New possibilities for the synthesis of high-molecular weight poly(2,5(6)-benzimidazole) (ABPBI) based on a new monomer, 3,4-diaminobenzoic acid monophosphate, in polyphosphoric acid medium at high monomer concentration under stirring and on a Teflon® surface without stirring are investigated. New complex solvent for ABPBI is found. This solvent is based on dimethyl sulfoxide, methanol, and potassium hydroxide. For the first time, molecular weight values are obtained by dynamic light scattering and viscometry in complex solvent. In addition, Kuhn segment values are estimated. Maximal molecular mass M_Dη value reaches 94 kDa. The values of macromolecular equilibrium rigidity A are estimated as 4.4–5.7 nm according to translational diffusion coefficient and 3.3 nm according to intrinsic viscosity measurements.