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
  3. draw brackets around the repeating unit so that the brackets go through the middle of the bonds at the end
  4. put the letter n in subscript after the brackets (n represents a very large number of the repeating unit)

Note that when drawing a polymerisation equation the letter n should go in front of the monomer to show there is a very large number of monomers.

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:

Structural formula of propene.

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

Repeating unit equation for the addition polymerisation of propene.

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

The final polymer you need to be aware of is poly(tetrafluoroethene), also known as PTFE or Teflon.

Repeating unit equation for the addition polymerisation of tetrafluoroethene.