CALCULATION OF THE EFFECTS OF PROTONATION ON RIGID-ROD POLYMERS

Semiempirical molecular orbital calculations have been used to investi gate the effects of protonation on the behaviour of the structural rep eat units of poly(p-phenylene benzobisthiazole), PBZT, and poly(p-phen ylene benzobisoxazole), PBO. Single and multiple protonations of the h eterocyclic rings at the nitrogen, oxygen and/or sulfur atoms were exa mined. It was found that protonation at the nitrogen was more favourab le than at the oxygen or sulfur atoms. Delocalization of the charge in to the phenyl ring resulted in a higher bond order for the inter-ring bond. While the energy barrier for torsional rotation increased with p rotonation, the resistance to bending the rings in an out-of-plane fas hion decreased somewhat. Results of the semiempirical calculations ind icated that protonation of the oxygen in PBO might result in opening o f the five-membered heterocyclic ring by breaking the carbon-oxygen bo nd. Ab initio calculations on relevant models, however, did not suppor t this result.