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

Abstract

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.