Bio-based poly(vinyl benzoxazine) derived from 3-hydroxycinnamic acid— An intrinsically green flame-retardant polymer free of both halogen and phosphorus

A novel bio-based benzoxazine monomer (VBOZ-7, 7-vinyl-3-phenyl-3,4-dihydro- 2H -1,3-benzoxazine) bearing polymeriable vinyl and 1,3-benzoxazine functionalities was synthesized from 3-hydroxycinnamic acid through successive decarboxylation reaction and Mannich condensation.The styrene-like bifunctional monomer underwent radical polymerization to obtain soluble P(VBOZ-7) which still remainining polymeriable benzoxazien pendent groups. The reactive P(VBOZ-7) possessed excellent solubility just like polystyrene, and also showed excellent thermal stability under 220 °C verified by differential scanning calorimeter (DSC) results. When the linear P(VBOZ-7) was heated over 240 °C, the remaining benzoxazines polymerized rapidly to form crosslinked structure. Thermogravimetric analysis (TGA) results showed that both linear and crosslinked P(VBOZ-7) possessed high char-formability at 800 °C. Cone calorimeter test (Cone test) proved that in situ crosslinking reaction strikingly improved the flame-retardancy of P(VBOZ-7). The total heat release of the crosslinked P(VBOZ-7) was only 6.7 MJ/m 2 , far below the value of polystyrene (109.4 MJ/m 2 ). Undoubtedly, the P(VBOZ-7) consisting of C, H, O, N atoms is an intrinsically green flame-retardant polymer with excellent solution processibility. • Bio-based bifunctional vinyl benzoxazine monomer prepared from renewable resource. • Soluble polybenzoxazine obtained by radical polymerization of vinyl functionality. • The halogen- and phosphorus-free polymers showed excellent fire retardancy. • The in situ benzoxazine polymerization benefited the thermal stability improvement.