The main types of steps in chain reaction are of the following types.
* Initiation (formation of active particles or chain carriers, often free radicals, in either a thermal or a photochemical step)
* Propagation (may comprise several elementary steps in a cycle, where the active particle through reaction forms another active particle which continues the reaction chain by entering the next elementary step). In effect the active particle serves as a catalyst for the overall reaction of the propagation cycle. Particular cases are:
** chain branching (a propagation step where one active particle enters the step and two or more are formed);
** chain transfer (a propagation step in which the active particle is a growing polymer chain which reacts to form an inactive polymer whose growth is terminated and an active small particle (such as a radical), which may then react to form a new polymer chain).
* Termination (elementary step in which the active particle loses its activity; e. g. by recombination of two free radicals).
The chain length is defined as the average number of times the propagation cycle is repeated, and equals the overall reaction rate divided by the initiation rate.
Some chain reactions have complex rate equations with fractional order or mixed order kinetics.