8.2 : Free-Radical Chain Reaction and Polymerization of Alkenes

The conversion of alkenes to the high-molecular-weight compounds, called ‘polymers’, is a commercially significant reaction. The smallest recurring structural fragment of the polymer is termed a repeat unit, and the subscript n denotes the number of such pieces along the chain.

For instance, propylene, in the presence of a radical initiator and heat or light, yields polypropylene by a free-radical mechanism.

The initiation step involves cleavage of an oxygen–oxygen bond in an initiator like benzoyl peroxide. The benzoyloxy radical further decomposes to a phenyl radical and carbon dioxide. An azo-radical initiator, such as azoisobutyronitrile, yields isobutyronitrile radicals and molecular nitrogen through an entropy-driven decomposition.

The initiator radical adds to the double bond of an alkene, like propylene, to form a new radical in the second step.

Once initiated, the chain propagates through the continuous addition of monomeric alkene units to the active sites on the polymer chain, and hence the process is called chain-growth polymerization.

The regiospecific addition of monomers to produce highly substituted radicals forms polymers with head-to-tail linkages.

The termination occurs when the two growing chain radicals combine head-to-head. The alternate termination process is the abstraction of the alpha hydrogen atom from the active radical by another chain radical, leading to the nonradical product chains.

The mechanism of ethylene polymerization features the formation of several butyl branches on the polymer chain.

The 1,5-hydrogen abstraction through a transition state adopting a cyclohexane chair-like conformation generates branching, and the active radical site is transferred from one chain to another.

Propagation through the new radical produces a branched polymer. Due to the massive branching, the polymer molecules are less closely packed, generating low-density polyethylene.

Polymers obtained by free-radical polymerization of alkenes can be found in various articles, like packaging materials made from transparent low-density polyethylene, carpet fibers and car tires produced from polypropylene, basketballs prepared using polyisobutylene, and others.