Synthesis and Characterization of Asymmetric Bio-Based Bisbenzoxazine with a Single Diethyl Phosphite Substituted Oxazine Ring

The severity of fire accidents and sustainable development concerns have made people increasingly aware of the importance of producing flame-retardant materials using biomass resources. Herein, we successfully synthesized an asymmetric biobisbenzoxazine monomer with a single diethyl phosphite-substituted oxazine ring (ST-fa-DEP) based on natural, renewable salicylaldehyde, tyramine, and furfurylamine. The structural characterization of ST-fa-DEP was performed using Fourier-transform infrared (FT-IR) and nuclear magnetic resonance (NMR) spectroscopies as well as high-resolution mass spectrometry (HR-MS), which confirms the successful inheritance of the single diethyl phosphite into the oxazine ring substituted bisbenzoxazine monomer. The ring-opening polymerization behavior of ST-fa-DEP was examined by using differential scanning calorimetry (DSC) and in situ FT-IR analysis. In addition, the resulting polybenzoxazine, poly(ST-fa-DEP), showed excellent thermal performance and flame retardancy with a Td10 of 413 °C, a Yc of 62.5,% and an HRC value of 53 J·g–1·K–1. Moreover, we used ST-fa-DEP as a property modifier for enhancing the comprehensive properties of the well-commercialized bisbenzoxazine resin (PH-ddm). Notably, the glass transition temperature of the resulting thermoset increased to 290 °C, and a V-0 rating in UL-94 testing was also achieved by adding only 3 wt % ST-fa-DEP in PH-ddm, which breaks the trade-off between the thermal stability and flame retardancy of other reported flame-retardant polybenzoxazines. The results obtained in the current work provide molecular-level insights into the structure–property relationship of polybenzoxazines as well as an angle for the design of highly thermal stable and intrinsically flame-retardant biothermosets.