Rational Optimization of Ammonium and Phosphonium Cations of Bifunctional Organoborane Catalysts for Copolymerization of Propylene Oxide with CO₂ to Afford Poly(propylene carbonate)

Ring-opening copolymerization (ROCOP) of CO2 and propylene oxide (PO) is a challenging task due to its tendency to generate a polyether linkage and cyclic carbonate. Our group recently reported a series of mononuclear organoborane catalysts for the efficient ROCOP of CO2 with cyclohexene oxide (J. Am. Chem. Soc., 2020, 142, 12245–12255), but only cyclic carbonate was obtained during the copolymerization of CO2 with PO (Angew. Chem. Int. Ed. 2020, 59, 23291–23298). By modulating the cationic part of the catalysts, herein, we upgraded our previous borinane-based and 9-BBN-based mononuclear organoborane catalytic systems and successfully realized the alternating CO2/PO copolymerization to produce poly(propylene carbonate) (PPC) with >99% selectivity. Optimal catalytic performance was achieved by catalysts bearing alfa-H (αH) atoms in Et3, nPr3, and nBu3 substituents for both ammonium and phosphonium cations. Notably, catalysts featuring a cation without an αH atom (even with beta-H, βH) exhibited inferior performance in both catalytic activity and PPC selectivity, suggesting the indispensable role of αH atoms of cations. An intramolecular αH atom-dominated interaction over βH, which is useful to suppress the backbiting side reaction and to facilitate chain propagation, was therefore proposed. Further, the 31P NMR spectra study indicated that the superior catalytic activity of phosphonium-based catalysts than its ammonium counterparts stems from the stronger Lewis acidity of the catalyst molecule imparted by the phosphonium cation. We believe the insights into the optimization of the cationic part of organoborane catalysts could inspire more advanced catalysts in the future.