Deduce polymerisation type from a polymer section
Inspect the repeated backbone and functional-group links to infer addition or condensation 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 polymer fragment most directly indicates condensation in the syllabus examples?
What you will learn
- Recognise alkene-derived carbon-chain repeats
- Recognise ester and amide condensation links
- Explain the inference while noting what a fragment cannot prove
Look for the chain backbone
A repeat made only from a saturated carbon backbone with alkene-derived substituents is consistent with addition polymerisation. Reverse the repeat by restoring C=C between the backbone carbons.
An alkene-derived addition repeat does not contain an ester or amide link created by eliminating a small molecule. The original monomer atoms remain in its repeating structure.
Look for functional-group links
A repeated –COO– group suggests a polyester, and –CONH– suggests a polyamide in the syllabus condensation examples. These links connect residues from bifunctional monomers or from a monomer carrying two different groups.
A short section may not reveal the exact monomer names, end groups or whether the precursor was a carboxylic acid or acyl chloride. State the reaction type supported by the visible bonds and avoid claiming details the fragment does not show.
Pause and explain
What can a short fragment containing –COO– establish?
Worked example
Infer the polymerisation type for (a) [–CH₂–CH(Cl)–]ₙ and (b) a chain containing repeated –CO–NH– links.
Show the worked solution
- The first has an alkene-derived carbon backbone.
- The second has amide links between residues.
- Connect each structure to its likely polymerisation route.
Answer (a) Addition polymerisation, consistent with chloroethene. (b) Condensation polymerisation, consistent with a polyamide.
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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.
A fragment is [–CH₂–C(CH₃)₂–]ₙ. Which polymerisation type is consistent with it?
Give me a hint
Restore a C=C between adjacent backbone carbons and inspect the retained substituents.
Compare my reasoning
- The backbone is saturated carbon only.
- Restoring C=C gives an alkene monomer pattern.
- No ester/amide link is shown.
Addition polymerisation is consistent: the repeat can arise when an alkene C=C opens, with the methyl groups retained.
Look for these in your work
- Backbone inspected
- Monomer logic used
- Inference qualified
A polymer fragment repeats –O–CO– links. Give the likely polymer class and route.
Give me a hint
Identify the functional group in the chain.
Compare my reasoning
- –COO– is an ester link.
- Repeated ester links define a polyester structure.
- These syllabus polyesters form by condensation.
It is a polyester, consistent with condensation polymerisation.
Look for these in your work
- Link identified
- Class named
- Route stated
Can a fragment with –COO– alone distinguish polymer made from a dicarboxylic acid from one made using a dioyl chloride?
Give me a hint
Compare the final link after each precursor loses its leaving group.
Compare my reasoning
- Both routes form the same ester connectivity.
- Water or HCl leaves during formation.
- Those by-products may not be present in the final fragment.
No. The final ester link does not by itself distinguish the acid route from the dioyl-chloride route.
Look for these in your work
- Both routes compared
- By-products distinguished
- Inference boundary stated
Common mistakes
- Inferring addition only from a polymer being a plastic
- Ignoring the –COO– or –CONH– link
- Claiming a fragment gives exact precursor reagents
- Treating each polymer section as a full molecule
What can you explain now?
Which polymer fragment most directly indicates condensation in the syllabus examples?
Compare with the explanation
Repeated –CONH– links
Amide links connect monomer residues through condensation.
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Make it stick
Which visible repeat-unit bonds indicate addition or condensation, and what can remain uncertain?
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.