Pyrolysis mechanisms deduced from anomalous rate constants for product formation

Abstract

In pyrolysis studies, product yields are usually measured as a fraction of the limiting available yield of the product under consideration. If the pyrolysis is conducted under conditions where first order behaviour is observed, it can be shown that the observed rate constants calculated from product evolution will be the same for all products, whatever the relative yields of the products, provided that the latter do not participate in secondary reactions. Thus the observed rate constant does not correspond to the rate constant for the measured product, and is in fact equal to the sum of the rate constants for the formation of all products. When this is observed, it may be used as a criterion that the measured products are primary products. Pyrolysis results from (butyl methacrylate)/(hexyl methacrylate) copolymers, differing in composition by a factor of about 70, have been analysed in this way, and it is deduced that the monomers are evolved by a direct parallel mechanism. This conclusion is confirmed by measurements of product ratios as a function of sample size. An important general consequence of the analysis presented in this paper is that true rate constants for the formation of individual pyrolysis products cannot be evaluated unless all products are detected and measured quantitatively.