Synthesis and Properties of Isoliquiritigenin-Based Main-Chain Type Benzoxazine Resins

In this study, two main-chain type benzoxazines (poly(ILG-ddm) main and poly(ILG-pdfa) main ) based on isoliquiritigenin were successfully synthesized, and their structures and properties were systematically characterized and evaluated. FT-IR and NMR analyses confirmed the chemical structures of benzoxazines, verifying the successful formation of the oxazine ring. DSC studies showed that both bio-based thermosetting systems exhibited lower curing temperatures compared to traditional petroleum-based benzoxazines. Notably, poly(ILG-pdfa) main displayed a significantly lower apparent activation energy (~127 kJ/mol) than poly(ILG-ddm) main (~148 kJ/mol), indicating its easier curing and superior processability. TGA results demonstrated that the resulting cured sample, poly(ILG-ddm) main X , possessed higher thermal stability ( T d5 = 352°C, char yield at 800°C = 58.4%). Although poly(ILG-pdfa) main X showed slightly lower thermal stability ( T d5 = 319°C, char yield = 49.8%), it still maintained excellent performance among fully bio-based polymeric materials. DMA analysis revealed that poly(ILG-pdfa) main X exhibited high storage modulus and a high glass transition temperature (266.2°C) due to the strong intermolecular forces and π–π stacking between the furan and chalcone structures. The isoliquiritigenin-derived polybenzoxazines exhibit an optimal balance between low polymerization temperature and high thermal stability, providing a robust matrix for their application in high-performance and sustainable composites.