Design and Synthesis of Diethyl Phosphite Functionalized Oxazine Ring Substituted Bio-Benzoxazine Resins with Enhanced Thermal Stability and Flame Retardancy via Hydrophosphonylation Strategy

As environmental pollution intensifies and fire incidents become increasingly frequent, there is an urgent need to develop environmentally friendly and intrinsically flame-retardant polymeric materials. Besides, in light of the restrictions from European Union on halogen-based flame retardants, phosphorus-based flame retardants are emerging as the preferred alternatives. Here we introduce an innovative molecular design strategy by incorporating diethyl phosphate (DEP) group into benzoxazines to achieve a new class of biobased thermosetting resins. Two DEP-functionalized oxazine ring-substituted benzoxazine monomers (PH-fa-[4-DEP] and PH-bfa-[4-DEP]) were successfully synthesized from naturally renewable biomass feedstocks via a one-pot synthetic strategy using ethanol as the solvent. The prepared polybenzoxazine samples, poly(PH-fa-[4-DEP]) and poly(PH-bfa-[4-DEP]), displayed outstanding thermal stability with 5% weight-loss temperatures of 318.3 and 352.3 °C, and intrinsic flame retardancy with UL-94 V-0 level. In addition, poly(PH-bfa-[4-DEP]) exhibited the char yield value as high as 67.8%. The DEP functionality as well as the oxygen and nitrogen from the thermoset structure collectively contribute to the excellent flame retardancy for the two newly developed biobased polybenzoxazines in this study.