Synthesis of In Situ PEDOT

In situ poly(3,4-ethylenedioxythiophene) (PEDOT) was the first example for PEDOT as synthesized under film-forming conditions. Up to now in situ PEDOT is the polythiophene with the highest achievable electric conductivity, although PEDOTPSS (poly(styrenesulfonate)) is catching up. That is why in situ PEDOT is of great practical and commercial value. These industrial aspects are described in Chapters 10 and 11. 

This chapter will focus on chemical issues, including some practical advice for performing oxidative in situ polymerization. There are several investigations of PEDOT covered by the term in situ in the literature, which deal with electrochemically synthesized PEDOT layers these studies are discussed in Chapter 14. Here the focus is on the preparation of doped PEDOT films from EDOT solutions with a chemical oxidant. 

The oxidative in situ polymerization is best carried out with ionic oxidants like iron-III manganese-IV, or similar metal ions in a suitable higher oxidation state. There are various other metal salt oxidants and also completely distinct compounds, for example, peroxides, suggested for oxidative polymerization in the literature or claimed in patents. These alternatives have been discussed in more detail in Chapter 6. Iron-III is the oxidant preferred in most experiments in literature and also by far predominant in industrial use. 

The solubility requirements for iron-III oxidants are determined by the limited solubility of EDOT in water in contrast to its miscibility with alcohols like ethanol or n-butanol in any ratio. So the well alcohol-soluble iron salts of sulfonic acids are preferred oxidants. Especially p-toluenesulfonate has been established as a very suitable anion with respect to solubility and reactivity of the corresponding iron-III salt. Iron-III toluenesulfonate has become the most widely used oxidant for EDOT in the preparation of in situ PEDOT layers, scientifically and commercially. 

The overall polymerization reaction can be separated into two steps. First, the oxidative polymerization of EDOT to the neutral, undoped polythiophene occurs. p-Toluenesulfonic acid and iron-II p-toluenesulfonate are formed as byproducts stoichiometrically. Therefore two mole equivalents of Fe-III tosylate are necessary to perform this reaction. In the second step, the neutral PEDOT is doped by the action of excess Fe-III tosylate. As every third or fourth thiophene moiety loses one electron to form the cationic structure, about 0.25 to 0.33 additional equivalents of iron(III) toluenesulfonate are needed for efficient doping. ITere also the corresponding stoichiometric amount of Fe-II tosylate is formed. The third tosylate anion is incorporated into the polymer structure as the coimterion. As the result, the in situ PEDOT (more exactly, PEDOT pTs) layers formed by this two-step reaction are initially containing about 2.3 moles of iron-II tosylate and 2.0 moles of free tolu-enesulfonic acid. Both can be removed by rinsing with water, for example. 

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