Aluminum porphyrins with quaternary ammonium halides as catalysts for copolymerization of cyclohexene oxide and CO2: metal–ligand cooperative catalysis. Issue 22 (26th May 2020) - Deng, Jingyuan
Abstract : Bifunctional Al porphyrins worked as excellent catalysts for the copolymerization of cyclohexene oxide (CHO) and CO2 . Abstract : Bifunctional Al III porphyrins with quaternary ammonium halides, 2-Cl and 2-Br, worked as excellent catalysts for the copolymerization of cyclohexene oxide (CHO) and CO2 at 120 °C. Turnover frequency (TOF) and turnover number (TON) reached 10 000 h −1 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 II and Zn II counterparts, 3-Cl and 4-Cl, as well as a binary catalyst system, 1-Cl with bis(triphenylphosphine)iminium chloride (PPNCl), showed poor catalytic performances. Kinetic studies revealed that the reaction rate was first-order in [CHO] and [2-Br ] and zero-order in [CO2 ], and the activation parameters were determined: Δ H ‡ = 12.4 kcal mol −1, Δ S ‡ = −26.1 cal mol −1 K −1, and Δ G ‡ = 21.6 kcal mol −1 at 80 °C. Comparative DFT calculations on two model catalysts, Al III complex 2′ and Mg II complex 3′, allowed us to extract key factors in the catalytic behavior of the bifunctional Al III 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 TS3b (Δ H ‡ = 13.3 kcal mol −1, Δ S ‡ = −3.1 cal mol −1 K −1, and Δ G ‡ = 14.4 kcal mol −1 at 80 °C).