Absolute Rate Constants for the Addition of Benzyl (PhĊH₂) and Cumyl (PhĊMe₂) Radicals to Alkenes in Solution

Abstract

Abstract Absolute rate constants and their temperature dependence were determined by time‐resolved electron spin resonance for the addition of the radicals PhĊH 2 and PhĊMe 2 to a variety of alkenes in toluene solution. To vinyl monomers CH 2 =CXY, PhĊH 2 adds at the unsubstituted C‐atom with rate constants ranging from 14 M −1 S −1 (ethoxyethene) to 6.7 · 10 3 M −1 S −1 (4‐vinylpyridine) at 296 K, and the frequency factors are in the narrow range of log ( A / M −1 S −1 ) = 8.6 ± 0.3, whereas the activation energy varies with the substituents from ca. 51 kJ/mol to ca. 26 kJ/mol. The rate constants and the activation energies increase both with increasing exothermicity of the reaction and with increasing electron affinity of the alkenes and are mainly controlled by the reaction enthalpy, but are markedly influenced also by nucleophilic polar effects for electron‐deficient substrates. For 1,2‐disubstituted and trisubstituted alkenes, the rate constants are affected by additional steric substituent effects. To acrylate and styrenes, PhĊMe 2 adds with rate constants similar to those of PhĊH 2 , and the reactivity is controlled by the same factors. A comparison with relative‐rate data shows that reaction enthalpy and polar effects also dominate the copolymerization behavior of the styrene propagation radical.