Thermal Swelling Facilitates Selective Monotransesterification of Dimethyl Carbonate with Biomass Alcohols over Metal-Free Flexible Ionic Polymers

Selective conversion of biomass-derived alcohols to unsymmetrical carbonates is of great significance. However, achieving this reaction in both high efficiency and selectivity is highly challenging, which requires the use of massive access of symmetric carbonates and metal-containing heterogeneous catalysts, arising serious environmental concerns. In this study, facile synthesis of a series of imidazolium-derived flexible ionic porous organic polymers via the Menshutkin reaction is presented. These iPOPs serve as metal-free catalysts for selective unsymmetrical transesterification of dimethyl carbonate with various common and biomass-derived alcohols to give unsymmetrical organic carbonates in high yields (up to 98%) and high selectivities (>99%) after a necessary thermal induction at 125 °C. The cryo-scanning electron microscopy reveals that a swelling behavior of these iPOPs, triggered by elevated reaction temperature, opens the “gates” of pore channels, which may facilitate substrate diffusion to access internal active ionic sites. Moreover, the confinement effect may impede a secondary transesterification reaction, thereby hinder the formation of symmetric carbonates and thus promote the desorption of the unsymmetrical organic carbonate products. Besides, a possible reaction mechanism of the synergistic substrate activation between the imidazolium cation and Br– anion for unsymmetrical transesterification is proposed based on the experimental results and theoretical calculations.