Structure–property relationships in bio‑based methyl salicylate‑derived copolyesters for energy‑absorbing polymer–metal coupler systems

A fully bio-based aromatic diester monomer, dimethyl 2,2′-((1,4-phenylenebis(methylene))bis(oxy))dibenzoate (PMBD), was synthesized from renewable methyl salicylate and 1,4-bis(chloromethyl)benzene and used to prepare copolyesters for energy-absorbing Shibata-type coupler buffers. Two copolyesters based on 1,3-propanediol (H1) or 1,7-heptanediol (H2) were obtained via melt polycondensation of PMBD with 1,4-cyclohexanedimethanol (CHDM). The materials exhibited moderate molecular weights (Mw = 3.96–4.28 × 104 g/mol, Đ = 2.2) and unimodal distributions. Structural characterization (FTIR, 1H NMR) confirmed successful incorporation of all monomers. H1 showed superior thermomechanical performance (Tg= 76 °C, Tm= 185 °C, Td,5% = 348 °C, modulus 1.63 GPa, yield strength 57 MPa, elongation 305%) compared to H2 (Tg = 58 °C, Tm = 170 °C, modulus 1.02 GPa, yield strength 50 MPa, elongation 228%), demonstrating the effect of diol chain length on stiffness and ductility. Both copolymers exhibited yielding and strain hardening with mechanical performance comparable to PET. Soil burial tests showed modest degradation (2.75%–4.21% mass loss over 30 wk), with low ecotoxicity (>80% survival). These results highlight PMBD-based copolyesters as promising sustainable alternatives to PET with tunable properties.