The Effect of Substituents in Radical Reactions: Reactivities of Substituted Cumenes to Attack by the Polystyryl Radical

The polymerizations of styrene in a series of nuclear-substituted cumenes, using α, α′-azobisisobutyronitrile as an initiator, have been carried out at 60°C, and the chain transfer constants, Cs, for the substituted cumenes have been determined. It has been found that the Cs values for cumenes with electron-repelling groups are smaller than that for an unsubstituted cumene, while those for cumenes with electron-attracting groups are greater. The plot of log (k⁄k0) against the corresponding Hammett’s σ-constants failed to give a linear relationship. The modified Hammett’s equation including the resonance effect for radical reactions, the equation which was proposed by the author and Azumi, i.e.,: log(k⁄k0)=ρσ+γER, could be adopted to the transfer with the usual precision, giving γ=1 and ρ=0.7. It is considered that the effects of substituents on the transfer reactivities of cumenes to the attack of the polystyryl radical depend on the contributions of polar and resonance factors.