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.

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:

References
  1. Chain reaction IUPAC Gold Book
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