Hydrophosphonylation strategy to achieve oxazine ring-substituted bio-benzoxazines showing both roles of thermal latent catalyst and flame retardant

Abstract

The increasing frequent occurrence of fire accidents alone with the desirable sustainable development have accelerated searching for more resilient and safer flame-retardant polymers derived from natural renewable resources. Here we have successfully designed and synthesized bio-based benzoxazine monomers (pH-fa-[2-pH,4-DEP] and pH-fa-[2-oOHph,4-DEP]) with oxazine ring substituted by diethyl phosphite (DEP) via hydrophosphonylation for the first time, thereby achieving non-flammable green thermoset materials. Specifically, pH-fa-[2-oOHph,4-DEP] with phenolic -OH group also shows a latent catalytic feature through its in-built intramolecular hydrogen bonding. As a result, we chose this newly synthesized monomer as both latent catalyst and flame retardant for enhancing the comprehensive properties of the well-commercialized benzoxazine resin (BA-a). The peak curing temperature of BA-a thermosetting system was decreased 23.3 °C and the glass transition temperature of resulting thermoset increased by 61.7 % with only adding 3 mol% of pH-fa-[2-oOHph,4-DEP]. Finally, the mechanistic insights into both advantages including low curing temperature and excellent flame retardancy have been discussed. With this work we demonstrate the possibilities for achieving bio-based non-flammable thermosets using benzoxazine chemistry, and offer molecular level insights into the functions, paving the way for new applications of green thermosetting resins for high-performance non-flammable composite materials.