Revision notes for AQA GCSE Chemistry Addition polymerisation. Open the guide for explanations and worked examples. Written against the AQA GCSE Chemistry (8462) specification, so the content matches what's examinable rather than general Chemistry background.
Addition polymerisation
What you'll learn
- How alkenes such as ethene and propene form polymers.
- What monomers, polymers and repeating units are.
- How to draw an addition polymer from an alkene monomer.
- How to work backwards from a repeating unit to the original monomer.
The key starting point: alkenes
A hydrocarbon is a compound made only from hydrogen and carbon atoms.
An alkene is a hydrocarbon that contains at least one carbon–carbon double bond, written as C=C. This C=C double bond is the important part of the molecule in addition polymerisation.
Definition
Alkene
An alkene is an unsaturated hydrocarbon containing a carbon–carbon double bond, C=C.
The C=C double bond is called the functional group of alkenes. A functional group is the atom or group of atoms that gives a family of compounds its characteristic reactions.
For addition polymerisation, you mainly need to recognise displayed or structural formulae like:
- ethene: H₂C=CH₂
- propene: CH₂=CH–CH₃
Key Idea
Why alkenes matter
The C=C double bond can “open up”, allowing many alkene molecules to join together into a long chain.
Monomers and polymers
A monomer is a small molecule that can join to many other similar molecules.
A polymer is a very large molecule made from many monomers joined together in a long chain.
Definition
Monomers and polymers
A monomer is a small molecule used to make a polymer. A polymer is a very large molecule made from many repeating monomer units joined together.
The word polymerisation means the reaction where monomers join to form a polymer.
You can think of the monomers as lots of small links, and the polymer as the long chain made by joining those links together.
Analogy
Chain links
A monomer is like one link in a chain. Polymerisation is the process of connecting many links together to make one long chain.
What addition polymerisation means
Addition polymerisation happens when many alkene monomers join together to form a polymer.
It is called addition because the monomers add together without forming any other product. No small molecule such as water is made.
Definition
Addition polymerisation
Addition polymerisation is a reaction in which many alkene monomers join together to form a polymer, with no other molecule produced.
For ethene, the overall idea is:
nnn H₂C=CH₂ → [–CH₂–CH₂–]ₙ
The symbol nnn means a very large number of repeating units. It is not a fixed number like 2 or 10.
The diagram shows the key idea: the C=C double bond in each ethene monomer opens, forming a long poly(ethene) chain.
Key Idea
No atoms are lost
In addition polymerisation, the repeating unit has the same atoms as the monomer because no other molecule is formed.
Ethene to poly(ethene)
Ethene has the formula H₂C=CH₂.
During addition polymerisation:
- the C=C double bond changes into a C–C single bond in the polymer chain
- each carbon atom forms a new bond to another monomer unit
- the atoms attached to each carbon stay the same
So ethene forms poly(ethene):
nnn H₂C=CH₂ → [–CH₂–CH₂–]ₙ
The name of the polymer is made by putting poly in front of the monomer name, with brackets:
- ethene → poly(ethene)
- propene → poly(propene)
Common Mistake
Do not change the atoms
A common mistake is to remove hydrogen atoms when drawing the polymer. In addition polymerisation, the atoms in the repeating unit are the same atoms as in the monomer.
The repeating unit
A repeating unit is the part of the polymer chain that repeats again and again.
Definition
Repeating unit
The repeating unit is the section of a polymer chain shown inside brackets, with a subscript nnn to show it repeats many times.
For poly(ethene), the repeating unit is:
[–CH₂–CH₂–]ₙ
The bonds sticking out through the brackets show that the chain continues on both sides.
This is important: the brackets do not mean there is just one unit. They mean that unit repeats many times.
Tip
Reading polymer brackets
When you see brackets with nnn outside, read it as “this unit repeats many times”.
How to draw an addition polymer from a monomer
This is one of the main GCSE skills for this topic.
The method
To draw the polymer from an alkene monomer:
- Find the C=C double bond.
- Change the C=C double bond into a C–C single bond.
- Keep all the atoms or groups attached to the same carbon atoms.
- Add single bonds sticking out from the two carbon atoms that used to be in the double bond.
- Put brackets around the repeating unit and write nnn outside.
The carbon atoms in the original C=C bond become the backbone of the polymer chain.
Example
Drawing poly(propene)
Draw the addition polymer formed from propene, CH₂=CH–CH₃.
