‑Heterocyclic Carbene-Based Group 4 Catalysts for the Terpolymerization of Cyclohexene Oxide and Cyclic Anhydrides with CO.

A series of bis-phenolate saturated -heterocyclic carbene (NHC) group 4 complexes ([κ-O,C,O]-NHC)-M-(OPr)-Cl-(THF) (M = Ti, ; Zr, ; Hf, ) in the presence of [PPN]Cl as cocatalyst were investigated and showed high activity in the tandem terpolymerization of phthalic anhydride (PA), cyclohexene oxide (CHO) with CO. The resultant terpolymers revealed a diblock pattern leading selectively to poly-(ester--carbonate). Subsequently, other titanium complexes ([κ-O,C,O]-NHC)-TiX bearing various coligands (X = Cl, ; OPr, ; OAc, ; OAc, ) also displayed high activity with a turnover frequency (TOF) up to 460 h that is comparable to . Using the same tandem approach, the nature of terpolymers was modulated with other mono- and tricyclic anhydrides alongside CHO with CO. Intrigued by the high rates of PA conversion observed experimentally in terpolymerization, complexes - as well as benzannulated and unsaturated NHC analogues of complex were investigated as a stand-alone reaction for the copolymerization of PA and CHO. Complex /[PPN]Cl displayed excellent catalytic activity (TOF ∼ 1600 h) and high selectivity (≥99%) toward polyesters comparable to other highly active heteronuclear (Al/K and Fe/K) catalysts and binary (salen)-MX systems. Kinetic studies performed on complexes and determined activation barriers ( ) consistent with the observed catalytic trend, ., : Ti < Hf.