Influence of protonation on the conformations of rigid-chain polymers

The conformations, internal rotation barriers, and the energies of bending deformation of molecules that model chain segments of the rigid-chain polymer poly(p-phenylenebenzobisoxazole) were calculated with the use of the AM1 semiempirical quantum-chemical method. The model molecules included several heterocycles and had a size larger than the repeat unit of the polymer, which includes one heterocyclic ring and one phenylene ring. For molecules in which all heterocyclic rings are diprotonated (at nitrogen atoms), the planar conformation is optimal (as in the case of uncharged molecules). At the same time, the internal rotation barriers in such molecules are reduced relative to the neutral molecules. However, when not all heterocycles are protonated in the molecule, the barriers turn out to be substantially higher than in the neutral molecule. For molecules in which all heterocycles are tetraprotonated, ab initio calculations of the optimal conformation were also performed. For these molecules, the conformation in which the phenyl rings and the heterocyclic rings are turned by almost 50° with respect to one another appeared to be optimal. In this case, the height of the rotation barriers is even lower than in molecules with diprotonated heterocycles. The protonation was found to have a weak effect on the bending rigidity of the poly(p-phenylenebezobisoxazole) chain.