Radicals formed on the polymer backbone by either hydrogen abstraction side-group elimination can cause the chain to break by beta scission. As a result the molecular weight decreases rapidly. As new free radicals with high reactivity are formed, monomers cannot be a product of this reaction, also intermolecular chain transfer and disproportion termination reactions can occur.
CH-CH-CH-CH-CH-CH-CH’→
CH-CH-CH=CH + CH-CH-CH’ or
CH’+CH=CH-CH-CH-CH-CH
As polymers approach their ceiling temperature scission starts to take place randomly on the backbone.