Radical addition to the unsaturated C=C bond is an important part of radical chemistry. Only the experimental data of radical polymerization and oxidation of unsaturated compounds are an extremely large block of information [ 1 ]. So, the question of reactivity is not discussed once. There were proposed both descriptive empirical schemes of relative reactivity and theoretical approaches. The main characteristic of this class of reactions is that functional groups at the reaction center cannot be considered just substituents. So, the usual linear correlations of Hammett type [ 2 ] had no success in the general case.

The first successful scheme of that kind was the Q–e scheme of Alfrey-Price for bulk copolymerization. Its equation for rate constant k_RM of addition of polymer radical R to monomer M looks like this:
ln k_RM = ln P_R + ln Q_M − e_Re_RM [ 3 ].

Although parameters P/Q were assigned the meaning of energies of stabilization, and e the measure of polar factor, this equation could be considered a 2-D variant of the usual Hammett equation: fix parameter e of the radical and get a linear equation for monomer reactivity; and the same but with radicals.

Hoyland proposed another scheme for this class of reactions, known as the model of electronegativities [ 4 ]. He used another mathematical representation of polar factor to improve agreement with experimental data:
ln k_RM = ln L_R + ln L_RM + |χ_R + χ_RM|

Another general descriptive scheme of radical reactions was proposed by Denisov [ 5 ], who made (and still does) significant work of classifying kinetic data of radical substitution and addition. His model, known as parabolic, was intended for calculating rate constants and activation energies of radical H-abstraction; it later was transferred on radical addition reactions. The author of this article would rather consider it another empirical scheme¹⁾.