Step-growth Polymerization
In a step-growth reaction, there are no free radicals and therefore, the chain does not have a highly reactive end. Instead, a normal chemical reaction takes place between the functional groups of two monomers and combining into one. Usually there is a side product as well, as shown in Figure 9.6.1, illustrating the step-growth polymerization of nylon 6,6, which results in an $\ce{HCl}$ as a byproduct. In that reaction, two different monomers—adipoyl chloride and hexamethylene diamine— react to form a single molecule. Because each of the monomers has the same functional groups at each ends, the two ends of the new longer molecule can react again with the other type of monomer, resulting in the chain growing "step by step" in both directions.
Figure 9.6.1 Step growth reaction of Nylon 6,6. It is called 6,6, because both of the monomers that create it have six carbon atoms.
Figure 9.6.2 shows a generic abstract representation of a step-wise polymerization process. The black circles represent one type of monomer and the white circles represent another kind. They react together step by step to eventually create a long polymer chain.
Figure 9.6.2 An abstract representation of a step-growth polymerization process between two types of monomers