Rational Optimization of Ammonium and Phosphonium\nCations of Bifunctional Organoborane Catalysts for Copolymerization\nof Propylene Oxide with CO<sub>2</sub> to Afford Poly(propylene carbonate)

Ring-opening\ncopolymerization (ROCOP) of CO<sub>2</sub> and propylene\noxide (PO) is a challenging task due to its tendency to generate a\npolyether linkage and cyclic carbonate. Our group recently reported\na series of mononuclear organoborane catalysts for the efficient ROCOP\nof CO<sub>2</sub> with cyclohexene oxide (<i>J. Am. Chem. Soc.,</i> <b>2020</b>, <i>142</i>, 12245–12255), but\nonly cyclic carbonate was obtained during the copolymerization of\nCO<sub>2</sub> with PO (<i>Angew. Chem. Int. Ed.</i> <b>2020</b>, <i>59</i>, 23291–23298). By modulating\nthe cationic part of the catalysts, herein, we upgraded our previous\nborinane-based and 9-BBN-based mononuclear organoborane catalytic\nsystems and successfully realized the alternating CO<sub>2</sub>/PO\ncopolymerization to produce poly(propylene carbonate) (PPC) with >99%\nselectivity. Optimal catalytic performance was achieved by catalysts\nbearing alfa-H (<sup>α</sup>H) atoms in Et<sub>3</sub>, <sup><i>n</i></sup>Pr<sub>3</sub>, and <sup><i>n</i></sup>Bu<sub>3</sub> substituents for both ammonium and phosphonium\ncations. Notably, catalysts featuring a cation without an <sup>α</sup>H atom (even with beta-H, <sup>β</sup>H) exhibited inferior\nperformance in both catalytic activity and PPC selectivity, suggesting\nthe indispensable role of <sup>α</sup>H atoms of cations. An\nintramolecular <sup>α</sup>H atom-dominated interaction over <sup>β</sup>H, which is useful to suppress the backbiting side reaction\nand to facilitate chain propagation, was therefore proposed. Further,\nthe <sup>31</sup>P NMR spectra study indicated that the superior catalytic\nactivity of phosphonium-based catalysts than its ammonium counterparts\nstems from the stronger Lewis acidity of the catalyst molecule imparted\nby the phosphonium cation. We believe the insights into the optimization\nof the cationic part of organoborane catalysts could inspire more\nadvanced catalysts in the future.