Kinetic chain length is defined as the average number of monomers that react with an active center such as a radical from initiation to termination.
[1] This definition is a special case of the concept of chain length in chemical kinetics.
[4] In chain-growth polymerization the propagation step is the addition of a monomer to the growing chain.
The word kinetic is added to chain length in order to distinguish the number of reaction steps in the kinetic chain from the number of monomers in the final macromolecule, a quantity named the degree of polymerization.
During the formation of a single chain, the reactant concentrations and therefore the propagation rate remain effectively constant.
The second form of the equation is valid at steady-state polymerization, as the chains are being initiated at the same rate they are being terminated (Ri = Rt).
[5] An exception is the class of living polymerizations, in which propagation is much slower than initiation, and chain termination does not occur until a quenching agent is added.
In such reactions the reactant monomer is slowly consumed and the propagation rate varies and is not used to obtain the kinetic chain length.
When the reaction goes to completion, [M] = 0, and then the kinetic chain length is equal to the number average degree of polymerization of the polymer.
The kinetic chain length is important in determining the degree of polymerization, which in turn influences many physical properties of the polymer.