High Molecular Weight Unsaturated Copolyesters Derived from Fully Biobased trans-β-Hydromuconic Acid and Fumaric Acid with 1,4-Butanediol: Synthesis and Thermomechanical Properties

This work describes the synthesis, structure, and physical and thermomechanical properties of fully biobased unsaturated copolyesters via melt-polycondensation of renewable monomers trans-β-hydromuconic acid and fumaric acid with 1,4-butanediol conducted by a two-step polymerization employing stannous octoate as a catalyst and 4-methoxyphenol as a radical inhibitor. The polymerization conditions, including catalyst amount, second-stage reaction temperature, and time, were optimized to prepare high molecular mass unsaturated copolyesters without isomerization and saturation reaction in the unsaturated double bonds. Consequently, a series of poly­(butylene hydromuconoate-co-butylene fumarate) (PBHBF) samples with weight-average molecular weight over 80 kg/mol were obtained. The chemical structures identified by ¹H and ¹³C NMR indicated that the notorious side reactions of isomerization and saturation of CC bonds in unsaturated polyesters were eliminated in the obtained PBHBF copolyesters. From the physical property analyses by wide-angle X-ray diffraction (WAXD), polarized optical microscopy (POM), and differential scanning calorimetry (DSC), semicrystalline nature was evidenced for all the samples; furthermore, isodimorphic behavior was observed in this copolyester system. The crystallinity degree and thermal properties including glass transition, melting, and crystallization temperatures could be effectively adjusted by controlling the comonomer composition. The resulting copolyesters exhibited excellent tensile properties, which were comparable or even superior to those of polyethylene and most well-known saturated aliphatic polyesters. Overall, these biobased unsaturated polyesters with superior thermomechanical properties and potential biodegradation appear to be the promising materials for practical applications.