Ti(IV)-Tris(phenolate) Catalyst Systems for the Ring-Opening Copolymerization of Cyclohexene Oxide and Carbon Dioxide

Abstract

Titanium(IV) complexes of amino-tris(phenolate) ligands (LTiX, X = chloride, isopropoxide) together with bis(triphenylphosphine)iminium chloride (PPNCl) are active catalyst systems for the ring-opening copolymerization of carbon dioxide and cyclohexene oxide. They show moderate activity, with turnover frequency values of ∼60 h^-1 (0.02 mol % of catalyst, 80 °C, 40 bar of CO₂) and high selectivity (carbonate linkages >90%), but their absolute performances are lower than those of the most active Ti(IV) catalyst systems. The reactions proceed with linear evolution of polycarbonate (PCHC) molar mass with epoxide conversion, consistent with controlled polymerizations, and evolve bimodal molar mass distributions of PCHC (up to M _n = 42 kg mol^-1). The stoichiometric reaction between [LTiO ⁱ Pr] and tetraphenylphosphonium chloride, PPh₄Cl, allows isolation of the putative catalytic intermediate [LTi(O ⁱ Pr)Cl]^-, which is characterized using single-crystal X-ray diffraction techniques. The anionic titanium complex [LTi(OR)Cl]^- is proposed as a model for the propagating alkoxide intermediates in the catalytic cycle.