13C NMR study of the effect of coordinating solvents on zirconocene-catalyzed propene/1-hexene copolymerization

The solvent effect observed in propene/1-hexene copolymerizations performed with the isospecific catalyst rac-Et(Ind)(2)ZrCl2/MAO is studied. A range of solvents with increasing donor character and steric hindrance has been tested, and their effect on copolymer yield, composition, and microstructure has been thoroughly analyzed. Our results demonstrate that the solvent can have a significant influence on the comonomer reactivities; even though the solvent polarity is not the relevant factor. At the same comonomer compositions in solution; polymerizations carried out in coorindating solvents (e.g., aromatic solvents), lead to the formation of products with considerably decreased content of 1-hexene. 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 polymerization site. These results give us valuable information regarding the mechanism of polymerization at the active centers.