Syndio-Specific Polymerization of Styrene Using Fluorinated Indenyltitanium Complexes *

Nine new fluorinated half-sandwich titanocene complexes (1b–9b) based on substituted alkylindenes were synthesized, by reacting Me3SnF with the corresponding chloride species, and employed as catalyst precursors for the syndio-specific polymerization of styrene. When activated with methylaluminoxane (MAO), the new precursors 1b–9b exhibited increased activities by factors of 15–40 compared with the corresponding chlorinated compounds and provided improved syndiotacticity, enhanced melting temperature, and higher polymer molecular weights. The activities of indenyl and methyl- or phenyl-substituted indenyl complexes were found to be higher by factors of 4–12.5 than those of CpTiF₃ and Cp*TiF₃. More importantly, the amount of MAO can be reduced to an Al: Ti molar ratio of 300 in the temperature range of 10°C–90°C. It is likely that Ti–F, more polarized than the Ti–Cl bond in the half-sandwich titanocenes, allows the formation of more active and stable active sites of Ti(III) complexes needed for the syndio-specific polymerization of styrene. Evidence in this direction is brought via the electron paramagnetic resonance (EPR) spectrum and redox titration. The higher activity and syndio-specificity of the fluorinated catalysts are attributable to a greater number, more stable Ti(III) active sites, and/or higher propagation rate constant.