In polymerization mechanisms, the termination of a polymer chain occurs exclusively when two free radicals collide, leading to the formation of a stable molecule.

Termination in radical polymerization can happen when two growing radicals combine, but it is not the only way. Two radicals can also collide and produce a stable molecule by combination, or they can undergo disproportionation, where one radical gives a hydrogen atom to the other, giving a saturated and an unsaturated chain. Additionally, termination can occur through chain transfer, where a radical reacts with a solvent or another molecule, ending the chain without a radical-radical collision. Thus, while radical collision is a common termination route, other mechanisms also exist. For example, in the polymerization of ethylene, a radical chain can be terminated by reacting with a hydrogen donor, producing a saturated polymer end and a new radical that continues the reaction.