radical chain-growth polymerization

How do you turn thousands of small, identical molecules into one long plastic chain? With a radical, repeated thousands of times. Radical chain-growth polymerization is the reaction that makes everyday plastics like polyethylene, polystyrene, PVC, and acrylics: a radical adds to a C=C double bond, which creates a new, longer radical, which adds to the next molecule, and so on, the chain literally growing one unit at a time.

It is a radical chain mechanism scaled up. Initiation: an initiator (such as a peroxide that undergoes homolysis on heating) makes the first radical, which adds to the double bond of a monomer (a small alkene like ethylene or styrene), turning the monomer into a radical itself. Propagation: that radical adds to the next monomer's double bond, regenerating a radical at the new chain end, and this repeats hundreds or thousands of times, so each added unit lengthens the chain by one monomer. Termination: two growing chains meet and combine (or one steals a hydrogen from another), ending growth. The key feature of chain-growth, as opposed to step-growth, is that long chains form quickly even early on, because only the few active radical ends react, not random pairs of monomers.

This single mechanism underlies a giant fraction of the world's plastics. Two practical levers come straight from the radical chemistry: initiators (which start more chains) and inhibitors or antioxidants (which kill stray radicals, which is exactly why a stabilizer is added to monomers so they do not polymerize on the shelf). Radical chains are fast and tolerant but hard to control precisely, so chain lengths come out as a distribution rather than one exact size, a real limitation of the method.