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

The synthesis of seven new bridged zirconocenes is described, which ma ke isotactic polypropylene when used in polymerizations of propylene w ith methylaluminoxane as cocatalyst. Their polymerization behavior in propylene and ethylene polymerizations is examined and discussed. Arom atic substituents in appropriate positions of the zirconocene ligand f rame result in catalysts with activities, stereospecificities, and pol ypropylene molecular weights much higher than those of any previously described metallocene system. By structure variations it is demonstrat ed that the effectiveness of these substitutions strongly depends on t heir position and on a nonincremental synergism with alkyl substituent s on the ligand frame. The high activities of the systems can be expla ined well by electronic effects, whereas steric effects obviously play the more important role for the high stereospecificities and high mol ecular weights of the polymers.