Redox Polymers and Metallopolymers

The analogous osmium polymers have also been studied in great detail. The synthetic procedures required for these metallopolymers are the same as those described above for ruthenium however, the reaction times are longer. The similarity between the analogous mononuclear and polymeric species is further illustrated by the fact that the corresponding osmium polymers have considerably lower redox potentials and are also photostable, as expected on the basis of the behavior observed for osmium polypyridyl complexes. 

The example considered is the redox polymer, [Os(bpy)2(PVP)ioCl]Cl, where PVP is poly(4-vinylpyridine) and 10 signifies the ratio of pyridine monomer units to metal centers. Figure 5.66 illustrates the structure of this metallopolymer. As discussed previously in Chapter 4, thin films of this material on electrode surfaces can be prepared by solvent evaporation or spin-coating. The voltammetric properties of the polymer-modified electrodes made by using this material are well-defined and are consistent with electrochemically reversible processes [90,91]. The redox properties of these polymers are based on the presence of the pendent redox-active groups, typically those associated with the Os(n/m) couple, since the polymer backbone is not redox-active. In sensing applications, the redox-active site, the osmium complex in this present example, acts as a mediator between a redox-active substrate in solution and the electrode. In this way, such redox-active layers can be used as electrocatalysts, thus giving them widespread use in biosensors. 

More recently, there has been growing interest in a new type of redox polymer that is a hybrid of materials from PTs and will be referred to as conjugated metallopolymers. The key feature of this class of material is that the metal is coordinated directly to the conjugated backbone of the polymer, or forms a link in the backbone, such that there is an electronic interaction between the electroactive metal centers and the electroactive polymer backbone. This can enhance electron transport in the polymer, enhance its electrocatalytic activity, and lead to novel electronic and electrochemical properties <1999JMC1641>. 

In Chapter 8, coauthored by Kelly and Vos, the electrochemical behavior of osmium and ruthenium poly(pyridyl) redox polymers is discussed in some detail. Vos has made significant contributions in this area. This chapter ties in well with the more general discussion presented by Lyons in Chapters 1 and 2, in that many of theoretical concepts addressed in the latter chapters are again discussed by Kelly and Vos with specific reference to redox-active metallopolymer materials. 

From what has been written above about ICPs and RPs, it is evident that, in principle, best performance may be reached once the virtues of the two classes of differently conducting polymers are possessed by a single molecule that includes both redox centers and jr-electron conjugated systems typical of ICP chains. Such a coupling is realized in so-called metallopolymers, where redox metal centers are co-ordinated to one or more ligands differently connected to the jt-electron conducting organic backbone [67-79]. 

In this section we briefly consider the application of the theory developed by Albery and Hillman and Andrieux and Saveant to redox polymer films. We concentrate on the metallopolymers [Os(bpy)2-PVPioCl]CP and [Ru(bpy)2PVP5Cl]Cl.< >... 

The standard potential for the Ru(II/III) redox transformation is 712 mV in aqueous perchlorate media, whereas the standard potential for the ferrocyanide/ferricyanide couple is 375 mV. Hence the driving force for the mediation is some 337 mV, which corresponds to an equilibrium constant of 5 X 10 at 298 K. Thus we see that equilibrium lies very much on the rhs. Typical RDE voltammograms for the oxidation of Fe(CN)e in 0.1 M HCIO4 at uncoated glass carbon and metallopolymer-coated glassy carbon electrodes are shown in Fig. 2.24. Note that the reduction of Fe(CN) is quite sluggish. This is to be expected due to the unfavorable thermodynamics. Two anodic oxidation waves are observed at the metallopolymer-coated electrode. The first occurs at a potential where Fe(CN)6 is oxidized at the bare electrode, so it corresponds to the direct unmediated oxidation of substrate at the inner electrode/polymer interface. The second wave is due to the mediated oxidation via the Ru(II) redox sites, as just discussed. This mediated wave exhibits linear Koutecky-Levich behavior. It is clear that we are dealing with Case C here, since the direct unmediated oxidation of substrate occurs at a less positive potential than the mediated oxidation via the Ru(III) sites in the film. 

In the first section, various kinds of functional polymer, in particular the most used conductive polymer, conjugated polymer (CP), redox polymer, metallopolymer. Selection of the correct functional polymer depends on the desired properties of the resulting nanocomposites. The second part of the chapter focuses on the basic approaches used in the preparation of polymeric nanoparticles. As mentioned earlier, there are two basic approaches in the recent literature to synthesize the polymeric nanoparticles. In this section, we focus on the discussion of the common and widely used preparation methods for various kinds of polymeric nanoparticles. The polymerization method is based on the encapsulation of nanoparticles through heterogeneous polymerization in dispersion media. This method can be further classified into emulsion, microemulsion and miniemulsion. Polymer encapsulated nanoparticles can also be prepared directly from preformed polymer, where this approach is based on the specific interactions between nanoparticles and the preformed polymer, such as electrostatic interactions, hydrophobic interactions and secondary molecular interactions or self-assembly method. 

Beginning in 1949, Cassidy and coworkers presented a series of papers demonstrating redox polymers [46 9]. In these early demonstrations, the redox sites were based on hydroquinones. If the redox site is a transition metal complex, however, the redox polymer is also a metallopolymer [50-53]. Ruthenium and osmium metal-lopolymers with polyvinyfimidazole backbones, for example, are shown in panels B and C of Table 9.1. By participating in oxidation-reduction reactions with other species and through self-exchange, a redox polymer in solution can conduct electrons... 

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