An addition polymer forms when many alkene monomers join end to end. The C=C bond in each monomer opens, and the carbons link into a chain.
This lesson is part of SPM Chemistry carbon compounds. It builds on the alkene reactions in writing characteristic organic reactions.
How do I draw the repeating unit?
Take the monomer and keep both carbons of the C=C. Turn the double bond into a single bond, and leave a free bond on each end.
Put the unit in brackets with n as a subscript. Every other atom stays attached where it was.
Worked example: three monomers
| Monomer | Repeating unit | Polymer |
|---|---|---|
| Ethene, CH₂=CH₂ | –[CH₂–CH₂]ₙ– | poly(ethene) |
| Propene, CH₂=CH–CH₃ | –[CH₂–CH(CH₃)]ₙ– | poly(propene) |
| Chloroethene, CH₂=CHCl | –[CH₂–CHCl]ₙ– | poly(chloroethene), PVC |
In each row the two carbons of the original double bond stay in the chain. The CH₃ or Cl hangs off as a side group.
Worked example: monomer from polymer
A polymer has the repeating unit –[CH₂–CH(CH₃)]ₙ–. To find the monomer, reverse the method: take the two carbons in the chain and put the double bond back.
The monomer is CH₂=CH–CH₃, which is propene. The side group CH₃ stays where it is.
How do I compare them by test?
A monomer contains C=C and a polymer does not. Shake each with bromine water.
Ethene turns the brown colour colourless. Poly(ethene) leaves it brown.
The mistake that loses marks
A student draws the repeating unit of poly(ethene) as –[CH₂=CH₂]ₙ– with the double bond still there. A double bond in the chain would mean the polymer still reacts with bromine water.
The fix is to ask what happened to the double bond. It was used to join the monomers, so the unit has a single C–C bond.
Check yourself
Draw the repeating unit of PVC from its monomer and state what happens when bromine water is added to the polymer.
Answer
The monomer is CH₂=CHCl. Open the double bond and keep both carbons: –[CH₂–CHCl]ₙ–.
Bromine water stays brown, because PVC has no C=C bond. The monomer, by contrast, would decolourise it.
What to study next
See how polymer steps sit inside a longer route in planning a reaction pathway. Then try the practice set.
Record any bond-drawing slips in the mistake log and paper-error review. For a teacher to check your drawings with you, see online one-to-one Chemistry tuition.