Electron-exchange polymers

At this point, we would like to mention again that the polymer PE-H in Table 3.14, poly-2,5-di(benzoyloxy)styrene, was first synthesized some 40 years ago as a precursor of an electron exchange polymer (Ezrin et al., 1953). In this study we were lucky to have found that this polymer is liquid crystalline at temperatures above its glass transition (Table 3.14). To our knowledge PE-H is probably the oldest thermotropic liquid crystalline polymer ever synthesized by chemists. 

A large number of redox polymers (electron-exchange polymers) have been reported. To a large extent, the use of these polymers has been limited to oxidation-reduction reactions of inorganic systems, such as Fe -Fe , Sn " -Sn, Ce " -Ce , and Ti " -Ti. A comprehensive account of such redox polymers has been published by Cassidy and Kun (1965), Cassidy (1972), Lindsey (1974), and Manecke (1974). 

Updegraff IH, Cassidy HG. Electron exchange polymers. 2. Vinylhydroquinone monomer and pol)mier. J Am Chem Soc 1949 71 407-410. 

Cassidy HG, Ezrin M, Updegraff IH. Electron exchange polymers. 4. Countercurrent applications. J Am Chem Soc 1953 75 1615-1617. 

Electron-exchange polymers, these are (co)polymers whieh include covalently fixed molecular formations with a reversible redox character and which do not possess ion-exchange properties, (these are unites like hydroquinone or eatechol). 

Figure 3. Cyclic voltammograms of 3-methylpyrrole-4-carboxylic acid in acetonitrile + 0.1 MEt4NC104.58 (Reprinted from P. G. Pickup, Poly-(3-methylpyrrole-4-carbox-ylic acid) An electronically conducting ion-exchange polymer, J. Electroanal. Chem. 225, 273-280, 1987, with kind permission from Elsevier Sciences S.A.)...

Like other ion-exchange polymers, conducting polymers have been used to immobilize electroactive ions at electrode surfaces. Often the goal is electrocatalysis, and conducting polymers have the potential advantage of providing a fast mechanism for electron transport to and from the electrocatalytic ions. 

The above statements are valid for monomolecular layers only. In the case of polymer films with layer thickness into the p-range, as are usually produced by electropolymerization, account must also be taken of the fact that the charge transport is dependent on both the electron exchange reactions between neighbouring oxidized and reduced sites and the flux of counterions in keeping with the principle of electroneutrality Although the molecular mechanisms of these processes... 

Pishko MV, Katakis I, Lindquist SE, Ye L, Gregg BA, Heller A. 1990. Direct electron exchange between graphite electrodes and an adsorbed complex of glucose oxidase and an osmium-containing redox polymer. Angew Chem 102 109-111. 

It was already established that pure ethyl-" " and f-butyllithium exist as six- and fourfold polymers, respectively, in benzene solution. Apparently, C—Li bond cleavage takes place in this solvent leading to an exchange of alkyl groups between polymeric organo-lithium molecules when both compounds are present. The products are believed to be electron-deficient polymers of the type (EtLi) (f-BuLi) , wha-e m is a small number such as 4 or 6. ... 

An enormous number of polymers have been used to prepare chemically modified electrodes. Some examples are given in Table 13.2 Albery and Hillman provide a more extensive list [8]. As indicated in Table 13.2, these polymers can be divided into three general categories—redox polymers, ion-exchange and coordination polymers, and electronically conductive polymers. Redox polymers are polymers that contain electroactive functionalities either within the main polymer chain or in side groups pendant to this chain. The quintessential example is poly(vinylferrocene) (Table 13.2). The ferrocene groups attached to the polymer chain are the electroactive functionality. If fer-... 

Electron hopping is not the only mechanism by which a multilayer film containing an electroactive species can be electrochemically oxidized or reduced. Consider an electrode surface that has been coated with a film of the cation-exchange polymer Nafion (Table 13.2). Let us assume that the electroactive cation Fe3+ has been ion-exchanged into this Nafion film (e.g., Eq. 13.4). In principle, the Fe3+ in this film can be reduced via the following redox reaction ... 

So, we have two possibilities for the case of the ion-exchange polymer film-coated electrode reduction could occur by physical diffusion of the Fe3+ through the film, or reduction could occur via electron hopping through the film. How can we know which process is operative Electrochemists have devoted a considerable amount of research effort to answering this question. The answer clearly depends on the nature of the polymer, the extent of swelling of the... 

Other aggregates can also allow for control of rates of electron exchange. Inclusion of a donor or acceptor within a cyclodextrin (277), a zeolite (278), a monolayer coating on an electrode (279), or on a polymer-coated metal colloid (280)... 

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