Photochemistry of hydrocarbon polymers

Pure polyethylene should not absorb ultraviolet radiation of wavelength above 200 nm since pure paraffins are transparent in that region of the spectrum. However, it is well established [ 20] that even carefully purified polyethylene differs from a simple high molecular weight straight chain paraffin in being to some extent unsaturated. The total unsaturation has been estimated to be about 0.25% C=C by weight [21]. Olefinic unsaturation of different types has been detected by infrared spectroscopy [21, 22] it seems to be mainly of the vinyl type in linear polyethylene, while most unsaturation is of the vinylidene type in branched polyethylene [22]. Attention has also been drawn to the fact that a structure seems to be present in low density polyethylene which leads to a triene on ultraviolet irradiation [23]. 

Most commercially available samples of polyethylene also exhibit in their ultraviolet spectrum a shoulder in the neighbourhood of 280 nm [24]. This must be attributed to trace amounts of aliphatic carbonyl groups as shown by the small infrared absorption peak [20] at 1720 cm-1. Those carbonyl groups have furthermore been identified with the luminescence centres responsible for the thermoluminescence observed when polyethylene samples irradiated with UV or 7 radiation at 77°K are allowed to warm up [25]. The oxygen content of commercial polyethylene has been measured it is about 0.15% [23]. 

Absorption of light by the ethylene—carbon monoxide copolymer results in a decrease in molecular weight accompanied by an evolution of 

The acyl radical subsequently decarbonylates with evolution of carbon monoxide 

The Norrish type II reaction requires the presence of a hydrogen atom on the carbon atom which is in the 7 position with respect to the carbonyl group. It yields a methyl ketone and a terminal vinyl group 

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