 Polymerization-active site

A polymerization-active site is the reactive location that enables the next chain-growth propagation step. In the schematic$M$ 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. ^[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.