Groups that are attached to the side of the backbone are held by bonds which are weaker than the bonds connecting the chain. When the polymer is heated, the side groups are stripped off from the chain before it is broken into smaller pieces.
For example, the PVC eliminates HCl, under 100–120 °C.
CH(Cl)CHCHCH(Cl)→CH=CH-CH=CH+2HCl
Side group elimination can also proceed in a radical manner. For instance, methyl groups in polypropylene are susceptible to homolysis at high temperatures, leaving radicals on the polymer backbone.