Dopants polyaniline electrochemical polymerization

A special feature of the polyanilines (28) is the fact that they are prepared by electrochemical polymerization in the presence of a chiral dopant (Scheme 1). Polymers prepared in the presence of enantiomeric counterions give rise to CD spectra with completely different signs. These signals are lost completely upon deprotonation of the polymer. This suggests that the helicity is maintained by hydrogenbonding and/or electrostatic interactions with the counterions. 

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... 

Composites of conducting polymers, e.g., polyaniline and PEDOT, with polyacids, e.g., poly (2-acrylamido-2-methyl-l-methyl-l-propanosulfonic add) (PAMPS), have been shown to be electro-chromic. The polyadd acts as a dopant for the polymer film with the optical properties of the composite being contributed by the conducting polymer. The composites are formed by dther chemical or electrochemical polymerization of the electrochromic component monomer in the presence of the polyacid. Films of polyaniline-PAMPS switch from yellow to green and finally to blue on oxidation [228,229]. Composite films of PEDOT and PAMPS show similar electrochromic properties to PEDOT with the films switching from dark blue in the neutral state to Kght sky blue in the oxidized state [140,230,231]. 

Ext ive investigations on polyaniline (PAn) and its derivatives have be carried out (i) since they possess a moderate conductivity upon doping with protonic acid and an excellent stability under ambient conations (2,3). PAn is simply prepared by the chemical and electrochemical oxidation of aniline or its derivatives in aqueous solution. In general, however, the chemical and electrochemical polymerization of aniline monomer lead merely to an insoluble powder and a thin brittle film, respectively. Hence, it is very difficult to process PAn for a practical use. In order to deal well with this problem, the improvement of processability of PAn has been studied by preparing polymer composites (4) and soluble PAn (5,6) and using plasma polymerization (7) and postsulfonation of PAn (8,9). Another approadi to the preparation of processible PAn is to apply a precursor polymer, e.g., PAn can be produced by the thermal treatment of poly(anthranilic acid) 0 ANA) (10). This mefliod is particularly useful for the preparation of processible PAn or its composites with other insulating polymers since it does not use external dopants that often cause an inconvenient situation associated with a practical use of the conducting polymer. 

Watanabe et al. [161] reported that doped polyaniline can be partially solubilized. A blue film, which was insoluble in organic solvents, was electrochemically polymerized from an aqueous solution of 0.1 M aniline and 0.1 M sulfuric acid. On placing the polyaniline in a 20% by weight aqueous sodium hydroxide-tetrahydrofuran solution, the polyaniline dissolved into the organic layer as indicated by the appearance of a blue color. However, for extraction times in excess of 1 h, the blue color of the tetrahydrofuran phase fades. Most likely, the sulfate dopant anions begin reacting with the sodium hydroxide. 

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