Addition polymerisation is used to make poly(ethene) from ethene monomers. Explain how the structure of ethene makes addition polymerisation possible, and describe what happens to the monomers during the reaction to form a polymer.

AQA A-Level Chemistry (7405) — 3.3.12 Polymers (A-Level only) · Explain · 5 marks · View as Markdown

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

Ethene is produced by cracking longer-chain hydrocarbons. It is an unsaturated hydrocarbon with the general formula C₂H₄.

Model answer (5 marks)

1. Ethene contains a carbon‑carbon double bond (C=C).
2. The double bond makes ethene unsaturated.
3. During addition polymerisation the C=C double bond opens or breaks.
4. The broken bonds allow many ethene monomers to join together, forming a long chain – a polymer.
5. No other product or small molecule is formed; addition polymerisation produces only the polymer.

Examiner tips

  • Use the exact terms ‘double bond’, ‘unsaturated’, ‘opens/breaks’, ‘join together’, ‘no other product’ to match the scheme.
  • Show the sequence: double bond → bond breakage → chain growth → only polymer produced.

Common mistakes

  • Confusing addition with substitution polymerisation; mentioning small molecules as by‑products.
  • Using vague language like ‘reacts’ without specifying the double bond opening.

Mark scheme (5 marks)

  1. Ethene contains a carbon-carbon double bond (C=C)
  2. The double bond makes ethene unsaturated
  3. The double bond opens / breaks during the reaction
  4. Many monomers join together (to form a long chain / polymer)
  5. No other product / small molecule is formed (addition polymerisation produces only the polymer)

Key terms in this question

addition polymerisation · monomer · polymer

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