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. Highlights TPA/OAP model enables qualitative catalyst screening. Cat-2 and Cat-3 boost AB-type PBO polymerization. A plausible N-acyl-imidazole pathway is proposed. Polymers show high thermal stability under N 2 .