Effect of Structural Isomerism in Imidazolium-Based Ionic Liquid on Polycarbonate Ethanolysis

Depolymerization of polycarbonate (PC) is an efficient and environmentally friendly approach to recycling waste PC. Recently, ionic liquids (ILs) have attracted attention as sustainable and recyclable catalytic solvents for PC depolymerization. In this study, we investigated the effect of alkyl chain isomerism in imidazolium-based ILs, focusing on four different isomers: n-butyl, sec-butyl, isobutyl, and tert-butyl. To elucidate the factors influencing PC depolymerization using ILs, excess enthalpy and hydrogen bonding energy of PC in the IL mixtures were calculated using the conductivity screening model for realistic solvent (COSMO-RS). Among the ILs examined, 1-butyl-3-methylimidazolium acetate ([n-BMIM][Ac]) exhibited the highest depolymerization efficiency in terms of PC conversion, bisphenol A (BPA) yield, and BPA purity and also showed the strongest hydrogen bonding energy of PC in IL-ethanol mixture. Even after five consecutive recycling cycles of PC ethanolysis, the IL maintained 100% PC conversion, 99% BPA yield, and 96% BPA purity. These results suggest that structural tuning of ILs can be a viable strategy for designing efficient and recyclable catalytic solvents for polymer depolymerization.