appears to be about one branch to every 50 carbon atoms% for example, for a molecular weight of 14,000 there are approximately 20 branches per molecule and the distribution of branches appears to be uniform for all molecules. Generally speaking the branching can be classified under two headings : short chain branching, when either methyl or ethyl groups are attached; this is especially in evidence when propylene or butylene are present in the ethylene before polymerization7 ; and a more complicated system of branching caused by the combination of two or more partially polymerized molecules, giving a web-like structure. This type of structure can be further complicated by cross linking between the molecules.It has been established that two different processes occur : cross linking, resulting in an increase in viscosity and an insolubility of the polymer in solvents such as hot tetralin; and degradation or chain breaking. Such degradation results in a decrease in the molecular weight of that portion of the aged polymer which is soluble. The relative rates at which these reactions occur depend to some extent upon the wavelengthintensity distribution of the illuminating source, the mercury arc producing slightly different relative rates from those in sunlight. The reaction is a photocatalysed oxidation, and infra-red analysis has shown that in an aged polythene sample, the following groups are formed : carboxyl, hydroxyl, carbonyl and RICH = CHR, in the polythene chains ; some carbon monoxide, water and low molecular weight aldehydes are also formed.This investigation has shown that it is probable that light absorption by the carbonyl groups present in the polythene is largely responsible for initiating the chain mechanism for the oxidative deterioration. Absorption of light quanta in the region zgoo to 3300 A. (85-95 kg.-cal.), would cause, by analogy with the behaviour of long-chain ketones, either an activation in the chain in the vicinity of the carbonyl group, or photodissociation of the ketone resulting in the temporary formation of a free radical. Either phenomenon could initiate combination with molecular oxygen ; the activation could initiate the processes of thermal oxidation and the free radical could combine directly with oxygen. The photodecomposition could also result in the formation of double bonds nv lo infra-red evidence for these is afforded by the appearance of the strong band a t 940 Em.-* (10-65 F.).
R-CH,-CH~-CH,-CO-CH~-R-J~R-CH= CH: 7-CH,-CO-CH,-R and carbon monoxide could be formed by reactions such as R-CH:-CO-CH, R-CH:-1 -COCHl -CO-CH, --* CO 7 --CH,A methyl group so produced would immediately combine with oxygen or remove hydrogen from a neighbouring polymer molecule giving a further free radical to combine with oxygen1* :R--L 0: -4 ROa-ROz---RH ---ROOH 4-R-ctc. These free radical reactions would presumably be termhated by thermal antioxidants, and at the temperatures at which the photocatalysed oxidation takes place it is unlikely that the molecules would have sufficient vibrational energ...
