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

This guide teaches section 3.10.2 Synthetic and naturally occurring polymers from the OxfordAQA International GCSE Chemistry (9202) specification, Version 6.2, for exams May/June 2018 onwards. The section has six statements, lettered a to f, and every one is covered below with worked examples. There are no tiers in this qualification, so you need all of it, and either of the two papers can test it.

Section 3.10.2 has no required practical of its own, but the bromine water test for a C=C double bond from 3.10.1.3 is used below to tell a monomer from its polymer.

All topics for this course are on the OxfordAQA chemistry hub, and the course checklist lists every statement for printing. To find weak spots before you start, try a free diagnostic. When you have finished, use the polymers revision notes to condense this page and the polymers practice questions to test yourself.

Statements in this unit

Statement	Skills you need
3.10.2 a	Describe polymerisation of alkenes such as ethene and propene; recognise monomer and polymer structures; represent the polymer formed from any given alkene monomer
3.10.2 b	Explain that polymer properties depend on the monomer and on the reaction conditions, using low-density (LD) and high-density (HD) poly(ethene)
3.10.2 c	Describe thermosoftening and thermosetting polymers; explain thermosoftening behaviour using intermolecular forces
3.10.2 d	Discuss how new polymer materials are developed and used (packaging, waterproof fabric coatings, dental polymers, wound dressings, hydrogels, smart materials)
3.10.2 e	Explain that many polymers are not biodegradable and the problems this causes in landfill and as litter
3.10.2 f	Describe bags made from polymers and cornstarch, and biodegradable plastics made from cornstarch

The specification says you need not recall named examples for statements d and e, and no further detail of polymerisation is required beyond statement a. The focus is drawing, explaining and evaluating.

3.10.2 a: Making polymers from alkenes

A monomer is a small molecule. A polymer is a very large molecule made when many monomers join together. The reaction is called polymerisation.

Alkenes can act as monomers because each has a C=C double bond. During polymerisation one of the two bonds in each C=C breaks, and each carbon uses that spare bond to join the next monomer. The result is a long chain of single C–C covalent bonds.

The specification shows the reaction for ethene like this:

     H   H              H   H
     |   |              |   |
 n   C = C     →     —( C — C )—
     |   |              |   |
     H   H              H   H   n

Method: drawing a polymer from any alkene

1. Draw the two carbons of the C=C in a row, with every other atom or group attached exactly as in the monomer.
2. Change the double bond to a single bond.
3. Add a bond line out of each end, through the brackets, to show the chain carries on.
4. Write n after the closing bracket, matching the n in front of the monomer: n monomers make one chain of n units.

Worked example 1: poly(propene)

Propene is CH₂=CH–CH₃. One carbon of the double bond carries two H atoms; the other carries one H and a CH₃ group, which stays as a side branch.

     H   CH₃            H   CH₃
     |   |              |   |
 n   C = C     →     —( C — C )—
     |   |              |   |
     H   H              H   H   n

Never draw all three carbons in a line inside the brackets; only the two carbons of the double bond form the chain.

Worked example 2: an unfamiliar monomer

You are given the monomer CH₃–CH=CHCl. One carbon of the double bond carries an H and a CH₃; the other carries an H and a Cl. The polymer is –( CH(CH₃)–CHCl )n–, drawn by the same method. Only the side groups change; you do not need the polymer’s name.

Working backwards: monomer from polymer

Given a section of chain, find the two-carbon repeating section, remove the bonds that cross the brackets, and put back a double bond between the two carbons. For a repeat of –CH₂–CH(CN)–, the monomer is CH₂=CH–CN.

Linking to the bromine water test

The monomer is unsaturated, so it turns bromine water from orange to colourless (3.10.1.3 d). The polymer chain has only single C–C bonds; it is saturated, so it leaves bromine water orange.

Worked example 3: masses and chain size

A chemist makes poly(propene) chains each containing 3200 propene units. Find the relative formula mass of one chain. Take Ar of C as 12 and of H as 1.

• Mr of propene, C₃H₆ = (3 × 12) + (6 × 1) = 42
• No atoms are lost when the units join, so each unit in the chain has Mr 42.
• Mr of chain = 3200 × 42 = 134 400

Because the polymer is the only product, the mass of polymer equals the mass of monomer that reacted: 50 kg of propene gives at most 50 kg of poly(propene).

3.10.2 b: Properties depend on monomer and conditions

• What it is made from. Different monomers give different side groups, which change how chains pack. Poly(ethene) and poly(propene) differ for this reason.
• How it is made. With the same monomer, a different catalyst, temperature and pressure changes the shape of the chains.

The specification’s example is poly(ethene), made from ethene in two different ways.

	LD poly(ethene)	HD poly(ethene)
How it is made	Very high pressure and high temperature, with a substance added to start the reaction	Much lower pressure and a milder temperature, with a special catalyst
Chain shape	Many branches along each chain	Mostly straight chains with few branches
Packing	Branches stop chains lying close together	Straight chains pack closely
Density	Lower	Higher
Typical behaviour	Soft, flexible, softens at a lower temperature	Stiffer, stronger, softens at a higher temperature

The reasoning chain: conditions, then branching, then packing, then strength of forces between chains.

