Water-soluble, self-doped polythiophenes

Water-soluble derivatives of polythiophene have been made allowing counterions bound to the polymer backbone to self-dope with the protons (e.g., lithium and sodium ions) injecting electrons into the pi-system. Thus, combinations of sodium salts and proton salts (e.g., prepared from poly-3-(2-ethanesulfonate)thiophene) have been prepared that are both water-soluble and conducting. 

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. 

The range of soluble polythiophenes has been growing rapidly to include side chains up to docosane 267), ether and amide links in the side chains 268), and water-soluble polymers with sulfonated side chains (Table 1) which are claimed to be self-doping in that the sulphonate may act as the counterion to the delocalized chain cation 269,270). In principle, these polymers can be p-doped and undoped by the transport of a proton or a small cation rather than a large anion, and so may respond more rapidly. By treatment of an aqueous solution with NOPF6, a doped solution can be made, which slowly degrades. 

Several water-soluble sulfonated polythiophenes such as 20 (x = 3 or 6) have been prepared by the FeClj oxidation of the corresponding monomers. " iR and NMR studies of these self-doped polymers in DjO/DMSO-dg or D20/acetonitrile-d3 showed a HT HH ratio of 4 1 in both cases. ... 

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