21.9 : 自由基链增长聚合：机理

Radical chain-growth polymerization consists of three stages: initiation, propagation, and termination.

In the initiation step, a free radical generated by the homolytic fission of the radical initiator reacts with the monomer's double bond. It generates a sigma bond between the initiator and the monomer. As a result, the radical site shifts to the monomer.

The propagation step begins as this new radical species adds to another monomer. This step repeats to add more monomers to grow the polymer chain.

The monomers preferentially bond such that the radical site is always present on the more substituted carbon.

Finally, chain termination can occur by the coupling of radicals in two growing polymer chains.

Alternatively, one chain's radical abstracts hydrogen from the carbon adjacent to the radical site in the other polymer chain. So, the polymerization stops and yields a pair of alkyl-terminated and alkenyl-terminated polymers.