OxfordAQA International GCSE Chemistry: Synthetic and naturally occurring polymers (9202) – Revision Notes

For full explanations and longer worked examples, read the polymers study guide first.

These notes condense section 3.10.2 Synthetic and naturally occurring polymers of the OxfordAQA International GCSE Chemistry (9202) specification, Version 6.2, for exams May/June 2018 onwards. Statements a to f are all here. With no tiers in this course, every line applies to you, and both papers may test it. The section has no required practical, though the bromine water test from 3.10.1.3 links in.

Use the OxfordAQA chemistry hub to reach other topics and the printable checklist to tick off statements. Test yourself afterwards with the polymers practice questions, or find weak topics with a free diagnostic.

Key words

Term	Meaning
Monomer	A small molecule that can join with many others to make a polymer
Polymer	A very large molecule formed from many monomers joined together
Polymerisation	The reaction in which monomers join to form a polymer
Unsaturated	Has a C=C double bond, as every alkene monomer does
Saturated	Only single C–C bonds (the polymer chain)
Thermosoftening polymer	Separate tangled chains; softens and melts on heating
Thermosetting polymer	Chains joined by cross-links; does not melt on heating
Cross-link	A covalent bond (or short bridge of atoms) joining one chain to another
Biodegradable	Can be broken down by microbes
LD / HD poly(ethene)	Low-density and high-density forms of poly(ethene), made under different conditions

3.10.2 a: Polymerisation of alkenes

What happens: one bond of each C=C opens, and the freed bonds link neighbouring monomers into one long chain. Nothing else is made.

Examples named in the specification: ethene → poly(ethene); propene → poly(propene).

Method in steps: monomer → polymer

1. Copy the two C=C carbons with all their attached atoms or groups.
2. Make the C=C a C–C.
3. Draw one bond out of each side.
4. Bracket the unit with the outgoing bonds crossing the brackets; write n after it.
5. Put n in front of the monomer in the equation.

Method in steps: polymer → monomer

1. Pick out the two-carbon repeating unit.
2. Delete the two bonds that cross the brackets, and the n.
3. Put a double bond between the two carbons.
4. Keep every side group where it was.

Worked reminder

Monomer: CH₂=CH–CN. Chain unit: –( CH₂–CH(CN) )n–. The CN group stays on its carbon as a side group.

Chain unit given as –( CH(CH₃)–CH₂ )n–. Monomer: CH(CH₃)=CH₂, which is propene written the other way round.

Numbers you can be asked

• Mr of one chain = number of units × Mr of monomer.
• Mass of polymer formed = mass of monomer used (only one product).
• Example: a poly(ethene) chain of 4000 units has Mr = 4000 × 28 = 112 000.

Monomer or polymer? Use bromine water

Sample	Bromine water
Alkene monomer	Orange → colourless
Polymer chain	Stays orange

3.10.2 b: Same monomer, different polymer

Properties depend on (1) the monomer used and (2) the conditions, including the catalyst.

	LD poly(ethene)	HD poly(ethene)
Monomer	Ethene	Ethene
Conditions	Very high pressure, high temperature, a starter substance	Lower pressure, milder temperature, a special catalyst
Chains	Highly branched	Mainly unbranched
Packing	Loose	Close
Forces between chains	Weaker	Stronger
Properties	Lower density, flexible, softens at lower temperature	Higher density, rigid, softens at higher temperature
Suits	Thin films, squeezable items	Rigid containers, pipes

Chain of reasoning to learn: conditions → branching → packing → strength of intermolecular forces → properties.

3.10.2 c: Thermosoftening vs thermosetting

	Thermosoftening	Thermosetting
Structure	Individual, tangled chains	Chains joined by cross-links
Holding chains together	Weak intermolecular forces	Strong covalent cross-links
On heating	Forces overcome, chains slide, polymer softens and melts	Cross-links hold chains in place; no melting (chars if heated strongly)
Reshape by heating?	Yes	No

Model sentence for the explanation

“The chains in a thermosoftening polymer are held to each other only by weak intermolecular forces. Little energy is needed to overcome these, so on heating the chains slide past one another and the polymer softens.”

What breaks: the forces between chains. What does not break: the covalent bonds within each chain.

3.10.2 d: New materials and uses

You do not need names. Be ready to match a property to a use.

Area	Property that matters
New packaging	Light, tough, keeps food dry and fresh
Waterproof fabric coatings	Keep rain out, let sweat vapour escape
Dental polymers	Hard, durable, safe in the mouth, set after shaping
Wound dressings	Absorb fluid, keep the wound moist, peel off cleanly
Hydrogels	Hold large amounts of water without dissolving
Smart materials, such as shape memory polymers	Change in response to their surroundings, for example returning to a set shape when warmed

How new materials are developed: identify the need → choose monomers and conditions → make and test samples → improve → consider cost, safety and disposal.

