Proton expulsion

Thus, it appears that the transition represented by the anodic peak in the cyclic voltammogram of polypyrrole is due to a changeover in the dominant carrier type and is accompanied by a dramatic contraction of the film. The authors strongly suspected that this contraction was due to electro-striction associated with bipolaron formation. As a further test they also carried out experiments intended to test if proton expulsion from the film occurred on oxidation. They found that it did indeed occur but monotonically at alt potentials > -0.6 F, in agreement with the extremely elegant work of Tsai et at. (1987), and so could not be responsible for the relatively sudden contraction at potentials > —0.2 V. 

One-electron oxidation of toluene results in the formation of a cation radical in which the donor effect of the methyl group stabilizes the unit positive charge. Furthermore, the proton abstraction from this stabilized cation radical leads to the conjugate base, namely, the benzyl radical. This radical also belongs to the it type. Hence, there is resonance stabilization in the benzyl radical. This stabilization is greater in the benzyl radical than in the tt cation radical of toluene. As a result, the proton expulsion appears to be a favorable reaction, and the acid-base equilibrium is shifted to the right. This is the main cause of the acidylation effects that the one-electron oxidation brings. 

Because Reaction (24) is reversible, eventually all the carbenium salt is consumed in irreversible Reaction (25). Similar behavior was observed for polymerization of tetrahydrofuran initiated with trityl salts. In this case, however, hydride transfer from tetrahydrofuran molecule is followed by proton expulsion to form 2,3-dihydrofuran, which complicates the initiation mechanism [27,28] ... 

Let us discuss in more detail all these factors. First of all, it should be stressed that onium ions (e.g., trialkyloxonium, tetraalkylammonium, trialkylsulfonium) are inherently stable different onium salts may be isolated and stored in a pure state, some of them are commercially available. This is in contrast to cationic vinyl polymerization, where carbenium ion species are not inherently stable due to the possibility of proton expulsion ... 

Storey et al. [140] point out that in living polymerizations of monomers like isobutylene that are co-initiated by TiCLt temperatures as low as —80°C, the livingness is limited not by chain transfer to monomer but rather by a unimolecular termination process. Unimolecular terminations often involve p-proton expulsions to produce polymers with terminal unsaturation. They claim, however, that this does not happen here. Rather the normal ferf-chloride chain ends of polyisobutylene formed by this type of polymerization gradually become depleted. They propose, therefore, that an isomerization mechanism takes place instead in the presence of an active Lewis acid, tmder monomer starvation conditions. It can be illustrated as follows [140] ... 

PBD is a very useful tool for identifying the ions that participate in the ion exchange processes that occur during the redox reactions of polymer film electrodes. For instance, the proton expulsion that occurs before anion insertion during the electrooxidation of PANI is clearly seen in the cyclic voltadeflectogram, which is almost... 

Henderson et al. [468] studied the behavior of Poll in an acidic perchloric acid solution with probe beam deflection and electrochemical quartz microbalance (EQMB) [468]. The degree of film hydration affected the relative contributions of protons and anions during the first redox cycle. During further cycles, a proton contribution was observed at electrode potentials around the first redox peak in the CV. During the second redox peak, electroneutrality was established by proton expulsion. 

The resulting mtermediate products of these reactions are thermodynamically unstable compounds that become stabili by proton expulsion to return to a resonating benzenoid structure ... 

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