AS Chemistry: Addition Polymerisation — Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Addition Polymerisation revision notes

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Section A

1. Explain why an alkene can undergo addition polymerisation but a saturated alkane cannot. [2]

2. Give the repeat unit of poly(propene) and of poly(tetrafluoroethene), showing the bonds extending through the brackets. [2]

Section B

3. A section of a polymer chain is shown as …–CH₂–CHCl–CH₂–CHCl–…

(a) Deduce and name the monomer. [2] (b) Draw or describe the repeat unit, stating the two conventions that must be shown. [3]

4. (Manufacturing conditions and catalyst are background — Topic 20.1 is limited to describing addition polymerisation, deducing repeat units/monomers and disposal, and does not cover manufacturing process detail; chain structure and properties below remain examinable.) Compare low-density and high-density poly(ethene) in terms of manufacturing conditions, chain structure and physical properties.

5. Explain, in terms of intermolecular forces between polymer chains, why poly(chloroethene) is more rigid than poly(ethene). [3]

6. Explain why addition polymers are chemically very unreactive, in terms of the bonds in their carbon backbone, and describe the environmental problem this same unreactivity causes. [4]

7. Explain what a plasticiser does and how it works at the molecular level. [3]

8. Poly(chloroethene) (PVC) waste is sometimes disposed of by incineration.

(a) Name the toxic and corrosive gas produced when PVC is burned, and explain why it forms. [2] (b) Explain why burning a plain poly(alkene) such as poly(ethene) does not produce this same gas. [2]

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Answers

1. The alkene has a C=C double bond, one bond of which can break to allow the monomer to join to others [1]. A saturated alkane has only single C–C bonds, so there is no bond available to open up and join to a neighbouring molecule [1].

2. Poly(propene): –[CH₂–CH(CH₃)]ₙ– [1]. PTFE (poly(tetrafluoroethene)): –[CF₂–CF₂]ₙ– [1]. In each case the repeat unit is derived directly from the monomer by opening the C=C double bond, with the bonds on either side extending through the square brackets and the subscript n written outside the bracket to show the chain continues indefinitely.

3. (a) CH₂=CHCl [1]; chloroethene [1]. (b) –[CH₂–CHCl]– [1] with the bonds drawn through the brackets to show the chain continues [1] and n written outside the bracket [1].

4. (Manufacturing detail below is background context, not AS-examinable.) LDPE — made at high pressure, giving branched chains that cannot pack closely together; the result is low density, flexible and with a lower softening point, used for bags and films [1]. HDPE — made with a Ziegler–Natta catalyst at low pressure, giving straight, unbranched chains that pack closely; the result is higher density, more rigid and with a higher softening point, used for pipes and crates [1]. (The examinable content is the link between chain branching and the resulting density/rigidity — the catalyst and pressure conditions are included only as background.)

5. The C–Cl bond is polar, so poly(chloroethene) chains have permanent dipole–dipole attractions between neighbouring chains [1], as well as London forces [1]. Poly(ethene) is non-polar with only London forces, so its chains slide past one another far more easily, making it flexible [1].

6. The backbone consists of strong, non-polar C–C and C–H bonds [1] with no polar bonds for nucleophiles, water or enzymes to attack [1]. Consequently the polymer is non-biodegradable and persists for hundreds of years in landfill [1], accumulating in the environment and in the oceans, where it harms wildlife [1].

7. A plasticiser makes a rigid polymer flexible and softer [1]. Its small molecules sit between the polymer chains, forcing them further apart [1], which weakens the intermolecular attractions between chains so they can slide over one another more easily [1].

8. (a) Hydrogen chloride gas [1]. It forms because PVC’s repeat unit contains chlorine (from the C–Cl bond), which is released during combustion — a gas not produced when burning a hydrocarbon polymer that contains only carbon and hydrogen [1]. (b) Poly(ethene) contains only carbon and hydrogen, so burning it produces the same products as burning any hydrocarbon — carbon dioxide, carbon monoxide and soot, depending on how complete the combustion is [1]; there is no chlorine present to form hydrogen chloride [1].

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Where marks are usually lost

• Leaving the double bond in the repeat unit.
• Forgetting the bonds must extend through the brackets, or forgetting the “n” outside them altogether.
• Attributing HDPE’s rigidity to stronger covalent bonds rather than closer packing of its unbranched chains.
• Saying a plasticiser breaks the polymer chains.
• Assuming all plastic combustion produces the same products — PVC and other chlorine- or nitrogen-containing polymers release additional toxic gases (HCl, and potentially other chlorinated by-products) that a plain hydrocarbon polymer does not.
• Explaining the disposal problem purely in terms of “plastic doesn’t rot” without linking it back to the specific bonds (strong, non-polar C–C and C–H) that make the backbone resistant to attack by everyday reagents in the first place.

Why the disposal problem and the chemical unreactivity are the same fact

Addition polymers are useful precisely because they are chemically inert — durable, resistant to rot and corrosion — and this is exactly the same property that makes them persist in landfill for a very long time. A strong answer on disposal links both halves of this together rather than treating “why they’re useful” and “why they’re a problem” as unrelated points: the saturated carbon backbone that most everyday reagents can’t attack is the single underlying cause of both.

For condensed recall notes on this topic, see the Addition Polymerisation revision notes; for the full explanation with additional worked examples, see the Addition Polymerisation study guide.