A chain-growth polymerization mechanism initiated by free radicals in which monomer molecules add sequentially to a radical chain end. The cycle proceeds through distinct initiation, radical propagation, chain transfer, and termination steps and typically yields polymers with broad molecular-weight distributions unless additional control methods are employed.

Principle

Radical centers (unpaired-electron species) add to unsaturated monomers to form a new radical at the chain terminus; the balance between propagation, chain-transfer, and termination events determines chain length and dispersity.

Demonstration

Polymerization of styrene initiated thermally by a peroxide or azo initiator: an initiator decomposes to radicals, a radical adds to styrene producing a phenethyl radical, repeated additions produce polystyrene chains until termination by recombination or disproportionation.

Misapplication

Treating free-radical polymerization as if it were step-growth (where all chains grow gradually by bimolecular combination) or assuming that monomer conversion alone controls molecular weight without accounting for transfer and termination events.

Consequence

Rapid chain growth enabling high polymerization rates and synthesis of a wide range of vinyl polymers; observable consequences include sensitivity to oxygen, radicals’ reactivity determining tacticity and chain defects, and typically high dispersity unless mediated.

Reversal

Replacing the process with a living/controlled radical method (e.g., reversible-deactivation radical polymerization) or with ionic polymerization inverts control over chain-ends and often yields narrower molecular-weight distributions and different end-group chemistry.

Boundary

Applies to chain-growth mechanisms propagated by free radicals acting at a carbon-centered (or heteroatom-centered) radical site; excludes purely ionic, condensation (step-growth) polymerizations and specific reversible-deactivation variants unless explicitly specified.

Semantic Tension

Distinguishing ‘free-radical polymerization’ from broadly ‘radical polymerization’ or from ‘controlled/living radical’ methods—terms overlap in everyday use but differ in mechanistic detail and in expectations for molecular-weight control.

Synthesis

A rapid chain-growth process driven by reactive radical chain ends that repeatedly add monomer; the final polymer properties reflect the competition between propagation, transfer, and termination, and modest control of molecular weight requires additional reversible-deactivation strategies.