21.10 : 自由基链增长聚合：链支化

Radical chain-growth polymerization always leads to branched polymers. For example, free-radical polymerization of ethylene yields low-density polyethylene with heavy branching.

During free-radical polymerization, the highly reactive radical may abstract hydrogen from a non-radical carbon. This abstraction can be either intramolecular—that is, from the same polymer chain—or intermolecular—that is, from another growing or terminated chain.

If the hydrogen abstraction occurs anywhere on a terminated chain or on a growing chain away from the α carbon of the radical site, the propagation site transfers from the end of the abstracting chain to that point within the polymer chain. This leads to chain branching.

The distance between the abstraction site and the nearest end of the chain determines the size of the branch in the polymer chains.

In intramolecular chain branching, 1,5-hydrogen abstraction is favored because of a relatively stable six-membered transition state resembling the chair conformation of cyclohexane. So, the resulting polymer has butyl branches.