Predict polymerisation type from the monomers
Use C=C bonds or complementary bifunctional groups to predict 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 monomer feature supports addition polymerisation in the syllabus examples?
What you will learn
- Recognise an alkene monomer suited to addition
- Recognise complementary groups suited to condensation
- Justify the predicted polymer link and by-product
An alkene can add across its double bond
A monomer with a C=C bond can undergo addition polymerisation when the double bond opens and links to other monomers. The product backbone is made from the alkene carbons and no small molecule is eliminated.
Substituents attached to the alkene remain on the same backbone carbon in the repeat. For example, propene gives a repeat containing –CH₂–CH(CH₃)–.
Complementary ends predict condensation
A diol with a dicarboxylic acid or dioyl chloride can form a polyester. A diamine with a dicarboxylic acid or dioyl chloride, or an aminocarboxylic acid, can form a polyamide.
Look for two reactive ends so a chain can grow. Predict ester or amide links and then identify water or HCl from the specific monomers supplied; do not infer a by-product before checking whether the carbonyl reagent is an acid or acyl chloride.
Pause and explain
A diol and a dioyl chloride are supplied. Which route should you predict?
Worked example
Predict the polymerisation type for (a) CH₂=CHCl and (b) HOOC–R–COOH plus H₂N–R′–NH₂.
Show the worked solution
- The first monomer has C=C.
- The second pair has carboxyl and amino ends.
- State polymer link and small molecule for each route.
Answer (a) Addition polymerisation gives a chlorine-substituted carbon backbone. (b) Condensation polymerisation gives amide links and eliminates water.
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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.
Predict the polymerisation type and repeat feature for CH₂=CHCH₃.
Give me a hint
Open the alkene double bond and keep the methyl substituent.
Compare my reasoning
- Identify C=C in propene.
- Use the two alkene carbons as the backbone segment.
- No small molecule is lost.
Addition polymerisation forms [–CH₂–CH(CH₃)–]ₙ with no small molecule eliminated.
Look for these in your work
- C=C identified
- Repeat feature stated
- By-product distinction
Predict the polymerisation type for HO(CH₂)₄OH plus ClCO(CH₂)₄COCl, and name the link.
Give me a hint
Match hydroxyl groups with acyl chloride groups.
Compare my reasoning
- The two bifunctional monomers can link repeatedly.
- Alcohol oxygen connects to the acyl carbon.
- Name the ester link and HCl by-product.
Condensation polymerisation forms a polyester through –COO– links and eliminates HCl.
Look for these in your work
- Route chosen
- Ester link named
- HCl identified
A monomer has both –NH₂ and –COOH. Predict a polymer route and justify it without inventing a second monomer.
Give me a hint
Use the two functional groups on the same molecule.
Compare my reasoning
- The amino group can react with another molecule's carboxyl group.
- The monomer is bifunctional.
- Repeated amide links can grow a chain with water eliminated.
It can self-condense to form a polyamide through –CONH– links, eliminating water; one monomer type supplies both ends.
Look for these in your work
- Groups matched
- Single monomer recognised
- Link and by-product justified
Common mistakes
- Assuming two monomer types always means condensation without checking groups
- Calling an alkene condensation polymerisation
- Using water for a dioyl chloride
- Omitting the structural reason for the prediction
What can you explain now?
Which monomer feature supports addition polymerisation in the syllabus examples?
Compare with the explanation
A carbon-carbon double bond
The C=C opens as alkene monomers join.
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Make it stick
Which monomer features let you predict a polymerisation type and its link?
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.