Ethene is an alkene produced by the cracking of crude oil fractions. Ethene can be used as a monomer to produce poly(ethene). Explain how the structure of ethene allows it to undergo addition polymerisation to form a long chain polymer.

OCR GCSE Chemistry A: Gateway Science (J248) — C6.2 Organic chemistry · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Poly(ethene) is one of the most widely used plastics in the world. It is produced from ethene, a small molecule obtained from cracking crude oil fractions.

Model answer (4 marks)

Ethene contains a C=C double bond, which is an unsaturated site.
The double bond opens during the addition reaction, breaking the C=C bond.
The opened bonds react with other ethene molecules, linking them together.
No small molecules are produced; the reaction gives only the long‑chain polymer.

Examiner tips

  • Mention the C=C double bond and its unsaturation first. Explain that the double bond breaks and the radicals add. Show that all monomers join, giving a single polymer chain. State that no by‑products are formed in addition polymerisation.

Common mistakes

  • Forgetting to mention the double bond. Saying the reaction produces small molecules. Using vague terms like "reacts" without describing the opening of the double bond.

Mark scheme (4 marks)

  1. Ethene contains a carbon–carbon double bond, making it unsaturated
  2. The double bond opens / breaks during the reaction
  3. Many ethene monomers join together to form a long chain
  4. No other molecules / small molecules are produced in addition polymerisation (no by-products lost)

Key terms in this question

addition polymerisation · monomer · alkene · long chain polymer

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