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IB DemystifiedMYP Sciences

Polymers and materials

Your water bottle, your clothes, the DNA in your cells and the starch in your lunch are all polymers: giant molecules built from thousands of small repeating units. Chemists can design polymers with almost any property, but the same durability that makes plastics useful makes them a lasting waste problem.

Recommended for MYP 5 · eAssessment priority · About 3 lessons · Criteria A, B, C and D

nCCHHHHethene (monomer)heat, pressure,catalystCCHHHHnpoly(ethene) (polymer)repeat unit
Figure 1. In addition polymerisation, the C=C double bonds open and monomers link into a long chain.
On this page
  1. Learning objectives
  2. Before you start
  3. Key vocabulary
  4. Understanding the ideas
  5. Addition polymers at a glance
  6. Polymers in the real world
  7. Worked examples
  8. In the eAssessment
  9. Check your understanding
  10. Practice questions
  11. Investigation
  12. Criterion-linked questions
  13. Challenge questions
  14. Topic check
  15. Review your mistakes
  16. Your progress

Learning objectives

By the end of this topic you should be able to:

  • explain addition polymerisation and draw repeat units from monomers
  • describe condensation polymers and natural polymers such as proteins, starch and DNA
  • relate the properties of polymers to their structure
  • compare thermosoftening and thermosetting plastics
  • analyse data on plastic strength and degradation
  • evaluate ways of reducing and managing plastic waste

Before you start

You will use these skills. If any feel shaky, review them first.

  • covalent bonding and intermolecular forces (see Covalent bonding and structures)
  • alkenes and cracking (see Hydrocarbons and fuels)
  • relative formula mass (see The mole and reacting masses)

Key vocabulary

Monomer
A small molecule that joins with others to form a polymer.
Polymer
A very large molecule made of many repeating units.
Addition polymerisation
Joining monomers with C=C double bonds, with no other product.
Condensation polymerisation
Joining monomers with two functional groups, releasing a small molecule such as water.
Thermosoftening / thermosetting
Plastics that soften when heated / have cross-links and do not melt.
Biodegradable
Able to be broken down by decomposers.

Understanding the ideas

  1. What is it?

    Polymers are long-chain molecules. Addition polymers, such as poly(ethene), poly(propene) and PVC, form when the C=C double bonds of alkene monomers open and link together. Condensation polymers, such as nylon, polyester, proteins and starch, form when monomers with two reactive groups join, releasing water.

  2. Why does it happen?

    A polymer's properties depend on its structure. Long chains held by weak forces make thermosoftening plastics that melt and can be remoulded; covalent cross-links make rigid thermosetting plastics. Unbranched chains pack closely, giving denser, stronger plastics such as HDPE.

  3. How do we know?

    Chemists test polymers' strength, flexibility, melting point and breakdown, and design new materials such as compostable PLA made from plant starch.

  4. Why does it matter?

    Plastics are light, strong, cheap and durable, which is why they are everywhere, and why waste plastic lasts for centuries, blocks drains, harms wildlife and breaks into microplastics.

  5. What does it connect to?

    Polymers link to bonding and fuels in chemistry, to proteins, DNA and digestion in biology, and to waste and sustainability in Earth science.

Addition polymers at a glance

  • Ethene CH₂=CH₂ → poly(ethene), repeat unit –CH₂–CH₂–: bags, bottles.
  • Propene CH₂=CHCH₃ → poly(propene), –CH₂–CH(CH₃)–: crates, ropes.
  • Chloroethene CH₂=CHCl → PVC, –CH₂–CHCl–: pipes, window frames.
  • Tetrafluoroethene CF₂=CF₂ → PTFE, –CF₂–CF₂–: non-stick coatings.
  • To draw a repeat unit: change C=C to C–C, keep the other atoms, add brackets, extension bonds and n.

Polymers in the real world

Informal waste pickers collect and sort much of the plastic that is recycled in South Asian cities. Islamabad banned thin plastic bags in 2019. Researchers are developing plastics from sugar cane, starch and even crop waste.

Worked examples

Example 1: drawing a repeat unit

Draw the repeat unit of the polymer made from propene, CH₂=CHCH₃.

  1. Change the C=C to a single bond.
  2. Keep the H atoms and the CH₃ group on the same carbons.
  3. Add brackets, extension bonds and n: –[CH₂–CH(CH₃)]ₙ–.

Example 2: number of repeat units

A PVC chain has Mr 125 000. How many repeat units does it have?

  1. Repeat unit –CH₂–CHCl– = 24 + 3 + 35.5 = 62.5.
  2. 125 000 ÷ 62.5 = 2000 repeat units.

In the eAssessment

Polymer questions combine structures, properties and waste decisions. Expect:

  • Draw and deduce repeat units and monomers.
  • Explain properties using chains, forces and cross-links.
  • Process data on strength and degradation.
  • Discuss bans, recycling, incineration and bioplastics.

Common ways to lose marks: keeping the double bond in a repeat unit; saying polymers melt because their covalent bonds break; assuming "biodegradable" means it breaks down anywhere; and forgetting the brackets and n.

Check your understanding

Quick questions on the ideas above. Try each one before using a hint.

Practice questions

Show

Investigation: sorting plastics by density

Partially guided investigation · about 40 minutes · pairs

Research question
Can small pieces of common plastics (PET bottle, HDPE milk bottle, PP bottle cap, PS cup, PVC pipe off-cut) be identified by whether they float or sink in water and in salt solutions of different densities?
Scientific background
Recycling plants sort plastics by density. Pieces float in a liquid that is denser than them and sink in one that is less dense. Poly(propene) and poly(ethene) are less dense than water; PET and PVC are denser.
Hypothesis
Write your own prediction, with a scientific justification.
Variables
Identify your independent, dependent and control variables, and explain how you will control them.
Apparatus
Small pieces (about 1 cm²) of labelled plastics, beakers, water, table salt, balance, measuring cylinder, stirring rod, a drop of washing-up liquid.
Method
  1. Test each piece in water with a drop of washing-up liquid (to remove air bubbles) and record whether it floats or sinks.
  2. Make salt solutions of known densities (for example 1.05, 1.10 and 1.20 g/cm³) by dissolving measured masses of salt in water.
  3. Test each piece in each solution.
  4. Use the results to place the plastics in order of density and compare with the recycling codes.

Safety. Cut plastic pieces carefully with scissors, not blades; wipe up spills at once; wash hands afterwards.

Then evaluate: which plastics could not be separated this way, and how could a recycling plant separate them?

Criterion-linked questions

Criterion B: inquiring and designing

Criterion C: processing and evaluating

Criterion D: reflecting on the impacts of science

Challenge questions

Harder problems in unfamiliar contexts. Plan before you calculate.

Topic check

Five questions picked at random from the whole topic. Take a new set whenever you like.

Review your mistakes

Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.

Your progress

Tracked separately for each skill, on this device only.

SkillCorrectStatus

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