Tertiary and Quaternary Phosphonium Borane Bifunctional Catalysts for CO2/Epoxide Copolymerization: A Mechanistic Investigation Using In Situ Raman Spectroscopy

Tertiary and quaternary phosphonium borane catalysts are employed as catalysts for CO2/epoxide copolymerization. Catalyst structures are strategically modified to gain insights into the intricate structure–activity relationship. To quantitatively and rigorously compare these catalysts, the copolymerization reactions were monitored by in situ Raman spectroscopy, allowing the determination of polymerization rate constants. The polymerization rates are very sensitive to perturbations in phosphonium/borane substituents as well as the tether length. To further evaluate catalysts, a nonisothermal kinetic technique has been developed, enabling direct mapping of polymerization rate constant (kp) as a function of polymerization temperatures. By applying this method, key intrinsic attributes governing catalyst performance, such as activation enthalpy (ΔH‡), entropy (ΔS‡), and optimal polymerization temperature (Topt), can be extracted in a single continuous temperature sweep experiment. In-depth analyses reveal intricate trends between ΔH‡, ΔS‡, and Lewis acidity (as determined using the Gutmann–Beckett method) with respect to structural variations. Collectively, these results are more consistent with the mechanistic proposal in which the resting state is a carbonate species, and the rate-determining step is the ring-opening of epoxide. In agreement with the experimental results, DFT calculations indicate the important contributions of noncovalent stabilizations exerted by the phosphonium moieties. Excitingly, these efforts identify tertiary phosphonium borane analogues, featuring an acidic phosphonium proton, as leading catalysts on the basis of kp and Topt. Mediated by phosphonium borane catalysts, epoxides such as butylene oxide (BO), n-butyl glycidyl ether (BGE), 4-vinyl cyclohexene oxide (VCHO), and cyclohexene oxide (CHO) were copolymerized with CO2 to form polyalkylene carbonate with >95% chemo-selectivity. The tertiary phosphonium catalysts maintain their high activity in the presence of large excess of di-alcohols as chain-transferring agents, affording well-defined telechelic polyols. The results presented herein shed light on the cooperative catalysis between phosphonium and borane.