Overoxidation polythiophenes

Therefore, if using constant-current or constant-potential polymerization, the product obtained will be a mixture of poly thiophene and overoxidized polythiophene. 

Hie electrochemical characteristics of overoxidation vary widely among polymers, solvents, and nucleophiles.129 Its rate depends on the degree of oxidation of the polymer (and therefore on the potential applied), and the concentration127 and reactivity of the nucleophile. Polypyrroles usually become overoxidized at lower potentials than polythiophenes because of their lower formal potentials for p-doping. In acetonitrile, the reactivity of the halides follows their nucleophilicity in aprotic solvents,... 

The overoxidation of polyanilines has been studied most extensively in aqueous solutions.33 It occurs much more slowly than the overoxidation of polypyrroles and polythiophenes, requiring extensive cycling through the second oxidation wave (at ca. +0.7 V in Figs. 6 and 8), or many minutes... 

This is particularly important because the development of systems utilizing thiophene have been thwarted by the polythiophene paradox. 2 It has been clearly shown that at potentials required to oxidize the thiophene monomer, the polymer itself becomes overoxidized. This overoxidation process proceeds according to Equation 6.2,3 and results in deterioration in the chemical and physical properties of the polymer. 

To achieve high conductivities, the polythiophene paradox must be overcome. The polymerization process and conductivity of the resultant material are influenced by the concentration of monomer used during polymerization116 because, if this is too low, the overoxidation reaction predominates, at least when galvanostatic polymerization is used. Synthesis at reduced temperatures will help avoid overoxidation and can be used to increase the conductivity of the resultant material.117... 

As stated earlier, the presence of alkyl functional groups on the monomer118 can be used to advantage in preventing overoxidation of the polythiophenes during synthesis. Conductivities as high as 7500 S cm-1 have been obtained for poly(methylthiophene).119... 

FIGURE 4.13 Electrochemical overoxidation techniques for use with polythiophenes, (a) A plotter pen modified to be used in an electrochemical cell, (b) Screen-printing electrochemical cell, (c) Photoresist patterned on a PEDOT PSS film protects areas of the film from overoxidation whereas the rest is patterned when wet by the liquid electrolyte. (Images from Tehrani, P., Remonen, T., Hennerdal, L.-O., Hall, J., Malmstrom, A., Leenders, L., Kugler, T., Robinson, N.D., Crispin, X., Fahlman, M., and Berggren, M., Smart Mater Struct 14, N21-N25. Copyright 2005, Institute of Physics Publishing. With permission.)... 

Tang, H., L. Zhu, Y. Harima, and K. Yamashita. 2000. Chronocoulometric determination of doping levels of polythiophenes Influences of overoxidation and capacitive processes. Synth Met 110 (2) 105-113. 

Tehrani, R, N.D. Robinson, T. Kugler, T. Remonen, L.-O. Hennerdal, J. Hall, A. Malmstrom, L. Leenders, and M. Berggren. 2005. Patterning polythiophene films using electrochemical overoxidation. Smart Mater Struct 14 N21-N25. 

Barsch, U., and F. Beck. 1996. Anodic overoxidation of polythiophenes in wet acetonitrile electrolytes. Electrochim Acta 41 (11-12) 1761-1771. 

Polythiophene deposition from oxidation of thiophene often results in poor materials due to overoxidation at the high potential required. The best results are obtained with 3-methylthiophene and bithiophene which are more easily oxidized [81-3]. The degree of polymerization is some hundreds for poly(3-alkylthio-phenes) [84] and 10-35 for polythiophene [85]. The polymer is mainly coupled at the 2 and 5 positions (Figure 4.5) [86]. 

Polythiophene is not stable at potentials used for electrochemical polymerization of thiophene. Thus, polythiophene deposited on the anode at the earlier stages of the polymerization is overoxidized and has deteriorated, while electrochemical polymerization... 

Other workers have minimized overoxidation during polymerization by using bithiophenes or terthiophenes as starting materials for the polymerization process. The polymerization potential has been shown to decrease according to terthiophene < bithiophene < thiophene. But, as reviewed by Roncali, polythiophenes produced from these starting materials generally have lower conductivity. ... 

Because polythiophene itself is prone to overoxidation during polymerization, most practical work has been carried out using alkylated thiophenes which have higher over-oxidation potentials. Synthesis of functionalized thiophenes (such as alkylated monomers) is much easier to achieve than with the pyrrole counterpart. The decreased activity of the sulfur group compared to that of the -NH group means that the laborious steps involved in protecting the heteratom during synthesis are not required for thiophene. 

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