THE INFLUENCE OF AROMATIC SUBSTITUENTS ON THE POLYMERIZATION BEHAVIOR OF BRIDGED ZIRCONOCENE CATALYSTS

The synthesis of seven new bridged zirconocenes is described, which make isotactic polypropylene when used in polymerizations of propylene with methylaluminoxane as cocatalyst. Their polymerization behavior in propylene and ethylene polymerizations is examined and discussed. Aromatic substituents in appropriate positions of the zirconocene ligand frame result in catalysts with activities, stereospecificities, and polypropylene molecular weights much higher than those of any previously described metallocene system. By structure variations it is demonstrated that the effectiveness of these substitutions strongly depends on their position and on a nonincremental synergism with alkyl substituents on the ligand frame. The high activities of the systems can be explained well by electronic effects, whereas steric effects obviously play the more important role for the high stereospecificities and high molecular weights of the polymers.