Build addition polymers from ethene and propene
Convert an alkene double bond into a repeating saturated polymer unit without losing atoms.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Apply the stated reagents and conditions, balance equations, and keep mechanistic claims within the named examples. Written practice is self-checked
Before you begin
- Use displayed and structural formulas, covalent bond polarity, balanced equations, and the organic reaction terms from Topic 13.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What happens to an alkene C=C during addition polymerisation?
What you will learn
- Identify addition polymerisation
- Draw a repeat unit from an alkene monomer
- Distinguish monomer and polymer structures
The double bond opens to link monomers
In addition polymerisation, many alkene molecules join as their C=C bonds become C–C single bonds in a long chain. No small molecule is eliminated.
Ethene forms poly(ethene), with repeat unit [–CH₂–CH₂–]ₙ. The bracket and subscript n show that the unit repeats many times; the polymer is not one finite molecule with a known n unless supplied.
Substituents stay attached to the same carbon
Propene forms poly(propene), [–CH₂–CH(CH₃)–]ₙ. The methyl side group remains attached to every second backbone carbon in the ideal repeat representation.
To derive the repeat unit, open the C=C, change it to C–C and place continuation bonds at both ends. Do not leave a double bond in the polymer backbone or discard the side group.
Pause and explain
Which is the repeat unit of poly(propene)?
Worked example
Draw the repeat unit formed from propene, CH₂=CHCH₃.
Show the worked solution
- Open the monomer C=C bond.
- Make a two-carbon backbone segment with single bonds.
- Keep CH₃ attached to the carbon it was on in the monomer and show continuation bonds.
Answer [–CH₂–CH(CH₃)–]ₙ, poly(propene).
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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 for poly(ethene) from CH₂=CH₂.
Give me a hint
Change the double bond to a single bond and show extension at both ends.
Compare my reasoning
- Retain both carbon atoms.
- Make the C–C backbone single.
- Put the repeating segment in brackets with n.
[–CH₂–CH₂–]ₙ.
Look for these in your work
- Two carbons retained
- Single-bond backbone
- Repeating notation used
A propene-derived repeat is written [–CH(CH₃)–CH₂–]ₙ. Is its connectivity equivalent to [–CH₂–CH(CH₃)–]ₙ?
Give me a hint
Compare the repeating unit after shifting the start position of the same chain segment.
Compare my reasoning
- Both contain alternating CH₂ and CH(CH₃) backbone atoms.
- Each repeat includes one methyl substituent on the substituted backbone carbon.
- Changing which end is written first does not change the polymer connectivity.
Yes. These are the same repeat connectivity written from opposite ends.
Look for these in your work
- Backbone compared
- Side group located
- Equivalent orientation explained
A student removes H₂O while polymerising ethene. Diagnose the error.
Give me a hint
Check whether ethene contains oxygen and whether the reaction is addition polymerisation.
Compare my reasoning
- Ethene contains only carbon and hydrogen.
- Addition polymerisation uses the C=C bond and eliminates no small molecule.
- The repeat unit is [–CH₂–CH₂–]ₙ.
No water is formed; ethene undergoes addition polymerisation to [–CH₂–CH₂–]ₙ.
Look for these in your work
- Atoms checked
- Reaction type stated
- Repeat unit corrected
Common mistakes
- Removing a small molecule as in condensation polymerisation
- Leaving C=C in the polymer repeat unit
- Moving the propene methyl group onto the wrong atom
What can you explain now?
What happens to an alkene C=C during addition polymerisation?
Compare with the explanation
It becomes a C–C single bond as monomers link
The π bond is used to form new links in the chain.
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Make it stick
How do you derive a polymer repeat unit from its alkene monomer?
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.