Biobased Polyesters Derived from 2-Methoxyhydroquinone: Impact of Cyclic and Alkyl Chain Segments on Their Thermomechanical Properties, Biodegradability, and Ecotoxicity

Abstract

To enhance the properties of bio-based polyesters, enabling them to more closely mimic the characteristics of terephthalate-based materials, a series of aliphatic-aromatic copolyesters (P₁–P₄) were synthesized via melt polycondensation. Diester monomers M and N were synthesized via the Williamson reaction, using lignin-derived 2-methoxyhydroquinone, methyl 4-chloromethylbenzoate, and methyl chloroacetate as starting materials. Hydroquinone bis(2-hydroxyethyl)ether (HQEE) and 1,4-cyclohexanedimethanol (CHDM) were employed as cyclic segments, while 1,4-butanediol (BDO) and 1,6-hexanediol (HDO) served as alkyl segments within the copolymer structures. The novel copolyesters exhibited molecular weights (M_w) in the range of 5.25×10⁴–5.87×10⁴ g/mol, with polydispersity indices spanning from 2.50–2.66. Evaluation of the structural and thermomechanical properties indicated that the inclusion of alkyl segments induced a reduction in both crystallinity and molecular weight, while significantly improving the flexibility, whereas cyclic segments enhanced the processability of the copolyesters. Copolyesters P₁ and P₂, due to the presence of rigid segments (HQEE and CHDM), displayed relatively high glass transition temperatures (T_g>80 °C) and melting temperatures (T_m>170 °C). Notably, P₂, incorporating CHDM, exhibited superior elongation properties (272%), attributed to the enhanced chain mobility resulting from its trans-conformation, while P₁ was found to be likely brittle owing to excessive chain stiffness. Biodegradability assessment using earthworms as bioindicators revealed that the copolyesters demonstrated moderate degradation profiles, with P₂ exhibiting a degradation rate of 4.82%, followed by P₄ at 4.07%, P₃ at 3.65%, and P₁ at 3.17%. The higher degradation rate of P₂ was attributed to its relatively larger d-spacing and lower toxicity, which facilitated enzymatic hydrolytic attack by microorganisms. These findings highlight the significance of optimizing the structural chain segments within aliphatic-aromatic copolyesters. By doing so, it is possible to significantly enhance their properties and performance, offering viable bio-based alternatives to petroleum-based polyesters such as polyethylene terephthalate (PET).