Conducting polymers oxidation-reduction

Kobayashi s group also conducted the oxidation-reduction polymerization (Scheme 44), where the germylene (178) acts as a reductant and the p-benzoquinone (179) as an oxidant - This polymerization occurs at 78°C to give high MW polymers (>10 ) within 1 h. These copolymers (180) were stable at room temperature and were resistant to moisture and air. The mechanism of this reaction was studied by ESR spectroscopy. It was foimd that the reaction proceeded via a biradical mechanism involving a germyl and a semiquinone radical. ... 

Reversible oxidation and reduction of polymers is commonly used to increase conductivity in these systems. Ions from the electrolyte are usually incorporated into the polymer as part of this process (see Electrically conducting polymers). 

Conducting Polymers Electronically conducting polymers (such as polypyrrole, polythiophene, and polyaniline) have attracted considerable attention due to their ability to switch reversibly between the positively charged conductive state and a neutral, essentially insulating, form and to incorporate and expel anionic species (from and to the surrounding solution), upon oxidation or reduction ... 

Later we will describe both oxidation and reduction processes that are in agreement with the electrochemically stimulated conformational relaxation (ESCR) model presented at the end of the chapter. In a neutral state, most of the conducting polymers are an amorphous cross-linked network (Fig. 3). The linear chains between cross-linking points have strong van der Waals intrachain and interchain interactions, giving a compact solid [Fig. 14(a)]. By oxidation of the neutral chains, electrons are extracted from the chains. At the polymer/solution interface, positive radical cations (polarons) accumulate along the polymeric chains. The same density of counter-ions accumulates on the solution side. 

Figure 20. Artificial muscle under work. In reduction (A) electrons are injected into the polymer chains. Positive charges are annihilated. Counter-ions and water molecules are expelled. The polymer shrinks and compaction stress gradients appear at each point of the interface of the two polymers. The free end of the bilayer describes an angular movement toward the left side. (B) Opposite processes and movements occur under oxidation. (Reprinted from T. F. Otero and J. Rodriguez, in Intrinsically Conducting Polymers An Emerging Technology, M. Aldissi, ed., pp. 179-190, Figs. 1,2. Copyright 1993. Reprinted with kind permission of Kluwer Academic Publishers.)...

On the other hand, Doblhofer218 has pointed out that since conducting polymer films are solvated and contain mobile ions, the potential drop occurs primarily at the metal/polymer interface. As with a redox polymer, electrons move across the film because of concentration gradients of oxidized and reduced sites, and redox processes involving solution species occur as bimolecular reactions with polymer redox sites at the polymer/solution interface. This model was found to be consistent with data for the reduction and oxidation of a variety of species at poly(7V-methylpyrrole). This polymer has a relatively low maximum conductivity (10-6 - 10 5 S cm"1) and was only partially oxidized in the mediation experiments, which may explain why it behaved more like a redox polymer than a typical conducting polymer. 

Electronically conducting polymers. The world-wide interest in electronically conducting polymers has been triggered in 1977/8, when Heeger, MacDiarmid, Shirakawa, and others have studied the oxidation and reduction reactions of poly acetylene,... 

Fig. 7. Schematic diagram showing the oxidation and reduction potentials of conducting polymers relative to oxygen reduction and water oxidation.

The thickness of the pure film can also be obtained by measuring the area under the oxidative or reductive waves of the CV of a conducting polymer, providing the extent of doping of the film (i.e. the number of charges in the film per monomer unit) at the anodic limit is known. 

The discovery of the metal-like properties of conducting polymers has once again focused attention on the oxidation and reduction characteristics of aromatic systems. It turns out that most of these conducting materials consist of chainlike connected carbocyclic or heterocyclic aromatics [94-97]. 

These ideas developed by chemists resemble the bipolaron model, which presents the solid-state physicist s view of the electronic properties of doped conducting polymers [96]. The model was originally constructed to characterize defects in inorganic solids. In chemical terminology, bipolarons are equivalent to diionic states of a system (S = 0) after oxidation or reduction from the neutral state. The transition from the neutral state to the bipolaron takes place via the polaron state (= monoion, S = 1/2,... 

The proposed mechanism of the doping processes in conducting polymers implies oxidation (p-doping) or reduction (n-doping) of the polymer with... 

Charge-transfer agent used to generate, by oxidation or reduction, positive or negative charges in an intrinsically conducting polymer. 

Note 1 The bulk electrical conductivity of an intrinsically conducting polymer is comparable to that of some metals and results from its macromolecules acquiring positive or negative charges through oxidation or reduction by an electron acceptor or donor (charge-transfer agent), termed a dopant. 

Transition metal compounds, such as organic macrocycles, are known to be good electrocatalysts for oxygen reduction. Furthermore, they are inactive for alcohol oxidation. Different phthalocyanines and porphyrins of iron and cobalt were thus dispersed in an electron-conducting polymer (polyaniline, polypyrrole) acting as a conducting matrix, either in the form of a tetrasulfonated counter anion or linked to... 

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