Repeating unit

A repeating unit is the smallest part of a polymer that repeats over and over along the chain. Because the whole chain is just this one part copied many times, the polymer is written as the repeating unit inside brackets with the subscript n, where n is a very large number. Malay: unit berulang · Chinese: 重复单元.

The repeating unit lets you describe an enormous molecule with a single short formula. For an addition polymer the repeating unit is the monomer after its carbon–carbon double bond has opened, so the double bond becomes a single bond and the unit gains a bond on each side ready to join its neighbours. For example, ethene CH₂=CH₂ gives the repeating unit –(CH₂–CH₂)– in polythene, and chloroethene gives –(CH₂–CHCl)– in PVC. Reading a repeating unit backwards to find the monomer, or forwards to draw a length of chain, is a standard exam skill.

Example. Polythene is written (–CH₂–CH₂–)ₙ. The part inside the brackets, –CH₂–CH₂–, is the repeating unit; the n outside tells you it repeats thousands of times to make the full chain.

Confusion to avoid. Do not treat the repeating unit as identical to the monomer. They contain the same atoms, but the monomer still has its double bond while the repeating unit has an opened single bond with a free bond at each end. Writing the double bond inside the brackets of a polymer is a common and costly error.

A useful check is to count the atoms: the repeating unit of an addition polymer contains exactly the same atoms as its monomer, only rearranged, so if your unit has gained or lost an atom you have made a mistake. In the exam you may be asked to draw three or four repeating units to show a length of chain, or to write the general formula of the polymer with the subscript n in place.

The repeating unit is how the chapter compresses a giant molecule into one line, so mastering it supports every drawing and deduction question in SPM Chemistry.