This happens in several ways. The simplest way is for two growing chain ends to find each other. The two unpaired electrons then join to form a pair, and a new chemical bond joining their respective chains. This is called coupling . Coupling is one of two main types of termination reaction .
Termination is the third and final step of a chain-growth polymerization.
Initiation and propagation are the first two steps, of course. Click here to see a movie of this reaction. This is a rather complicated way in which two growing polymer chains solve the problem of their unpaired electrons. In disproportionation, when two growing chain ends come close together, the unpaired electron of one chain does something strange. Rather than simply joining with the unpaired electron of the other chain, it looks elsewhere for a mate.