= Chain-growth polymerization
{wiki}
= Chain polymerization
{synonym}
In <chain-growth polymerization>, each growth step joins a <monomer> to a reactive site on a <polymer> chain and regenerates that reactive site. This is the precise mechanistic restriction in IUPAC's definition: growth occurs exclusively through these monomer–chain-site reactions. The word chain refers to the succession of site-regenerating <chemical reactions>. It does not merely mean that the product looks like a long chain. <Chain-growth initiation> starts this succession; <chain-growth propagation> continues it; <chain-growth termination> and <chain transfer in chain polymerization> can change its fate.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book) by International Union of Pure and Applied Chemistry>[chain polymerization, C00958, definition and notes 1–2; source recommendation p. 2306]{ref}
The contrast with <step-growth polymerization> concerns the permitted joining partners. There, suitable reactive groups can join two existing chains, for example a two-unit chain and a three-unit chain. In <chain-growth propagation>, a free <monomer> adds to a <polymerization-active site>. Release of a small byproduct is a separate classification question: IUPAC explicitly allows condensative <chain polymerization>.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book) by International Union of Pure and Applied Chemistry>[chain polymerization, C00958, notes 3–6]{ref} The useful consequence is that a few growing chains can incorporate many <monomer molecules> while much of the feed remains unreacted. Their <degree of polymerization> depends on how those units are shared among chains, as <finite chain-growth bookkeeping> makes explicit.
\Image[/chain-growth-active-site-bookkeeping.png]
{title=<Chain-growth initiation>, <chain-growth propagation>, and two possible <chain-growth termination> counts}
{id=image-chain-growth-active-site-bookkeeping}
{description=Original schematic for linear chains with one <polymerization-active site> each. Blocks count <monomeric units>; stars identify reactivity, not <electric charge>. $I^*$ is a free initiating species, excluded from the count of polymer-bound sites. The two termination rows are alternative outcomes for two active chains, not consecutive reactions. Joining two chains preserves their incorporated-unit total while reducing the number of <polymer> <molecules>. Neither row specifies a universal <reaction mechanism> or rate.}
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