Polythiophenes paradox

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... 

This problem is sometimes designated the polythiophene paradox. 

Table 20.8 contains a compilation of literature entries on the voltammetry of conducting polymer films. The scope of these studies is similar to that of the transient experiments discussed in Section V.A in terms of the types of electrodes and media employed. Both cyclic and hydrodynamic voltammetry have been used as shown in Table 20.8. Other aspects under discussion include the mathematic modeling of cyclic voltammo-grams [277,278], the occurrence and origin of prewaves in the cyclic voltammograms [319], the use of very fast scan rates [220], structural relaxation effects and their manifestation in voltammetry [304,317,320], the inactivation of polymer electroactivity when driven to extreme potentials, and the so-called polythiophene paradox [225,226,306,321]. Unusual media and cryogenic temperatures have also been employed for the volta-mmetric observation of doping phenomena [322-325]. Dual-electrode voltammetry (Section II.1) has been performed on derivatized polypyrrole [290] in an attempt to deconvolute the electronic and ionic contributions to the overall conductivity of the sample as a function of electrode potential. Finally, voltammetry has been carried out in the solid state , i.e., in the absence of electrolyte solutions [215,323].

This is particularly important because the development of systems utilizing thiophene have been thwarted by the "polythiophene paradox". It... 

They found that all monomers could be oxidized to form conducting polymers but that the oxidation potentials increased according to MTh < ETh < Th. The fact that ETh was harder to oxidize than MTh was attributed to steric effects. This lowering of the oxidation potential by addition of alkyl groups at a P carbon position avoids the "polythiophene paradox" during polymerization. 

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