Efficient synthesis of isosorbide-based polycarbonate with scalable dicationic ionic liquid catalysts by balancing the reactivity of the endo-OH and exo-OH

Abstract

In this study, a series of high-activity imidazole-based dicationic ionic liquids (DILs) were designed and prepared as efficient catalysts for balancing the reactivity between the endo-hydroxyl group (endo-OH) and the exo-hydroxyl group (exo-OH) of isosorbide (ISO) to synthesize high molecular weight poly(isosorbide carbonate) (PIC). Meanwhile, the thermal performance of PIC was precisely optimized by regulating the chain configuration. The results showed that when the trace amounts (4.5 × 10⁻⁵ based on the ISO molar amount) of bis-(3-methyl-1-imidazole)-ethylene dibromide ([C₂(Min)₂][Br]₂) were used, the weight average molecular weight (M_w) of PIC reached 98 700 g mol⁻¹. It could be concluded from the results of the experiment and the stimulation that the high catalytic activity of DILs was attributed to the strong electrostatic interaction between the cation and the substrate and the effective balance of the reactivity of the endo-OH and the exo-OH. Furthermore, we found that the reduction of hydroxyl groups in the terminal groups and the increase of endo–endo (a₁) structure in the repeating unit improved the thermal properties of PIC. Finally, ¹H NMR, Fourier infrared spectroscopy, and density functional theory (DFT) calculations were used to verify the reaction process through anion and cation multi-site synergistic effect and a possible electrophilic–nucleophilic reaction mechanism was successfully obtained.