AbstractThe radical polymerization of styrene and of thep‐substituted styrenesp‐methoxy‐,p‐methyl‐,p‐fluoro‐,p‐chloro‐,p‐bromo‐, andp‐cyanostyrene, was kinetically studied. The styrenes with more electron‐attracting substituents show a greater overall rate of polymerization. At 30,0°C., the individual rate constants of propagation (kp) and termination (kt) were determined by a rotating sector and by usual inhibitor method using diphenylpicrylhydrazyl (DPPH) orp‐benzoquinone. It was found that the following HAMMETTrelationship is established in the homopolymerization ofp‐substituted styrenes:\documentclass{article}\pagestyle{empty}\begin{document}$${\rm k}_{\rm px} /{\rm k}_{\rm pH} = {\rm k} \cdot {\rm exp}\left( {\rho ' \cdot \sigma } \right)$$\end{document}where σ is the HAMMETTconstant, and ρ′ is a reaction constant which was calculated as +0.6. This relation is interpreted from a participation of ionic structures in the transition state of the propagation reaction. For confirmation, MULLIKEN's electronegativity of thep‐substituted styrene and thep‐substituted benzyl radical was calculated by simple LCAO MO method. The relationship between the rate constants of termination, kt, and the σ‐values is expressed by two lines, crossing atp‐fluorostyrene which is at the maximum point. These two lines are explained by two ways; one is the diffusion controlled termination and the other is the reco