= Step-growth polymerization
{wiki}
In <step-growth polymerization>, suitable reactive groups on <molecules> of different chain lengths can join. The linear count and its assumptions are given in <polymerization>.
A one-unit <monomer>, a two-unit chain, and a twenty-unit chain can all be joining partners if their <functional groups> are compatible and accessible. Schematically, $P_j+P_k\longrightarrow P_{j+k}$: the subscripts count <monomeric units>, and end-group chemistry is suppressed. In <chain-growth propagation>, by contrast, $P_j^*+M\longrightarrow P_{j+1}^*$ requires a free <monomer> and a <polymerization-active site>. An arbitrary pair of existing chains is not a propagation pair in that mechanism. The practical consequence of <step-growth polymerization> is that most <monomers> can have joined something while the average chain is still short.
Whether a joining step releases a small <molecule> is a separate question. <Polycondensation> releases small products during the joining chemistry, as in the <Nylon 6,10> demonstration in <polymerization>. <Polyaddition> joins suitable chains without that elimination: reacting a <diol> with a molecule bearing two <isocyanate> groups can build a <polyurethane>. Thus <step-growth polymerization> and <polycondensation> are not synonyms.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book) by International Union of Pure and Applied Chemistry>[polyaddition, P04720, definition and note 1; polycondensation, P04722, definition and growth-step equation; source recommendation p. 2307]{ref}
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