Synthesis and Characterization of Aliphatic-Aromatic Copolyesters Based on Daidzein

A novel diol monomer, 7-(2-hydroxyethoxy)-3-(4-(2-hydroxyethoxy)phenyl)-4H-chromen-4-one (M), was synthesized by hydroxyethylation of daidzein, a lignin-derived compound that can be extracted from leguminous plants. A diester monomer, dimethyl 4,4′-((1,4-phenylenebis(methylene))bis(oxy))bis(3-methoxybenzoate) (N), was obtained through a Williamson etherification reaction of methyl vanillate, potentially derived from plants such as Hovenia dulcis. The reaction of the two biobased monomers coupled with aliphatic diols (ethylene glycol, 1,4-butanediol, 1,6-hexanediol, or 1,8-octanediol) in a two-step melt polymerization process yielded a series of novel aliphatic-aromatic copolyesters. Structure–property relationships of these materials were established through Fourier transform infrared spectroscopy, NMR spectroscopy, gel permeation chromatography, differential scanning calorimetry, thermogravimetric analysis, and dynamic mechanical analysis. The weight-average molecular weight (Mw) of the samples ranged from 62,600 to 69,900 g/mol, and varying the carbon chain length in the aliphatic diol monomers not only induced changes in molecular weights but also synergistically tuned the thermal stability and mechanical properties of the copolymers. Thermal analysis yielded glass transition temperatures (Tg) of 115–141 °C, melting temperatures (Tm) of 271–308 °C, and 5% decomposition temperatures (Td,5%) of 397–460 °C. The mechanical analysis exhibited high yield strength (50–115 MPa) and elongation at break (235–300%) values. After 32 weeks of degradation in soil, the copolyesters experienced mass losses of up to 4.1%. Ecotoxicity studies showed that the 14-day survival rate of earthworms exposed to the copolyesters was above 80%, suggesting a low toxicity in the environment. Overall, the excellent thermal and mechanical properties, as well as significant biodegradability, of these copolyesters make them suitable to replace petroleum-based commercial polyesters, thereby offering innovative solutions to the problem of environmental pollution.