Why are polymerization reactions sometimes called dehydration synthesis reactions?

Reactions that involve condensation and step-growth polymerization and in which water is the molecular group removed are sometimes called "dehydration synthesis" reactions.

As an illustration, consider the esterification reactions that link the different monomer molecules together to form polyesters through the reaction of polyols and dibasic acids. These reactions can be thought of as "dehydration" reactions because water is eliminated during the process.

This is not applicable to condensation polymerization (step growth) or chain growth polymerization (radical polymerization) in which the group removed is not water.

Since the process of polymerization reactions involves the removal of water molecules, they are also known as dehydration synthesis reactions. During the formation of larger polymer chains, monomers join together, and a water molecule is eliminated as a byproduct. This process of eliminating water molecules is what gives polymerization reactions their name.