Association of Electroactive Counterions with Polyelectrolytes. 6. Electrostatic and Coordinative Binding of Ru^(3+/2+) Centers to a Copolymer of Acrylic Acid and Acrylonitrile

Abstract

A random copolymer of acrylic acid and acrylonitrile was prepared in which the functionality on the polymer chain consisted of 93% carboxylic acid groups, 4.1% nitrile groups, and 2.9% acid anhydride groups. The nitrile groups were used to coordinate Ru(NH_3)_5^(2+) centers to the copolymer. The anionic carboxylate groups produced at pH values above ∼3.5 were used to bind Ru(NH_3)_6^(3+) counterions to the resulting polyanionic copolymer. The electrochemical oxidation or reduction of the two redox centers bound to the copolymer were used to estimate diffusion coefficients for the copolymer. Reasonable agreement was obtained between the values obtained from both electroactive redox groups. The diffusion coefficient exhibited a strong dependence on pH that reflected changes in the structure of the copolymer as it was converted from its neutral to its polyanionic form. The rate of the reaction between the Ru(NH_3)_5OH_2^(2+) complex and the nitrile groups of the copolymer increased significantly with pH because electrostatic binding of the cation to the polyanionic copolymer increased the concentration of the complex in the vicinity of the nitrile groups. Adsorption of the copolymer on the surface of the electrodes interfered with the oxidation of Ru(NH_3)_5^(2+) groups coordinated to the dissolved copolymer but not with Ru(NH_3)_6^(3+) counterions electrostatically bound to the polyanion. A possible explanation for the contrasting behavior is suggested.