Polymers
Addition polymerisation, condensation polymers, and the environmental impact of plastics.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 11.8.1Describe addition polymerisation and deduce the repeat unit from a monomer.
- 11.8.2Discuss the environmental problems caused by non-biodegradable plastics.
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Polymers
A polymer is a very large molecule made from thousands of small repeating units, called monomers, joined end to end into a long chain — plastics are the most familiar example.
Addition polymerisation
Addition polymers are made from monomers that each contain a C=C double bond (i.e. alkenes). During polymerisation, every double bond opens up into a single bond, freeing a bond on each carbon to join the next monomer — so the monomers link up with nothing left over: no by-product is released, unlike condensation polymerisation.
- Ethene (CH₂=CH₂) → poly(ethene), repeat unit −(CH₂−CH₂)−.
- Propene (CH₂=CHCH₃) → poly(propene), repeat unit −(CH₂−CHCH₃)−.
The general rule: draw the monomer, replace the double bond with a single bond, and add extending bonds on both ends of the repeat unit to show it continues.
Naming from the monomer, and vice versa
To go from monomer to polymer name: take the monomer's name, put it in brackets, and add "poly" in front, e.g. propene → poly(propene). To go the other way — from the repeat unit back to the monomer — reverse the process: identify the repeating −C−C− backbone, then reinstate the double bond.
The environmental problem
Most addition polymers are non-biodegradable: microorganisms cannot break the strong C−C backbone down, so plastics persist in landfill and the natural environment for decades or longer, and can harm wildlife. Options for dealing with waste plastic include recycling (melting and reshaping), incineration (burning for energy, though this can release toxic gases if not carefully controlled), and using biodegradable plastics designed to break down naturally.
Think of it like this
Addition polymerisation is like a long line of people linking arms: each person (monomer) has one "hand" free on each side because they let go of holding both their own hands together (the double bond opening) — nothing is dropped, they just link up.
Worked examples
Method, step by step
Chloroethene, CH₂=CHCl, is the monomer used to make PVC (poly(chloroethene)). Deduce the repeat unit.
- 1Identify the double bond: between the two carbons.
- 2Open the double bond into a single bond.
- 3Add extending bonds on both ends to represent the repeating chain.
The repeat unit is −(CH₂−CHCl)−, with a bond extending from each end.
Common misconceptions
- Forgetting the extending bonds when drawing a repeat unit — a repeat unit without bonds sticking out on both ends looks like a single, isolated molecule rather than part of a chain.
- Thinking the double bond survives inside the polymer — it does not; the C=C becomes a C−C single bond as soon as polymerisation happens, which is exactly what frees up the bonds needed to link to neighbouring monomers.
- "Recyclable" and "biodegradable" are not the same thing — a plastic can be recyclable by industrial processing while still not breaking down naturally if left in the environment.
In the exam
- When deducing a repeat unit from a monomer, always show two things: the correct atoms in brackets, and a bond extending from each end of the bracket.
- For "explain why non-biodegradable plastics are a problem" questions, name a specific consequence (e.g. harm to wildlife, landfill taking up space for decades) rather than just restating that they "don't break down".