What is a key characteristic of condensation polymerization compared to addition polymerization?

Condensation polymerization joins monomers by forming covalent bonds while ejecting a small molecule, such as water or methanol, as a by‑product. In contrast, addition polymerization links monomers without releasing any small molecules. This loss of a small molecule means the reaction must balance the removal of that species, often driving the reaction forward. For example, when ethylene glycol reacts with terephthalic acid, water is released each time a new ester bond is formed, creating polyethylene terephthalate. Thus, the hallmark of condensation polymerization is the simultaneous formation of a polymer chain and the elimination of a small molecule.