Six poly(benzobisimidazoles) (PBIs) have been synthesized by the solution polycondensation of 1,2,4,5-tetraaminobenzene tetrahydrochloride in poly(phosphoric acid) (PPA) with systematically varied diacids to demonstrate the solubilization of the aromatic heterocyclic rigid-rod polymers. The role of PPA is identified and the effects of phosphorous pentoxide and water on PBI during polycondensation are discussed. Polymer properties such as the inherent viscosity, decomposition temperature are correlated to the polymer concentration. Finally, the effect of diacid architecture on the synthesis and microstructure of PBI is studied. The results are further discussed in terms of resonance, symmetry, and solubilization of the diacids.