Multiple redox polymers prepared by covalent and electrostatic binding to poly(vinylpyridine-co-styrene): cooperative binding and potential-induced electrostatic binding

Abstract

Binding of [Ru^III(edta)(OH₂)]^–(edta = ethylenediaminetetraacetate) to pyridine (py) sites in poly(vinylpyridine-co-styrene)(PUPS; 10% styrene) is induced by exposing a PUPS-coated glassy carbon (GC) electrode to dilute solutions of [Ru^III(edta)(OH₂)]^–. This polymer is capable of incorporating [Mo^IV(CN)₈]^4–, and the amount of anion incorporated is augmented by holding the electrode in the Ru^II state in line with expectations due to charge balance. Rotating-disc electrode experiments confirm that the current observed due to the MV²⁺(MV²⁺= 1,1′-dimethyl-4,4′-bipyridinium) ion arises from membrane diffusion through the polymer rather than from pinhole effects. Similarly, [Ru^II(NH₃)₅(OH₂)]²⁺ was covalently bound to PVPS to give a polymer which can bind [Mo^IV(CN)₈]^4– by ion-exchange, the amount depending on the Ru oxidation state. Co-operative binding of both [Ru^III(edta)(OH₂)]^– and [Ru^II(NH₃)₅(OH₂)]²⁺ to PVPS results in a single broad wave composed of the two overlapping Ru^III/^II redox couples present in a 2:1 ratio consistent with their respective charges. The {[Ru^III(edta)(OH₂)(py)]₂[Ru^II(NH₃)₅(py)]} polymer can incorporate redox-active ions from either MV²⁺ or [Mo^IV(CN)₈]^4– containing solutions, but at lower levels than observed for the pure [Ru^III(edta)(py)]^– and [Ru^II(NH₃)₅(py)]²⁺ polymers. The diffusion coefficient for charge transfer, D_ct, is higher for the mixed polymer, reflecting its greater concentration of redox sites. The mixed polymer is capable of binding both MV²⁺ and [Mo^IV(CN)₈]^4– species, yielding a redox polymer which can be switched into six different redox states.