Design and Synthesis of Flavonoid-Based Mono-, Bis-, and Tri-Benzoxazines: Toward Elucidating Roles of Oxazine Ring Number and Hydrogen Bonding on Their Polymerization Mechanisms and Thermal Properties

Abstract

Designing smart chemical structures and gaining insight into the structure–property relationship can effectively guide the development of advanced green thermosetting resins. Herein, four flavonoid-based biophenols were used as sustainable raw materials for achieving novel mono-, bis-, and trifunctional benzoxazine monomers (HYD-a, CHR-a, API-a, and LUT-a). These monomers were developed to discover the roles of the oxazine ring number and hydrogen bonding on their polymerization mechanisms and thermal properties. Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance, and high-resolution mass spectrometry were employed to confirm the successful synthesis of benzoxazine monomers. Polymerization behavior of each benzoxazine was investigated using differential scanning calorimetry, thermogravimetric analysis (TGA), and in situ FT-IR. The thermal properties of the resulting thermosets were evaluated by TGA and microscale combustion calorimetry. With the designed varieties of flavonoid-based benzoxazine structures, it has been found that the intramolecular hydrogen bonding can act as a latent curing agent for reducing the curing temperature while maintaining excellent shelf life. In addition, the onset curing temperature decreases progressively with increasing the number of oxazine rings in the benzoxazine monomer. Interestingly, the polybenzoxazine derived from bis-benzoxazine (API-a) rather than trifunctional monomer shows the best thermal stability (T_d10: 438 °C, Yc: 62%) and flame retardancy (HRC: 11.4 J g^–1 K^–1, THR: 2.22 kJ g^–1), indicating the advantage of designing high-performance thermosets based on flavonoid-based bis-benzoxazines.