A Novel Benzimidazole-Containing Stabilized AB-Type Polybenzoxazole Monomer: Synthesis, Polymerization, and Performance

The development of high-performance polybenzoxazole (PBO) materials relies on novel AB-type monomers to overcome challenges in stoichiometric control and enhance material processability. However, significant challenges remain in the molecular structure innovation and efficient synthesis of such monomers. This study successfully developed a novel AB-type self-condensing monomer based on a benzimidazole structure—2-(3-amino-4-hydroxyphenyl)-5-carboxylic acid-1 H -benzimidazole (ACBP). Starting from 3,4-diaminobenzoic acid and 3-nitro-4-hydroxybenzaldehyde, the monomer was efficiently synthesized through cyclization condensation and reduction reactions. This method offered advantages such as simplicity, environmental benignity, and low energy consumption, and achieved a high yield. Experimental results demonstrated that ACBP exhibited excellent storage stability. Furthermore, the monomer was subjected to a programmed temperature-controlled polycondensation reaction in polyphosphoric acid and successfully yielded poly[2,5-(1 H -benzimidazol-2-yl)benzoxazole] (ABPBIO) with an intrinsic viscosity as high as 5.9 dL/g. The structures of the monomer and polymer were systematically characterized using liquid chromatography-mass spectrometry (LC–MS) and nuclear magnetic resonance (NMR), which confirmed their molecular configurations. Performance studies revealed that ABPBIO not only possessed outstanding thermal stability, with 5% weight-loss temperatures ( T 5% ) of 551.6°C in air and 615.2°C in N 2 , and char yields of 44.1% and 76.6%, respectively, surpassing those of similar materials reported in the literature, but also exhibited good gas-phase water adsorption capabilities. Based on these superior properties, the polymer shows promising potential for specialized applications in fields such as firefighting and chemical protection.