Screening of Imidazole‐Based Catalysts Through Model Reactions for Efficient Polymerization of AB ‐Type Polybenzoxazole Monomers and Investigation of the Catalytic Mechanism

In traditional polyphosphoric acid (PPA) systems, polybenzoxazole (PBO) synthesis suffers from long reaction times and high energy consumption. To address this, a rapid model‐reaction–based screening protocol using the terephthalic acid (TPA)/o‐aminophenol (OAP) system was developed to qualitatively rank catalyst activity. Fourteen imidazole‐based derivatives were evaluated, revealing structure–activity trends and enabling selection of two optimal catalysts, 4,5‐dimethyl‐2‐phenyl‐1 H ‐imidazole (Cat‐2) and 2‐methyl‐1 H ‐benzimidazole (Cat‐3). Both accelerated polymerization of aromatic AB‐type monomers (3,4‐AHBA, 4,3‐AHBA, 3,4‐AHPA·HCl, ACBP), yielding polymers with higher intrinsic viscosities within the same reaction times. The resulting PBOs maintained intact benzoxazole backbones and exhibited excellent thermal stability under nitrogen ( T 5% = 598°C–654°C; char yield 67%–72%). Solid‐state 13 C NMR, FT‐IR, XPS, and TGA, together with viscosity measurements, support a plausible mechanism in which imidazole catalysts may form N‐acyl‐imidazole intermediates, facilitating acyl transfer and lowering the early condensation barrier. Overall, this work proposes an efficient catalytic strategy and mechanistic insights for the investigated aromatic AB‐type PBO monomers, providing guidance for catalyst design in related PBO and heterocyclic polymer systems.