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 ) 5 OH 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.In recent reports from this laboratory, the electrochemical behavior of electroactive counterions in aqueous solutions of a variety of water soluble polyelectrolytes has been described. [1][2][3][4][5][6] The present study is an extension of a previous one on poly-(acrylic acid) homopolymers. 6 A random copolymer of acrylic acid and acrylonitirle was synthesized and characterized. The copolymer contained a few nitrile groups interspersed along chains of poly(acrylic acid). The two functional groups (-COO -and -CN) available in this copolymer made it possible to obtain both electrostatic and coordinative binding of suitable metal complexes to the polyelectrolyte. When electroactive metal complexes were used, the resulting copolymer-metal complex molecules exhibited electrochemical responses which offered a simple and convenient means to estimate diffusion coefficients for polymers that are otherwise much more tedious to measure. Quantitative comparisons of the magnitude and pH dependence of the responses obtained from both electrostatically and coordinatively bound redox groups provided information about the diffusion coefficients and structures of the copolymer molecule in solution. One objective was to verify that polymer diffusion coefficients evaluated from the electrochemical responses of electrostatic or covalently attached redox groups were self-consistent. In addition, a kinetic study of the rate of the reaction between Ru(NH 3 ) 5 OH 2 2+ cations and nitrile groups of the copolymer revealed a significant rate enhancement when the reaction proceeds within the poly...

Pdop layer exhibiting zwitterionicity: a simple electrochemical interface for governing ion permeability

“…[1][2][3][4][5] For example, Yoshikawa et al reported that a polyelectrolyte membrane made with polyacrylic acid and poly(4-vinylpyridine) showed electrostatic binding of electroactive counterions. 6,7 These materials are usually prone to the effects of the external environment, resulting in change in charge density and thereby altering wettability, permeability towards ions, and stiffness of films. [8][9][10][11][12][13][14] Potential applications of such ''smart'' membranes include gates to control the permeability in microfluidic devices, surface coatings in chromatographic separations and controlled release systems, and artificial muscles to drive mechanical motion.…”

Controlled Motion of Solvent-Driven Gel Motor and Its Application as a Generator

“…The prolonged motion of the gel on the water surface is chemically driven and it is different from the motion observed in an oil/water system where repetitive changes in the interfacial tension at oil−water interface causes a self-movement of the oil drop. , The driving force of the present case is the surface spreading of the organic solvent released from the gel.…”

Diffusion Coefficients of Intrinsically Electroactive Polyelectrolytes and of Counterions Bound to Them

“…Relatively simple electrochemical techniques offer convenient means for estimating diffusion coefficients of electroactive molecules − With concentrations of the polyelectrolyte sufficient to cause essentially all of the multiply charged electroactive counterions to bind to the polyelectrolyte, the electrochemically evaluated diffusion coefficients of the counterions were assumed to match that of the polyelectrolyte. Although considerable circumstantial evidence in support of this assumption has been obtained, it remained an assumption.…”