In this paper the results from time-resolved experiments (pulse radiolysis, laser photolysis) on the formation of radical cations derived from styrene type olefins in solvents of different polarity are presented. The free olefin radical cations formed by charge transfer (in cyclohexane), two photon ionization or one electron oxidation (in aqueous solutions) dimerize to distonic radical cations or react with nucleophiles to produce benzyl type radicals. The decay of the dimer cation ArC.HCH2CH2C+HAr seems to be determined by their ionic state: cationic polymerization in the case of free radical cations (one electron transfer ionization of styrenes), and intramolecular rearrangement to molecular dimer structures in the presence of counter ions [triplet sensitized electron transfer as reported by Schepp and Johnston (1994) J. Am. Chem. Soc. 116, 6895].

The triplet states of styrene, p -methyl-, p -methoxy- and p -chlorostyrene produced by direct excitation (255 nm) or by sensitization with triplet benzophenone in cyclohexane solution, were identified and their absorption spectra and their reactivity with triplet quenchers were determined. The absorption spectra are characterized by a band at about 280 nm (ϵ þe range 14000–24000 M −1 cm −1 ) and a weaker band (or shoulder) at about 320 nm ( ϵ ≈ 5000 M − cm − ). For direct excitation, the quantum yield for triplet formation are 0.2–0.5. The rate constants for reaction of the triplets with O 2 , 1,3-cyclohexadiene or ferrocene are of the order or 10 9 M −1 s −1 .

Radical cations of the substituted styrenes (M)1–10 have been produced by pulse radiolysis of solutions of M in cyclohexane. The monomer radical cations, M˙+, are formed by fast electron transfer from the solute, M, to the solvent radical cation produced by the ionizing radiation. The decay of M˙+ proceeds by an ion–molecule reaction with the parent styrene (dimerization; k= 0.6–1.0 × 1010 dm3 mol–1 s–1) resulting in head-to-head connected distonic dimer radical cations which show properties of carbenium ions. At high styrene concentrations a trimerization which represents the start of the free cationic polymerization takes place with a rate constant of ca. 106 dm3 mol–1 s–1. In aqueous or alcoholic solutions, radical cations were generated by direct two-photon ionization (indicated by the presence of the solvated electron) of the styrenes or by reactions with strong oxidants, such as SO4˙– and Tl2+, generated by laser UV photolysis as well as by pulse radiolysis. Only for 4-methoxystyrene 6 and trans-anethole 7 could the (monomeric) radical cations be directly observed in the nanosecond to millisecond range. In the other cases, the radical cations decay within the sub-nanosecond timescale by reaction with H2O forming β-hydroxybenzyl type radicals.