Chemical oxidation of pyrroles

The first chemical oxidation of pyrrole 106 (scheme 26) was achieved as early as 1916 by using hydrogen peroxide to obtain an amorphous powder known as pyrrole black [158]. The room temperature conductivity of PPy 20 prepared with acid or peroxide are in the range of 10"to 10 " S/cm, which can be inereased by halogen doping to 10 S/cm [159]. This low conductivity is due to the high degree of saturation of the pyrrole monomer units caused by defects. In the last... 

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... 

Important processing methods Langmuir-Blodgen teclinique of monolayer production, solution polymerization over the substrate, electrochemical anodic polymerization, chemical oxidation of pyrrole in carbon black suspension... 

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. ° ... 

Shen and Wan193 have shown that chemical oxidation of pyrrole (by APS) in the presence of p-naphthalenesulfonic acid (P-NSA) results in the formation of tubules down to ca. 200 nm in diameter. The tubular structures had reasonable conductivity (10 S cm-1) and were soluble in m-cresol. In later work,194 it was reported that the morphology was significantly influenced by the concentration of the p-NSA. Granular morphologies were present when [p-NSA] was less than 0.2 M, whereas on increasing the p-NSA concentration, fiber formation became the dominant morphology. 

Others196 have used lipid tubules as templates during chemical oxidation of pyrrole to form nanofibers with diameters between 10 and 50 nm and lengths reaching up to several hundred microns. 

A. Pron and K. Wojnar, Characterization of Polypyrrole-Polyvinyl Alcohol Composite Prepared by Chemical Oxidation Of Pyrrole, Springer-Verlag, Berlin, 1987. 

Another approach to the formation of conductive biocomposites involves chemical oxidation of pyrrole [28] or sulfonated pyrrole [29] in the presence of the biomolecule. Polypyrrole chitosan composites have been formed via chemical oxidation of pyrrole in the presence of chitosan [30]. Chitosan is another commonly used biomaterial to which polyaniline has been chemically grafted to produce materials with conductivities as high as 10 S cm (Figure 11.7) [31]. 

Chemically prepared PPy can also be processed in various shapes, and especially a wide variety of micro- and nanostructured composites have been prepared. An important amount of work is due to Armes and coworkers, who systematically studied the formation of PPy composites formed by chemical oxidation of pyrrole, essentially by iron salts, in various colloidal solutions. This results into the formation of colloidal suspensions of the host substrate (sihca, polystyrene, silica-coated magnetite, etc.) impregnated with PPy. However, the conductivity of pressed pellets of these composites is low, mostly below 1 S/cm. More recently, a wood saw dust/PPy composite has been reported, and applied to the selective removal of chromium (VI) from waste water. PPy/Mn02 composite (see above) has been shown to be effective for use as an active material in supercapacitors. 

Polypyrroles. The first reported polymerization of pyrrole was in 1916 (97). Polypyrrole was prepared by the chemical oxidation of pyrrole using hydrogen peroxide. An amorphous black powder known as pyrrole black was obtained, which... 

Low temperatures (0-5°C) are the most appropriate for obtaining the best conductivities in aqueous solutions of ferric salts [22]. A similar behaviour of the chemical oxidation of pyrrole in organic solvents has been reported [23]. This seems to suggest that lowering the reaction rate results in an increasing conductivity of the polymer. 

Because the conductivity of polypyrrole prepared by the chemical oxidation of pyrrole with FeCl3 in methylene chloride is 1 x 10 S cm , the conductivities of the copolymers are very low. The copolymer with 55 mole% pyrrole units, for example, has a conductivity of 3.4 x 10 S cm . ... 

Blends of polypyrrole and Teflon were also prepared by the chemical oxidation of pyrrole in an emulsion of Teflon containing the ferric / -toluene sulfonate salt [69]. Pyrrole was added to the ferric salt aqueous solution and mixed with the Teflon emulsion stabilized by Triton X-100. After stirring for 3 hours a finely dispersed blend was obtained. This emulsion was coagulated with ethanol. Conductivity was reported to reach 1 S cm with a percolation behavior. This method could be used for large-scale production of the blend. 

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