Biodegradable Copolyesters Derived from 2,5-Thiophenedicarboxylic Acid for High Gas Barrier Packaging Applications: Synthesis, Crystallization Properties, and Biodegradation Mechanisms

2,5-Thiophenedicarboxylic acid (TDCA) is a biobased building block for aromatic–aliphatic copolyesters. This study synthesized poly(propylene succinate-co-thiophenedicarboxylate) (PPSTh) and poly(propylene adipate-co-thiophenedicarboxylate) (PPATh) via two-step melt polycondensation. PPATh70 exhibits the highest melting temperature at 144.8 °C. Crystallization kinetics indicate that diol-TDCA segments primarily form crystalline phases in PPXThs, with long aliphatic units enhancing crystallization. PPXThs containing over 50 mol % TDCA have a higher tensile modulus than poly(butylene adipate-co-terephthalate) (PBAT) and possess excellent gas barrier properties, outperforming PBAT by over 200 times. Dynamic mechanical analysis links the superior gas barrier properties to reduced free volumes. PPAThs degrade faster than PPSThs, with hydrolytic differences explained by Fukui function analysis and DFT calculations. Molecular dynamics simulations clarified the degradation mechanism catalyzed by Candida antarctica lipase B, showing that residues at the entrance interact with PPXTh50 residues, hindering the carbonyl carbon from approaching the catalytic nucleophile, while the flexible PPXTh40 more easily achieves an ideal Bürgi–Dunitz angle for nucleophilic attack.