Absolute rate constants and their temperature dependence were determined by time‐resolved electron spin resonance for the addition of the radicals PhĊH2 and PhĊMe2 to a variety of alkenes in toluene solution. To vinyl monomers CH2=CXY, PhĊH2 adds at the unsubstituted C‐atom with rate constants ranging from 14 M−1S−1 (ethoxyethene) to 6.7 · 103 M−1S−1 (4‐vinylpyridine) at 296 K, and the frequency factors are in the narrow range of log (A/M−1S−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, PhĊMe2 adds with rate constants similar to those of PhĊH2, 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.

No abstract

The rate constant for the addition of benzyl radicals to c 6 0 in toluene is determined by time-resolved ESR spectroscopy as k298 = (1.4 f 0.2) X lo7 M-I s-l with log AIM-' s-I = 9.5 f 0.3 and Ea = 13.5 f 0.2 W/mol. This agrees with a previous correlation between log k/M-' s-I and electron affinities for benzyl addition to alkenes and shows that the addition of C 6 0 is strongly facilitated by charge transfer in the transition state.

Nonplanar radical centers in the transition state are responsible for the slow rate of addition of benzyl and para‐substituted benzyl radicals to monosubstituted and 1,1‐disubstituted alkenes. This was shown by ESR spectroscopy, since the para‐substituents do not significantly affect the rate of the addition. The conclusion is in agreement with the calculated transition‐state structures for the addition of simple alkyl radicals to alkenes.

Nichtplanare Radikalzentren im Übergangszustand sind die Ursache dafür, daß Benzyl und para‐substituierte Benzylradikale, wie ESR‐spektroskopisch gezeigt wurde, nur sehr langsam an mono‐ und 1,l‐disubstituierte Alkene addieren; dies folgt unter anderem daraus, daß die para‐Substituenten keinen signifikanten Einfluß auf die Geschwindigkeit der Addition haben. Diese Befunde sind in Einklang mit für die Addition einfacher Alkylradikale an Alkene berechneten Strukturen.

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ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Absolute rate constants and thzir temperature dependence were determined by time-resolved electron spin resonance for the addition of the radicals PhcH, and PhcMe, to a variety of alkenes in toluene solution. To vinyl monomers CH,=CXY, PhCH, adds at the unsubstituted C-atom with rate constants ranging from 14 M-Is-'(ethoxyethene) to 6.7. lo3 M-'s-' (Cvinylpyridine) at 296 K, and the frequency factors are in the narrow range of log ( A /M-'s-') = 8.6 & 0.3, whereat$ the activation energy varies with the substituents from ca. 51 kJ/mol to en. 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 c'mstants are affected by additional steric substituent effects. To acrylate and styrenes, PhcMe, adds with rate constants similar to those of PhcH,, 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.