Electropolymerization, electrolytic reactions

Perchlorate salts are used as electrolytes in - electropolymerization reactions involving monomers such as aniline, benzidine, azulene, biphenyl, di-vinylbenzene, and indole. 

In choosing the electrolyte for electropolymerization, an important requirement is that both the anion and cation are inert to electrochemical reactions at the potentials used for polymerization. Typical electrolytes used in nonaqueous solutions are tetraalkylammonium salts such as tetrabutylammonium hexafluorophosphate, tetrafluoroborate, perchlorate, and corresponding lithium salts. 

The ideal electropolymerization scheme (Eq. (5.5.39)) is further complicated by the fact that lower oligomers can react with nucleophilic substances (impurities, electrolyte anions, and solvent) and are thus deactivated for subsequent polymerization. The rate of these undesired side reactions apparently increases with increasing oxidation potential of the monomer, for example, in the series ... 

Electrolytic polymerization or electrolytically initiated polymerization, or shortly electro-initiated polymerization or electropolymerization, generally means initiation by the electron transfer processes which occur at the electrodes of an electrolytic cell containing monomer and electrolyte, in that by controlling the electrolysis current it is possible to control the generation of initiating species. Under appropriate conditions it may proceed by a free radical, anionic or cationic mechanism. In addition to the electrolytic addition polymerization, production of polymers through condensation reaction by electrolytic means should also be covered. Examples of each of these propagation mechanisms have now been reported in the literature. 

Effects of various experimental parameters on PAn growth and its properties have been studied by a number of investigators [43,90-101]. These parameters include the solution pH, electrolytes, aniline concentrations, temperature, solvents, substituents on the aniline molecule, and the presence of foreign materials. In this section, the effects of these parameters primarily on the electropolymerization reaction of aniline are discussed briefly. 

As we now have a functional estimation of the electric field rate we may inject it directly in equations (2.5) and (2.8), and terminate with the determination of our model potential meant to compute the lifetime of chemisorbed anions. This is the dedicated purpose of another paper [28]. Let us merely note that the electropolymerization reactions we are interested in are carried out in acetonitrile (e = 36.5), using a supporting (1 1) electrolyte at a concentration of 5.10 M [1,2]. According to equation... 

No racemization was observed when the electrode potential was scanned only to a value where the dianion is formed. Upon formation of the tetraanion, subsequent chemical reactions were found. With a slightly different electrolyte salt (Mc4NBF4 instead of BU4NF6), reversibility without racemization was found even up to the tetraanion formation. Further examples include the spectroelectrochemistry of vitamin D2 [139], which has been studied with a long pathlength cell similar to the design described by Zak et al. [44]. Optical rotary dispersion and CD of optically active polybithiophene that has been electropolymerized in a cholesteric electrolyte have been studied [140]. The optical rotation of this chiral polymer could be controlled via the electrode potential. 

Potential Electropolymerization is carried out at moderate potentials to prevent the oxidative decomposition of the solvent, electrolyte and polymer film. The polymerization potential also determines the stability of intermediate species. The formation of a polypyrrole film, for example, occurs via cation intermediates whose stability favours the radical coupling reaction. The reactive cations may also react with solvent and other nucleophiles in the vicinity of the electrode surface, minimizing the polymer forming reaction. Some of the monomers which have been electropolymerized are listed in Table 2.3 along with their respective peak potentials and the apparent electrochemical stoichiometry of the reaction. 

Electrolytes and solvents. The electropolymerization reaction may be sensitive to the nucleophilic nature of the solvent and electrolyte. For this reason, many of the films are prepared in aprotic solvents, such as acetonitrile, which are poor nucleophiles. Electro-oxidative polymerization in the presence of small anions simultaneously incorporates the anions which render the polymer film conductive. Upon reduction, the anions are released from the film. Cycling the film through oxidation and reduction leads to insertion and release in the respective parts of the cycle. Simultaneous incorporation or removal of the solvent and/or cations may also occur, as shown by measurements on the quartz crystal microbalance [51-52]. Polymerization in the presence of large anions such as poly(vinylsulphonate) and poly(4-styrene sulphonate) (PSS") also incorporates the anion during growth [53-56]. Subsequent cycling, however, does not release the anions which are trapped because of their... 

Reaction 4 attempts to recover the considerable influence of electrolytes in empirical kinetics, through its discharge on the polymer. The formed radical is immediately transferred to a monomer molecule. This indirect initiation can explain the behaviour of pyrrole electropolymerization at high potentials in water [51]. The other electrolyte influences act on the metal oxide production [49], by stabilization of the monomeric radical cations [130] and through polymer oxidation. 

A general problem in the synthesis of those composites is the appearance of conductivity gradients in their thickness direction, which are caused by inhomogeneities in the polypyrrole structure, as a result of difficulties in diffusion of the electrolyte across the host polymer. A method for overcoming these problems has been developed by Wang et al. [375]. PPy/PVC composites can significantly improve uniformity in electrical conductivity and resistance to mechanical delamination when a certain amount of electrolyte is mixed with PVC prior to the electrochemical reaction. After electropolymerization of pyrrole in the presence of electrolyte blended PVC for 20 minutes, the conductivity across the film thickness showed more than ten orders of magnitude difference with respect to film obtained from unblended PVC. 

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