text: Polyolefins such as polyethylene and polypropylene are susceptible to photo-oxidation and around 70% of light stabilizers produced world-wide are used in their protection, despite them representing only around 50% of global plastic production. Aliphatic hydrocarbons can only adsorb high energy UV-rays with a wavelength below ~250 nm, however the Earth’s atmosphere and ozone layer screen out such rays, with the normal minimum wavelength being 280–290 nm.
  The bulk of the polymer is therefore photo-inert and degradation is instead attributed to the presence of various impurities, which are introduced during the manufacturing or processing stages. These include hydroperoxide and carbonyl groups, as well as metal salts such as catalyst residues. 
  All of these species act as photoinitiators.
  The organic hydroperoxide and carbonyl groups are able to absorb UV light above 290 nm whereupon they undergo photolysis to generate radicals. Metal impurities act as photocatalysts, although such reactions can be complex. It has also been suggested that polymer-O charge-transfer complexes are involved. Initiation generates radical-carbons on the polymer chain, sometimes called macroradicals (P•).
  Chain initiation
  Chain propagation
  Chain branching
  Termination
  Classically the carbon-centred macroradicals (P•) rapidly react with oxygen to form hydroperoxyl radicals (POO•), which in turn abstract an H atom from the polymer chain to give a hydroperoxide (POOH) and a fresh macroradical. Hydroperoxides readily undergo photolysis to give an alkoxyl macroradical radical (PO•) and a hydroxyl radical (HO•), both of which may go on to form new polymer radicals via hydrogen abstraction. Non-classical alternatives to these steps have been proposed. The alkoxyl radical may also undergo beta scission, generating a acyl-ketone and macroradical. This is considered to be the main cause of chain breaking in polypropylene.
  Secondary hydroperoxides can also undergo an intramolecular reaction to give a ketone group, although this is limited to polyethylene.
  The ketones generated by these processes are themselves photo-active, although much more weakly. At ambient temperatures they undergo Type II Norrish reactions with chain scission. They may also absorb UV-energy, which they can then transfer to O, causing it to enter its highly reactive singlet state. Singlet oxygen is a potent oxidising agent can go on to form cause further degradation.