Organic chemistry · GCSE Chemistry
Polymers and addition polymerisation
GCSE Chemistry revision on addition polymerisation: alkene monomers, repeat units, poly(ethene) and poly(propene), and why plastics are useful, persistent in landfill, and awkward to recycle.
Alkene monomers, C=C, open and join. One product: the polymer. Draw the repeat unit by turning the double bond into a single bond in a bracket with n. Poly(ethene), not “polyethene” in AQA brackets.
The important bits
What you need to know
- 1
A monomer is a small molecule that can join to others. An addition polymer is a very long chain made when many alkene monomers add together. The polymer is the only product.
- 2
In addition polymerisation the C=C double bond in each monomer opens, and carbon atoms join to carbon atoms in neighbouring monomers. The displayed repeat unit shows a single C–C bond in the chain.
- 3
Name addition polymers as poly(monomer): poly(ethene) from ethene, poly(propene) from propene, poly(chloroethene) / PVC from chloroethene. AQA expects the brackets.
- 4
To draw the repeat unit from a monomer: open the double bond, put the two carbons in the backbone, keep side groups (H, CH₃, Cl) on the same carbons, and add a bracket with n.
- 5
To identify the monomer from a repeat unit: reverse the process — put C=C back between those two backbone carbons and remove the bracket and n.
- 6
Addition polymers are usually unreactive, flexible or mouldable, and waterproof, which is why they are used for packaging, pipes, windows and fibres. Properties depend on chain length and branching.
- 7
They are not biodegradable, so they persist in landfill. Incineration releases energy but also carbon dioxide and, for some plastics, toxic gases unless the plant is well designed. Recycling saves raw materials but needs sorting.
- 8
Higher / Triple may contrast addition polymers (one product, from alkenes) with condensation polymers, which make a small molecule such as water as well. This page is the addition story.
Quotations worth analysing
Short evidence. Real method.
“Many small alkene molecules (monomers) join to form a long-chain polymer. The polymer is the only product.”
Monomer, polymer, and “only product” are the scoring words. Condensation polymerisation also makes water or HCl — do not mix the two.
“The repeating unit of poly(ethene) is –CH₂–CH₂–.”
Show the single bond in the chain, trailing bonds through the brackets, and n. Do not leave a double bond in the repeat unit.
“Addition polymers are not biodegradable, so they cause disposal problems.”
Name landfill persistence, incineration (energy but CO₂ / possible toxins), and recycling (sorting needed). “Plastic is bad” scores nothing.
Go deeper
The drawing is the whole topic
Start with ethene: C=C, two hydrogens on each carbon. In the polymer those two carbons are in the chain with a single bond, hydrogens still attached, bonds left and right to the next units, bracket, n. Propene has a CH₃ on one carbon; that methyl group stays on the same carbon in poly(propene). Chloroethene has Cl on one carbon for PVC. If you move the side group to the wrong carbon, the polymer is wrong. Trailing bonds must go through the brackets, otherwise you have drawn a small molecule, not a repeat unit. Count carbons in the backbone of the repeat unit: two, matching the two carbons of the alkene double bond. Never put n inside the molecule as an atom.
Go deeper
Useful because they are dull chemistry
Poly(ethene) bags and bottles last because the C–C backbone has no C=C left, so there is no easy addition reaction and the chains are unreactive at room temperature. That is a design feature for pipes and food wrap, and a problem when you want the material to rot. Biodegradable means microorganisms can break the material down; addition polymers generally do not allow that. Recycling works only if you sort by type, because different polymers melt at different temperatures and do not mix well. Burning can recover energy, but carbon that was locked in crude oil returns as CO₂. A balanced evaluation names a use, a property that matches the use, and one disposal method with a drawback.
Go deeper
n is large, not a mole ratio from a small equation
Writing “C₂H₄ → poly(ethene)” is acceptable as a word equation. A displayed equation shows n ethene molecules on the left and the bracketed repeat unit with n on the right. n is a very large number; you do not calculate it at GCSE. Atom economy of addition polymerisation is 100% in theory because every atom in the monomer ends in the polymer — a quantitative-chemistry link. That does not mean plastic production is environmentally free: the ethene came from cracking fossil hydrocarbons, and the polymer may last centuries. If a question gives a displayed monomer with a side chain, copy the side chain into the repeat unit. Do not convert it into poly(ethene) out of habit.
See the idea in action
Draw the repeat unit of the polymer from propene, CH₂=CHCH₃. Open the double bond. The backbone is two carbons. One of them still carries CH₃; the other carries two hydrogens. Trailing bonds go left and right through a bracket, with n outside. Name: poly(propene). The monomer could be recovered on paper by putting the double bond back between those two carbons. Bromine water would be decolourised by propene but not by the polymer, because the C=C has gone.
Exam technique
Turn knowledge into marks
Use the words monomer, addition, and only product. Draw the repeat unit with a single C–C in the chain, side groups copied across, trailing bonds through the brackets, and n. For evaluation, match a property to a use, then give a specific disposal problem, not “pollution”.
Common mistakes
Do not give these marks away
- 01
Leaving C=C in the repeat unit, or omitting trailing bonds and n.
- 02
Calling the process condensation polymerisation, or writing a small-molecule by-product for addition.
- 03
Writing “plastic is non-recyclable” as a blanket statement instead of landfill / incineration / sorting issues.
What is produced when ethene undergoes addition polymerisation?
APoly(ethene) and water
BPoly(ethene) only
CEthane and hydrogen
DA mixture of shorter alkanes
Show the answer
Poly(ethene) only. Addition polymerisation joins alkene monomers into a long chain. There is no other product, unlike condensation polymerisation which also makes a small molecule.
Quick questions
If this is the bit you searched
What is addition polymerisation GCSE?
Many unsaturated alkene monomers join when the C=C double bonds open. A long-chain polymer forms and it is the only product.
How do you draw the repeat unit from an alkene monomer?
Change the double bond to a single bond, keep the side groups on the same carbons, show bonds through the brackets to the next units, and write n.
Why are addition polymers hard to dispose of?
They are not biodegradable, so they remain in landfill. Incineration produces carbon dioxide and sometimes toxic gases. Recycling requires sorting different plastics.
What is the difference between a monomer and a polymer?
A monomer is a small molecule, here an alkene. A polymer is the very large chain made when many monomers join. Poly(ethene) is the polymer of ethene.