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^[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:

${\displaystyle (-M-)_{n}(polymer)+M(monomer)\rightarrow (-M-)_{n+1}}$

• IUPAC Gold Book definition: chain-propagating reaction