Worked example 4: choosing the right grade

Which grade suits thin, stretchy food wrap, and which suits rigid crates washed in hot water?

• Food wrap: LD poly(ethene). Branched chains cannot pack closely, so forces between chains are weaker and the chains slide past each other.
• Crates: HD poly(ethene). Closely packed straight chains have stronger forces between them, so the plastic is rigid and softens at a higher temperature.

3.10.2 c: Thermosoftening and thermosetting polymers

Thermosoftening polymers are made of individual polymer chains, tangled together like cooked spaghetti. Each chain is held together by strong covalent bonds, but there are no bonds joining one chain to the next. The chains are held to each other only by weak intermolecular forces.

When you heat a thermosoftening polymer:

1. The chains gain energy.
2. The weak intermolecular forces between chains are overcome.
3. The chains can slide past each other, so the polymer softens and melts.
4. The covalent bonds inside each chain are not broken.

On cooling, the forces re-form and the polymer sets again, so it can be reshaped by heating.

Thermosetting polymers have cross-links between their chains. A cross-link is a covalent bond, or a short chain of atoms, joining one chain to another. These are strong bonds, so the chains cannot slide apart. Heating does not make a thermosetting polymer melt. If heated strongly enough it breaks down or chars instead.

Worked example 5: a model explanation

Question: A bowl made from a thermosoftening polymer goes soft in a hot oven, but a bowl made from a thermosetting polymer does not. Explain the difference.

Answer: In the thermosoftening polymer, the chains are separate and held together only by weak intermolecular forces. Heat overcomes these forces so the chains move past each other and the bowl softens. In the thermosetting polymer, the chains are joined by strong covalent cross-links, which do not break at oven temperature, so the chains stay in place and the bowl keeps its shape.

The answer names the force or bond in each case and says what happens to it.

3.10.2 d: New materials and their uses

The specification lists new packaging materials, waterproof coatings for fabrics, dental polymers, wound dressings, hydrogels, and smart materials including shape memory polymers. You do not need names; you need to link a material’s properties to its use and think about how new materials are developed.

Use	Property needed
Packaging	Light, strong, keeps out water and air
Waterproof fabric coating	Stops liquid water getting in but lets water vapour from sweat escape
Dental polymer	Hard, not attacked by mouth chemicals, can be shaped then set
Wound dressing	Keeps the wound moist, does not stick, lets oxygen through
Hydrogel	Holds a lot of water; its cross-linked network swells but does not dissolve
Shape memory polymer	Returns to a set shape when warmed

How new materials are developed

A new polymer starts from a need. Chemists choose monomers and conditions that should give the right chain structure, make small samples, test properties such as strength, flexibility and water uptake, then adjust the recipe. Safety, cost and disposal are judged too.

Worked example 6: suggesting properties

A hospital wants a dressing for burns. Suggest two properties the polymer should have and explain why.

• It should absorb and hold water, like a hydrogel, so the wound stays moist and fluid from the burn is soaked up.
• It should not stick to the wound, so it can be removed without damaging new skin.

3.10.2 e: Polymers and waste

Many polymers are not biodegradable: they are not broken down by microbes. A discarded item made from one of these polymers can therefore last for a very long time, whether it is buried or left in the open.

This causes problems with waste disposal:

• Landfill sites. Polymer waste does not rot, so it fills space that is running out, and new sites use more land.
• Litter. Discarded plastic lasts for years, spoils streets, beaches and countryside, and harms animals that swallow it or get tangled in it.

3.10.2 f: Cornstarch and biodegradable plastics

Two approaches reduce the problem:

1. Bags made from polymers and cornstarch. Starch is a naturally occurring polymer that microbes can digest. When the starch in the bag is broken down, the bag falls apart more easily into smaller pieces.
2. Biodegradable plastics made from cornstarch. Here the plastic itself is made from cornstarch, so the whole material can be broken down by microbes.

When you evaluate these, give points on both sides:

• For: the material breaks down more quickly, so less waste builds up in landfill or as litter; cornstarch comes from plants, a renewable resource, whereas most polymers come from crude oil.
• Against: in a polymer–cornstarch bag, the polymer part may still not biodegrade, leaving small fragments behind; land used to grow maize for starch cannot grow food; breakdown may need the right conditions (microbes, moisture, air), which a sealed landfill may not provide.

Common errors

• Leaving the C=C in the polymer chain, or forgetting the bonds through the brackets.
• Putting a side group, such as CH₃, into the main chain.
• Saying heat breaks the covalent bonds in a thermosoftening polymer. It overcomes the intermolecular forces between chains.
• Calling cross-links “intermolecular forces”. Cross-links are covalent bonds.
• Saying LD and HD poly(ethene) come from different monomers. Both come from ethene; the conditions differ.
• Claiming polymer–cornstarch bags vanish completely.

Where to go next

• Polymers revision notes for the condensed version
• Polymers practice questions with fully worked answers
• Bonding, structure and properties for more on intermolecular forces
• Quantitative chemistry for relative formula mass
• The chemistry course hub, plus a checklist you can print

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, with the alkene test from 3.10.1.3 used for support.