Free-radical copolymerization of styrene and diethyl fumarate. Penultimate-unit effects on both propagation and termination processes

The propagation and termination processes of the bulk copolymerization of styrene (1) and diethyl fumarate (2) at 40 degreesC were examined on the basis of a complete set of experimental data obtained by rotating sector and other experiments. The composition curve conformed to the terminal model with a standard deviation of about 1.5%. However, an exceptionally strong penultimate-unit effect (PUE) for the propagation of the terminal radical 1 (s(1) = k(211)/k(111) = 0.055) was observed. The measured termination rate constants were describable by the penultimate termination model with a geometric mean approximation, showing that a strong PUE exists also on the termination of the terminal radical 1 (delta (1) = (k(t21.21)/k(t11.11))(1/2) - 0.067). The geometric mean approximation was justified by the shielding effect arguments. These analyses suggest that the frequency factors of the propagation and termination rate constants are somehow correlated with each other. On the basis of these results, a general, yet simple two-parameter rate-of-copolymerization equation with PUEs on both propagation and termination was proposed, which described the experiments extremely well. This new rate equation includes the classical Russo-Munari equation as a special case.