A one-component phosphonium borane Lewis pair serves as a dual initiator and catalyst in the ring-opening alternating copolymerization of anhydrides and epoxides

Abstract

Lewis pairs as synergistic catalysts have demonstrated versatile adaptabilities towards different monomers. Herein, we converted a bi-component Lewis pair into a one-component Lewis pair by integrating a Lewis acid and ionic Lewis base within one molecule to enhance the synergistic effect on the polymerization reaction. To exemplify the design strategy, one-component phosphonium borane Lewis pairs 1–6 were rationally designed, readily synthesized, and employed as metal-free catalysts for the ring-opening alternating copolymerization (ROAC) of anhydrides and epoxides. Detailed MALDI-ToF MS analysis illustrated the microstructure of the obtained copolymers and verified the presence of two types of initiating species derived from Br⁻ and cyclohexane-1,2-diol (CHD) in the polymerization set-up. Kinetic studies and in situ¹¹B{¹H} NMR experiments further clarified the mechanism of polymerization. Strikingly, Lewis pair 3 featuring pentamethylene –(CH₂)₅– linked triphenylphosphonium bromide (Lewis base) and a borabicyclo[3.3.1]nonane moiety (Lewis acid) showed high activity (TOF = 1920 h⁻¹), thermal robustness (120 °C) and good alternating selectivity (>92%) in the ROAC of phthalic anhydride (PA) and cyclohexene oxide (CHO). Lewis pair 3 achieved a feed ratio of CHO/PA/catalyst = 15 000 : 10 000 : 1 in 96% conversion within 5 h at 150 °C. The higher molecular weight fraction of the produced P(PA-alt-CHO) had an M_n value of 51.8 kg mol⁻¹ with a polydispersity Đ = 1.10. The lower molecular weight fraction was ascribed to Br⁻ initiation. In particular, P(PA-alt-CHO) with unimodal distribution can be generated by deliberate addition of water. The relatively larger radius parameter of the phosphonium countercation led to a weak electrostatic interaction, thus yielding a loose Lewis pair to allow for much easier dissociation of carboxylate active species, accounting for the high catalytic and synergistic behavior of the designed phosphonium borane Lewis pair in the ROAC of PA and CHO. One-component Lewis pairs will not only diversify organocatalyst structures, but also deepen our understanding of the polymerization mechanism and provide a possibility to realize controlled copolymerization of anhydrides and epoxides.