Synthetic Polymers
Organic Chemistry
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 5% of your exam marks.
Addition polymerisation (drawing repeat units, identifying monomers, common polymers + uses) and disposal / environmental impact regularly tested.
Why polymers are difficult to dispose of
- The chains in an addition polymer are held together by strong C−C single bonds and they contain no chemically reactive functional groups
- This makes the polymers and non-biodegradable: bacteria and other decomposers in soil cannot break the chains down
- Once thrown away, the polymer persists in the environment for centuries — a serious downside of one of the chemistry industry's most useful product families
Landfill
- Most household plastic waste ends up in sites
- Landfill takes up large amounts of land and the polymer waste does not degrade, so each site fills up quickly and a new one has to be excavated
- Plastic in landfill can leach additives (plasticisers, pigments) into groundwater over time
Incineration
- Burning waste polymers in an incinerator is an alternative, and it can recover energy for heating or power
- Drawbacks:
- Burning hydrocarbons releases large amounts of carbon dioxide, a greenhouse gas, contributing to climate change
- Burning polymers that contain chlorine (PVC) releases toxic hydrogen chloride (HCl) gas, which is corrosive and contributes to acid rain
- Incomplete combustion in a poorly run incinerator releases carbon monoxide, the toxic gas that cuts the blood's ability to transport oxygen around the body (see topic 22 section 2)
Recycling
- Many addition polymers can be sorted, melted and re-moulded into new products — the most environmentally friendly disposal route for those polymers that can be recycled
- Drawbacks: sorting different plastics is labour-intensive; mixed-polymer waste reduces the quality of the recycled product; thermosetting polymers cannot be melted and re-formed
Problems caused by disposing of addition polymers
What comes up: state or discuss problems caused by the disposal of addition polymers (landfill or burning), often worth 1–4 marks.
Write: for landfill — polymers remain in landfill indefinitely (they are inert and non-biodegradable, so they do not decompose, and landfill sites fill up). For incineration — burning produces toxic gases: hydrogen chloride (HCl) when chlorine-containing polymers such as PVC are burned, and carbon monoxide from incomplete combustion; burning also produces carbon dioxide, a greenhouse gas contributing to climate change.
Watch out: saying polymers "break down slowly" is allowed, but saying they "break bonds" or are "broken down by bacteria" without specifying they are NOT biodegradable will not score the mark. The mark scheme also penalises incorrect claims about the ozone layer.