Why should the monomers used in addition polymerisation through free radical pathway be very pure?

In addition polymerization, peroxy free radicals are generated in chain initiation step. This free radical combines with an unsaturated monomer(containing double/triple bond) to form larger free radical.

If the monomer is mixed with even traces of impurities, these impurities may act as inhibitors. This leads to the sudden termination of chain reaction or the formation of polymers with shorter chain length. Thus, we cannot get the desired polymer.