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.
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 kp.

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.


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:


  1. chain reaction IUPAC Gold Book

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