Mechanism Study on Synthesis of Biobased Polycarbonate from Diphenyl Carbonate and Isosorbide with Metal-Free Catalysts by Identifying Reactivity of endo-OH and exo-OH

Isosorbide (ISB), a promising biobased monomer with two asymmetric hydroxyl groups, however, exhibits an unsatisfying reactivity in the poly(isosorbide carbonate) (PIC) synthesis with metal-free catalysts. In this work, we employed density functional theory (DFT) calculations to address the existing disputes surrounding the catalytic mechanisms, focusing on two potential reaction pathways catalyzed by tetraethylammonium hydroxide ([TEA][OH]). Energy profiles reveal that the synergistic catalytic mechanism represents the energetically favorable route for the PIC synthesis. Meanwhile, calculations disclose the quaternary ammonium base as the catalyst precursor with tetraethylammonium phenol ([TEA][Ph]) acting as the effective catalyst in the catalytic cycle. Seeking enhanced catalytic efficiency, cation modification of the quaternary ammonium base was conducted, and hexadecyl trimethylammonium hydroxide ([HTMA][OH]) exhibits exceptional performance with low energy barriers of 29.1 and 30.0 kcal/mol for exo -OH and endo -OH, respectively. This aligns with kinetic experiments, emphasizing the superior catalytic efficiency of [HTMA][OH]. The proposed mechanism model enables accurate predictions of the terminal hydroxyl group distribution. A thorough examination of steric hindrance and electronic effect impact on reaction pathway and catalytic efficiency during transesterification for PIC offers significant insights for efficient catalyst design not only for biobased polyester synthesis but also for the chemical recycling strategy of wasted ones.