Ethylene polymerization by rigid benzene ring centered dinuclear metallocene (Ti, Zr)/MAO systems

Two rigid benzene centered dinuclear metallocene complexes C6H2[(CH2C5H4)2MCl2]2, M = Ti (1), Zr (2) have been prepared by treating two equivalents of TiCl4 and ZrCl4 with the tetralithium salt of the ligand C6H2(CH2C5H5)4-1,2,4,5 in toluene and characterized by 1H NMR and elemental analysis. Both complexes are effective catalysts for ethylene polymerization in the presence of methylaluminoxane (MAO). The influence of [MAO]/[Cat] molar ratio, catalyst concentration, polymerization temperature and time has been tested in detail. The catalytic activity of complex 2 is more than two times higher than that of complex 1, which is still more active than that of the tetranuclear titanocene C6H2[CH2C5H4Ti(C5H5)Cl2]4-1,2,4,5 (5). On the other hand, the catalytic activities of 1 and 2 is slightly lower than that of the dinuclear metallocene complexes C6H4[CH2C5H4Ti(C5H4CH3)Cl2]2-1,3 (3) and C6H4[CH2C5H4Zr(C5H5)Cl2]2-1,3 (4), respectively, which is related to the limited intermolecular rotation of the metallocene units in 1 and 2. The melting points above 130 °C indicate a polyethylene formed by complexes 1 and 2 with highly linear and highly crystalline. GPC spectra show that polyethylene produced by complexes 1 and 2 has a broad and even bimodal molecular weight distribution (MWD).