Learning objectives
By the end of the activity, students should be able to:
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Describe addition polymerisation as the joining of many alkene monomers to form a long-chain polymer.
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Explain how carbon–carbon double bonds open to form new carbon–carbon single bonds.
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Identify monomers, polymers and repeating units from displayed structural formulae.
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Draw repeating units using correct brackets, continuation bonds and the symbol n.
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Explain why polymers are solids at room temperature in terms of covalent bonds and intermolecular forces.
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Compare the structures and properties of LDPE and HDPE.
Using the simulation
1. Build a polymer chain
Begin with ethene and ask students to predict what happens to the C=C double bond during addition polymerisation. Run the animation to show monomers joining together through new covalent bonds. Repeat with propene, chloroethene or tetrafluoroethene to demonstrate that different monomers produce different polymers.
Emphasise that no atoms are lost during addition polymerisation and that no small molecules are produced.
2. Understanding repeating units
Switch to the repeating-unit diagrams and explain the purpose of the square brackets, continuation bonds and n. Show how the two carbon atoms from the original C=C double bond become part of the polymer backbone.
Use the Find the monomer activity to challenge students to work backwards from a polymer repeating unit. Encourage them to draw their answers before revealing the original monomer.
3. Explaining polymer properties
Use the molecular structure visualisation to distinguish between strong covalent bonds within individual polymer chains and intermolecular forces acting between separate polymer molecules.
Demonstrate heating and ask students to explain why the polymer chains can move past one another without the covalent bonds along the polymer backbone breaking.
Reinforce that polymer molecules are very large and that the overall intermolecular attractions between chains are relatively strong, meaning polymers are generally solids at room temperature.
4. Comparing LDPE and HDPE
Display the branched LDPE chains alongside the more linear HDPE chains. Use the packing controls to demonstrate why HDPE chains can pack more closely together.
Ask students to explain why:
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HDPE has a higher density than LDPE.
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HDPE generally has a higher melting temperature.
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Greater branching reduces how closely polymer chains can pack.
Emphasise the distinction between the two explanations: density depends on the mass of material within a given volume, while melting behaviour depends on the energy required to overcome intermolecular attractions.
5. Checking understanding
Use the interactive questions as a class plenary or independent assessment. Students should practise recognising polymers from diagrams, identifying repeating units, predicting monomers and explaining polymer properties using appropriate scientific terminology.
Common misconceptions
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The double bond disappears completely: The C=C double bond becomes a C–C single bond in the polymer backbone.
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Atoms are removed during polymerisation: All atoms from the monomers are retained in the addition polymer.
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The repeating unit is the entire polymer: The repeating unit represents the smallest structural section that repeats along the chain.
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Melting breaks covalent bonds: Melting involves overcoming intermolecular attractions, not breaking the covalent bonds within polymer chains.
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HDPE has stronger covalent bonds than LDPE: The main differences arise from branching, chain packing and intermolecular attractions, rather than fundamentally different backbone covalent bonds.
Suggested classroom use
Suitable for: GCSE Chemistry – AQA Combined Science and Separate Chemistry.
Recommended duration: 15–25 minutes.
Use the simulation during initial teaching of addition polymerisation, as a visual explanation of polymer structure and properties, or for revision before examination questions involving repeating units and the comparison of LDPE and HDPE.
For maximum impact, pause the animations regularly and ask students to predict what will happen before revealing the next stage. Encourage students to reproduce the structural diagrams on mini-whiteboards, using the correct examination notation.