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

Abstract

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-tertbutylimino-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.

Sequence-Controlled Copolymerization of Salicylic O-Carboxyanhydrides by Organocatalysts

“…The former interaction is relatively loose between DBU, TU, monomer, and BnOH, while the latter can produce catalytic center resembling metal complex. Based on our previous work, 67 additional TU-3 or TU-4 is assumed to be situated around the anionic thiomidates through C−H•••F and π−π stacking interactions. The increased steric hindrance of the entire catalytic system and the compressed space of the activation cavity (Scheme 2B) are favorable for enhancing the selectivity of monomers for ROP, contributing to the sequence regulation of the resulting copolymers.…”

“…Salicylic acid (SA) and its derivatives are a class of natural components mainly found in various plants, such as willow bark, holly bark, and white pearl leaves, which possess antibacterial, antifungal, and anti-inflammatory properties. − With significant advancements in the synthesis technology of poly­(salicylic acid)-based (PSA) polymers, − including our developed pathway via ring-opening polymerization of salicylic acid-based O -carboxyanhydride (SAOCA), − PSA-based polyesters present a promising category of environmentally friendly materials suitable for applications in nanomedicine and sustainable plastic packaging. − In this investigation, 1,8-diazabicyclo[5.4.0] undec-7-ene (DBU) serves as the base, combined with TUs of varying acidities to create a set of catalyst pairs, to explore an effective method for achieving precise ROCOP of the mixed SAOCA-series monomers in a one-pot reaction, generating copolymers with defined compositions ranging from random to block sequences. Herein, we systematically explore how the catalytic ability, influenced by changes in acidity and the feed ratio of TU, differentiates the ROP rate of three similar OCA monomers: SAOCA, 5-methylsalicylic acid O -carboxyanhydride (5-MeSAOCA), and 4-fluor-salicylic acid O -carboxyanhydride (4-FSAOCA) (Scheme B).…”