Synthesis and isomeric effect of ethynyl-containing sesamol-based monobenzoxazine resins

The objective of this paper is to conduct a comparative analysis of sesamol-based benzoxazine monomers derived from anilines with an ethynyl substituent in a different position and to obtain some laws to guide the design of thermosets. The synthesis of bio-benzoxazine monomers (S-a, S-oea, S-mea, and S-pea) via a one-step method from sesamol, a biomass monophenol, in the presence of polyformaldehyde by combining four different amines (aniline, ortho-ethynyl aniline, meta-ethynyl aniline, and para-ethynyl aniline), respectively. The structures of four new benzoxazine monomers were characterized by ¹H NMR, ¹³C NMR, 2D-HMQC, and FT IR analysis. The FT IR spectra calculated by Gaussian 16 exhibited a high degree of consistency with the experimental data. The DSC scanning results of the monomers demonstrated that the order of the maximum exothermic peak temperature of polymerization was as follows: S-oea (198.3 °C) < S-pea (202.1 °C) < S-mea (216.8 °C) < S-a (240.2 °C), which is in general agreement with the energy level order of HOMO-LUMO, as determined by Gaussian analysis. In addition, the reaction mechanisms of the oxazine ring and the ethynyl moiety were proposed in conjunction with an investigation of the polymerization process by in-situ FT IR. The TGA and MCC test results showed that the polymers (poly(S-oea), poly(S-mea), and poly(S-pea)) derived from the corresponding monomers with ethynyl substituent in various site exhibit both excellent thermal stability and flame retardancy. Notably, poly(S-pea) exhibited the highest carbon yield (70.2 %) at 800 °C. The incorporation of the ethynyl group not only reduces the polymerization process of benzoxazine monomers but also enhances the performance of resulting polybenzoxazines. This work offers a valuable reference for the preparation of optimal benzoxazine structures in future research.