Analysis of the ESR line width in time-resolved (TR-ESR) experiments is shown to be a suitable tool for the measurement of addition constants of phosphinoyl and substituted benzoyl radicals. Compared with kinetic investigations, which make use of the resonance intensity or integral, observation of the line width as a function of monomer concentration has the advantage that the experimental parameter is not affected by spin-polarization processes and, therefore, a lengthy determination of these polarization parameters is avoided. The resulting addition constants are discussed with respect to the experimental hyperfine coupling constants and the geometry of the radicals. TR-ESR experiments simultaneously provide rate constants and ESR parameters and allow structure−reactivity relationships to be established.

The addition rate constants of five differently substituted benzoyl radicals to butyl acrylate were determined by continuous-wave time-resolved EPR (TR-EPR at X-band (9 GHz) and at W-band (95 GHz)) spectroscopy. Remarkably, the reactivity of the benzoyl radicals, is divided into two domains: One is established at low (<1.25 M) concentrations of butyl acrylate with rate constants in the range of 10 6 mol -1 s -1 . At alkene concentrations above ca. 1.25 M, the addition reaction slows down by ∼1 order of magnitude. This decrease of the reactivity coincides with a dramatic increase of the macroscopic viscosity in the reaction medium.