Relevance of numerical values

Earlier on in this chapter it was pointed out, in some detail, that over the last few years the dominant feature in studies of cationic polymer- 

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. 

In comparison to these two sets of data, the results from polymerizations initiated by molecular iodine present a dramatic contrast, and one which is characteristic of many monomers. The rate coefficient for 

A similar though less comprehensive comparison is possible for the polymerization of styrene. Here much more data is available from chemically initiated polymerizations, but unfortunately none of these is conclusive with respect to the nature of the active centre (Table 6). 

As with isobutyl vinyl ether the result from 7-ray irradiation must represent the reactivity of naked free ions and this fep is at least 10  

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