A NEW DUAL-PARAMETER FOR REACTIVITIES OF VINYL MONOMERS TOWARD FREE-RADICALS

A new dual‐parameter of vinyl monomers for the reactivities toward free‐radicals has been evaluated on the basis of kinetic data obtained by the flash photolysis method. As a model reaction system, the reversible addition reactions of the substituted phenylthiyl radicals to vinyl monomers are selected. The absolute rate constants and the relative equilibrium constants for these reactions have been determined. The Hammett reaction constants (ρ) for the addition reaction rates were evaluated by the variation of the substituents at the para‐position of the arylthiyl radicals. From the values, the polar parameters (P‐ values) have been evaluated. The P values of various vinyl monomers correlate with the e values of the Q‐e scheme proposed by Alfrey and Price. The resonance stabilization terms of the propagating radicals were evaluated from the equilibrium constants (K) which correlate with the Q values. The K‐P dual‐parameter can be used in order to predict the reactivities of vinyl monomers for the primary radicals (initiation step). The monomer reactivities from the copolymerization data are also correlated with the dual parameter. There are some advantages in the new dual‐parameter compared with the Q‐e scheme; the separation of the resonance term from the polar term is complete and the steric factor is not included in the polar term. Copyright © 1990 John Wiley & Sons, Inc.