Reactivity of polyenes in solid-state autoxidation

The reactivity of polyenes (beta-carotene, canthaxanthin, lycopene and some retinyl polyenes as related compounds) in the solid state rewards molecular oxygen at various temperatures and oxygen pressures was studied. The specimens were thin (ca 0.1 mu m) amorphous films prepared by evaporation of one or two drops of appropriate solution on quickly rotating supports. The autoxidation was monitored by recording the changes in the electronic and infrared spectra of oxidizing films and was demonstrated to be a chain free-radical process with extremely high chain-initiation rates (10(-5)-10(-6) mol l(-1) s(-1)). For various polyenes the kinetic parameters of the initiation reaction were measured. In the films of several polyenes (retinyl acetate, retinal, methyl retinoate, beta-carotene), the formation of free radicals proceeds in the absence of oxygen and the mechanism of the process was suggested. The observed ratios of the propagation and termination rate constants depend on the oxygen pressure, indicating the involvement of polyenyl radicals in the chain termination process. The observed trends are explained using the concepts of the stabilization energy of radicals, the reversibility of oxygen addition to polyenyls and morphological features of polyene films. Copyright (C) 1999 John Wiley & Sons, Ltd.