Distinguish addition from condensation polymerisation
Use the monomer's reactive groups and the polymer link to decide whether a polymer forms by addition or condensation.
Section 35 · Polymerisation
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 35. Track the functional groups, repeat-unit connectivity and small-molecule by-products. Written practice is self-checked
Before you begin
- Recognise alkenes, carboxylic acids, acyl chlorides, alcohols, amines, amides and ester linkages.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which feature is characteristic of condensation polymerisation in the syllabus examples?
What you will learn
- Recognise alkene addition polymerisation
- Recognise condensation between complementary functional groups
- Use a polymer section to justify the reaction type
Start with the monomers
An alkene can undergo addition polymerisation: its C=C bond opens, monomers join into a carbon-chain backbone, and no small molecule is eliminated. The repeat unit retains the atoms of the alkene monomer.
Condensation polymerisation joins suitable functional groups, commonly forming ester or amide links and eliminating a small molecule such as water or HCl. A diol with a dicarboxylic acid, for example, forms a polyester; a diamine with a dicarboxylic acid forms a polyamide.
Read the polymer when monomers are not shown
A saturated carbon backbone with substituents inherited from alkene monomers is evidence for addition polymerisation. Ester (–COO–) or amide (–CONH–) links between residues indicate condensation polymerisation in the examples in this syllabus.
Use the structure provided rather than relying on a polymer name alone. A fragment may identify the link type without showing the complete chain or its original reagents.
Pause and explain
A polymer fragment contains repeated –COO– links between residues. Which type is indicated?
Worked example
Classify the formation of poly(ethene) and a polyester made from a diol and a dicarboxylic acid.
Show the worked solution
- Ethene has a C=C bond that opens to form the chain.
- No small molecule leaves poly(ethene)'s addition repeat.
- The diol and diacid form ester links and eliminate water.
Answer Poly(ethene) forms by addition polymerisation. The polyester forms by condensation polymerisation because ester links form with water eliminated.
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.
Classify the formation of poly(propene) from propene and give the structural clue.
Give me a hint
Look for the monomer's alkene bond and the atoms retained.
Compare my reasoning
- Propene contains C=C.
- That bond opens as the chain forms.
- No small molecule leaves the repeat.
Addition polymerisation: the C=C bond opens and propene's atoms remain in the repeat unit.
Look for these in your work
- Monomer feature identified
- Bond change stated
- By-product distinction made
A diol and a dicarboxylic acid form a chain containing ester links. Classify the process and name the by-product.
Give me a hint
Track what happens when an –OH group and –COOH group link.
Compare my reasoning
- The functional groups form an ester link.
- The linking reaction eliminates water.
- A small molecule is removed as the chain grows.
Condensation polymerisation; water is eliminated as ester links form.
Look for these in your work
- Link type named
- Reaction type named
- By-product stated
A polymer section has –CH₂–CH(CH₃)– repeat units and no functional-group links. What evidence supports an addition route?
Give me a hint
Reverse the repeat unit to its likely alkene monomer.
Compare my reasoning
- Restore C=C between the two backbone carbons.
- Keep CH₃ on the same carbon to obtain propene.
- An alkene's atoms are retained when the C=C opens.
The section is consistent with addition polymerisation of propene: its C=C opens and the repeat contains no condensation link or lost small molecule.
Look for these in your work
- Monomer recovered
- Backbone evidence used
- Conclusion justified
Common mistakes
- Calling every polymerisation addition
- Looking only at the polymer name
- Claiming water is eliminated when an alkene undergoes addition polymerisation
- Treating the small-molecule by-product as part of the repeat unit
What can you explain now?
Which feature is characteristic of condensation polymerisation in the syllabus examples?
Compare with the explanation
A functional-group link forms and a small molecule is eliminated
Ester or amide links form by condensation in these examples.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
Which monomer feature and polymer-link feature help distinguish addition from condensation polymerisation?
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.