Self-Doped Polypyrrole

Significant variations in the properties of polypyrrole [30604-81-0] ate controlled by the electrolyte used in the polymerization. Monoanionic, multianionic, and polyelectrolyte dopants have been studied extensively (61—67). Properties can also be controlled by polymerization of substituted pyrrole monomers, with substitution being at either the 3 position (5) (68—71) or on the nitrogen (6) (72—75). An interesting approach has been to substitute the monomer with a group terminated by an ion, which can then act as the dopant in the oxidized form of the polymer forming a so-called self-doped system such as the one shown in (7) (76—80). 

Figure 13. (a) Substituted polypyrrole, (b) self-doped polypyrrole, (c) heteroaromatic polymer showing the monomer unit, (d) composite polypyrrole-polyelectrolyte, and (e) hybrid material. (Polyaniline macroion photo supplied by Gomez-Romero and M. Lira.)... 

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]. 

Fig. 3.10. Some of the more commonly encountered organic conductor materials (a) polypyrrole, (b) polyaniline, and (c) poly(3,4-ethylenedioxythiophene) (PEDOT). When combined with water soluble organic acids (e.g. sulfonic acids like benzosul-fonic acid) many of these polymers can form doped complexes which are highly conductive and can be dispersed into suspension. Substituted versions of these polymers which are self-doped have also been developed.

The introduction of sulfonic acid groups in the alkyl side chains affords water-soluble self-doped polypyrrole. Using the sodium salt of the 3-alkylsulfonic acid pyrrole, the monomer acts as electrolyte for the electrochemical synthesis as well. A second long alkyl chain on the opposite side of the pyrrole ring affords a highly ordered lamellar polymer also soluble in chloroform [96]. 

Yin, W., and E. Ruckenstein. 2001. A water-soluble self-doped conducting polypyrrole-based copolymer. J Appl Polym Sci 79 86. 

Havinga, E.E., W. ten Hoeve, E.W. Meijer, and H. Wynberg. 1989. Water-soluble self-doped 3-substituted polypyrroles. Chem Mater 1 650-659. 

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. 

Figure 1.19 Various self-doped polypyrrole derivatives.

Figure 1.22 The structure of self-doped polypyrrole graft copolymer (Reprinted with permission from Macromolecules, 38, 1044. Copyright (2005) American Chemical Society.)...

The highest conductivity of 65 S/cm for electrochemically prepared self-doped ring-sulfonated polypyrrole was reported by Sahin et al. [203]. However, conductivity decreases from 65 S/cm to 6.5 S/cm with... 

Similarly to polyaniline and polythiophene, several other self-doped conducting polymers have been explored including polypyrrole, poly(3,6-(carbaz-9-yl) propanesulfonate), poly(p-phenylene), poly(indolecar-boxylic acid), polyphenylenevinylene, and poly(2-ethynyl-N-(4-sulfo-butyl)pyridinium betaine). The details of synthesis and properties are described in the following sections. 

A new class of water soluble polypyrrole has been prepared by selfdoping of the polymer. These self-doped polypyrroles can be prepared electrochemically or chemically, using various dopant anions covalently bound to the polymer backbone. The self-doped sulfonated polypyrrole is most commonly synthesized electrochemically in nonaqueous media. Electrochemical synthesis in aqueous media and chemical synthesis are not typically used, presumably due to issues with overoxidation. The postpolymerization modification of polypyrrole, in a manner similar to that used to form sulfonated polyaniline is rare [41]. The various synthetic approaches and properties of the polymer are discussed in the following sections. 

Self-doped polypyrrole was first prepared by Reynolds et al. [42] and Havinga et al. [43] in 1987. Their approach to self-doping of polymers was based on monomers that were easier to polymerize electrochemically. Reynolds et al. prepared the N-substituted pyrrole copolymer, poly(pyrrole-co-(3-(pyrrol-l-yl)propanesulfonate)) (Figure 5.1) in acetonitrile containing tetrabutyl ammonium tetrafluoroborate as a supporting electrolyte on a platinum electrode. The monomer, potassium... 

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