Stereochemical control in propylene polymerization catalyzed by unbridged metallocene catalysts

Two unbridged metallocene catalysts, bis(2,4,7-trimethylindenyl)zirconium d ichloride (met-I) and bis(2,4,6-trimethylindenyl)zirconium dichloride (met- II), which are different in the position of substituents on the six-membere d ring of the indenyl ligands were synthesized. The effect of substituents in the two metallocenes on the propylene polymerization was studied in the presence of methylaluminoxane (MAO) and triisobutylaluminium (TIBA). From t he analysis of microstructure determined by C-13-NMR, it was demonstrated t hat the polymers produced by met-II have higher [mmmm] isotactic sequences than that of met-I. Using a mechanism based on model statistical analysis, it was found that chain-end model was dominant for met-I. However, met-II o beys the concurrent two-sites model during polymerization, which can be att ributed to the existence of "racemic-like" conformer in its system.