Investigation of the polymerization behavior of polysiloxane-bridged dinuclear zirconocenes as model compounds for a heterogenized metallocene at the silica surface

The polymerization of ethylene was studied by using a series of polysiloxane-bridged dinuclear zirconocenes [(SiMe₂O)_nSiMe₂ (C₅H₄)₂] [(C₉H₇)ZrCl₂]₂ (7, n = 1; 8, n = 2; 9, n =3), the corresponding mononuclear zirconocene (C₅H₅)(C₉H₇)ZrCl₂, 10, and the pentamethylene-bridged dinuclear zirconocene [(CH₂)₅(C₅H₄)₂][(C ₉H₇)-ZrCl₂]₂, 13. From the polymerization studies using these catalysts it was found that (i) activities of the polysiloxane dinuclear zirconocenes 7-9 were lower than that of the corresponding mononuclear zirconocene 10, (ii) molecular weights of polyethylenes produced by the dinuclear metallocenes are greater than that of polyethylene produced by the mononuclear metallocene, (iii) the complex 9 holding the longest bridging ligand exhibited the highest activity but produced a polymer having the smallest molecular weight among the polysiloxane-bridged dinuclear zirconocenes, and (iv) the pentamethylene-bridged dinuclear metallocene 13 showed higher activity than the complexes 7-9 and the mononuclear zirconocene 10. The formation of the lowest molecular weight of polyethylene by 9 was attributed to the influence of electron withdrawal caused by the Lewis acid-base interaction between the acidic aluminum of the cocatalyst and the basic oxygen at the polysiloxane linkage as well as the lack of a steric problem. An increase in steric congestion around the metal center led to not only a decrease in catalytic activity due to preventing facile monomer access to the active site but also an increase in the molecular weight of polyethylenes due to supressing β-H elimination.