Rotating Benzyl Substituent in ansa-Bis(indenyl)zirconocenes Controls Propene Polymerization

A series of benzyl-substituted, dual-site ansa-metallocenes were synthesized and characterized. Their isolated rac- and meso-diastereomers were studied in propene polymerization after methyl-aluminoxane or borate activation. Catalysts' polymerization behavior were investigated in various polymerization conditions, and produced polypropenes (PPs) were characterized with NMR, GPC, and DSC. The rac- and meso-diastereomers of these unsymmetric catalysts bearing a SiMe(2) bridge produced PP with similar activity, tacticity, and molar mass. According to quantum chemical calculations, the benzyl group in the catalysts can rotate, having significant energy minima. The reason that the diastereomers produce PP with similar molar mass is linked to these local energy minima and is further discussed.