13C NMR studies of olefin copolymer microstructure:: A tool for understanding active center behavior in ansa-zirconocene-based catalysis

In the present paper the effects of the solvent, a Lewis base and of nonpolymerizable olefin on copolymerization of propene with higher alpha-olefins performed with isospecific ansa-zirconocene catalysts are investigated. A wide range of solvents with increasing polarity, donor character and steric hindrance has been tested, and their effects on copolymer yield, composition and microstructure thoroughly analyzed. Our results demonstrate that the solvent can have a significant influence on the copolymer composition, even though the solvent polarity is not the relevant factor. Polymerizations carried out in coordinating solvents, e.g. aromatic solvents, lead to the formation of products with considerably decreased higher alpha-olefin content. The reduced incorporation of the higher alpha-olefin is explained in terms of competition between the nucleophilic medium and the olefin monomer for coordination to the active polymerisation site. The dependence of polymer microstructure on the solvent nucleophilicity is further elucidated by copolymerization studies performed in the presence of diisopropyldimethoxysilane as a Lewis base. The addition of the non-polymerizable olefin, 2-methyl-1-pentene, which does not significantly affect comonomer composition, seems to slightly modify comonomer distribution. The sum of the results obtained give us valuable information regarding the mechanism of polymerisation at the active centers.