Tailoring Molecular Weight of Bioderived Polycarbonates via Bifunctional Ionic Liquids Catalysts under Metal-Free Conditions

Synthesis of bioderived high-molecular-weight polycarbonates over metal-free catalysts is of great importance but also challenging. In this work, a series of 1-butyl-3-methylimidazolium (Bmim) ionic liquids (ILs) were prepared as catalysts for a melt polycondensation reaction of isosorbide and diphenyl carbonate. By modifying the structures of ILs’ anions, the number-average molecular weight (<i>M</i><sub>n</sub>) of poly­(isosorbide carbonate) (PIC) was effectively tailored. In the presence of a trace amount (0.05 mol % based on isosorbide) of bifunctional [Bmim]­[CH<sub>3</sub>CHOHCOO], the synthesized PIC possessed high <i>M</i><sub>n</sub> of 61,700 g/mol and a glass transition temperature of 174 °C, both the highest so far to the best of our knowledge. Besides, it was found that the anions with stronger electronegativity and hydrogen bond formation ability were more efficient for the formation of PIC with higher <i>M</i><sub>n</sub>. To modify the flexibility of PIC, poly­(aliphatic diol-<i>co</i>-isosorbide carbonate)­s with <i>M</i><sub>n</sub> ranging from 34,000 to 75,700 g/mol were also formulated by incorporating with various aliphatic diols. Additionally, based on the experimental results and nuclear magnetic resonance spectroscopy, a possible mechanism of cooperative nucleophilic–electrophilic activation through hydrogen bond formation and electrostatic interactions by the ILs catalyst was proposed.