Ring-Opening Copolymerization of Pyridinium Salts with Piperazine To Give Ionic Polymers with Expanded π-Conjugation System Derived from Through-Space Interaction in a Piperazinium Ring

Abstract

The reactions of N-(2,4-dinitrophenyl)-4-arylpyridinium chlorides (aryl (Ar) = phenyl and 4-pyridyl) with piperazine caused ring-opening of the pyridinium ring to yield polymers that consisted of the 5-piperazinium-3-aryl-penta-2,4-dienylideneammonium chloride unit [−N(CH2CH2)2N+(Cl−)═CH−CH═C(Ar)−CH═CH−] (Ar = phenyl, unit A, and 4-pyridyl, unit B). Copolymers having both the unit A and unit B and the model compounds were also obtained. The 1H NMR spectra suggested that the π-electrons of the penta-2,4-dienylideneammonium group of the polymers, the copolymer, and the model compounds were delocalized. UV−vis measurements revealed that the π-conjugation system expanded along the polymer chain due to the orbital interaction between the electrons on the two nitrogen atoms of the piperazinium ring, and its conversion from the boat form to the chair form via the half-chair form occurred stepwise accompanied by the decrease of the π-conjugation length. Kinetic studies of the conversion of the piperazinium ring was carried out. The surface of the pellets molded from the polymers and the copolymer showed metallic luster. The polymers and the copolymer received electrochemical oxidation in a solution.