Radical Chain-Growth Polymerization: Chain Branching

During radical chain-growth polymerization, the highly reactive radical abstracts hydrogen from non-radical carbons in polymer chains. This hydrogen abstraction transfers the radical site from the chain end to an internal position, creating a branch point. The part of the chain between the abstraction site and the original chain end becomes a branch. This branching mechanism is inherent to radical polymerization and unavoidable.

Intramolecular hydrogen abstraction occurs when the radical abstracts hydrogen from the same growing polymer chain, creating branches within that chain. Intermolecular hydrogen abstraction happens when the radical abstracts hydrogen from a different growing or terminated chain, terminating one chain's growth while initiating branching in another. Intramolecular abstraction is more probable than intermolecular abstraction.

Free-radical polymerization of ethylene produces low-density polyethylene through extensive chain branching. The reactive radical abstracts hydrogen from growing or terminated polyethylene chains, shifting the radical site to internal positions. This creates numerous branch points throughout the polymer structure, resulting in the characteristic low density and heavily branched skeletal structure of this polyethylene grade.

1,5-hydrogen abstraction is favored because it proceeds through a relatively stable six-membered transition state that resembles the chair conformation of cyclohexane. This favorable transition state geometry makes the reaction kinetically preferred over other abstraction distances. The result of 1,5-hydrogen abstraction is the formation of butyl branches, which are commonly found in branched polymers produced by radical polymerization.

The distance between the hydrogen abstraction site and the nearest end of the polymer chain determines branch size. When abstraction occurs close to the chain end, small branches form. When abstraction occurs far from the end, larger branches result. Short-chain branching is more common than long-chain branching because the ends of chains are more accessible to the reactive radical site.

When the radical abstracts hydrogen from a non-radical carbon, the radical site transfers from its original position at the chain end (a primary carbon center) to the abstraction point within the chain (a secondary carbon center). This shift in radical position allows polymerization to continue from the new site, with the segment between the new radical site and the original chain end becoming a branch.

During intermolecular hydrogen abstraction, the radical abstracts hydrogen from another polymer chain, terminating that chain's growth at the abstraction point. Simultaneously, a new radical site forms at the abstraction location in the other chain. Polymerization then continues from this new radical site, creating a branch that grows from the middle of the previously growing chain.