Physico-chemical aspects of polyethylene processing in an open mixer. Part 30: Formal kinetics of γ-lactone formation at a constant rate

Abstract

There are only few mechanisms susceptible to explain γ-lactone formation at a constant rate. The formal kinetics based on these mechanisms proves to be a useful tool in the attempt to estimate the likeliness and possible relative amount of their contribution. The α,γ-keto-hydroperoxides formed in 4-position to hydroxyl groups are decomposed very rapidly at the temperatures of the experiments. The decomposition yields a carboxylic acid group in 4-position to the alcohol group and is first choice for explaining γ-lactone formation at a constant rate. However, the activation energy deduced from the formal kinetics developed for this mechanism is rather small with about 3.6 kcal/mol and hardly in agreement with the experimental value of 29.8 kcal/mol. This leads to the re-examination of the experimental data. Separate fitting of the data for the low temperature range yields the value of 4.1 kcal/mol. This value is sufficiently close to the value deduced from the formal kinetics to be compatible with it. The formal kinetics indicates also that on passing from air to pure oxygen the rate should increase by a factor of about 1.7. This is sufficiently close to the experimental value of about 2 for agreement. It is concluded that the mechanism examined can account for the bulk of the γ-lactone formed at a constant rate. The calculations for 1-peroxy-2,5-di-hydroperoxides and 1,4-keto-hydroperoxides do not yield conclusions that are as straightforward as those for the α,γ-keto-hydroperoxides in 4-position to hydroxyl groups. Although the estimated activation energies are roughly compatible with the experimental value for the low temperature range, the increase with the oxygen concentration is significantly larger than that observed experimentally. Hence, the contribution of these intermediates to the constant rate of γ-lactone formation can only be minor.