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2.4.4Additional lessons

Polymers — IB Chemistry HL Revision Notes & Topical Questions

5 min read 6 parts Lesson 46 of 172

📚 IB Chemistry HL Hub / Topical Revision
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Part 1 of 6Monomers and Polymers

2.4.4 Polymers

Polymers are very large molecules built from many small monomer units.

Monomer
  • A small molecule that can join to many others to form a polymer.
Polymer
  • A macromolecule made of many repeating units joined by covalent bonds.
Repeating unit
  • The smallest section that, repeated, makes up the whole chain.
Degree of polymerisation
  • The number of repeating units, n, in a polymer chain.

Part 2 of 6Types of Polymerisation

Types of Polymerisation
Addition
  • Monomers with C=C double bonds join end to end and no other product forms.
Condensation
  • Monomers with two functional groups join and a small molecule such as water is lost.
Polyamides
  • Formed from a diamine and a dicarboxylic acid, such as nylon-6,6 from hexane-1,6-diamine and hexanedioic acid.
Polyesters
  • Formed from a diol and a dicarboxylic acid, with water released.

Part 3 of 6Structure and Properties

1
Longer chains have stronger London forces between them, so the polymer is stronger and has a higher melting point.
2
Branched chains pack poorly, so low-density poly(ethene) is softer than linear high-density poly(ethene).
3
Linear chains pack closely, giving higher density, rigidity and strength.
4
Cross-links between chains, as in thermosetting plastics, make the material rigid and unable to be remoulded.
5
Polar groups, such as the amide links in nylon, give hydrogen bonding between chains and extra strength.

Part 4 of 6Uses and Concerns

Packaging
  • Poly(ethene) is used for bags and films because it is light, flexible and cheap.
Fibres
  • Nylon and polyesters are spun into strong fibres for clothing and ropes.
Environment
  • Many polymers do not biodegrade, so they persist as waste and need recycling.

Part 5 of 6Worked Example

Question

  • Poly(ethene) has 500 repeating units.
  • Calculate its relative molecular mass, using C = 12.01 and H = 1.01.

  1. The repeating unit is C₂H₄2 × 12.01 + 4 × 1.01 = 28.06.
  2. Multiply by the number of units28.06 × 500.

Answer Mr ≈ 1.40 × 10⁴.

Part 6 of 6Exam Checklist & Tips

2.4.4 – Key Points for the Exam

  • Monomer A small molecule that joins with others to form a polymer.
  • Polymer A long chain molecule made of repeating units.
  • Condensation polymerisation Polymerisation in which a small molecule is eliminated at each link.
  • Thermosetting A polymer that sets rigid and cannot be reshaped by heating.

Common Mistakes

  • Thinking polymers have a single fixed molar mass; chains vary in length.
  • Saying condensation polymers form from alkenes.
  • Forgetting to remove water when forming condensation links.
  • Claiming all plastics soften on heating.

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