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Cambridge IGCSE Chemistry · 0620

Chapter 11: Organic Chemistry — Part 8

Topic 11.8 · Polymers

Fundamentals of Polymerisation

  • Polymers: Large molecules built from many smaller monomers joined by covalent bonds.
  • Addition Polymerisation: Occurs when unsaturated monomers (alkenes) join together to form a long chain; the double bond breaks to allow connection. Example: Poly(ethene) from ethene monomers.
  • Condensation Polymerisation: Occurs when monomers with two different functional groups react, producing a polymer and a small molecule (usually water).

Exam Traps

  • Do not call nylon or PET addition polymers — they are condensation polymers because water is released during formation.
  • Do not say the C=C bond remains in addition polymers — it must break so monomers can link; show [CH2–CH2]n with no double bonds in the backbone.

Specific Condensation Polymers

  • Nylon (Polyamide): Formed from a dicarboxylic acid and a diamine, connected by amide linkages (-CONH-).
  • PET (Polyester): Formed from a dicarboxylic acid and a diol, connected by ester linkages (-COO-). PET can be converted back into monomers by hydrolysis.
  • Proteins: Natural polyamides formed from amino acid monomers (containing an amine, carboxyl, and 'R' group).
Diagram showing the amide linkage in polyamides and the ester linkage in polyesters
Diagram 1: Diagram showing the amide linkage in polyamides and the ester linkage in polyesters

Exam Traps

  • Do not mix up monomer pairs — nylon needs a diamine; PET needs a diol (alcohol with two −OH groups).
  • Do not confuse amide linkages (−CONH−) in nylon with ester linkages (−COO−) in PET — the linkage type defines the polymer class.
  • Do not say proteins are polyesters — they are polyamides formed from amino acid monomers with amine and carboxyl groups.

Environmental Impact of Plastics

The durability of plastics leads to significant environmental challenges:

  • Disposal in landfill sites due to slow degradation.
  • Accumulation in oceans, harming aquatic life.
  • Formation of toxic gases when incinerated (e.g., HCl from PVC).

Exam Traps

  • Do not claim all plastics biodegrade quickly — their durability is the main environmental problem.
  • Do not name HCl from PET or nylon — toxic HCl on incineration is specifically associated with PVC (contains chlorine).

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