Elucidating the role of acetylene in ortho-phthalimide functional benzoxazines: design, synthesis, and structure–property investigations

Two novel acetylene-containing ortho-phthalimide functional benzoxazine monomers have been designed and synthesized in this work. We systematically investigated the effect of an additional acetylene moiety on the characteristics of ortho-phthalimide functional benzoxazines, with a particular focus on the structural configuration, polymerization behaviors and the associated thermal stability and flame retardancy of polybenzoxazine thermosets. We first employed NMR, FT-IR, elemental analysis, and density functional theory (DFT) calculations to study the structural details of benzoxazine, followed by DSC and in situ FT-IR spectroscopy to investigate the polymerization processes of each benzoxazine monomer. Specifically, the thermoset based on mono-benzoxazine bearing two acetylene groups exhibits extremely high thermal stability (Td5 of 404 °C; Tg of 297 °C) and excellent flame retardancy (HRC < 10 J g−1 K−1), although expected benzoxazole cyclization at a much higher temperature (∼400 °C) did not take place. Our work suggests that the incorporation of acetylene moieties in the ortho-phthalimide mono-benzoxazine is an effective strategy to improve the comprehensive properties of polybenzoxazine thermosets.