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This study focuses on the rheological behavior and fiber formation from synthesis solutions of poly(2,5(6)-benzimidazole (ABPBI) in polyphosphoric acid (PPA) at a polymer concentration of 12.5 wt%. The synthesized ABPBI solutions demonstrate characteristic features of associative polymer systems, including pronounced shear thinning and high elasticity. The activation energy of viscous flow for ABPBI in PPA is a low and increases with polymer concentration, reaching 29 kJ/mol at 12.5 wt%, significantly lower than in phosphoric acid solutions of comparable or lower concentrations. This indicates more efficient solvation and chain mobility in PPA. Comparative analysis with two superbase-based solvent systems further highlights the role of the solvent's nature in determining the flow mechanism and associative behavior of the solutions. A comprehensive set of model coagulation experiments was conducted to investigate the influence of non-solvent composition on fiber morphology and solidification dynamics. The optimized coagulation conditions enabled the successful formation of homogeneous monolithic fibers with a good mechanical property. The results contribute to a deeper understanding of the physicochemical principles underlying the formation of high-performance ABPBI fibers and establish polyphosphoric acid as a promising solvent for their preparation.