Three different series of pyridine based polybenzimidazole (PyPBI) random copolymers consisting of meta-para pyridine linkages were synthesized from various stoichiometric mixtures of meta (2,4; 2,6 and 3,5) and para (2, 5) structure pyridine dicarboxylic acids (PDA) with 3, 3', 4, 4'- tetra-aminobiphenyl (TAB) by solution polycondensation in polyphosphoric acid (PPA). The influences of the structural isomers of PDA on the PyPBI copolymerization and properties were elucidated by characterizing the resulting copolymers. The solubility of PDA monomers in PPA and the overall monomer concentration in the polymerization were found to be the deciding factors. The higher molecular weight PyPBI were obtained for higher para content copolymers due to the low solubility of para PDA in PPA. The introduction of para structure had enhanced the conjugation along the polymer chain. NMR study showed that the reactivity ratio of para PDA was not identical for all the three sets of PyPBI copolymers, it varied upon the positions of the dicarboxylic acids in the pyridine ring of meta PDAs (structural isomeric effect) with which 2,5 PDA is forming the copolymer. Introduction of para structure and meta PDAs structural isomers affected the thermal stability, flexibility and the photophysical properties of the PyPBI copolymers. (C) 2010 Elsevier Ltd. All rights reserved.