Scalable Bifunctional Organoboron Catalysts for Copolymerization of CO₂ and Epoxides with Unprecedented Efficiency

Abstract

The metallic catalyst-dominated alternating copolymerization of CO₂ and epoxides has flourished for 50 years; however, the involved multistep preparation of the catalysts and the necessity to remove the colored metal residue in the final product present significant challenges in scalability. Herein, we report a series of highly active metal-free catalysts featured with an electrophilic boron center and a nucleophilic quaternary ammonium halide in one molecule for copolymerization of epoxides and CO₂. The organocatalysts are easily scaled up to kilogram scale with nearly quantitative yield via two steps using commercially available stocks. The organocatalyst-mediated copolymerization of cyclohexane oxide and CO₂ displays high activity (turnover frequency up to 4900 h^-1) and >99% polycarbonate selectivity in a broad temperature range (25-150 °C) at mild CO₂ pressure (15 bar). At a feed ratio of cyclohexane oxide/catalyst = 20 000/1, an efficiency of 5.0 kg of product/g of catalyst was achieved, which is the highest record achieved to date. The unprecedented activity toward CO₂/epoxide copolymerization for our catalyst is a consequence of an intramolecular synergistic effect between the electrophilic boron center and the quaternary ammonium salt, which was experimentally ascertained by reaction kinetics studies, multiple control experiments, ¹¹B NMR investigation, and the crystal structure of the catalyst. Density functional theory calculations further corroborated experimental conclusions and provided a deeper understanding of the catalysis process. The metal-free characteristic, scalable preparation, outstanding catalytic performances along with long-term thermostability demonstrate that the catalyst could be a promising candidate for large-scale production of CO₂-based polymer.