Facile Synthesis of Aromatic Polyesters with High Molecular Weights via Lewis Pair-Mediated Ring-Opening Polymerization of Salicylic Acid-Derived O‑Carboxyanhydrides

Compared to fossil resources, which display limited degradability, exploring novel green polyesters as more sustainable alternatives is an extremely challenging task. In this study, we have developed an effective pathway from natural origin salicylic acid (SA) to poly(salicylic acid) (PSA) with high molecular weight. This was achieved by employing Lewis pair-mediated ring-opening polymerization of salicylic acid O-carboxyanhydrides (SAOCA). We identified two metal-free combinations, namely, 1,3-bis(2,6-diisopropylphenyl)imidazole (NHC-1)/N-[3,5-bis(trifluoromethyl)phenyl]-N′-cyclohexyl thiourea (TU-1) and 2-tert-butylimino-2-diethylamino-1,3-dimethyl-perhydro-1,3,2 diazaphosphorine (BEMP)/TU-1, that mediated ROP of SAOCA at room temperature within 100 min and successfully produced PSA with molecular weight greater than 150 kDa. We observed that the phase state of the polymerization solution switched from clarification to turbidity to phase separation when the PSA chain was growing up. Based on the NMR results of the interaction between organic base and TU-1, the analysis of polymerization kinetics, and DFT calculations, we found that the excess addition of TU-1 could effectively restrain unfavorable results in which very fast propagation of the polymerization chain caused phase separation in a short time. Furthermore, it improved the controllability of ROP reactions and the resulting chain length of PSA. The generality of this synthetic strategy was evaluated by the convenient preparation of other PSA-series polyesters via the ROP of SAOCA analogues varying the substituents. The obtained PSA-series polyesters exhibited good thermal stability and high glass transition temperature, outperforming some commodity polyolefins. This synthetic method paves the way for a better understanding of PSA-based polyesters as a class of future green materials applicable in the field of biomedicine and sustainable plastic packaging.