text: Susceptibility to photo-oxidation varies depending on the chemical structure of the polymer. Some materials have excellent stability, such as fluoropolymers, polyimides, silicones and certain acrylate polymers. However, global polymer production is dominated by a range of commodity plastics which account for the majority of plastic waste. Of these polyethylene terephthalate (PET) has only moderate UV resistance and the others, which include polystyrene, polyvinyl chloride (PVC) and polyolefins like polypropylene (PP) and polyethylene (PE) are all highly susceptible. 
  Photo-oxidation is a form of photodegradation and begins with formation of free radicals on the polymer chain, which then react with oxygen in chain reactions. For many polymers the general autoxidation mechanism is a reasonable approximation of the underlying chemistry. The process is autocatalytic, generating increasing numbers of radicals and reactive oxygen species. These reactions result in changes to the molecular weight (and molecular weight distribution) of the polymer and as a consequence the material becomes more brittle. The process can be divided into four stages:
  :Initiation the process of generating the initial free radical. 
  :Propagation the conversion of one active species to another
  :Chain branching steps which end with more than one active species being produced. The photolysis of hydroperoxides is the main example.
  :Termination steps in which active species are removed, for instance by radical disproportionation
  Photo-oxidation can occur simultaneously with other processes like thermal degradation, and each of these can accelerate the other.