Comparison of the C1-symmetric diastereoisomers of a zirconocene-based catalyst in ethylene polymerization: a benzyl substituent as a regulator in branch formation

C1-symmetric diastereoisomers of a zirconocene dichloride, SiMe2(3-benzylindenyl)(indenyl)ZrCl2, known as catalyst precursors used to produce polypropylenes with similar molecular weights and tacticities, have been investigated in ethylene polymerization. Activated by methylaluminoxane, they produce microstructurally different polymers: high-density polyethylene and linear low-density polyethylene, the latter characterized by the presence of ethyl branches. The formation of branches is relevant in the complex having a sterically more crowded (inward) site. A comparison with the complex without substituents, meso-SiMe2(indenyl)2ZrCl2, shows that the presence of a benzyl group on only one of the two indenyl moieties can regulate the number of branches and the molecular weight of the macromolecule. Actually, the unsubstituted complex is able to give double the number of branches and lower molecular weights, whereas the C1-symmetric disubstituted complexes previously reported generally give linear polyethylene.