Repeating units

Polymer molecules are very large compared with most other molecules, so the idea of a repeat unit is used when drawing a displayed formula. When drawing one, you need to:

  1. change the double bond in the monomer to a single bond in the repeat unit
  2. add a bond to each end of the repeat unit
After polymerisation: Ethene monomer, repeating unit of polyethene, single bonds between carbon atoms. Chloroethene monomer, repeating unit of polychloroethene, single bonds between carbon atoms.Addition polymerisation

It can be tricky to draw the repeat unit of poly(propene). Propene is usually drawn like this:

Displayed formula for propene.

It is easier to construct the repeat unit for poly(propene) if you redraw the monomer like this:

Displayed formula for ethene.

You can then see how to convert this into the repeat unit.