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 s tyrene (1) and diethyl fumarate (2) at 40 degreesC were examined on the bas is of a complete set of experimental data obtained by rotating sector and o ther experiments. The composition curve conformed to the terminal model wit h a standard deviation of about 1.5%. However, an exceptionally strong penu ltimate-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 con stants were describable by the penultimate termination model with a geometr ic mean approximation, showing that a strong PUE exists also on the termina tion 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 effec t arguments. These analyses suggest that the frequency factors of the propa gation and termination rate constants are somehow correlated with each othe r. 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.