PHOTODEGRADATION OF POLYPROPYLENE FILMS .3. PHOTOLYSIS OF POLYPROPYLENE HYDROPEROXIDES

Hydrogen-bonded hydroperoxide groups are believed to be the chief primary product in the photooxidation of polypropylene films in the near ultraviolet. The kinetics and mechanism of the photolysis of these hydroperoxides have been studied at 3650 Å in the solid state under high vacuum. A combination of infrared, glpc and chemical analysis showed that the primary photolytic step involved hydroperoxide cleavage into t-alkoxy and •OH radicals. The major volatile photolysis product was water, which resulted from hydrogen abstraction by •OH radicals. Little evidence was found for radical-induced hydroperoxide decomposition. The main polymeric products which were produced by reaction or decomposition of the alkoxy radicals have been tentatively identified. The t-alkoxy radicals are believed to undergo extensive β scission by two competing processes both of which produce ketonic products. One of these processes leads to scission of the polymer backbone, and is probably responsible for the large drop in intrinsic viscosity which resulted from hydroperoxide photolysis. The kinetics of hydroperoxide photolysis were followed by infrared analysis and by the continuous measurement of weight loss from each film sample during irradiation. An experimental overall quantum yield of about 4 was found for hydroperoxide photolysis, although the true value probably lies closer to unity. Hydroperoxide photolysis by sunlight in the presence of air is believed to represent a major source of free radicals and backbone scission during the photodegradation of polypropylene. This photolysis is probably prevalent both in the very early stages of and in the advanced stages of photodeterioration of commercial polypropylene articles. © 1969, American Chemical Society. All rights reserved.