N-Heterocyclic Carbene-Based Group 4 Catalysts for the Terpolymerization of Cyclohexene Oxide and Cyclic Anhydrides with CO2

Abstract

A series of bis-phenolate saturated N-heterocyclic carbene (NHC) group 4 complexes ([κ3-O,C,O]-NHC)M(OiPr)Cl(THF) (M = Ti, 1; Zr, 2; Hf, 3) 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 CO2. The resultant terpolymers revealed a diblock pattern leading selectively to poly(ester-b-carbonate). Subsequently, other titanium complexes ([κ3-O,C,O]-NHC)TiX2 bearing various coligands (X = Cl, 4; OiPr, 5; OAc, 6; OAcF, 7) also displayed high activity with a turnover frequency (TOF) up to 460 h-1 that is comparable to 1. Using the same tandem approach, the nature of terpolymers was modulated with other mono- and tricyclic anhydrides alongside CHO with CO2. Intrigued by the high rates of PA conversion observed experimentally in terpolymerization, complexes 1-3 as well as benzannulated and unsaturated NHC analogues of complex 1 were investigated as a stand-alone reaction for the copolymerization of PA and CHO. Complex 1/[PPN]Cl displayed excellent catalytic activity (TOF ∼ 1600 h-1) 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 1 and 3 determined activation barriers (Ea) consistent with the observed catalytic trend, i.e., Ea: Ti < Hf.