Photo- and radiation-induced chemical generation and reactions of styrene radical cations in polar and non-polar solvents

Abstract

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.