyst is referred to as coordination polymerisation , when each polymerisation step involves the complexation of the monomer before its enchainment at the active site of the catalyst. The active site in each coordination catalyst comprises the metal atom (Mt), surrounded with ligands, one of which (X) forms a covalent active bond (Mt X) with this metal atom. This implies that the growing polymer chain is covalently bound to the metal atom. A characteristic feature of coordination polymerisation is the mutual activation of the reacting bonds of both the monomer (M) and the active site (Mt-X) through the complexation of the monomer with the metal atom at this site, which results in the cleavage of these bonds in the concerted reaction. 

monomer enchainment into the metal-bound polymer chain is preceded by, and activated by, the coordination of the monomer with the metal atom. It must be noted in this connection that only monomers capable of forming labile complexes with the active site of the catalyst may undergo coordination polymerisation. 

The coordination step proposed in many polymerisation systems with coordination catalysts has not been unequivocally established. Thus, the more 

Monomer coordination at the active site of the catalyst may occur in varied ways, essentially reducing to a twofold mechanism which is dependent on both the kind of monomer and the catalyst. This appears to encompass cases where unsaturated hydrocarbon monomers coordinate at the metal of transition metal-based catalysts, involving % complex formation, as well as cases where heterocyclic and heterounsaturated monomers are subjected to polymerisation with various coordination catalysts in which monomer complexation proceeds via a bond formation between the heteroatom and the metal atom. 

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