condensation polymer

A condensation polymer is built by joining monomers through ordinary functional-group reactions, with a small molecule (usually water) expelled at every link. Nylon, polyester (PET), and the proteins and polysaccharides of life are all condensation polymers. The hallmark is the lost byproduct: where an addition polymer keeps every atom, a condensation polymer throws out a little water (or HCl, etc.) each time two monomers bond.

It works because each monomer carries two reactive functional groups, one at each end, and those groups join by the same condensations you already know — esterification (acid + alcohol giving an ester + water) or amide formation (acid + amine giving an amide + water). Picture a monomer with a -COOH on one end and an -OH (or -NH2) on the other. Its acid end reacts with the alcohol end of a neighbour to form an ester, releasing water; now the joined-up piece still has a free -COOH on one side and a free -OH on the other, so it keeps reacting outward in both directions. Any two pieces with the right ends can fuse — short chains combine into longer chains into very long chains — which is why this is called step-growth: the polymer builds up in steps from fragments, not by a single zipping chain reaction. Nylon links acids to amines (amide bonds); polyester links acids to alcohols (ester bonds).

Condensation polymers matter because they include the toughest synthetic fibers and, strikingly, the polymers of life. A protein is amino acids joined by peptide (amide) bonds with water expelled — a condensation polymer. A polysaccharide is sugars joined by glycosidic bonds with water expelled — also a condensation polymer. So nylon and silk, polyester and cellulose are cousins built by the same logic: repeat a water-losing condensation thousands of times. And because each link can be hydrolyzed back, many condensation polymers can be broken down (digested, or recycled), unlike the inert backbones of addition plastics.