Deduce a repeat unit from an alkene
Convert a monomer C=C into a two-carbon chain segment while retaining all substituents and atom counts.
AS Level; Cambridge International Chemistry 9701, syllabus for 2028–2030. The taught examples are addition polymers of alkenes; do not generalise their disposal behaviour to every polymer. Written practice is self-checked
Before you begin
- Recognise C=C bonds and displayed, structural and repeat-unit formulae from Topics 13–14.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What should happen to the alkene C=C when drawing an addition-polymer repeat unit?
What you will learn
- Open the monomer double bond
- Draw the repeat-unit brackets and continuation bonds
- Check that all substituents and atoms are retained
A reliable three-step drawing method
First identify the two carbon atoms of the alkene C=C. Replace that double bond with a single bond. Then add a continuation bond at each end of the two-carbon segment to show how neighbouring monomers connect.
Finally, copy every substituent onto the same carbon as in the monomer. The polymer is written as a bracketed repeat unit with continuation bonds crossing the brackets and n outside.
Use atom counts as a check
Addition polymerisation does not remove atoms. If the monomer contains chlorine, oxygen or a carbon branch, that feature must remain in the repeat unit. Check that the formula and the connectivity both match the monomer.
A double bond in the polymer's main chain usually signals that the monomer has not been converted to an addition-polymer repeat unit correctly.
Pause and explain
Where should a substituent remain when a repeat unit is drawn?
Worked example
Deduce the repeat unit from propene, CH₂=CHCH₃.
Show the worked solution
- Open the C=C between the first two carbons.
- Keep CH₃ attached to the second alkene carbon.
- Add continuation bonds at each end.
Answer [–CH₂–CH(CH₃)–]ₙ.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Draw the repeat unit of propene, CH₂=CHCH₃.
Give me a hint
Open the C=C, preserve the methyl branch, and show continuation bonds.
Compare my reasoning
- The backbone has two carbons from the double bond.
- The CH₃ group stays on the second backbone carbon.
- Bracket the chain segment and add n.
[–CH₂–CH(CH₃)–]ₙ.
Look for these in your work
- Double bond opened
- Methyl branch position preserved
- Brackets and continuation shown
Deduce the repeat unit formed from CH₂=CHBr.
Give me a hint
Only the bond order changes; keep Br attached to its original carbon.
Compare my reasoning
- Use the two alkene carbons as the backbone segment.
- Change C=C to C–C.
- Retain Br on the second carbon.
[–CH₂–CH(Br)–]ₙ.
Look for these in your work
- Backbone correct
- Br retained in position
- Repeat-unit notation correct
A proposed repeat unit for CH₂=CCl₂ is [–CH₂–CCl₂–]ₙ. Check it against atom count and connectivity.
Give me a hint
Open the double bond without removing either chlorine atom.
Compare my reasoning
- The first carbon remains CH₂.
- The second carbon retains two Cl substituents.
- Add chain continuation bonds to the backbone carbons.
[–CH₂–C(Cl)₂–]ₙ. The proposed atom arrangement is correct if the bracket ends show continuation bonds; both chlorine atoms remain on the second carbon.
Look for these in your work
- Both Cl atoms retained
- Substituent carbon identified
- Continuation bonds required
Common mistakes
- Moving a branch to a different carbon
- Leaving the original C=C unchanged
- Omitting continuation bonds or brackets
- Losing an atom despite addition polymerisation retaining all monomer atoms
What can you explain now?
What should happen to the alkene C=C when drawing an addition-polymer repeat unit?
Compare with the explanation
Replace it with a C–C single bond
The pi bond opens and each carbon links to neighbouring repeat units.
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Make it stick
How do bond order, substituent position and atom count help you check a repeat unit?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.