Borinane-based organoboron catalysts for alternating copolymerization of CO2 with cyclic ethers: improved productivity and facile recovery

Abstract

The design and synthesis of organoboron catalysts for effective alternating ring-opening copolymerization (ROCOP) of CO₂ with cyclic ethers are described. Such organoboron catalysts include an ammonium salt which is connected to a boron center, separated from the ammonium cation by a few carbon–carbon bonds. The type of boron center whether it was carried by borinane or by 9-BBN, the distance separating the boron center from the ammonium cation, and the size of the substituents of the cation were the various parameters considered when assessing the performance of these organoboron catalysts. Of all the boron-based catalysts, the borinane-based catalyst 5 enabled the copolymerization of CO₂ and epoxides with the highest productivity: 271.5 g of poly(propylenecarbonate) per g of catalyst and 5.7 kg of poly(cyclohexene carbonate) per g of catalyst, respectively. In addition, 5 could be easily recycled via a simple precipitation process and also used as a difunctional initiator when combined with a diacid. Density Functional Theory computation gives a deep insight into the mechanism underlying the bifunctional catalyst-mediated CO₂ copolymerization and reveals the importance of the type of boron centers in the overall performance.