= Polymerization-active site
A <polymerization-active site> is the reactive location that enables the next <chain-growth propagation> step. In the schematic
$$
P_j^*+M\longrightarrow P_{j+1}^*,
$$
$M$ is one free <monomer molecule>, $j$ is the chain's <degree of polymerization> in incorporated <monomeric units>, and the star marks the <polymerization-active site>. The site's reactive character survives the growth step, although the <chemical atoms> carrying it can change. The star is not an extra unit, a literal particle, or necessarily a positive or negative <electric charge>. Actual <reaction mechanisms> can involve an unpaired <Electron>, a <cation>, an <anion>, or a metal-associated site; the abstract notation does not identify which. <Organic Chemistry by John McMurry/31.1 Chain-Growth Polymers>[John McMurry, Organic Chemistry, §31.1: successive reactive intermediates and radical, cationic and anionic initiation]{ref}
For bookkeeping below, each linear active chain has exactly one <polymerization-active site>. This is a stipulated model, not a property of every <chain polymerization>: a chain can have several reactive sites, and a reversibly inactive chain can remain capable of future growth. Count currently active sites separately from all chains capable of later reactivation.
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