Overview

The conversion of alkenes to macromolecules called polymers is a reaction of high commercial importance. The structure of the polymer is defined by a repeating unit, while the terminal groups are considered insignificant. The average degree of polymerization represents the number of repeating units in the polymer molecule and is denoted by the subscript n.

Alkenes undergo polymerization via a free-radical mechanism involving three steps: initiation, propagation, and termination. 

Radicals are generated in the initiation step by heating a small quantity of free-radical initiators such as benzoyl peroxide. The benzoyloxy radical loses CO₂ and forms a phenyl radical (Ph·), which adds to the double bond of the alkene to initiate the polymerization process. One of the electrons of the alkene π bond pairs with one electron from the phenyl radical to form a new C–C bond.

In the propagation step, the carbon radical generated in the initiation step adds to another molecule of alkene to generate a new radical. The continuous addition of alkene monomers at the radical site yields the polymer.

The termination step of the free-radical mechanism occurs via recombination or disproportionation. In a recombination step, two growing chains form a bond at their radical sites.

In termination by disproportionation, a β-hydrogen atom is transferred from one radical to another radical center, resulting in two non-radical products.

The free-radical polymerization of ethylene yields low-density polyethylene (LDPE), where the low density is a consequence of significant branching in the polymer chains. Articles made from polymers surround us in various forms, such as food packaging materials, plastic bags, bottles, stationery, and automotive parts.

Procedure

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.