Structure of polymer–acid complexes in solution and crystal-solvate phases of rigid-rod heterocyclic polymer - poly(p-phenylene benzobisoxasole)

Investigations of the structures of the complexes formed by di-protonated oligomer molecules of poly (p-phenylene benzobisoxasole) (PBO) with charged and neutral molecules of orthophosphoric acid (OPA) by quantum-chemical and molecular mechanics methods are described. The complexes with partially compensated, fully compensated, and overcompensated PBO molecular charges as models of the polymer–acid complexes in solution and in crystal-solvate phases (Phase I and Phase II) are considered. It is shown that the structures with one anion associated with two heterocycles are most energetically favorable for the complexes with a partially compensated polycation charge. The polymer molecule bending resulting from this association can contribute to an enhanced PBO chain flexibility in solution. The PBO chains in Phase I are protonated due to the presence of neutral acid molecules. The protonated state of the PBO molecules in Phase II can be explained by the presence of excess anions.