Why do some polymerization reactions release water molecules?

Polymerization reactions that release water molecules are known as condensation reactions. In these processes, the formation of a covalent chemical bond between two monomers or parts of a molecule is accompanied by the elimination of a small molecule, most commonly water.123

This occurs because the reacting monomers contain specific functional groups—such as carboxylic acids, alcohols, or amines—that react to form new functionalities. For instance, when a carboxylic acid reacts with an alcohol to form an ester, or with an amine to form an amide, a water molecule is produced as a by-product.4

This mechanism is a hallmark of step-growth polymerization, which is used to create important materials like nylon-6,6 and various aldehyde condensation polymers such as phenol-formaldehyde resin. Unlike addition polymerization, where monomers combine without forming by-products, condensation polymerization relies on this loss of small molecules to link the repeating units into a larger chain or network.1543