Describe addition polymerisation
Connect alkene monomers into long chains by opening their C=C bonds, with no small molecule eliminated.
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
Which bond opens during addition polymerisation of an alkene?
What you will learn
- Describe addition polymerisation
- Recognise poly(ethene) and PVC from their monomers
- Distinguish a monomer from the polymer chain
The alkene double bond opens to link monomers
In addition polymerisation, many alkene molecules join as the C=C double bonds open and new C–C bonds link the monomers into a long chain. The polymer contains the atoms from the monomers; no small molecule is eliminated.
Ethene forms poly(ethene), with repeat unit [–CH₂–CH₂–]ₙ. Chloroethene, CH₂=CHCl, forms poly(chloroethene), also called PVC, with repeat unit [–CH₂–CH(Cl)–]ₙ. The chlorine substituent remains attached to the chain carbon.
Read the repeating pattern
The bracketed section is the smallest unit that repeats along the chain. The subscript n indicates many repeats; it is not a fixed number of atoms in every polymer sample.
Unlike condensation polymerisation, addition polymerisation does not eliminate a small molecule such as water. The name ‘addition’ refers to joining the alkene monomers after their double bonds open.
Pause and explain
Which atoms are lost when ethene forms poly(ethene) by addition polymerisation?
Worked example
Name the polymer formed from CH₂=CHCl and write its repeat unit.
Show the worked solution
- The monomer is chloroethene.
- Open the C=C bond to form two single bonds in the chain.
- Keep Cl attached to the same carbon.
Answer Poly(chloroethene), PVC: [–CH₂–CH(Cl)–]ₙ.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
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.
Write the repeat unit of poly(ethene) and explain how it forms from ethene.
Give me a hint
Open C=C and connect the two former double-bond carbons to neighbouring units.
Compare my reasoning
- Ethene is CH₂=CH₂.
- Each carbon forms new single bonds along the chain.
- Keep two CH₂ groups in the repeating unit.
nCH₂=CH₂ → [–CH₂–CH₂–]ₙ. The double bond opens and the molecules link by new C–C bonds.
Look for these in your work
- Correct repeat unit
- C=C opened
- No atoms removed
A polymer has repeat unit [–CH₂–CH(Cl)–]ₙ. Identify its monomer and polymer name.
Give me a hint
Reverse the chain-forming step while retaining the side group.
Compare my reasoning
- Restore a C=C between the two backbone carbons.
- Keep Cl attached to the second carbon.
- Name the monomer and the polymer.
Monomer: CH₂=CHCl, chloroethene. Polymer: poly(chloroethene), PVC.
Look for these in your work
- Double bond restored
- Cl position retained
- Names correct
A learner says PVC is made by polymerising ethene and then adding chlorine to the finished chain. Correct the explanation.
Give me a hint
Compare the repeat-unit substituent with the atom arrangement in the monomer.
Compare my reasoning
- The chlorine atom is already in chloroethene.
- The C=C bond opens during polymerisation.
- The chain forms with Cl remaining on the same carbon.
PVC forms by addition polymerisation of chloroethene, CH₂=CHCl. Its C=C opens and the Cl substituent remains attached in [–CH₂–CH(Cl)–]ₙ.
Look for these in your work
- Correct monomer identified
- Correct bond change
- Substituent retention explained
Common mistakes
- Deleting the chlorine substituent
- Leaving a C=C bond in the polymer backbone
- Adding water as a by-product
- Treating n as a specific sample size
What can you explain now?
Which bond opens during addition polymerisation of an alkene?
Compare with the explanation
C=C
The alkene double bond opens as monomers link together.
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Make it stick
What changes and what stays in place when an alkene monomer becomes an addition-polymer 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.