Cambridge IGCSE Chemistry Polymers: Repeat Units
Revise Cambridge IGCSE Chemistry polymers, monomers and repeat units with worked examples, exam wording and targeted practice.
Polymers in Cambridge IGCSE Chemistry are large molecules made when many small molecules join together. The small molecules are called monomers, and the repeating section in the chain is the repeat unit.
This topic is often tested with short wording questions and simple structure diagrams, so the key is using the terms accurately and showing the repeat unit clearly.
Quick Answer
| Term | Meaning | Example |
|---|---|---|
| Monomer | Small molecule that joins to form a polymer | Ethene |
| Polymer | Large molecule made of many repeat units | Poly(ethene) |
| Repeat unit | The section of the polymer chain that repeats | -CH2-CH2- |
From Monomer to Polymer
In addition polymerisation, many alkene monomers join together. The carbon-carbon double bond opens, and the monomers form a long chain.
For example, ethene can form poly(ethene). You do not need to make this more complicated than it is: many small molecules join to make one large molecule.
Exam-Style Worked Example: Ethene to Poly(ethene)
Question: Ethene can form poly(ethene). Describe what happens to the ethene molecules during addition polymerisation.
Worked answer:
Many ethene monomers join together. The carbon-carbon double bond in each ethene molecule opens, allowing the molecules to link into a long chain. The polymer formed is poly(ethene), made of many repeat units.
How marks are awarded:
| Mark | What the examiner is looking for |
|---|---|
| 1 | Uses the word monomer or many ethene molecules |
| 1 | States that the C=C double bond opens |
| 1 | Describes formation of a long-chain polymer or repeat units |
This is stronger than only writing "ethene becomes plastic" because it explains the bonding change.
How to Spot the Repeat Unit
The repeat unit is the smallest section of the polymer chain that repeats. In displayed formula questions, brackets usually show the repeating part, and n means there are many repeats.
Check that the bonds continue out of the bracket. Those open bonds show that the polymer chain keeps going.
What Students Usually Confuse This With
| Confusion | Fix |
|---|---|
| Monomer vs polymer | Monomer is the small starting molecule; polymer is the long chain |
| Repeat unit vs monomer | The repeat unit is the section shown inside the polymer chain |
| Addition vs general joining | Addition polymerisation usually starts from an alkene double bond |
Mini Practice Set
| Question | Quick check |
|---|---|
| What is the small molecule that forms a polymer called? | Monomer |
| What does n mean outside a polymer bracket? | Many repeating units |
| What happens to the C=C bond in addition polymerisation? | It opens so monomers can join |
Practice This Topic
Try this exam-style question:
State the name given to the small molecules that join together to form polymers, and explain what a repeat unit represents.
Answer guide:
- The small molecules are called monomers.
- Monomers join together to form a long-chain polymer.
- A repeat unit is the section of the polymer chain that repeats many times.
- In addition polymers, the monomer double bond opens to form the chain.
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FAQ
What is a polymer in Cambridge IGCSE Chemistry?
A polymer is a large molecule made from many repeating units. Polymers form when many small monomer molecules join together in a chain.
What is a monomer?
A monomer is a small molecule that can join with other monomers to form a polymer. In addition polymerisation, alkene monomers such as ethene join together.
What is a repeat unit?
A repeat unit is the section of the polymer chain that repeats many times. In displayed formula questions, it is usually shown inside brackets with an n outside.
What happens to the double bond in addition polymerisation?
The carbon-carbon double bond in the alkene monomer opens. This allows many monomers to join together into a long-chain polymer.
How do you draw a polymer repeat unit?
Draw the smallest repeating section of the chain, place it inside brackets and put n outside the brackets. The bonds should continue through the bracket edges to show the chain extends.
Final Takeaway
Keep the vocabulary clean: monomer = small starting molecule, polymer = large chain, repeat unit = repeating section.
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