Constitutional repeat unit

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Repeating unit of polyethylene
Structural formula of ethene

Repeat unit or repeat unit is a term used in organic chemistry or from the polymer - Materials Science . The constitutional repeating unit (CRU) is the smallest repeating unit within a polymer.

During the polymerization , the monomers from which the polymer is produced are subjected to a change. In the simplest case, e.g. B. Ethylene to polyethylene , their double bonds are broken and they become the chain-forming bonds, as shown in the example image. Formally, the chain consists of methylene groups , but since these can never occur in an odd number, the sum formula of the repeating unit is C 2 H 4 and the molar mass of the repeating unit is 28.05 g · mol −1 .

In the case of other polymers, such as polyethylene terephthalate (PET), molecules are split off during the polymerization, so that the composition of the repeating unit differs from the sum of the monomers.


  • Hans-Georg Elias: Macromolecules . Chemical structure and syntheses. 6th edition 1999. p. 5.