How can two growing polymer chains terminate by combining?

In radical chain-growth polymerization, the growth of polymer chains is halted during the termination step. When two growing polymer chains terminate by combination, the two reactive chain ends—each possessing an unpaired electron—react with one another to form a single, stable covalent bond.1

This process effectively links the two growing chains together into one longer polymer molecule, thereby neutralizing the radical sites on both ends and ending the propagation of the chain.1

Alternatively, termination can occur through a disproportionation reaction, where a hydrogen atom is transferred from one growing chain to another, resulting in two separate, non-reactive polymer chains. Because these termination events occur randomly, polymer chains do not all reach the same length, which is why chemists typically refer to a polymer's average molecular weight.1