Zwitterionic ring-opening polymerization of macrocyclic ethyleneoxy-substituted carbonate: Access to cyclic PEG-like polycarbonate

The innovation in polymer design to rival conventional polyethylene glycol (PEG) is an important approach to achieving a more sustainable society. Here, cyclic PEG-like polycarbonates having high molecular weight (4.4–49.5 kg/mol) were enabled through zwitterionic ring-opening polymerization (ZROP) of macrocyclic carbonates (MCs) mediated by N-heterocyclic carbene (NHC). The thermodynamic behavior of polymerization depends on the ring size of monomers. During this process, the ZROP of 11-membered MC was driven by the change of enthalpy (ΔH_p) which differed from the ZROP of 14-membered MC driven by the entropic change (ΔS_p). Cyclic polycarbonates depicted improved thermostability (T_d5% ≥ 204 °C) and higher glass transition temperatures (T_g > ‒40 °C) in comparison to their linear analogues (T_d5% ≤ 185 °C, T_g ∼‒50 °C). In addition, the mechanism of ZROP of MC was addressed through computational study. A distinct mechanism of polymerization distinguishable from the well-known NHC-mediated ZROP of cyclic esters was revealed, where the zwitterion from nucleophilic addition to MC, i.e. tetrahedral intermediate, cannot be ring-opened probably due to the delocalization of negative charge on the carbonate group, but serves as an active center for the polymerization. In comparison to PEG, the attained polymer demonstrated comparable hydrophilic and biocompatible properties, as revealed by the results of contact angle and in vitro cytotoxicity studies, suggesting that cyclic polycarbonate hold the promise as the alternative of PEG.