Properties of Polythiophenes

Because poly thiophene itself is prone to overoxidation during polymerization, most practical work has been carried out using alkylated thiophenes, which have higher overoxidation potentials. Synthesis of functionalized thiophenes (such as alkylated monomers) is much easier to achieve than that of its 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 heteroatom during synthesis are not required for thiophene. 

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 

As with polypyrroles, the counterion used during electropolymerization influences the conductivity of polythiophenes.120121 Electrochemically produced copolymers122 of 3-dodecylthiophene (DTh) and 3-methylthiophene (MTh) have been shown to exhibit conductivities intermediate to the two homopolymers. The actual value depends on the ratio of MTh to DTh in the polymer. 

The conductive properties of alkylated thiophenes are known to be unstable, particularly at elevated temperatures. The mechanism of thermal undoping has been associated with thermal mobility. Consequently, various workers126 have considered synthesis of random copolymers (e.g., thiophene and 3-octylthiophene), with well-distributed octyl side groups leaving space around the main chains to accommodate dopants. 

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The development of polythiophenes since the early 1980s has been extensive. Processible conducting polymers are available and monomer derivathation has extended the range of electronic and electrochemical properties associated with such materials. Problem areas include the need for improved conductivity by monomer manipulation, involving more extensive research using stmcture—activity relationships, and improved synthetic methods for monomers and polymers alike, which are needed to bring the attractive properties of polythiophenes to fmition on the commercial scale. 

Figure 5. Cyclic voltammograms of (a) 2,5"" -di-methyl-a-hexathiophene and (b) poly(2,2 -bithio-phene) films in acetonitrile containing 0.1 M E NCIO 103 (Reprinted from G. Zotti, G. Schia-von, A. Berlin, and G. Pagani, Electrochemistry of end-ca )ed oligothienyls-new insights into the polymerization mechanism and the charge storage, conduction and capacitive properties of polythiophene, Synth. Met. 61 (1-2) 81-87, 1993, with kind permission from Elsevier Science S.A.)...

As might be expected, the properties of polythiophene show many similarities with those of polypyrrole. As with polypyrrole, polythiophene can be prepared via other routes than electrochemical oxidation both as the neutral material [390-392] or in the p-doped form [393]. This material is produced as an infusible black powder which is insoluble in common solvents (and stable in air up to 360°C), with conductivities ranging from approximately 10 11 Scm-1 in the neutral form [390] to 102 Scm-1 when doped [19, 393, 394]. Early work on thiophene polymers showed that the p-doped material is air-sensitive in that the conductivity decreases on exposure to the atmosphere [20, 395] although no evidence of oxygen-containing species was seen in XPS measurements [19],... 

Table 1.14 Electrochromic colour change properties of polythiophenes (Reproduced with permission of Wiley-UCH)...

Toward an understanding of the conduction properties of polythiophenes (26) and polypyrroles (25) a large number of soluble oligomers has been prepared. Oligothiophenes, indeed, represent the most common model compounds for electrically conducting polymers [149]. Thereby, lower oligomers... 

Studies of the chemical properties of polythiophenes have been limited. As with polypyrroles, a hydrophobic backbone is formed, and the polymer has ion-exchange properties. Modification of chemical properties by incorporation of appropriate counterions is not so readily addressable because polymerization must be carried out from nonaqueous solution and occurs at more anodic potentials compared to pyrrole. 

Optical Properties of Polythiophenes Electronic Band Structure and UV-Visible Spectra... 

Mardalen, J. et al.. Structure and electronic-properties of polythiophene and poly(3-alkylthiophene) films electropolymerized on indium tin oxide (Ito) glass at elevated potentials, Macwmol. Chem. Phys. 194, 2483-2495, 1993. 

G. Tourillon and F.Garnier, Structural effect on the electrochemical properties of polythiophene and derivatives, y. Electroanal. Chem., 161 51 (1984). 

Hiraishi, K., Masuhara, A., Nakanishi, H., Oikawa, H., Shinohara, Y, 2009. Evaluation of thermoelectric properties of polythiophene films synthesized by electrolytic polymerization. Jpn. J. Appl. Phys. 48, 071501. 

Roux, C., and M. Leclerc. 1994. Thermochromic properties of polythiophene derivatives Formation of localized and delocalized conformational defects. Chem Mater 6 (5) 620-624. 

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