Propagation kinetics of para-substituted styrenes: a test of the applicability of the Hammett relationship to free-radical polymerization

In this work, pulsed laser polymerization measurements of the homopropagation rate coefficient (k_p) for a series of para-substituted styrene monomers (4-X-styrene: X = OCH₃, CH₃, F, Cl, Br) at 20, 30, and 40°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 (× 10⁷ 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.