Chain propagation

Chain propagation (sometimes referred to as propagation) is a process in which a reactive intermediate is continuously regenerated during the course of a chemical chain reaction. For example, in the chlorination of methane, there is a two-step propagation cycle involving as chain carriers a chlorine atom and a methyl radical which are regenerated alternately:

^•Cl + CH₄ → HCl + ^•CH₃

^•CH₃ + Cl₂ → CH₃Cl + ^•Cl

IUPAC definition

(in a chain polymerization) Chemical reaction between a chain carrier
and a monomer that results in the growth of a polymer chain and the
regeneration of at least one chain carrier.

Note 1: The recommended symbol for the rate constant for chain
propagation in a homopolymerization is k_p.

Penczek S.; Moad, G. Pure Appl. Chem., 2008, 80(10), 2163-2193

The two steps add to give the equation for the overall chain reaction:

CH₄ + Cl₂ → CH₃Cl + HCl.

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