Solvent-free Synthesis of Alkynyl-Based Biobased Benzoxazine Resins with Excellent Heat Resistance

The synthesis of high-performance biomass benzoxazine resin by a solvent-free method is of great significance for environmental sustainability and practical application in the industrial field. In this paper, biobased eugenol and magnolol with additional allyl cross-linking sites were taken as phenol sources, and 3-aminophenylacetylene with alkynyl cross-linking sites was used as an amine source to prepare two biomass benzoxazine monomers E-apa and M-apa, and the corresponding cured resins poly(E-apa) and poly(M-apa) were prepared through a certain curing process. The comprehensive properties of the cured resins were studied. The results show that the glass-transition temperature ( T g ) of poly(M-apa) is as high as over 400 °C, which is higher than that of poly(E-apa) (215 °C), as well as the T g values of the typical biobased heat-resistant benzoxazine resins reported so far. At the same time, the initial thermal decomposition temperature ( T di ) of poly(M-apa) (441 °C) is higher than that of poly(E-apa) (373 °C), showing better thermostability. In addition, poly(M-apa) also exhibits better mechanical properties including higher storage modulus and tensile strength (5.03 GPa, measured at 25 °C, and 48.0 ± 2.2 MPa, respectively) than poly(E-apa) (3.93 GPa and 37.5 ± 1.5 MPa, respectively). Besides, poly(M-apa) performs better in flame retardancy. The better comprehensive properties of poly(M-apa) are based on the combined action of multiple cross-linking sites and rigid phenol sources.