High-Molecular-Weight Biobased Polycarbonate Preparation Using Metal-Free Deep Eutectic Solvents: Efficient Activation of the Hydroxyl Groups

Abstract

Activating hydroxyl groups in monomers is crucial for preparing high-molecular-weight bioderived polycarbonates. Herein, we developed metal-free deep eutectic solvents (DESs) to activate endo- and exo-hydroxyl groups in isosorbide (ISB) through hydrogen bonding interactions, facilitating the preparation of poly(isosorbide carbonate) (PIC) by melt polycondensation with diphenyl carbonate (DPC). Catalytic evaluation results indicated that the Brønsted acid–base properties of DESs had a greater impact on the M_n of PIC than the molecular size and Lewis acid–base properties. The DESs with a pH value close to 7, a smaller molecular size, and a higher Lewis acidity yielded PICs with a higher M_n. The formation of hydrogen bonds between the DES and the monomer was conducive to the preparation of high-molecular-weight PIC. Among all DES catalysts, the synthesis of PIC catalyzed by choline chloride–ethylene glycol (n:n = 1:2) had the highest M_n, reaching 116,100 g·mol^–1. Obtained PIC maintained a high transmittance (>90%) and low yellowness (b* < 0.40). Mechanistic study demonstrated the synergistic activation of ISB and DPC by hydrogen bonding sites of the hydrogen bonding acceptor and hydrogen bonding donor in the DES to prepare PIC. This strategy not only provides a low-cost and efficient method for synthesizing PIC but also offers catalyst design guidance for hydroxyl activation.