Homo-polymerization of α-olefins and co-polymerization of higher α-olefins with ethylene in the presence of CpTiCl2(OC 6H4X-p)/MAO catalysts (X = CH3, Cl)

Abstract

Cyclopentadienyl-titanium complexes containing -OC6H 4X ligands (X = Cl, CH3) activated with methylaluminoxane (MAO) were used in the homo-polymerization of ethylene, propylene, 1-butene, 1-pentene, 1-butene, and 1-hexene, and also in co-polymerization of ethylene with the α-olefins mentioned. The -X substituents exhibit different electron donor-acceptor properties, which is described by Hammett's factor (σ). The chlorine atom is electron acceptor, while the methyl group is electron donor. These catalysts allow the preparation of polyethylene in a good yield. Propylene in the presence of the catalysts mentioned dimerizes and oligomerizes to trimers and tetramers at 25°C under normal pressure. If the propylene pressure was increased to 7 atmospheres, CpTiCl2(OC 6H4CH3)/MAO catalyst at 25°C gave mixtures with different contents of propylene dimers, trimers and tetramers. At 70°C we obtained only propylene trimer. Using the catalysts with a -OC 6H4Cl ligand we obtained atactic polymers with M w 182,000 g/mol (at 25°C) and 100,000 g/mol (at 70°C). The superior activity of the CpTiCl2(OC6H4Cl)/MAO catalyst used in polymerization of propylene prompted us to check its activity in polymerization of higher α-olefins (1-butene, 1-pentene, 1-hexene) and in co-polymerization of these olefins with ethylene. However, when homopolymerization was carried out in the presence of this catalyst no polymers were obtained. Gas chromatography analysis revealed the presence of dimers. The activity of the CpTiCl2(OC6H4Cl)/MAO catalyst in the co-polymerization of ethylene with higher α-olefins is limited by the length of the co-monomer carbon chain. Hence, the highest catalyst activities were observed in co-polymerization of ethylene with propylene (here a lower pressure of the reagents and shorter reaction time were applied to obtain catalytic activity similar to that for other co-monomers). For other co-monomers the activity of the catalyst decreases as follows: propylene >1-butene > 1-pentene ≫ 1-hexene. In the case of co-polymerization of ethylene with propylene, besides an increase in catalytic activity, an increase in the average molecular weight Mw of the polymer was observed. Other co-monomers used in this study caused a decrease of molecular weight. A significant increase in molecular weight distribution (Mw/M n) evidences a great variety of polymer chains formed during the reaction. © 2005 by MDPI.