Termination occurs in free radical polymerization by one of two mechanisms, coupling or disproportination. Either mechanism involves the reaction between two growing chain ends. These are present in very low concentration (ca. 10-8 M), so termination reactions occur infrequently even though they are intrinsically very rapid processes. Some monomers terminate exclusively by coupling, some by disproportionation, some by both mechanisms.
Two radicals at the chain termini simply join to form a single bond, as shown here in an example with styrene:
The radical at the end of one chain attacks a hydrogen atom at the second-to-last carbon atom in the second chain, as shown here in an example with methyl methacrylate (MMA):
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