Chain propagation

In chemistry, chain propagation (sometimes just 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 ^[1] which are regenerated alternately:

·Cl + CH₄ → HCl + ·CH₃

·CH₃ + Cl₂ → CH₃Cl + ·Cl

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

CH₄ + Cl₂ → CH₃Cl + HCl

In a chain-growth polymerization reaction, the reactive end-groups of a polymer chain react in each propagation step with a new monomer molecule transferring the reactive group to the last unit. Here the chain carrier is the polymer molecule with a reactive end-group, and at each step it is regenerated with the addition of one monomer unit M: $${\displaystyle {\bigl [}{\ce {-M -}}{\bigr ]}_{n}+{\ce {M}}\rightarrow {\bigl [}{\ce {-M -}}{\bigr ]}_{n+1}}$${\displaystyle {\bigl [}{{-}\mathrm {M} {}-{}}{\bigr ]}_{n}+{\mathrm {M} }\rightarrow {\bigl [}{{-}\mathrm {M} {}-{}}{\bigr ]}_{n+1}}

• IUPAC Gold Book definition: chain-propagating reaction

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