In polymer chemistry, a repeat unit or repeating unit (or mer) is a part of a polymer whose repetition would produce the complete polymer chain (except for the end-groups) by linking the repeat units together successively along the chain, like the beads of a necklace. IUPAC definition In condensation polymers (see examples below), the repeat unit contains fewer atoms than the monomer or monomers from which it is formed. In copolymers there are two or more types of repeat unit, which may be arranged in alternation, or at random, or in other more complex patterns.