Stereospecific propylene polymerization using half-sandwich metallocene/MAO systems: a mechanistic insight

The propylene polymerizations with different half-sandwich metallocene comp lexes of the general formula, Me2Si[Me4CpNR]TiCl2, result in the formation of polymers with increased isotacticity as compared to atactic polypropylen e. This was observed if a sterically demanding aromatic system at the amido -ligand coordinates with the polymerization active metal center. In this ca se, the metal center is partly shielded and the steric demand of the ligand is similar to an ansa-metallocene. The monomer partly coordinates stereosp ecifically at the metal center. Additionally, the solvent polarity has a gr eat influence on the microstructure of the produced polymers. Using the com plex Me2Si[Me4CpNCHCH3-naphthyl]TiCl2 in toluene, ortho-dichlorobenzene and hexane polypropylenes of similar microstructures were formed (mmmm approxi mate to 45%) whereas in methylene chloride as solvent, the isotacticity of the polypropylene was decreased by 50%. (C) 2000 Elsevier Science B.V. All rights reserved.