Aluminum porphyrins with quaternary ammonium halides as catalysts for copolymerization of cyclohexene oxide and CO: metal-ligand cooperative catalysis.

Bifunctional Al porphyrins with quaternary ammonium halides, and , worked as excellent catalysts for the copolymerization of cyclohexene oxide (CHO) and CO at 120 °C. Turnover frequency (TOF) and turnover number (TON) reached 10 000 h and 55 000, respectively, and poly(cyclohexene carbonate) (PCHC) with molecular weight of up to 281 000 was obtained with a catalyst loading of 0.001 mol%. In contrast, bifunctional Mg and Zn counterparts, and , as well as a binary catalyst system, with bis(triphenylphosphine)iminium chloride (PPNCl), showed poor catalytic performances. Kinetic studies revealed that the reaction rate was first-order in [CHO] and [] and zero-order in [CO], and the activation parameters were determined: Δ = 12.4 kcal mol, Δ = -26.1 cal mol K, and Δ = 21.6 kcal mol at 80 °C. Comparative DFT calculations on two model catalysts, Al complex and Mg complex , allowed us to extract key factors in the catalytic behavior of the bifunctional Al catalyst. The high polymerization activity and carbonate-linkage selectivity originate from the cooperative actions of the metal center and the quaternary ammonium cation, both of which facilitate the epoxide-ring opening by the carbonate anion to form the carbonate linkage in the key transition state such as (Δ = 13.3 kcal mol, Δ = -3.1 cal mol K, and Δ = 14.4 kcal mol at 80 °C).