Rational Optimization of Ammonium and Phosphonium Cations of Bifunctional Organoborane Catalysts for Copolymerization of Propylene Oxide with CO<sub>2</sub> to Afford Poly(propylene carbonate)
Cheng‐Kai Xu, Chenjie Lu, Shuo Zhao, Guan‐Wen Yang, Wei Li, Jingdai Wang, Guang‐Peng Wu
Abstract
Ring-opening copolymerization (ROCOP) of CO 2 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 CO 2 with cyclohexene oxide ( J. Am. Chem. Soc., 2020, 142, 12245–12255), but only cyclic carbonate was obtained during the copolymerization of CO 2 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 CO 2 /PO copolymerization to produce poly(propylene carbonate) (PPC) with >99% selectivity. Optimal catalytic performance was achieved by catalysts bearing alfa-H ( α H) atoms in Et 3, n Pr 3, and n Bu 3 substituents for both ammonium and phosphonium cations. Notably, catalysts featuring a cation without an α H atom (even with beta-H, β H) exhibited inferior performance in both catalytic activity and PPC selectivity, suggesting the indispensable role of α H atoms of cations. An intramolecular α H atom-dominated interaction over β H, which is useful to suppress the backbiting side reaction and to facilitate chain propagation, was therefore proposed. Further, the 31 P 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.