text: For polystyrene the complete mechanism of photo-oxidation is still a matter of debate, as different pathways may operate concurrently and vary according to the wavelength of the incident light.
  Regardless, there is agreement on the major steps. 
  Pure polystyrene should not be able to absorb light with a wavelength below ~280 nm and initiation is explained though photo-labile impurities (hydroperoxides) and charge transfer complexes, all of which are able to absorb normal sunlight. Charge-transfer complexes of oxygen and polystyrene phenyl groups absorb light to form singlet oxygen, which acts as a radical initiator.  Carbonyl impurities in the polymer (c.f. acetophenone) also absorb light in the near ultraviolet range (300 to 400 nm), forming excited ketones able to abstract hydrogen atoms directly from the polymer. Hyroperoxide undergoes photolysis to form hydroxyl and alkoxyl radicals.
  These initiation steps generate macroradicals at tertiary sites, as these are more stabilised. The propagation steps are essentially identical to those seen for polyolefins; with oxidation, hydrogen abstraction and photolysis leading to beta scission reactions and increasing numbers of radicals. 
  These steps account for the majority of chain-breaking, however in a minor pathway the hydroperoxide reacts directly with polymer to form a ketone group (acetophenone) and a terminal alkene without the formation of additional radicals.
  Polystyrene is observed to yellow during photo-oxidation, which is attributed to the formation of polyenes from these terminal alkenes.