Bimolecular reactions like-charge ions

The majority of radical ion reactions are bimolecular in nature, although some of these are merely variations of the unimolecular reactions discussed above, and many occur as pair reactions, albeit with a modified partner. Radical ions may react with polar or nonpolar neutral molecules, with ions, with radicals, or with radical ions of like or opposite charge (Scheme 6.3). Alkene radical ions undergo a particularly rich variety of reactions, including additions and cycloadditions. 

At the time of writing this review, the major question remains in the understanding of the reasons for absence of the M.I.R. in most photo-induced e.t. processes. Observations of the M.I.R. in thermal charge shift and charge recombination reactions have now become commonplace, especially in intramolecular e.t. as well as in formally bimolecular geminate ion pair neutralizations — here the molecules which form the ion pair may form a supermolecule , something like an exciplex. 

There seems little doubt that in radiation induced polymerizations the reactive entity is a free cation (vinyl ethers are not susceptible to free radical or anionic polymerization). The dielectric constant of bulk isobutyl vinyl ether is low (<4) and very little solvation of cations is likely. Under these circumstances, therefore, the charge density of the active centre is likely to be a maximum and hence, also, the bimolecular rate coefficient for reaction with monomer. These data can, therefore, be regarded as a measure of the reactivity of a non-solvated or naked free ion and bear out the high reactivity predicted some years ago [110, 111]. The experimental results from initiation by stable carbonium ion salts are approximately one order of magnitude lower than those from 7-ray studies, but nevertheless still represent extremely high reactivity. In the latter work the dielectric constant of the solvent is much higher (CHjClj, e 10, 0°C) and considerable solvation of the active centre must be anticipated. As a result the charge density of the free cation will be reduced, and hence the lower value of fep represents the reactivity of a solvated free ion rather than a naked one. Confirmation of the apparent free ion nature of these polymerizations is afforded by the data on the ion pair dissociation constant,, of the salts used for initiation, and, more importantly, the invariance, within experimental error, of ftp with the counter-ion used (SbCl or BF4). Overall effects of solvent polarity will be considered shortly in more detail. 

Termination will occur when the carbocation undergoes reaction with nucleophilic species other than monomer to produce a dead chain and no re-initiation. Since cationic polymerizations are carried out with high-purity reagents and under rigorous conditions this reaction is much less likely than chain transfer to monomer. The mutual repulsion of the charged polymerization sites ensures that bimolecular termination cannot occur (unlike in free-radical polymerization, where this is the most probable termination route). Recombination of the cation with the counter-ion will occur, and these termination reactions are often very specific to the chemistry of the initiator. 

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