3.10.2 e and f: Waste and cornstarch

Problem. Many polymers are not biodegradable, so microbes do not break them down.

• Landfill: waste persists for a long time and sites fill up.
• Litter: lasts for years, looks unsightly, harms wildlife.

Two responses in the specification.

	Polymer + cornstarch bag	Biodegradable plastic from cornstarch
What it is	An ordinary polymer mixed with cornstarch	A plastic made from cornstarch itself
What happens	Microbes break down the starch, so the bag falls apart more easily	The whole material can be broken down by microbes
Drawback	Polymer fragments may remain	Needs land to grow the crop; breakdown needs suitable conditions

Cornstarch comes from plants, so it is renewable; most synthetic polymers come from crude oil.

Model answers to learn

Why HD poly(ethene) is more rigid than LD poly(ethene). “HD poly(ethene) is made with a different catalyst and under different conditions, so its chains have few branches. Unbranched chains pack closely together, so the intermolecular forces between them are stronger and the chains are harder to move.”

Evaluating a polymer–cornstarch bag. “Microbes digest the starch, so the bag breaks into pieces more quickly than an ordinary plastic bag and causes less long-lasting litter. However, the polymer pieces left behind may still not biodegrade.”

Must-know distinctions

• Monomer vs polymer: the monomer has C=C and decolourises bromine water; the polymer does not.
• Intermolecular forces vs cross-links: weak forces between separate chains vs strong covalent bonds joining chains.
• Thermosoftening vs thermosetting: melts and can be remoulded vs does not melt.
• LD vs HD poly(ethene): same monomer, different conditions and catalysts, different branching.
• Breaks down vs biodegrades: a polymer–cornstarch bag falls into pieces; a biodegradable plastic is digested by microbes.

Quick self-test

1. Define the term polymer.
2. Name the polymer made from propene.
3. Draw the chain unit formed from CH₂=CHBr.
4. Give the monomer of the chain unit –( CHF–CF₂ )n–.
5. A poly(propene) chain has Mr 105 000. How many propene units does it contain? (Ar: C = 12, H = 1)
6. A factory uses 3.6 tonnes of ethene and all of it polymerises. What mass of poly(ethene) forms?
7. Why do LD and HD poly(ethene) have different densities when both are made from ethene?
8. What holds the chains together in a thermosoftening polymer?
9. Why does a thermosetting polymer not melt when heated?
10. What does biodegradable mean?
11. Give one advantage and one disadvantage of bags made from polymers and cornstarch.
12. State what you would see when bromine water is shaken with a sample of poly(ethene).

Answers

1. A very large molecule made from many small molecules (monomers) joined together.
2. Poly(propene).
3. –( CH₂–CHBr )n–, with single C–C, bonds through the brackets and n after.
4. CHF=CF₂.
5. Mr of C₃H₆ = 42; 105 000 ÷ 42 = 2500 units.
6. 3.6 tonnes: the polymer is the only product, so mass is conserved.
7. Different conditions and catalysts give chains with different amounts of branching; less branching lets chains pack more closely, so HD is denser.
8. Weak intermolecular forces.
9. Strong covalent cross-links join the chains so they cannot slide past each other.
10. It can be broken down by microbes.
11. Advantage: breaks down more easily, so less long-lasting waste. Disadvantage: the polymer part may remain as fragments (or land is used to grow the crop).
12. It stays orange: the chain is saturated.

Where marks are usually lost

• Drawing the polymer with C=C still in the chain.
• Missing the bonds through the brackets, or missing n.
• Placing a side group, such as Cl or CH₃, in the main chain.
• Saying heating breaks covalent bonds in a thermosoftening polymer.
• Describing cross-links as weak forces, or intermolecular forces as bonds within a chain.
• Writing that HD and LD poly(ethene) use different monomers.
• Listing uses of new materials without linking each to a property.
• Saying a polymer–cornstarch bag disappears completely.
• Defining biodegradable without mentioning microbes.

Next steps

• Polymers practice questions
• Back to the polymers study guide
• Intermolecular forces: bonding and structure notes for intermolecular forces
• Relative formula mass and moles: quantitative notes
• Carbon compounds as fuels for cracking and alkenes
• All free 10-minute diagnostics

Official syllabus

OxfordAQA International GCSE Chemistry (9202) specification, Version 6.2, for exams May/June 2018 onwards, published by OxfordAQA. Unit covered: 3.10.2 Synthetic and naturally occurring polymers, statements a to f.