The synthesis of various polybenzimidazoles (PBIs) in condensation media such as polyphosphoric acid and Eaton's reagent was accomplished. Molecular weight distribution of the synthesized polymers was examined with respect to the process conditions and the chemical structure of the monomers. It was found that during the synthesis of PBIs a side acylation reaction occurs which leads to the formation of branched macromolecules with Mw up to 2.3 x 10(6) and higher and even crosslinking, resulting in gelation of the polymers during synthesis. Fraction of the side reaction in the overall process appeared to be dependant on both the chemical structure of the monomers and the condensation media used. The orientation of the acylation reaction was determined by quantum-mechanical calculations. (C) 2010 Elsevier Ltd. All rights reserved.