Polyvinylferrocene

We now use this model to explain our AC impedance data from polyvinylferrocene. Our experimental results are similar to those obtained by Rubinstein et who interpreted their data in terms of the classical transmission line with a single resistance, as shown in Fig. 4.1. However, we show that it is possible to measure both / and Rx as 

FIGURE 4.5. Behavior of the real (Zr ) and imaginary (-Zi ) components of the impedance for the dual transmission line. Values of p, defined in Eqn. 18, are given by each line p varies from 0, when resistances are very unequal (classical line), to p = 1/4, when resistances are equal. 

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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... 

By setting the ratio of the oxidized and reduced forms of a redox couple in an electrode coating film to unity, the potential of this electrode in an inert electrolyte is poised at the half-wave potential of the couple. This has indeed been shown for platinum wires coated with polyvinylferrocene or ferrocene modified polypyrrole But the long term stability of these electrodes during cell connection... 

The redox active polymer films might bear the mediator group attached either covalently to the polymer backbone (polyvinylferrocene, Ru(II) complexes of polyvinylpyridine, etc.) or electrostatically within the ion-exchange polymer (e.g. in Nation, cf. Section 2.6). 

Ferrocene-based Linear Polymers. The first derivative that was studied from the electrochemical point of view was polyvinylferrocene (PVF). As illustrated in Figure 25, it displays a single oxidation process, which in some solvents is affected by problems of adsorption of the oxidation product (though not of the ideal Langmuir isotherm type discussed in Chapter 2, Section 1.6). 

The landmark discovery of ferrocene by Kealy and Paulson in 1951 marked the beginning of modern organometallic chemistry. The first organometallic addition polymer was polyvinyl-ferrocene synthesized by Arimoto and Haven in 1955. While polyvinylferrocene (structure 11.32) had been synthesized it was about another decade until the work of Pittman, Hayes, and George, and Baldwin and Johnson allowed a launch of ferrocene-containing polymers. 

Many other types of polymer have been prepared which exhibit semiconductivity. All obey the equation a = a0exp — E/kT. These include xanthene polymers (109, 110), polymerized phthalocyanines (111, 112), epoxides and polydiketones (86, 113), polypentadienes (114), polydicyanoacetylenes (115), polyvinylferrocene and substituted ferrocene (116, 117, 118, 119), polymeric complexes of tetracyanoethylene and metals (120), poly(vinyl chloride) and poly(vinylidene chloride) (121), polyvinylene and polyphenylene (122) and poly(Schiff s bases) (123, 124). 

Figure 13.8 Curve A Cyclic voltammogram of polyvinylferrocene (PVF) on gold in 0.1 M KPF6. Scan rate 10 mV s1. Curve B EQCM frequency curve obtained simultaneously with Curve A. [From Ref. 84.]...

Similar words of caution belong with redox polymers. Polyvinylferrocene typifies the type of polymeric compound where the redox centre is an integral part of the polymer. In view of ferrocene s well tried mediation talents this would seem an ideal choice of model, but the heterogeneous charge transfer rate constant for this redox polymer is typically 102 - 103 lower than the solution species (47) and although catalytic activity can usually be partially retained the kinetics of electron transfer are undesirably slow.. 

Adsorption. Adsorption can be reversible or irreversible. This method has been used particularly for the preparation of polymer-modified electrodes. A solution of polymer is either painted on the electrode and the solvent evaporated, or the electrode is immersed in a solution of the polymer. Relevant examples are polymers that let charge pass through the film polyvinylpyridine (PVP), polyvinylferrocene (PVF), porphyrins, and phthalocyanines. Direct deposition in the gas phase or sputtering are also possible. 

The polyvinylferrocene-supported ephedrines 19a—e were applied as chiral catalysts to the asymmetric ethylation of benzaldehyde in a heterogeneous system (hexane, room temperature). A quantity of 5 mol% of total ephedrine unit per benzaldehyde was used and the effect of the content of ephedrine unit in the polymers on enantioselectivity was investigated (Table 3-3). All chiral polymers 19a —e afforded (S)-l-phenylpropanol. Chiral polymer 19c, containing 32.8 mol% ephedrine units. 

Whilst it is agreed that electron transport within polymer supported ferrocene involves ferrocene-ferrocenium electron hopping and requires no participation from the organic framework, recent studies of electron transfer to a substrate in solution indicate that this need not be mediated by iron(II/III) hopping. Thus X-ray photoelectron spectroscopy gave no evidence for unexposed platinum on an electrode coated with polyvinylferrocene, but scannii electron microscopy revealed channels in the polymer layer. It was concluded that the reactant could either diffuse through such channels or pinholes to the electrode surface, or indeed diffuse through the polymer matrix. These possibilities are illustrated in equation (33). 

The introduction of mixed valence states can also induce semiconductivity. Cowan and Kaufman (10, 11) demonstrated that the conductivity of biferrocene [Fe(II)Fe(III)] picrate, 4, was six orders of magnitude greater than that of biferrocene, 3, itself. Could this be extended to polymeric systems We previously reported 12) the synthesis of mixed valence [Fe(II)Fe(III)] polyvinylferrocene and polyferrocenylene systems with significantly enhanced conductivities (10 -10 Q cm ) as compared with the [Fe(II)Fe(II)] analogs (10 Q cm ). 

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