How are polymers made, and how do we compare materials and their life cycles?
Polymers and materials: addition polymerisation of alkenes, condensation polymerisation, the properties and uses of polymers, comparing materials, and life cycle assessment.
A focused answer to Edexcel GCSE Chemistry topic 9 (separate chemistry), covering addition polymerisation of alkenes and drawing repeating units, condensation polymerisation, the properties and uses of common polymers, comparing materials such as polymers, ceramics, composites and metals, and life cycle assessment.
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What this dot point is asking
Edexcel wants you (in separate chemistry) to describe addition polymerisation of alkenes and draw repeating units, describe condensation polymerisation, give the properties and uses of polymers, compare different materials, and describe life cycle assessment. Addition polymerisation and life cycle assessment are the central marks.
Addition polymerisation
For example, many ethene molecules () join to form poly(ethene). To draw the repeating unit, take the monomer, open the double bond to a single bond, and draw bonds extending from each end: with the brackets and the extending bonds and an outside.
Condensation polymerisation
Condensation polymerisation joins monomers that each have two functional groups. Each time two monomers join, a small molecule (often water) is released. This is different from addition polymerisation, which makes no by-product. Condensation polymers include polyesters.
Properties and uses of polymers
Polymers are widely used because they are cheap to produce, light, and easily moulded into shape, and many are flexible and good insulators. Common examples include poly(ethene) for bags and bottles, poly(propene) for crates and ropes, and poly(chloroethene) (PVC) for pipes and window frames.
Their main drawback is disposal:
Comparing materials
Different materials suit different jobs because of their properties:
- Polymers: light, cheap, mouldable, good insulators, but can be weak and hard to dispose of.
- Ceramics (such as clay and glass): hard, strong in compression, heat-resistant and electrical insulators, but brittle.
- Composites (such as fibreglass and reinforced concrete): combine two materials to get the best properties of each, for example strong and light.
- Metals: strong, malleable and good conductors, but dense and can corrode.
Life cycle assessment
An LCA helps compare products, but it is not always straightforward: some impacts (energy, water) are easy to measure, while others require a judgement (such as the effect of pollutants), so an LCA can be subjective and may be biased to support a particular conclusion.
Try this
Q1. State what is formed, besides the polymer, in addition polymerisation. [1 mark]
- Cue. Nothing; no other product is formed.
Q2. Give one reason why poly(ethene) is difficult to dispose of. [1 mark]
- Cue. It is not biodegradable, so it does not rot and persists in landfill (or burning releases toxic gases).
Q3. Name the four stages a life cycle assessment considers. [2 marks]
- Cue. Raw materials, manufacture, use, and disposal.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20194 marksPoly(ethene) is made from ethene by addition polymerisation. Explain what is meant by addition polymerisation, draw the repeating unit of poly(ethene), and state why poly(ethene) is difficult to dispose of.Show worked answer →
A 4-mark addition-polymerisation question.
In addition polymerisation, many small unsaturated monomer molecules (alkenes) with carbon-carbon double bonds join together to form one long polymer chain, with no other product formed (1 mark). The repeating unit of poly(ethene) is drawn as two carbon atoms each with two hydrogen atoms, joined by a single bond, with bonds extending out of each side (1 mark). Poly(ethene) is difficult to dispose of because it is not biodegradable (1 mark), so it does not rot away and persists in landfill, and burning it can release toxic gases (1 mark).
Markers reward "many alkene monomers join with no other product", a correct repeating unit with the extending bonds, and a disposal problem.
Edexcel 20214 marksA life cycle assessment is carried out to compare a plastic carrier bag with a paper carrier bag. Describe what a life cycle assessment considers, and explain why comparing the two bags is not always straightforward.Show worked answer →
A 4-mark life cycle assessment question.
A life cycle assessment considers the environmental impact at each stage of a product's life: extracting and processing the raw materials, manufacturing, using the product, and disposal at the end of its life (1 mark for raw materials and manufacture, 1 mark for use and disposal). Comparing the bags is not straightforward because some impacts are easy to measure (such as energy and water used) but others involve a judgement, such as the effect of pollutants or how many times each bag is reused (1 mark), so the conclusion can depend on the assumptions made and may be biased (1 mark).
Markers reward listing the life-cycle stages and explaining that some impacts require judgement, so assessments can be subjective.
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Sources & how we know this
- Pearson Edexcel GCSE (9-1) Chemistry (1CH0) specification — Pearson Edexcel (2016)