[1][2] Simply put, even in the absence of air, polymers will begin to degrade if heated high enough.
These reactions result in changes to the molecular weight (and molecular weight distribution) of the polymer and can affect its properties by causing reduced ductility and increased embrittlement, chalking, scorch, colour changes, cracking and general reduction in most other desirable physical properties.
For instance, methyl groups in polypropylene are susceptible to homolysis at high temperatures, leaving radicals on the polymer backbone.
[6] Radicals formed on the polymer backbone by either hydrogen abstraction side-group elimination can cause the chain to break by beta scission.
As polymers approach their ceiling temperature scission starts to take place randomly on the backbone.
Where this is the case thermal oxidation is to be expected, leading to the formation of free radicals by way of hydroperoxides.