Free-Radical Chain Reaction and Polymerization of Alkenes

A repeat unit is the smallest recurring structural fragment of a polymer chain. It represents the basic building block that repeats throughout the polymer structure. The subscript n denotes the number of repeat units along the chain, also called the degree of polymerization, which determines the polymer's molecular weight and properties.

The initiation step begins when a free-radical initiator, such as benzoyl peroxide, is heated and undergoes cleavage of its oxygen-oxygen bond. The benzoyloxy radical decomposes further to produce a phenyl radical and carbon dioxide. This phenyl radical then adds to the double bond of an alkene monomer, forming a new carbon-centered radical that starts the polymerization process.

During propagation, the carbon radical generated in initiation continuously adds to the double bond of incoming alkene monomers. Each addition creates a new radical at the end of the growing chain, allowing the process to repeat. This chain-growth mechanism produces polymers with head-to-tail linkages, where monomers add regiospecifically to form highly substituted radicals.

Termination occurs through two mechanisms. In recombination, two growing chain radicals combine head-to-head to form a single nonradical polymer. In disproportionation, an alpha hydrogen atom is abstracted from one active radical by another chain radical, producing two nonradical product chains with different end groups.

Ethylene polymerization generates low-density polyethylene through extensive branching of the polymer chains. The 1,5-hydrogen abstraction via a cyclohexane chair-like transition state transfers the active radical site from one chain to another, creating butyl branches. This massive branching prevents tight molecular packing, resulting in a lower-density polymer suitable for flexible packaging materials.

Radical initiators generate the initial free radicals needed to start polymerization. Common initiators include benzoyl peroxide and azoisobutyronitrile, which decompose when heated or exposed to light. Benzoyl peroxide yields phenyl radicals and CO2, while azo-initiators produce isobutyronitrile radicals and nitrogen through entropy-driven decomposition.

Free-radical polymerization produces numerous commercial products including transparent low-density polyethylene used in packaging materials and plastic bags, polypropylene carpet fibers and car tires, and polyisobutylene used in basketballs. These polymers are valued for their versatility in applications ranging from food packaging to automotive components and consumer goods. Understanding polymer synthesis connects to broader alkene chemistry, such as oxidation of alkenes syn dihydroxylation with potassium permanganate, which explores alternative alkene transformations.