Triethyl borane-regulated selective production of polycarbonates and cyclic carbonates for the coupling reaction of CO2 with epoxides

Abstract

The copolymerization of carbon dioxide (CO₂) with epoxides via organocatalysis is still a big challenge. This work reports the selective copolymerization of CO₂ and epoxides bearing a phenyl group, i.e., phenyl glycidyl ether (PGE) and styrene oxide (StO), catalyzed by a combination of Lewis bases (LBs) and excess triethyl borane (TEB). The resultant CO₂/PGE copolymers presented a molar fraction of carbonate units (F_CO2) of more than 99%, with a regioregularity of ca. 92%. The weight percentage of the cyclic carbonate (W_CC) was less than 1 wt%. The turnover frequency (TOF) was as high as 38 h⁻¹ for producing the CO₂/PGE copolymer with a number-average molecular weight (M_n) of 16.6 kg mol⁻¹ and a dispersity (Đ) of 1.2. What is of importance is that the dosage of TEB had a strong impact on the selectivity, where the regioregular copolymer or cyclic carbonate could be selectively produced by simply tuning the TEB/LB molar ratios, as revealed by in situ FT-IR spectroscopy. It is proposed that excess TEB could either promote the equilibrium to generate the TEB-coordinated growing anion or activate the epoxide for enhancing the reactivity. Both copolymers are promising optical materials as colorless solids with a high refractive index (n_d) of 1.55–1.56 (590 nm, 20 °C, cast film).