EVALUATION OF ADDITION RATES OF THE THIYL RADICALS TO VINYL MONOMERS BY FLASH-PHOTOLYSIS .2. SUBSTITUENT EFFECT ON ADDITION OF SUBSTITUTED BENZENETHIYL RADICALS TO METHYL-METHACRYLATE OR STYRENE

Rate constants for the addition reactions (k1) of the para-substituted benzenethiyl radicals to methyl methacrylate (MMA) or styrene (St) were determined by means of flash photolysis. Rate constants for reverse reaction (k-1) and equilibrium constants (K = k1/k-1) were estimated as ratios of the rate constant for the reaction between the carbon centered radical and oxygen (k2). Fairly good linear correlations were obtained for the Hammett plots of log k1 vs. σ+ for both monomers (ρ+ (k1,MMA) = 1.05 ± 0.05 and (ρ+ (k1,SL) = +1.37 ± 0.08). For MMA, log k-1/k2 and log Kk2 also gave the linear correlations to σ+ yielding ρ+ (k-1,MMA) = -0.36 ± 0.05 and ρ+ (kMMA) = +1.35 ± 0.05. Decreasing order of log Kk2 is closely related to increasing order of the thermodynamic stabilities of the thiyl radicals. For MMA which has an electron-poor double bond and St which has an electron-rich double bond, we attempted to assess the relative importance of stabilities of the radicals and of the possible contribution of polar structures to the transition state. © 1979, American Chemical Society. All rights reserved.