How are alkenes turned into plastics, and what are the disposal issues?
Addition polymerisation of alkenes to form polymers, drawing the repeating unit, naming common addition polymers and their uses, and the problems of plastic disposal.
A CCEA GCSE Chemistry answer on addition polymers, covering how alkene monomers join to form polymers, how to draw the repeating unit from a monomer, common addition polymers such as poly(ethene) and their uses, and the environmental problems of disposing of plastics.
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What this dot point is asking
CCEA wants you to explain addition polymerisation of alkenes, draw the repeating unit from a monomer, name common addition polymers and their uses, and discuss the environmental problems of plastic disposal.
Addition polymerisation
Only molecules with a double bond (alkenes) can undergo addition polymerisation, because it is the opening of the double bond that lets the monomers join. This links the topic straight back to the reactivity of alkenes.
Drawing the repeating unit
The repeating unit always has the same atoms as the monomer, because nothing is added or lost; only the double bond is changed to single bonds joining the chain.
Common addition polymers and uses
These plastics are cheap, light, waterproof and durable, which is exactly why they are so widely used, and also why they cause disposal problems.
The disposal problem
Worked example
Examples in context
Example 1. The plastic waste crisis. Because poly(ethene) bags do not biodegrade, they accumulate in oceans and landfill for decades, which is why many countries now charge for or ban single-use bags. The non-biodegradable nature studied here is the root of a global environmental issue.
Example 2. Recycling sorting. Plastics are sorted by their polymer type so they can be recycled into new products, reducing the need for fresh crude oil. Understanding that each plastic is a specific addition polymer explains why sorting is necessary before recycling.
Try this
Q1. State what type of small molecule can undergo addition polymerisation. [1 mark]
- Cue. An alkene (a molecule with a carbon-carbon double bond).
Q2. State why addition polymers are difficult to dispose of. [1 mark]
- Cue. They are not biodegradable, so they persist for many years.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20184 marksEthene undergoes addition polymerisation to form poly(ethene). Describe what happens to the monomers and draw the repeating unit of poly(ethene).Show worked answer →
Markers want the addition process and a correct repeating unit.
In addition polymerisation, many ethene monomers, each with a carbon-carbon double bond, join together. The double bonds open up and the carbon atoms bond to the next monomer, so the molecules add together to form one long chain (the polymer) with no other product.
The repeating unit of poly(ethene) is two carbons each with two hydrogens, with the bonds extending out at each side:
drawn with the brackets and a bond passing through each side and an n outside.
Markers reward the double bonds opening and monomers adding with no other product, and a correct repeating unit with the continuation bonds.
CCEA 20214 marksExplain why the disposal of addition polymers is a problem, and give two ways of reducing the environmental impact.Show worked answer →
The marks are for the disposal problem and two solutions.
Addition polymers are not biodegradable (microorganisms cannot break them down), so they last for many years in landfill, taking up space. Burning them can release toxic gases and carbon dioxide.
Two ways to reduce the impact:
- Recycle the plastics so they are used again rather than thrown away.
- Reuse items, or develop biodegradable plastics that break down naturally.
(Reducing the amount of plastic used is also accepted.)
Markers reward non-biodegradable so it persists/takes landfill space (and toxic gases if burnt), plus two sensible solutions such as recycling and reuse or biodegradable plastics.
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Sources & how we know this
- CCEA GCSE Chemistry specification (1110) — CCEA (2017)