Absolute Rate Constants and Arrhenius Parameters for the Addition of the Methyl Radical to Unsaturated Compounds:  The Methyl Affinities Revisited

Abstract

Absolute rate constants and their Arrhenius parameters are reported for the addition of the methyl radical to 21 monosubstituted and 1,1-disubstituted alkenes and to 6 benzenes in 1,1,2-trifluoro-1,2,2-trichloroethane solution. They are used to convert relative reaction rates known as methyl affinities from the work of M. Szwarc and others for about 250 additional unsaturated compounds to the absolute scale. An analysis shows that the addition rates depend on the reaction enthalpy but also indicates a moderate nucleophilic polar effect for the liquid-phase reactions. It is pointed out that this polar effect may be smaller in the gas phase.

Reactivities of ω‐Unsaturated Methacrylate Oligomers toward tert‐Butoxy Radicals: Investigation of the Effect of Degree of Polymerization and Ester Alkyl Group

“…[33,34] The resulting approximate values of E add % 31 kJ Á mol À1 and A add % 5.4 Â 10 9 L Á mol À1 Á s À1 fall well within the range of values previously reported for similar addition reactions of Me Á to various alkenes. [36] The value of E add is also of a reasonable magnitude in comparison with the activation energy for propagation of MMEA (E add ¼ 33.6 kJ Á mol À1 ), [2] which is a SHAP-type monomer. The values of E H and A H were also estimated from all data points for MMA-n, giving E H % 21 kJ Á mol À1 and A H % 2.2 Â 10 8 L Á mol À1 Á s À1 (Figure 3), similar to the reported values for hydrogen abstraction from hydrocarbons.…”

“Charge transfer” polymerization—and the absence thereof!

“…Similar behavior has recently been demonstrated for the electron-rich methyl radical, which reacts 100 times faster with acrylonitrile compared with ethylene. 12 In contrast, the same researchers demonstrated that electron-poor trifluoroacetonyl radical adds faster to electron-rich olefins such as ␣-methylstyrene than to electron-poor olefins such as methyl acrylate, although the effect is less pronounced in these cases, as summarized in Table II. 13 Taking inspiration from the organic chemistry results, Jones et al…”

Mechanistic Perspectives on Stereocontrol in Lewis Acid-Mediated Radical Polymerization

“…At w25 C, the rate of addition of the 2-hydroxy-2-propyl radical to the "electrophilic" alkene AN is three orders magnitude faster than the comparable addition of the cyanomethyl radical. 66,67 Conversely, the addition of the cyanomethyl radical to the "nucleophilic" alkene ethyl vinyl ether is over two orders of magnitude faster than the addition of the 2-hydroxy-2-propyl radical. 66,67 Analogous polar effects have also been noted in transfer reactions.…”

“…66,67 Conversely, the addition of the cyanomethyl radical to the "nucleophilic" alkene ethyl vinyl ether is over two orders of magnitude faster than the addition of the 2-hydroxy-2-propyl radical. 66,67 Analogous polar effects have also been noted in transfer reactions. 68,69 The importance of polar effects in radical reactions is exemplified by the copolymerization of styrene (STY) and maleic anhydride.…”

“…In this instance, polar effects are counteracted by radical stability, which favors the formation of the relatively stable product radical derived from addition to AN, rather than the less stable product Figure 3 Illustrative examples of polar and radical stability effects in radical addition reactions. Addition rate coefficients (k add ) are given in L mol À1 s À1 and were measured at w25 C. 66,67 radical derived from addition to ethyl vinyl ether. Indeed, the addition of the 2-hydroxy-2-propyl radical to the AN forms a stabilized radical species and is thus favored by both polar and radical stability effects.…”