To find the repeating unit of an addition polymer, keep the two carbons of the double bond, make the bond single, and show the open ends. The side groups stay exactly where they were.
This lesson is part of polymers. It follows distinguishing addition and condensation polymerisation.
What is the three-step method?
Use the same steps for every addition monomer.
- Draw the monomer with its C=C double bond.
- Change the double bond to a single bond and add an open bond at each end of the two carbons.
- Put the unit in square brackets and add n.
The atoms inside the brackets are the same as the atoms in one monomer molecule.
Worked example: chloroethene
Chloroethene is CH₂=CHCl. The two carbons are CH₂ and CHCl, joined by a double bond.
Change it to a single bond and open both ends: –CH₂–CHCl–. The polymer is poly(chloroethene), written [–CH₂–CHCl–]ₙ.
The chlorine atom stays on the same carbon. Moving it to the other carbon would give a different arrangement from the one in the monomer.
Worked example: counting the units
A sample of poly(chloroethene) has a relative molecular mass of 62 500. (C = 12, H = 1, Cl = 35.5.)
The monomer C₂H₃Cl has relative molecular mass 24 + 3 + 35.5 = 62.5. So n = 62 500 ÷ 62.5 = 1000 repeating units.
| Polymer | Monomer | Repeating unit | Mr of monomer |
|---|---|---|---|
| Poly(ethene) | CH₂=CH₂ | –CH₂–CH₂– | 28 |
| Poly(propene) | CH₂=CH–CH₃ | –CH₂–CH(CH₃)– | 42 |
| Poly(chloroethene) | CH₂=CHCl | –CH₂–CHCl– | 62.5 |
The mistake that costs marks
The common slip is to keep the double bond in the unit. Another is to draw the brackets but leave the bonds at the ends closed, so the unit looks like a separate molecule.
| Drawing | Problem |
|---|---|
| [–CH₂=CH₂–]ₙ | A double bond remains, which is not possible in the chain |
| CH₃–CH₃ | Closed ends, this is ethane |
| [–CH₂–CH₂–]ₙ | Correct, single bonds and open ends |
Check the drawing by counting atoms. The repeating unit should have exactly the atoms of one monomer. The units and significant figure checker helps you keep a mass calculation tidy.
Check yourself
Tetrafluoroethene is CF₂=CF₂. Draw the repeating unit, and find n for a polymer with relative molecular mass 50 000. (C = 12, F = 19.)
Answer
The repeating unit is –CF₂–CF₂–, with the double bond changed to a single bond and open ends.
Mr of the monomer C₂F₄ = 24 + 76 = 100. So n = 50 000 ÷ 100 = 500.
What to study next
Go on to connecting polymer structure with properties, or mix the skills in the polymers practice set.
For a teacher to watch you draw and correct the detail, see online one-to-one Chemistry tuition.