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2.4.4Additional lessons

Polymers — IB Chemistry SL Revision Notes & Topical Questions

5 min read 6 parts Lesson 36 of 98

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

2.4.4 Polymers

Polymers are the basis of plastics, fibres and many natural materials.

Monomer
  • A small molecule that can join to many others to form a long chain.
Polymer
  • A very large molecule made of many repeating units joined by covalent bonds.
Repeat unit
  • The smallest section of the chain that repeats along the molecule.
Degree of polymerisation
  • This is the number of repeat units in a chain, which can be thousands.

Part 2 of 6Types of Polymerisation

Types of Polymerisation
Addition
  • Monomers with a C=C double bond open the double bond and join together without losing any atoms.
Condensation
  • Monomers join and release a small molecule such as water at each link, as in nylon and polyesters.
Natural polymers
  • Proteins, starch and DNA are polymers made by living organisms.
Synthetic polymers
  • Plastics such as poly(ethene) and PVC are made industrially from petrochemicals.

Part 3 of 6Structure and Properties

1
Polymer chains are long and may be tangled together, which makes the material flexible and strong.
2
Chains are attracted to one another by London dispersion forces, which add up over a long chain.
3
Chains with polar groups also have dipole-dipole forces and sometimes hydrogen bonds, which makes them stronger.
4
Long chains with many attractions give higher melting points than small molecules of similar composition.
5
Because there are no free ions or electrons, most polymers are electrical insulators.
PropertyReason
Flexiblechains slide and uncoil
Insulatorno free charge carriers
Low densitylight elements in loosely packed chains
Unreactivestrong C–C and C–H bonds

Part 4 of 6Environmental Issues

Persistence
  • Many synthetic polymers are not biodegradable because microorganisms cannot break the strong C–C bonds.
Waste
  • Large quantities of plastic accumulate in landfill and oceans, harming wildlife.
Microplastics
  • Plastic fragments break into tiny particles that enter food chains.
Responses
  • Recycling, reuse, reduced use and development of biodegradable alternatives help reduce the problem.

Part 5 of 6Worked Example

Question Explain why poly(ethene) is unreactive and persists in the environment.

  1. Poly(ethene) consists of long chains containing only C–C and C–H bonds.
  2. These bonds are strong and non-polar, so they are not easily attacked by reagents or by the enzymes of microorganisms.
  3. Consequently the plastic does not biodegrade quickly and remains for a very long time.

Answer Its strong, non-polar C–C and C–H bonds resist chemical and biological attack.

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 large molecule made from many repeating units.
  • Repeat unit The smallest group of atoms that repeats in a polymer chain.
  • Biodegradable Able to be broken down by living organisms.

Common Mistakes

  • Saying monomers and polymers have the same properties.
  • Confusing the repeat unit with the monomer, which may differ in bonding.
  • Forgetting that stronger intermolecular forces raise the melting point.
  • Assuming all plastics are biodegradable.

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