Poly oxidation-reduction cycle

An example of an ICP is poly(-3,4-ethylenedioxythiophene) with the short name PEDOT. The polymer can be prepared in acetonitrile or in aqueous electrolytes. The polymer films formed have quite different oxidation-reduction properties. For the film prepared in dry acetonitrile, the change of the half width of the quartz resonance frequency in the oxidation-reduction cycle is negligible (Figure 11.12). ... 

Figure 11.13 Mass change per electrode area Am/A during an oxidation-reduction cycle of poly (-3,4-ethylenedioxythiophene) in 0.1 mol-dm NaC104/acetonitrile.

Other successful electrochromic devices have been realized by Kelly et al. using polyaniline-impregnated fibres [79]. In situ electrochemical polymerization of polyaniline is used to bind poly aniline to a PET or viscose spacer fabric. The fabric is then impregnated with an electrolyte and sandwiched between two electrodes. For the bottom electrode, carbon black or silver ink can be printed directly on the fabric. Polyaniline colour changes from green to blue through oxidation—reduction processes. However, the lifetime of this structure is also short and does not exceed dozens of oxidation—reduction cycles. 

Fig. 13.14. Poly(vinylferrocene) break-in completion of first oxidation half cycle, by a second reconfiguration followed by a second solvation step, to form the most stable reduction product (doubly solvated, doubly reconfigured O ).

Fig. 18.8. Doping and undoping of a poly(pyrrole) film during oxidation and reduction cycles. When a poly(pyrrole) coated electrode is cycled between the reduction and oxidation potentials, the current observed at the oxidation potential is related to the ability of anions to enter the polymer film and dope the polypyrrole.

The electrochemical behavior of poly(pyrrole) films prepared and cycled in an AICI3 [C2mim][Cl] melt was investigated in detail and improvements in reproducibility and the rate of oxidation and reduction of these films were observed compared to films prepared under similar conditions in acetonitrile [49]. This was postulated to be a result of an increase in the porosity of poly(pyrrole) films deposited from the melt compared to those from acetonitrile, although attempts to describe this porosity using porous electrode models were not totally conclusive. 

Our published studies [18, 19] of poly(thionine) redox switching showed that, in acetic acid buffer, the leuco (reduced) form (L) is oxidized, losing two electrons and two protons, to generate the oxidised form (7 ). Furthermore, acetic acid is coordinated only to the 7-form so that reduction causes 7 to lose an acetic acid molecule. However, reduction of 7 to L follows a different path than the reverse of the oxidation path. Water transfer is also involved in the redox cycle. We can (in simplified form) write the overall redox switching process as ... 

The RRDE can also be used to study electrochemical processes at electrodes modified with thin polymer films (Chapter 14). In this application, the polymer film is prepared on the disk, and the ring monitors the flux of ions from the film during a potential sweep. For example, the flux of the cation, 1,3-dimethylpyridinium, from a film of polypyrrole/poly(styrenesulfonate) was monitored at the ring electrode, as the disk was cycled in an acetonitrile solution over the potential region where reduction and oxidation of the film occurred (27). 

Polypropylene imine dendrimers with covalently attached perfluorinated poly-(propylene oxide) end-groups have been employed for the stabilization of palladium colloids in Heck reactions in fluorous solvents by Crooks et al. [36] (Table 2). Relatively low activities were obtained, which were further reduced upon re-use of the fluorous phase in a second cycle. From the results of repeated Heck reactions without an added base, it can be asstuned that the reduction in activity upon recycling is due to protonation of the dendrimer scaffold, serving as a base. No leaching of palladium from the fluorous phase was detected (< 0.01 ppm) however, this value was not related to the overall palladium loading (cf. also Section 4.2). 

In most cases, stability and oxidation potential issues mean that polythiophenes are prepared, studied, and used in non-aqueous media, and the influence of the solvent can be significant. The extent of solvent swelling is determined by polymer-solvent interactions, which can be manipulated both by choice of solvent and introduction of substituents, here in the 3- and 4-positions. Electropolymerization of alkyl-substituted thiophenes provides good examples of the effects one can observe. EQCM (with PBD) measurements of potentiodynamic deposition from acetonitrile benzonitrile (4 1) of poly(didodecylterthiophene) (with the substituents in the 3, 4 - or 3,3"-positions) showed that film deposition occurred primarily during the reductive half-cycle, in the form of short chain oligomers of the... 

It was observed that the anodic peak current of Fc was enhanced in the presence of the [FeCCbOg] " " redox probe. This effect was rationalized by a possible communication between the [FeCCbOg] " redox probe in close proximity to the Fc/Fc couple. Thus, the oxidation Fc- Fc + e is followed by a reduction process which involves [Fe(CN)g] , and which allows another reduction process to the Fc to continue the CV cycle. Similar observations have been reported for immobilized Fc-peptides and Fc-functionalized poly(propylene imine dendrimers [41,42]. The differential in the... 

Electrolytes and solvents. The electropolymerization reaction may be sensitive to the nucleophilic nature of the solvent and electrolyte. For this reason, many of the films are prepared in aprotic solvents, such as acetonitrile, which are poor nucleophiles. Electro-oxidative polymerization in the presence of small anions simultaneously incorporates the anions which render the polymer film conductive. Upon reduction, the anions are released from the film. Cycling the film through oxidation and reduction leads to insertion and release in the respective parts of the cycle. Simultaneous incorporation or removal of the solvent and/or cations may also occur, as shown by measurements on the quartz crystal microbalance [51-52]. Polymerization in the presence of large anions such as poly(vinylsulphonate) and poly(4-styrene sulphonate) (PSS") also incorporates the anion during growth [53-56]. Subsequent cycling, however, does not release the anions which are trapped because of their... 

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