Preparation and properties of copolymer resins based on phenolphthalein benzoxazine–benzoic acid and bisoxazoline

The novel phenolphthalein benzoxazine–benzoic acid (PBB) was synthesized. The structure of the monomer was supported by FTIR, 1 H-NMR, and elemental analysis. The curing behavior of PBB–bisoxazoline (1,3-PBO) resin was monitored by FTIR and differential scanning calorimetry. It was found that PBB–bisoxazoline resin exhibited two-stage polymerization mechanism. The thermogravimetry showed that PBB–bisoxazoline resin had good heat resistance due to the rigidity of PBB polymer chains and the high cross-linking density of copolymer. Furthermore, it seemed that the reaction between PBB and 1,3-PBO also led to an additional cross-linking, which increased the cross-linking density and delayed the decomposition. The cured resin had low water absorption.