Tunable protic ionic liquid catalysts for the efficient one-step synthesis of isosorbide-based polycarbonates

Using CO₂-derived dimethyl carbonate (DMC) instead of diphenyl carbonate (DPC) as a carbonyl source for synthesizing bio-based polycarbonates is a green and cost-effective route. However, the synthesis of high-performance polycarbonates via the DMC route remains challenging due to the poor reactivity and selectivity of DMC compared to DPC. Herein, we designed a series of highly active protic ionic liquid (PIL) catalysts for the synthesis of poly(isosorbide carbonate) (PIC) from DMC with ISB. The influences of the structures of anion and cation on the catalytic activity of PILs were systematically studied. Compared with the reported aprotic IL catalysts, the unique reactive hydrogen of the cation in PILs could form a strong hydrogen bond interaction with the carbonyl group of DMC, resulting in higher reactivity of the carbonyl carbon of DMC. Moreover, the nucleophilicity of the anion could be easily tuned by adjusting the pK_a value, which effectively realized the balance of the reactivity difference between exo-OH and endo-OH in ISB. Among them, [DBUH][Im] showed the highest catalytic activity, and the weight-average molecular weight (M_w) and glass transition temperature of PIC reached 55,700 g/mol and 160 °C, respectively. Combined with NMR analyses and DFT calculations, the mechanism that exhibited the synergetic catalytic effect of anion-cation for the polymerization of DMC and ISB was presented.