EVALUATION OF ADDITION RATES OF PARA-CHLOROBENZENETHIYL RADICAL TO VINYL MONOMERS BY MEANS OF FLASH-PHOTOLYSIS

By flash photolysis of di(p-chlorobenzene) disulfide (RSSR) in cyclohexane at room temperature, the p-chlorobenzenethiyl radical (RS η) was generated. Enhancement of the decay rate of RS η was observed by the addition of vinyl monomer (CH2̿CHY) only when oxygen was added. This indicates that the reaction proceeds by a mechanism involving an equilibrium: RS η + CH2̿CHY (k1) ⇋ (k-1) RSĊH2CHY (k2, O2) →. Low reactivity of RS η toward oxygen was confirmed and high reactivity of oxygen toward the carbon-centered radical is well known. The radical species, RSCH2CHY, was characterized by the flash photolysis of the degassed system containing both benzenethiol and 1, 1-diphenylethylene. A transient band at 335 nm was attributed to PhSĊH2C(Ph)2. In the presence of oxygen, the decay rate of RS η increased linearly with increasing vinyl monomer concentration and was also affected by added oxygen concentration. Extrapolation to infinite oxygen concentration gives the absolute addition rate constant k1 and the ratio k-1/k2. The relative rate constants for various monomers calculated from our observed absolute rate constants agree with reported values obtained using the spin trapping method. A plot of log k1 vs. log K(K = k1/k-1), in which k-1 was estimated by assuming k2 = 109 M-1 s-1, gives a linear relationship. The results suggest that increasing stability of RSĊH2CHY radical decreases the activation energy for the addition reaction. © 1979, American Chemical Society. All rights reserved.