Repeating units
A large molecule formed from many identical smaller molecules known as monomers. molecules are very large compared with most other A collection of two or more atoms held together by chemical bonds., so the idea of a A part of a polymer that would make a complete polymer molecule if many of them were joined end to end. is used when drawing a displayed formula. When drawing one, you need to:
- change the double bond in the Small molecule, usually containing a C=C bond, that can join end to end with other monomers to form a polymer molecule. to a single bond in the repeat unit
- add a bond to each end of the repeat unit
- draw brackets around the repeating unit so that the brackets go through the middle of the bonds at the end
- put the letter n in A letter or number written below the line. 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.
It can be tricky to draw the repeat unit of poly(propene). Propene is usually drawn like this:
It is easier to construct the repeat unit for poly(propene) if you redraw the monomer like this:
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.