Carboxylate Salts as Ideal Initiators for the Metal-Free Copolymerization of CO2 with Epoxides: Synthesis of Well-Defined Polycarbonates Diols and Polyols

Tetrabutylammonium carbonate (TBAC) which is obtained by treating CO2 with tetrabutylammonium hydroxide is shown to perform as an ideal difunctional initiator for the copolymerization of carbon dioxide (CO2) and propylene oxide (PO) in the presence of triethylborane (TEB). In this system, CO2 thus serves as the initiating moiety of its own copolymerization with epoxides when used in the form of a carbonate salt. Based on this remarkable result, mono-, tri-, and tetrafunctional ammonium carboxylate initiators and also other difunctional carboxylate initiators were synthesized and used for the synthesis of well-defined omega-hydroxyl-polycarbonates with linear and star structures: Well-defined telechelics, three- and four-armed star samples of molar mass varying from 1 kg/mol to 10 kg/mol, with around 95% carbonate content, were successfully synthesized. The structure of the obtained polycarbonate omega-polyols were characterized by H-1 NMR, MALDI-TOF, and GPC. The terminal hydroxyl functionality of polycarbonate diol was further used for polycondensation with diisocyanates to afford polyurethanes. Finally, taking TBAC as an example, the recyclability of this ammonium-based initiator is demonstrated for the preparation of polycarbonate diols.