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Figure 5. Cyclic voltammogram of a PBT/S-PHE coated platinum electrode obtained after immersion in a propylene carbonate solution containing 0.5 M LiCIO4, scan rate = 100 mV s-1. PBT/S-PHE was prepared galvanostatically (1 mA cm-2 for 3 min) using a solution containing 0.2 M 2,2'-bithiophene and 2% S-PHE.

Figure 5. Cyclic voltammogram of a PBT/S-PHE coated platinum electrode obtained after immersion in a propylene carbonate solution containing 0.5 M LiCIO4, scan rate = 100 mV s-1. PBT/S-PHE was prepared galvanostatically (1 mA cm-2 for 3 min) using a solution containing 0.2 M 2,2'-bithiophene and 2% S-PHE.

be oxidation of the aniline monomer to give the radical cation of aniline. This is followed by coupling of the radicals, predominantly N and para forms, and subsequent re-aromatization to give a dimer. This undergoes more facile oxidation than the aniline monomer, leading to chain propagation and eventual deposition of the emeraldine salt from solution.

The nature of the dopant anion incorporated along the growing polymer chain during polymerization has a profound effect upon the morphology [52], conductivity [53], switching characteristics, and solubility of the resulting polyaniline salts. Incorporation of polyelectrolyte dopant anions during either chemical or electrochemical oxidation of aniline has been an area of intense recent interest. These large anions can often be preferentially incorporated even when an acid such as HClO4 is present in large excess [54]. As will be discussed further in Section 3, this can induce water "solubility" onto the resultant emeraldine salt when the anion is a polyelectrolyte such as poly(styrenesulfonate) or polyacrylate [55-57]. This approach has helped in overcoming one of the problems previously associated with polyanilines, namely their insolubility in most common solvents. It has also provided a convenient route to aqueous nanosized dispersions of conducting emeraldine salts (see Section 4). Biological polyelectrolytes such as DNA have also been incorporated into emeraldine salts using this approach [58, 59].

On the other hand, organic solvent solubility has been induced in polyanilines via the incorporation of surfactant-type dopant anions such as dodecylbenzenesul-fonate, racemic 10-camphorsulfonate (CSA-) [60], and dinonylnapthalene sulfonate [61]. Another significant development has been the facile generation of optically active emeraldine salts such as PAn(+)-HCSA or PAn(-)-HCSA by the simple expedience of employing chiral acids such as (+)- or (-)-HCSA as the electrolyte during electrochemical polymerization [62, 63]. These chiral conducting polyanilines are believed to preferentially adopt single-handed helical structures for their polymer chains depending on which hand of the dopant HCSA acid is employed.

A large range of substituted anilines are available. Polymerization of these has given emeraldine salts whose properties differ significantly from those of the parent unsubstituted PAn • HA salts. For example, the presence of alkyl or alkoxy substituents [64] on the aniline rings imparts enhanced solubility in organic solvents on the resultant polymers. On the other hand, sulfonate groups lead to water solubility, as in poly(2-methoxyaniline-5-sulfonic acid) [65].

Postpolymerization modification of polyanilines has also been pursued to introduce added functionality into the polymer for a variety of applications. For example, treatment of the emeraldine base (EB) or leucoemeraldine base (LB) forms of polyaniline with fuming sulfuric acid has led to water-soluble, sulfonated polyanilines (SPANs) in which 5075% of the aniline rings bear sulfonate groups [66]. These latter emeraldine salts are self-doped (i.e., ring-bound sul-fonate groups provide the dopant anion for the radical cation nitrogen sites along the chain). An exciting recent development has been the synthesis of poly(aniline boronic acid) [67]. This promises to be a convenient precursor for

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