What is Step and chain growth?

In a step-growth polymerisation, any two molecules carrying complementary functional groups (monomers, oligomers or long chains) can react, and the chains lengthen by joining one another: polyesters, polyamides. In a chain-growth polymerisation, a reactive centre (radical, ion or metal–carbon bond) adds monomer molecules one at a time to the end of a growing chain: the polymers of alkenes.