Self-doped polymers carboxylate

Self-doped PANI are very interesting due to their unique electrochemical behavior unlike PANI, the self-doped polymer remains in its doped state in near neutral or alkaline media [28]. Fully self-doped PANIs are not easy to synthesize due to the lower reactivity of acid-functionalized anilines. Kim et al. [29, 30] introduced an alternative approach in the template-assisted enzymatic polymerization of aniline. Previously, only polyanionic templates had been used for PANI synthesis. However, acid-functionalized anilines bear a net anionic charge in aqueous solution, and attempts to use SPS as template with carboxyl-functionalized aniline resulted in red-brown colored polymers with no polaron transitions, regardless of the synthetic conditions. The use of polycationic templates, such as those shown in Figure 8.2 allowed the synthesis of linear and electrically conductive PANIs with self-doping ability due to the doping effect of the carboxyl groups present in the polymer backbone. 

Soluble conducting polymers can be solvent cast to form coatings. The addition of appropriate substituents to the polymer backbone or to the dopant ion can impart the necessary solubility to the polymer. For example, alkyl or alkoxy groups appended to the polymer backbone yield polypyrroles [117,118], polythiophenes [118], polyanilines [119,120], and poly(p-phenylenevinylenes) [97] that are soluble in common organic solvents. Alternatively, the attachment of ionizable functionalities (such as alkyl sulfonates or carboxylates) to the polymer backbone can impart water solubility to the polymer, and this approach has been used to form water-soluble polypyrroles [121], polythiophenes [122], and polyanilines [123]. These latter polymers are often referred to as self-doped polymers as the anionic dopant is covalently attached to the polymer backbone [9]. For use as a corrosion control coating, these water-soluble polymers must be cross-linked [124] or otherwise rendered insoluble. 

Following the discovery of the unique electronic properties of polypyrrole, numerous polymers of pyrrole have been crafted. A copolymer of pyrrole and pyrrole-3-carboxylic acid is used in a glucose biosensor, and a copolymer of pyrrole and A-methylpyrrole operates as a redox switching device. Self-doping, low-band gap, and photorefractive pyrrole polymers have been synthesized, and some examples are illustrated [1,5]. 

Electrochemical polymerization of 3-alkylsuIfonate pyrroles with various length of alkyl chain (61) has given water-soluble self-doped polypyrroles [297]. A self-doped polypyrrole based on the carboxyl group was prepared from anodic coupling of 3-carboxy-methylpyrrole (62) [298]. The polymer was subsequently tested as a proton pump electrode [299]. 

Elemental analysis and X-ray fluorescence measurements of films electrochemically oxidized in aqueous KCIO4 solutions, at pH values below the pK of the carboxylic acid indicate the absence of perchlorate counterions in the films [25]. This shows that the oxidized polymer backbone is charge compensated by covalently bound carboxylate anions and confirms the self-doping mechanism of poly( -(3-thienyl)octanoic acid). In contrast to films oxidized in aqueous solutions,... 

Above pH 6.5, virtually all the carboxylic acid groups are deprotonated so that the formal redox potential of the polymer film becomes independent of pH. Above pH 11 the polymer films are electrochemically inactive probably due to deprotonation of the N-H group on the pyrrole group. Evidence for self-doping of these polymers was not explicitly provided. The elTect of electrostatic binding of cationic species on the electrochemistry of the films has been reported in detail. 

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