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How does a peroxide initiator help start a polymer chain?

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Diagram illustrating a homopolymer arrangement using polyvinyl chloride as an example, showing a chain of identical repeating vinyl chloride units with the chemical structure highlighted. — polymer Industrial polymers are synthesized from simple compounds joined together to form long chains. For example, polyvinyl chloride is an industrial homopolymer synthesized from repeating units of vinyl chloride.

A peroxide initiator facilitates the start of a polymer chain through the generation of highly reactive free radicals, which are chemical species containing an unpaired electron.1

The process typically follows these steps:

Decomposition: The peroxide—a compound featuring two oxygen atoms joined by a single covalent bond—decomposes to form free radicals (represented as $R O •$ ).12
Initiation: These reactive radicals react with a monomer to create an intermediate compound that possesses an unpaired electron.31
Propagation: This intermediate compound is then capable of linking successively with a large number of other monomers, effectively building the polymer chain.31

This sequence is a fundamental part of chain reactions in polymerization, where the product of each step serves as the reagent for the next until a termination step concludes the process.1