Scalable Bifunctional Organoboron Catalysts for Copolymerization of CO2 and Epoxides with Unprecedented Efficiency.J Am Chem Soc. 2020 Jul 15; 142(28):12245-12255.JA
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.