The first observation of 3,1-benzoxazine polymerization for developing high performance thermosets

1,3-Benzoxazine has long been considered as a unique polymerizable benzoxazine species to prepare high performance polymers. Herein, polymerization of 2H,4H-3,1-benzoxazine is observed for the first time and the thermal properties of the obtained material are studied. A phenolic hydroxyl-containing mono-functional 3,1-benzoxazine monomer has been synthesized. With additional phenol substitution, 3,1-benzoxazine is able to undergo ring-opening polymerization. The reaction kinetics and variation in the molecular structure during polymerization were systematically investigated. Besides, quantum chemistry simulation showed that the C-O and C-N bond strength reduction in the 3,1-oxazine ring induced by adjacent phenol substitution is the critical factor that endows the monomer with polymerization ability. A polymerization mechanism of 3,1-benzoxazine has been proposed. The reported 3,1-benzoxazine monomer has good evaporation resistance even at 200 °C for 2 hours. More importantly, the cured material shows attractive thermal performance, with a glass transition temperature of 217 °C and a decomposition temperature above 400 °C together with a char yield of 54%. This study offers a new way to develop polymerizable benzoxazines with high performance for numerous emerging applications.