Polyanilines, synthesis electrochemical polymerization

Completely different monomers were called for. Before long, three of today s workhorses had been identified pyrrole, aniline and thiophene. In Japan, Yamamoto [38] and in Germany, Kossmehl [39] synthesized polythiophene doped with pentafluoroarsenate. At the same time, the possibilities of electrochemical polymerization were recognized. At the IBM Lab in San Jose, Diaz used oxidative electrochemical polymerization to prepare polypyrrole [40] and polyaniline. [41] Electrochemical synthesis forms the polymer in its doped state, with the counter-ion (usually an anion) incorporated from the electrolyte. This mechanism permits the selection of a wider range of anions, including those which are not amenable to vapor-phase processes, such as perchlorate and tetra-fluoroborate. Electrochemical doping also overcomes an issue associated with dopants... 

In addition to sulfonic acid groups, carboxylic acid groups as ring substituents results in self-doping of polyaniline and influence properties such as solubility, pH dependent redox activity, conductivity, thermal stability, etc. Sulfonated polyanilines are typically obtained by postpolymerization modifications such as electrophilic and nucleophilic substitution reactions. However, carboxylic-acid-functionalized polyanilines are typically synthesized directly by chemical and electrochemical polymerization of monomer in the form of homopolymer or copolymer with aniline. In contrast to sulfonated polyaniline, very few monomers are available for the synthesis of carboxyl acid functionalized polyaniline. Anthranilic acid (2-aminobenzoic acid) is an important monomer and is often used for the synthesis of carboxyl acid functionalized polyanilines. 

Similarly, self-doped PABA can be prepared using excess of saccharide and one equivalent of fluoride to monomer. Complexation between saccharides and aromatic boronic acids is highly pH dependent, presumably due to the tetrahedral intermediate involved in complexation [25]. Because the pKa of 3-aminophenylboronic acid is 8.75, complexation requires pH values above 8.6. This pH range is not compatible with the electrochemical synthesis of polyaniline, which is typically carried out near a pH value of 0. However, Smith et al. have shown that the addition of fluoride can stabilize the complexation of molecules containing vicinal diols with aromatic boronic acids [23]. Based on this work, it was postulated that the electrochemical polymerization of a saccharide complex with 3-aminophenylboronic acid in the presence of one molar equivalent of fluoride at pH values lower than 8 is possible if a self-doped polymer is produced in the process. 

The electrochemical stability of PAn film at higher potentials is essential for its use as an electrochromic material and as a catalytic electrode. Similarly, soluble polyaniline is also required for many applications to facilitate post-synthesis processing. Electrochemical polymerization can be carried out by 0.1 M monomer in 1 M of aqueous solution of aromatic sulphonic acid. Prior to polymerization, the solution should be deoxygenated by passing argon gas for 30 minutes. 

Leclerc [125] has shown that poly(2-methyl aniline) has properties similar to that of polyaniline. Dao et al. [127] have carried out extensive investigations on chemical and electrochemical polymerization of substituted polyanilines. These authors report that chemical synthesis yields a polymer having a higher molecular weight. Some of their results are given in Table 12.13. Their study further shows that there is no polymer film on an electrode surface in the presence of the following substitutions ... 

Electroactive polyaniline films were synthesized by the catalysis of biUru-bin oxidase (BOD, a copper-containing oxidoreductase). The polymerization of aniline was carried out on the surface of a sohd matrix such as glass sUde, plastic plate, or platinum electrode to form homogeneous films [33]. The BOD was immobilized on the surface by physical absorption. The optimum pH was around 5.5. Some aniline derivatives such as p-aminophenol and p-phenylenediamine were good substrates for BOD. Structural analysis suggested the BOD synthesized polyanihne possessed partially 1,2-substititued structures. Cyclic voltammetric studies demonstrated that the PANl films were electrochemically reversible in redox properties, but differed from that of chemically or electrochemically synthesized PANl. The difference was attributed to the partial 1,2-substitution. Laccases are known to oxidize phenolic compounds in nature in the presence of oxygen and are capable in polyaniline synthesis in vitro [34-36]. 

The HRP-catalyzed polymerization of phenols was found to be a convenient way to produce redox polymers and conducting (electronically conducting and ionically conducting) polymers. Besides the interest in electronic conductive polyanilines [121], many efforts have been made to produce ionically conductive phenol polymers for battery applications. A classic effort is the synthesis of poly(hydroquinone) for use as a redox polymer. Typically, poly(quinone)s are prepared via chemical or electrochemical methodologies [122,123]. Both processes produce a large amount of by-products and lead to complex polymer structiues. The first alternative pathway to produce poly(hydroquinone) by peroxidase catalysis was based on a multienzymic... 

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