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Identify the two carbon atoms in the C=C double bond: in propene, they are the CH₂ carbon and the CH carbon attached to CH₃.
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Change the C=C double bond into a C–C single bond, because the double bond opens during addition polymerisation.
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Keep the CH₃ group attached to the same carbon as before, giving the repeating unit –CH₂–CH(CH₃)–.
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Put the repeating unit in brackets and add nnn outside:
[–CH₂–CH(CH₃)–]ₙ
Common Mistake
Forgetting the bonds through the brackets
In a polymer repeating unit, the bonds should pass through the brackets on both sides. These bonds show the repeating unit is joined to more repeating units in the chain.
How to work backwards: polymer to monomer
You may also be asked to identify the monomer from a polymer repeating unit.
For addition polymers, this is the reverse of drawing the polymer.
The method
To find the monomer from a repeating unit:
- Look at the two carbon atoms in the polymer backbone.
- Remove the brackets and the nnn.
- Replace the single bond between those two backbone carbons with a C=C double bond.
- Keep all side groups attached to the same carbon atoms.
So:
[–CH₂–CH₂–]ₙ becomes H₂C=CH₂
That monomer is ethene.
Example
Finding the monomer from a repeating unit
Find the monomer used to make the polymer with repeating unit [–CH₂–CH(CH₃)–]ₙ.
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Identify the two backbone carbon atoms in the repeating unit: CH₂ and CH(CH₃).
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Remove the brackets and the nnn, because you are going back to one monomer molecule.
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Change the single bond between the two backbone carbons into a double bond, while keeping the CH₃ group attached to the same carbon.
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The monomer is CH₂=CH–CH₃, which is propene.
Recognising addition polymers and monomers
In exam questions, you may be shown several structures and asked which one is a monomer, which one is a polymer, or which monomer forms a given polymer.
An alkene monomer will contain a C=C double bond.
An addition polymer will usually be shown as a repeating unit in brackets, with nnn outside.
Example
Spotting monomers and polymers
You are given these structures: CH₂=CH₂, CH₃–CH₃ and [–CH₂–CH₂–]ₙ. Identify which could be an addition polymer monomer and which is the polymer.
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CH₂=CH₂ contains a C=C double bond, so it is an alkene and can act as a monomer for addition polymerisation.
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CH₃–CH₃ contains only a C–C single bond, so it is not an alkene and would not be used as the monomer in addition polymerisation.
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[–CH₂–CH₂–]ₙ has brackets and nnn, showing a repeating unit, so it represents the polymer poly(ethene).
Tip
Quick exam check
If the monomer has C=C, the polymer repeating unit should have C–C. Addition polymerisation turns the double bond into single bonds in a chain.
Why the repeating unit has the same atoms as the monomer
This is a very important idea in this chemistry-only topic.
In addition polymerisation, the monomers join together directly. No atoms are removed, and no small molecule is produced.
For example:
- ethene monomer: C₂H₄
- poly(ethene) repeating unit: C₂H₄
The arrangement of bonds changes, but the atoms in each repeating unit match the atoms in one monomer.
Key Idea
Same atoms, different bonding
The monomer and repeating unit contain the same atoms, but the C=C double bond in the monomer becomes part of a C–C single-bonded polymer chain.
Representing the polymer chain
There are two common ways to represent addition polymers.
You might see a short section of the chain:
–CH₂–CH₂–CH₂–CH₂–CH₂–CH₂–
Or you might see the repeating unit:
[–CH₂–CH₂–]ₙ
The repeating unit version is usually clearer because it shows the smallest section that repeats.
Common Mistake
Different orientations can still be correct
A repeating unit may be drawn flipped around. For example, [–CH(CH₃)–CH₂–]ₙ represents the same polymer as [–CH₂–CH(CH₃)–]ₙ. The chain has just been drawn from the other direction.
Exam technique
In the exam
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Start by finding the C=C double bond in the monomer; those two carbon atoms become the polymer backbone.
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When drawing the repeating unit, change C=C to C–C, keep all side groups the same, and put bonds through the brackets.
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If working backwards from a polymer, remove the brackets and nnn, then put the C=C double bond back between the two backbone carbons.
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Remember: addition polymerisation makes one product only — the polymer. No small molecule is formed.
Self review
Check yourself
- What feature must a monomer have to form an addition polymer?
- Draw the repeating unit formed from CH₂=CH–CH₃.
- Why does the repeating unit contain the same atoms as the monomer?
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