Initiation

Abstract: A kinetic analysis is presented of “living” polymerizations initiated and controlled by reactions involving persistent radicals. It is shown how a simple self-regulation operates which is known as the persistent radical effect in other areas of radical chemistry. Predictive analytical formulas are derived for the time dependent concentrations of the radical intermediates and the dormant and the final polymer products. They are supported by numerical simulations and agree with experimental findings. In particular, we consider the initiation by homolysis of a stoichiometric initiator into a transient and a persistent radical in the absence and in the presence of self-initiation and the use of conventional initiators together with a persistent species.

Abstract: Lipid oxidation has long been accepted as a classic free radical chain reaction mechanism with initiation, propagation, and termination stages. This view is not inaccurate, but it is incomplete. Initiation is generally treated as if it was spontaneous or random, but in reality, radical initiation is quite specific and varies with the source. Similarly, propagation is considered to involve only hydrogen abstraction where as internal cyclization, addition, scission, and disproportionation reactions are competitive or even dominant under some conditions, leading to variations in oxidation kinetics and, in particular, product mixes. Alternative reaction paths have been recognized for some time, but remain largely ignored in measurements of lipid oxidation. This chapter reviews initiation processes with a focus on differentiating rates, mechanisms, and specificity of radical formation; and it presents evidence for multiple propagation mechanisms that lead to different product mixes. Finally, a new integrated view of lipid oxidation is proposed to account for multiple propagation pathways. Practical application of this information should enable development of analyses that more accurately reflect the true extent of lipid oxidation and also antioxidant approaches that more effectively extend shelf life and preserve desirable food qualities. Keywords: lipid oxidation; initiation; propagation; termination; hydrogen abstraction; cyclization reactions; addition reactions; scission reactions; oxidation products