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

• Published in: Organometallics 2020-05, Vol.39 (9), p.1619-1627
• Main Authors: Raman, Sumesh K, Deacy, Arron C, Pena Carrodeguas, Leticia, Reis, Natalia V, Kerr, Ryan W. F, Phanopoulos, Andreas, Morton, Sebastian, Davidson, Matthew G, Williams, Charlotte K
• Format: Article
• Language: English
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/acs.organomet.9b00845

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.