Facile Synthesis of Polycarbonate Diol via Copolymerization of CO2 and Cyclohexene Oxide Catalysed by a Combination of One-Component Phosphonium Borane Lewis Pair and Water

Well-defined polycarbonate diol was successfully synthesized through a strategy using a combination of organocatalyst and water. Such strategy was less developed in organocatalyzed polymerization and frequently regarded as side reactions. Herein, one-component phosphonium borane Lewis pairs PB1-PB8 were successfully applied in the copolymerization of CO2 and cyclohexene oxide (CHO) to generate poly(CHO-alt-CO2) carbonate (PCHC). Parameters of linker length and counter anion effects on the catalyst activity were investigated. It was found that Lewis pair PB3 served as a dual initiator and catalyst in the copolymerization of CHO and CO2 with or without the presence of water. In contrast, Lewis pair PB8 can serve as a true catalyst for the preparation of well-defined alpha,omega)-hydroxyl PCHC diols. This was achieved by introducing a labile CF3COO group as counter anion through anion exchange reaction while water molecules acted as chain transfer agents. The function of trifluoroacetate group in the polymerization process was investigated in detail and possible mechanism was proposed. Upon changing the amount of water and catalyst loading, PCHC diols with varied molecular weight (1.5 kg/mol to 7.5 kg/mol), low dispersities (D<1.2) and carbonate content (>99%) could be easily obtained. The low molecular weight PCHC diol was used as a bifunctional macroinitiator for the ring-opening polymerization of L-lactide (LLA) to afford ABA triblock copolymer in one-pot synthesis.

Automated summary

A well-defined polycarbonate diol was successfully synthesized using a strategy involving organocatalyst and water. One-component phosphonium borane Lewis pairs were applied in the copolymerization of CO2 and cyclohexene oxide, and Lewis pair PB3 was found to serve as a dual initiator and catalyst. Lewis pair PB8, on the other hand, acted as a true catalyst for the preparation of well-defined alpha,omega)-hydroxyl PCHC diols through anion exchange reaction with water acting as a chain transfer agent.