Evidence is presented that the formation of aryl vinyl sulfide cation radicals from the corresponding neutral precursors via reaction with tris(4-bromophenyl)aminium hexachloroantimonate in the context of a cation radical Diels−Alder addition to 1,3-cyclopentadiene does not occur via outer sphere electron transfer but by a mechanism involving strong covalent interaction between the aminium salt acting as an electrophile and the aryl vinyl sulfide substrate acting as a nucleophile.

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Both experimental and theoretical studies confirm that the formation of aryl vinyl ether and aryl vinyl sulfide cation radicals from the corresponding neutral substrates correlates with the Brown ' parameters as opposed to Hammett ' values. Peak oxidation potentials for both classes of substrates correlate preferentially with ' , as do gas-phase ionization energies calculated by both semi-empirical and ab initio methods. In contrast, the protonation energies of the same substrates, which relate to carbocation formation, correlate preferentially with ' values, as do rates of protonation and other electrophilic additions. These observations permit a sharp distinction between electrophilic and electron transfer reactions of these two common classes of electron-rich substrates. Using this criterion, the cycloadditions of tetracyanoethylene to these substrates are found to proceed via an electrophilic mechanism, rather than by a previously proposed electron transfer mechanism.

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Cation radical chain cycloaddition polymerization, a fundamentally new addition polymerization method involving cation radical intermediates in each propagation step, is described and demonstrated. The cycloaddition reactions of appropriately constituted difunctional monomers, catalyzed by tris(4‐bromophenyl)aminium hexachloroantimonate in dichloromethane solvent at 0 °C, is shown to afford polymers having average molecular weights of up to 150 000. Both cyclobutanation and Diels–Alder polymers were obtained in this way. The surprising efficiency of these polymerization reactions is believed to be the result of rapid intramolecular hole transfer from the site at which the hole is originally generated in the cycloaddition step to a reactive, terminal alkene moiety. Consequently, chain propagation is much more efficient than in the cycloadditions of corresponding monofunctional compounds, which necessarily involve intermolecular hole transfer. Copyright © 1999 John Wiley & Sons, Ltd.