Pyrrole oxidative polymerization

The polypyrrole molecular interface has been electrochemically synthesized between the self-assembled protein molecules and the electrode surface for facilitating the enzyme with electron transfer to the electrode. Figure 9 illustrates the schematic procedure of the electrochemical preparation of the polypyrrole molecular interface. The electrode-bound protein monolayer is transferred in an electrolyte solution containing pyrrole. The electrode potential is controlled at a potential with a potentiostat to initiate the oxidative polymerization of pyrrole. The electrochemical polymerization should be interrupted before the protein monolayer is fully covered by the polypyrrole layer. A postulated electron transfer through the polypyrrole molecular interface is schematically presented in Fig. 10. 

LiMii204) electrochemical kinetics ensure that all of the surfaces of the nanotubules remain accessible to solvent and electrolyte. The polypyrrole coat was deposited by simply applying 5 al of a solution that was 1 M in HCIO4 and 0.2 M in pyrrole to the LiMn204 surface. This results in oxidative polymerization of all of the pyrrole, yielding 0.065 mg of polypyrrole per cm of Pt substrate surface [125]. 

Oxidative polymerizations of thiophene and pyrrole and their derivatives are also under study to obtain polymers with conducting and optoelectronic properties [Jin et al., 2000 ... 

In general, polypyrrole can be prepared via electrochemical or chemical oxidative polymerization of pyrrole involving different highly reactive intermediates. Also, the pyrrole/cyclodextrin complex can be polymerized in aqueous solution under oxidative conditions by adding potassium peroxodisulfate as an ox-... 

Electrically conducting polymer particles such as polypyrrole and polyaniline could also be prepared by dispersion polymerization in aqueous ethanol (31). The oxidation polymerization of pyrrole and aniline has been carried out at the electrode surfaces so far and formed a thin film of conducting polymer. On the other hand, polypyrrole precipitates as particles when an oxidizing reagent is added to a pyrrole dissolved ethanol solution, which contains a water-soluble stabilizer. In this way electrically conducting polymer particles are obtained and, in order to add more function to them, incorporation of functional groups, such as aldehyde to the surface, and silicone treatment were invented (32). 

The oxidative polymerization of pyrrole can also be induced in solvent-free systems and several papers have described synthesis via exposure of a matrix saturated with an Fe(III) salt to pyrrole vapour 90,91>. Mohammadi et al. 92> described synthesis on planar substrates by sequential exposure to FeCl3 and pyrrole and have also discussed a similar synthesis using H202 as the oxidant93). 

Vanadiaxerogels are especially interesting for their redox properties, which promote the catalytical activity of enzymes such as glucose oxydase [644] or induce the oxidative polymerization of organic monomers, such as aniline, pyrrol, and thiophene [873],... 

The oxidative polymerization has been proposed to proceed via a radical coupling that involves the coupling of neutral radicals or cation radicals. The former case corresponds to the oxidative polymerization of phenols and dithiols in which the neutral radical is formed by one-electron transfer after dissociation of a hydron from the monomer, or by the elimination of a hydron after the oxidation. The latter case takes place when the cation radical formed by one-electron oxidation exists as a stable species. The cation radicals then couple with each other, and the dimer is formed through solvent-catalyzed hydron elimination from the intermediate dication. Oxidative polymerization of pyrrole and thiophene uses this mechanism [57-62]. 

When considering the synthesis of phospholes, one has to forget most of the classical and powerful methods employed for the preparation of thiophenes and pyrroles. For example, Paal-Knorr condensation, direct ortho-lithiation, halogenation with NBS or I2/Hg2+ and Vilsmeier-Haack formylation are not operative in phosphole chemistry. Likewise, no chemical or electrochemical oxidative polymerization... 

As an example, thermosensitive PVME microgels can be used as template and stabilizer for the synthesis of composite polypyrrole (PPy) particles [32], The PVME microgels were obtained by electron beam irradiation above the phase transition temperature as described in Sect. 3.2.1 [3,4], Pyrrole (Py) was polymerized by oxidative polymerization with ferric chloride in the presence of the PVME microgels in water/ethanol mixtures as reaction medium. For comparison, the synthesis was also carried out in the presence of uncross-linked PVME. 

Although some mechanistic details are still controversial, it has been established that the oxidative polymerization (chemically or electrochemically) of pyrrole and pyrrole derivatives proceeds via an E(CE) mechanism which involves cation-radical propagating species. The most commonly accepted mechanism of polypyrrole formation is illustrated in Fig. 57 [237,242]. The polymerization begins with the one-electron oxidation of pyrrole to produce cation radical 399. This cation radical has been... 

Finally, one should at least mention the all important heterocycle-based conducting materials, which are most often obtained by oxidative polymerization, e.g. of pyrrole [58]. 

A peculiar example of heterocoupling has been found in the electroreduction of a mixture of pyrrole (60) and 2,4,7-trinitrofluoren-9-one (59), which leads to the formation of two Meisenhemer anions (the major isomer (structure 61) from attack at position 3, the minor one from attack at position 1) arguably via coupling of the two radical anions (see Scheme 42) [131], Electro-oxidation of such anions gives a black polypyrrole film which differs from that obtained by the normal electro-oxidative polymerization of pyrrole (compare Section 6.3.4). 

Chart 17 Possible structural defects of oxidatively polymerized poly(pyrrole). 

Polypyrrole (PPy) has been studied in the form of thin films deposited on electrode surfaces, by electrochemical oxidative polymerization of pyrrole with anions (e.g. C104, HSO4 ) present in solution, resulting in relatively air-stable, highly conducting films. Chemical oxidation of pyrrole with Cu(II) or Fe(III) salts in solution has also been reported.58,59,60,61... 

In earlier works we reported on the preparation and structural analysis of 1 1 host-guest compounds of several types of CDs with pyrrole or 3,4-ethylenedioxythiophene (EDT), and their oxidative polymerization in water. These complexes were prepared simply by adding one equivalent of monomer to one equivalent of CD in water (Fig. 3) [26],... 

Fig. 3 Pyrrole or 3,4-ethylenedioxythiophene (EDT)/CD complex formation in water and oxidative polymerization of CD complexes...

Figure 6.26 gives information on the effect of carbon black loading on the polymerization efficiency of pyrrole. The polymerization rate was reduced as the loading of carbon black was increased. The reduced rate is caused by the oligomer coupling on the surface of the carbon black and by the absorption of the chemical oxidant needed for polymerization."" ... 

In suspensions of carbon black in pyrrole, anodic polymerization takes advantage of the fact that carbon black particles are negatively charged on their surface which makes it possible for them to migrate to a positively charged anode where they become embedded within a growing polypyrrole matrix This production method is suitable for production of materials for sensors, supercapacitors, fuel cells, etc. The effect of carbon black on the chemical oxidation of pyrrole in carbon black suspensions is shown in Figure 6.26. ° ... 

Aizawa and Wang have reported123 that the copper-containing enzyme, bilirubin oxidase (BOX), catalyzes the oxidative polymerization of pyrrole to give thin films of PPy on substrates such as glass, plastic, or platinum plates. The BOX was first adsorbed onto the matrix support from an aqueous acetate buffer solution (pH 5.5), followed by incubation with the pyrrole monomer (0.2 M) in acetate buffer (pH 6) for several hours at room temperature. The deposited PPy film was reported to have similar properties to PPy made by conventional chemical or electrochemical methods. 

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