Redox polymers properties

Intensive research on the electrocatalytic properties of polymer-modified electrodes has been going on for many years Until recently, most known coatings were redox polymers. Combining redox polymers with conducting polymers should, in principle, further improve the electrocatalytic activity of such systems, as the conducting polymers are, in addition, electron carriers and reservoirs. One possibility of intercalating electroactive redox centres in the conducting polymer is to incorporate redoxactive anions — which act as dopants — into the polymer. Most research has been done on PPy, doped with inter alia Co 96) RyQ- 297) (--q. and Fe-phthalocyanines 298,299) Co-porphyrines Evidently, in these... 

The application of two successive redox polymer layers at an electrode surface gives rise to rectifying properties because the electron transport between the electrode and the outer layer has to be mediated by the inner redox polymer Among several conbeivable situations, the one where the inner layer possesses two reversible redox potentials (e.g. a Ru"(bipy)j polymer) and the outer layer has one redox transition with a potential between the former ones (e.g. polyvinylferrocene) is most interesting gjj electrode device has two opposite-sign rectifying... 

A basic property of all conducting polymers is the conjugation of the chain-linked electroactive monomeric units, that is, the monomers interact via a jt-electron system. In this respect, they are fundamentally different from redox polymers. Although redox polymers also contain electroactive groups, the polymer backbone is not conjugated and the interaction between the isolated redox counters is weak. Consequently, redox polymers are nonconductors [17]. They will not be discussed in this context. 

Redox polymers are electroactive polymers for which the redox centers are localized on pendent, covalently attached redox centers. The electrochemical properties of such materials depend not only on both the loading and the nature of the redox-active center but also on the type of polymer backbone. The electroactive groups are typically metal complexes, which are covalently attached to a polymer... 

These results indicate that the properties of the redox polymers, such as redox potentials and spectroscopic properties, can be varied systematically and, more importantly, can be predicted from those observed for mononuclear model compounds. As an example of the transfer of photochemical properties from monomeric analogues to the corresponding polymers, the photochemical behavior of the redox polymer [Ru(bpy)2(PVP)sCl]Cl will be considered. This polymer contains one metal center for every five-monomer units. Photolysis of a thin layer of this material on a glassy carbon surface leads to a change in the redox potential of the material from about 650 to 850 mV (See Figure 4.17) [32]. The voltammetric process affected is associated with a metal-center-based Ru(ll/m) redox process. By analogy to the behavior observed for the mononuclear species [Ru(bpy)2(py)Cl]+ (py = pyridine),... 

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. 

Additional deviations from the Nernst law [Eq. (4)] can come from kinetic effects in other words, if the potential scan is too fast to allow the system to reach thermal equilibrium. Two cases should be mentioned (1) ion transport limitation, and (2) electron transfer limitation. In case 1 the redox reaction is limited because the ions do not diffuse across the film fast enough to compensate for the charge at the rate of the electron transfers. This case is characterized by a square-root dependence of the current peak intensity versus scan rate Ik um instead of lk u. Since the time needed to cross the film, tCT, decreases as the square of the film thickness tCT d2, the transport limitation is avoided in thin films (typically, d < 1 xm for u < 100 mV/s). The limitation by the electron transfer kinetics (case 2) is more intrinsic to the polymer properties. It originates from the fact that the redox reaction is not instantaneous in particular, due to the fact that the electron transfer implies a jump over a potential barrier. If the scan... 

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>. 

Polymers with useful electronic properties can be subdivided into two classes (i) redox polymers [23] and (ii) conducting polymers [24]. Redox polymers contain redox-active subunits, which are linked by saturated spacers, and thus exist as electronically independent building blocks, while conducting polymers are characterized by an extended ir-conjugation (chart 2). 

Ferrocene modified flexible polymeric electron transfer systems Ferrocene and its derivatives are readily available and commonly used organometalUc redox mediators, so it is quite natural that they were selected first to synthesize mediator modified polymeric electron transfer systems. Siloxane pol5uners are flexible but aqueous insoluble pol3nmers. As previously indicated, a flexible polymer backbone allows close contact between the redox center(s) of the enzyme and the mediator, and the water insoluble property of the polymer prevents not only redox polymer from leaching into bulk media but also prevents enzyme diffusion away fi-om the electrode surface by entrapping it in the polymer/carbon paste matrix. Therefore, ferrocene and... 

Our approach to the color-variable LEDs presented here has a number of important advantages (1) The two redox polymers modify the charge-inj ection properties of the polymer/metal interfaces, allowing the use of high-workfunction metals as electrodes. This potentially reduces the aging problems associated with conventional polymer LEDs, which must use reactive low-workfunction metals to... 

The related fully sulfonated, self-doped polymer poly(2-methoxyaniline-5-sulfonic acid) (PMAS 9) may be prepared under normal atmospheric pressure by the oxidation of 2-methoxyaniline-5-sulfonic acid (MAS) monomer with aqueous (NH4)2S208 in the presence of ammonia or pyridine (to permit dissolution of the MAS monomer).141 The polymerization pH was therefore >3.5. Subsequent studies showed that the product consisted of two fractions a major fraction with Mw of ca. 10,000 Da whose electrical conductivity and spectroscopic and redox switching properties were consistent with a PAn emeraldine salt, as well as a nonconducting, electroinactive oligomer (Mw ca. 2,000 Da).143 144 Pure samples of each of these materials can be obtained using cross-flow dialysis.145... 

PNIPAAm has been synthesized from N-isopropylacrylamide (NIPAAm) by a variety of techniques, the most widely used being free-radical initiation of organic solutions [147] and redox initiation in aqueous media [148]. Redox polymerization of NIPAAm in aqueous media typically uses ammonium persulfate or potassium persulfate as the initiator and either sodium metabisulfite or N,N,N N -tetramethylethylenediamine (TEMED) as the accelerator. In addition, the solutions are usually buffered to constant pH since in the absence of buffer much greater polydispersity is obtained. Whether one polymerizes NIPAAm in organic or aqueous solution also affects polymer properties [149]. 

Although "polyaniline" has been known for about 150 years, it was not until the mid-1980 s that intense interest in it and its derivatives, as a completely different type of conducting polymer, really began,h3. resulted not only from its ease of synthesis and derivatization, but also from its novel non-redox doping properties and from its potential technological importance. 

Among the most common unsaturated units, there are mono (poly) cyclic aromatic hydrocarbons, heterocycles, benzofused systems, and olefinic and acetylenic groups, typically paired with various fullerene derivatives. The extent of conjugation/interaction between these units determine the polymer solution/solid-state electronic structure, which in turn control polymer properties, such as, optical absorption/emission, redox... 

---