Repeat unit

A repeat unit or repeating unit 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.

A repeat unit is sometimes called a mer or mer unit. "Mer" originates from the Greek word "meros," which means part. The word polymer derives its meaning from this, which means "many mers." A repeat unit (or mer), is not to be confused with the term monomer, which refers to the small molecule from which a polymer is synthesized.

One of the simplest repeat units is that of the addition polymer polyvinyl chloride, -[CH₂-CHCl]_n-, whose repeat unit is -[CH₂-CHCl]-. In this case the repeat unit has the same atoms as the monomer vinyl chloride CH₂=CHCl. When the polymer is formed, the C=C double bond in the monomer is replaced by a C-C single bond in the polymer repeat unit, which links by two new bonds to adjoining repeat units.

In condensation polymers (see examples below), the repeat unit contains fewer atoms than the monomer or monomers from which it is formed.