Structural characteristics and polymerization properties of polysiloxane bridged ansa-metallocene as a model compound for the heterogenized metallocene at silica surface

Three polysiloxane bridged ansa-metallocenes, tetramethyldisiloxanediylbis (cyclopentadienyl)zirconium dichloride, [TMDSCp₂]ZrCl₂, hexamethyltrisiloxanediylbis (cyclopentadienyl)zirconium dichloride, [HMTSCp₂]ZrCl₂, and tetramethyldisiloxanediylbis (cyclopentadienyl)zirconium dimethylate, [TMDSCp₂]ZrMe₂, have been prepared and their polymerization studies have been pursued as a model compound for the heterogenized metallocene onto silica surface. Polymerization results show that [TMDSCp₂]ZrCl₂ having disiloxane (Si-O-Si) linkage between two Cp has unexpectedly high activity for ethylene polymerization while [HMTSCp₂]ZrCl₂ having trisiloxane (Si-O-Si-O-Si) linkage between two Cp does not polymerize ethylene at all. The above results can be rationalized by structural characteristics of [TMDSCp₂]ZrCl₂. It turns out that [TMDSCp₂]ZrCl₂ has an wide active site around metal center due to the big angle of Cl-Zr-Cl, which consequently does not exert much steric disturbance during monomer coordination and insertion. Another feature to be noted is that molecular weight of the polyethylene from disiloaxne bridged metallocene is extremely sensitive to the change of polymerization temperature.