In this work, pulsed laser polymerization measurements of the homopropagation rate coefficients (k(p)) for a series of para-substituted styrene monomers ( 4-X-styrene: X = OCH3, CH3, F, Cl, Br) at 20, 30, and 40 degrees C are reported. On the basis of nonlinear least-squares fits of the Arrhenius model to these data, the following point estimates for the activation energy (kJ mol(-1)) and frequency factor (x 10(7) L mol(-1) s(-1)) for each of the above five monomers were obtained: 34.9 and 6.35, 32.4 and 2.84, 32.0 and 3.50, 32.1 and 4.48, 33.9 and 9.57, respectively. The k(p) data were also used to test the applicability of the Hammett relationship. It was found that neither the Hammett relationship nor the extended Hammett relationship could quantitatively describe the substituent effects on the rate coefficients, though both could provide a reasonable qualitative description of the trends in the data.