Thermally Activated Structural Changes of a Norbornene–Benzoxazine–Phthalonitrile Thermosetting System: Simple Synthesis, Self-Catalyzed Polymerization, and Outstanding Flame Retardancy

Phthalonitrile polymers exhibit many attractive advantages including high thermal and mechanical properties, low water absorptivity, and excellent flame retardancy. However, the poor processability requiring high temperature polymerization for long duration seriously impedes their further applications. In this work, a benzoxazine monomer bearing both norbornene and phthalonitrile functionalities (oHPNI-apn) has been synthesized, and its chemical structure has been confirmed by NMR, FT-IR, and elemental analysis. In addition, the polymerization behavior of oHPNI-apn has been systematically studied using DSC, in situ FT-IR and solid-state 13C NMR analysis. The phthalonitrile in this developed thermosetting system can be thermally induced polymerized at a relatively lower temperature with no added catalysts, which indicates the existence of self-catalyzed effect during the terpolymerization of oHPNI-apn. Moreover, oHPNI-apn-based thermoset is found to possess excellent thermal stability and outstanding flame retardancy, with extremely high Tg temperature (419 °C), very high char yield value (68%, at 800 °C in N2), and extraordinarily low heat release capacity (25.1 J/(g K)) in the absence of any added antiflammable additives.