Evidence for polarity effects in propagation reactions

The idea of charge separation in the transition state of the propagation step of free radical polymerization reactions, as suggested by Price,was discounted by Mayo and Walling and many subsequent workers. Their rejection of this idea was based upon the absence of any unambiguous correlation between the reactivity ratios of a system and the dielectric constant of the solvent. For instance, in the copolymerization of STY with MMA, it was reported that the reactivity ratios were independent of small quantities of water, ethyl benzene, dodecylmercaptans or hydroquinone, or the presence or absence of air and were thus unaffected by the dielectric constant of the system. In contrast, other studies have found a relationship between dielectric constant and the reactivity ratios in specific systems.  

In fact, recent theoreticaP and experimental studies of small radical addition reactions indicate that charge separation does occur in the transition state when highly electrophilic and nucleophilic species are involved. It is also known that copolymerization of electron donor-acceptor monomer pairs are solvent sensitive, although this solvent effect has in the past been attributed to other causes, such as a Bootstrap effect (see Section 13.2.3.4). Examples of this type include the copolymerization of styrene with maleic anhydride and with acrylonitrile. Hence, in these systems, the variation in reactivity ratios with the solvent may (at least in part) be caused by the variation of the polarity of the solvent. In any case, this type of solvent effect cannot be discounted, and should thus be considered when analyzing the copolymerization data of systems involving strongly electrophilic and nucleophilic monomer pairs. 

---

Numerous studies have been devoted to the addition of RSO2X to olefins. The propagation steps for these chain processes are shown in Scheme 13. The main factors controlling the reactivity of sulfonyl radicals are polar effects and the reversibility of the addition. Sulfonyl radicals exhibit an electrophilic character with respect to addition to olefins [98]. Evidence for the reversibility came early from the observation of the isomerization of cis- and rran -2-butenes during their copolymerization with sulfur dioxide and their Cu(I)-mediated reaction with benzenesulfonyl chloride [99]. Therefore, the chain transfer of alky] adduct radica]s has to compete effectively with -elimination of sulfonyl radicals (see Scheme 13). Selenosulfonates, sulfonyl halides, and sulfonyl cyanides are the most popular substrates that meet this property. 

---