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 in which the chain carriers are a chlorine atom and a methyl radical 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
Polymerization 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:
[ − M − ] n + M → [ − M − ] n + 1 {\displaystyle {\bigl [}{\ce {-M -}}{\bigr ]}_{n}+{\ce {M}}\rightarrow {\bigl [}{\ce {-M -}}{\bigr ]}_{n+1}}
External links IUPAC Gold Book definition: chain-propagating reaction
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