Quantum-Chemically Guided Insights on Ionic Liquid Catalysts for Biobased Polycarbonate Synthesis─Mechanistic Exploration and Cation-Functionalized Optimization

The advancement of biobased polymers is intrinsically linked to the optimization of polymerization catalyst design. This necessitates a comprehensive understanding of the structure–effect relationship and the control mechanisms inherent in these catalysts. Recent studies have demonstrated the potential of ionic liquid (IL) catalysts to activate glucose-derived diol isosorbide (ISB), which could overcome the challenge of low and imbalanced hydroxyl activity of ISB. However, the microscopic mechanism of IL-catalyzed melt polycondensation remains elusive, lacking a well-established foundation for the rational design of IL catalysts. Using the melt polycondensation of ISB as a model, this study combines computational and experimental investigations to reveal that IL-catalyzed melt polycondensation follows an addition–elimination mechanism. ILs can alter the mechanistic pathway by forming tetrahedral intermediates, thereby reducing the energy barrier for both the exo- and endo-OH routes. Additionally, cationic functionalization can significantly regulate the electrophilicity and nucleophilicity of ILs, as well as stabilize transition states by C–H bonds and π electrons in the cationic ring. The optimal catalyst 1-ethyl-2-methylpyridinium bromide ([1-C₂-2-C₁Py]Br) achieves the product with M_w of 133.9 kg/mol, which is due to the modification of cations by a weak electron-donating group enhancing π···π weak interactions. This research contributes a theoretical foundation for IL-catalyzed polycondensation processes and catalyst designs, thereby offering a perspective for quantum chemistry-driven investigations in biobased polymer science, and further promotes the development of sustainable, eco-friendly materials and processes in the context of green chemistry and renewable resources.