Relative reactivities of radicals derived from several p-dimethylanilines toward the polymerization of butyl acrylate have been calculated. It was observed that the reactivity of the radicals increases as the electron-withdrawing character of the groups attached in the para position increases. The experimental efficiency factor (f) has been found to be proportional to the sigma(p) value defined in the Hammett equation. The dependence of the reactivities on the nature of the radicals has also been explained through their frontier orbital characteristics. The high similarity of the behavior of f versus sigma(p) and f versus E(SOMO) led us to conclude that the simple perturbative molecular orbital calculations can be used to predict the relative reactivity of initiating radicals with similar structure toward the polymerization of acrylic monomers.