Question

what do you think happens to F , Cl, or other heteroatom groups on the polymer chain as the backbone decays?

Answer 1

The most typical processes when the backbone of polimer decays are combustion and oxidation.
When organic compounds burn, halogens form hydro halic acids.
RCl +O2->HCl
Also oxidation and combustion are radical processes.
The halogens will form radicals which in term can further react and form haloalkanes.
RCl->Cl∙
CH4+Cl∙->CH3∙+HCl
CH3∙+Cl∙->CH3Cl
The last point is the formation of oxohalogen derivatives.
RCOnClm+O2->COCl2