Lin--Copolymerization of CO2 and Epoxides: Bifunctional Catalysts, Mechanisms, and Polymer Applications

Abstract: Tethered phosphonium-borane compounds have been employed as catalysts for CO₂/epoxide copolymerization. Catalyst structures are strategically modified to gain insights into the intricate structure–activity relationship. To quantitatively compare these catalysts, the copolymerization reactions were monitored by in situ Raman spectroscopy. A non-isothermal kinetic technique has been developed, enabling direct mapping of polymerization rate constant (k_p) 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 (T_opt), 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 catalyst resting state is a carbonate species, and the rate-determining step is the ring-opening of epoxide. Excitingly, these efforts identify tertiary phosphonium borane analogues, featuring an acidic phosphonium proton, as leading catalysts on the basis of k_p and T_opt. 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. These results shed light on the cooperative catalysis between phosphonium and borane. The polymer properties and potential applications will also be discussed.

Speaker Bio: Tzu-Pin was trained as a synthetic organometallic/inorganic chemist by Prof. Francois Gabbai at Texas A&M University. After completing his Ph.D., he accepted a postdoc position at Caltech with Prof. Jonas Peters and subsequently Prof. Bob Grubbs. With the background spanning the fields of main group, base-metal catalysis, olefin metathesis, and polymerization, Tzu-Pin began his independent career at ExxonMobil in 2017 as a research chemist. He is particularly interested in polyolefin catalysis, olefin metathesis, CO₂/epoxide copolymerization, and battery materials.