AbstractThe rate constants (k2) of the thermal dimerization of seven substituted trifluorostyrenes inn‐hexane solution have been determined in the temperature range 110°‐160°c. In general, no good correlation can be obtained with various Hammett type substituent parameters,e.g., σ,σ+,σ−,σ°, σR,σR+,σR_, and σR°etc.These results accord with the view that in addition to the polar effects, the abilities of the substituent to delocalize the spin also play important roles in affecting the rates of dimerizations. Rationalizations for previously reported successful correlations of polar substituent constants with radical reacti