Half-Titanocene-Based Catalysts in the SyndiospecificPolymerization of Styrenes: Possible Oxidation States ofthe Titanium Species and Geometries of the Active Sites

Half-titanocenes (CpCH2CH2O)TiCl2 (1) and (CpCH2CH2OCH3)TiCl3 (2), (Cp ) cyclopentadienyl), activated by methylaluminoxane (MAO), have been tested in the polymerization of styrene, p-methylstyrene, and p-chlorostyrene. Both catalysts produce syndiotactic polystyrene and poly-p-methylstyrene in the whole range of explored temperatures and concentrations. In the polymerization of p-chlorostyrene, however, catalyst 1 affords a syndiotactic polymer only at the low temperature and the high monomer concentrations; and catalyst 2 gives only an atactic product. The results have been compared to those obtained in the presence of the catalytic system CpTiCl3 (3)/MAO and have been discussed considering the possible oxidation states of the titanium species and the geometries of the active sites