Conducting polymers radical cation coupling

Although several examples of anodic dimerizations involving the coupling of cation radicals have been described,34 the most systematic mechanistic analyses have concerned electropolymerizations leading to conducting polymers.35 They involve a series of successive cation radical-cation radical coupling steps as represented in Scheme 2.33 for the case of pyrrole. Deprotonation of the first... 

Summaries on the synthesis, properties, and uses of polythiophenes are included in two general reviews on poly thiophenes [259,260]. A synopsis of important aspects of polythiophenes are also included in several reviews on various aspects of conducting polymers [221-226], Cation radicals are the propagating species in both electrochemical and chemical oxidative polymerizations of thiophene and its derivatives. The polymer obtained by this method is linked primarily by a,a-linkages. However, other types of linkages (a,f3 and /3,/3) are present in varying amounts (Fig. 59). Substituted thiophene derivatives can couple in a head-to-tail or head-to-head manner. 

It should be noted here for triarylamine networks and dendrimers [104], respectively, the radical cations have interesting properties like the formation of high-spin polyradicals with ferromagnetic coupling [105] or conducting polymers [106]. Very often, triarylamines have been used as the hole-transport layer in electroluminescent devices [107]. 

Compared with chemical oxidation polymerization, electrochemical polymerization is performed at an electrode (conductive substrate) using the positive potential [97-104]. Whereas the powder forms are obtained by chemical polymerization, the electrochemical method leads to films deposited on the anode. When a positive potential is apphed at the electrode, pyrrole monomer such as a heterocychc compound is oxidized to form a delocalized radical cation, which includes the possible resonance forms. Radical-radical coupling reaction produces the dimerization of the monomer radicals at the a-position. Removal of 2H+ ions consequently forms the neutral dimer. Next step is chain propagation which includes the oxidation of the neutral dimer to form the dimer radical. The resultant radical can react with other monomer or dimer and this radical coupling and the electrochemical oxidation processes repeat in order to extend the polymer chain. The final step involves the termination of chain growth and the resultant PPy film is formed on the anodic electrode. 

Among all the techniques listed above, chemical polymerization is the most used for preparing large amounts of conductive polymers, since it is performed without electrodes (Malinauskas 2001). Chemical polymerization (oxidative coupling) is followed by the oxidation of monomers to a cation radical and their coupling to form dications. The repetition of this process generates a polymer, and... 

A last point remains unresolved. Does the coupling result from the direct reaction between two radical-cations, as generally accepted for other conducting polymers poly (pyrrole), poly(thiophene), poly(ani-line)), or from a mixed reaction between benzene and the corresponding radical-cation ... 

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