Condensation polymerisation (HT only) — AQA GCSE Chemistry
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Condensation polymerisation (HT only) explained
Condensation polymerisation (HT only) covers 4 specification points, including Condensation polymerisation involves monomers… and When these types of monomers react they join….
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Open a point below for its explanation and revision guidance.
Condensation polymerisation involves monomers with two functional groups.
Condensation polymerisation is a type of polymerisation in which monomers with two functional groups react together. Each monomer must have two reactive groups, such as two carboxylic acid groups or two alcohol groups, or one of each. When they react, a small molecule such as water is eliminated for each new bond formed between monomers. For example, a dicarboxylic acid and a diol react to form a polyester and water. The two functional groups allow the monomer to link in two directions, creating a long chain. This is different from addition polymerisation, where monomers have a C=C double bond and no small molecule is eliminated. Students should be able to identify the functional groups in monomers and draw the repeating unit of a condensation polymer.
When these types of monomers react they join together, usually losing small molecules such as water, and so the reactions are called condensation reactions.
In condensation polymerisation, monomers that each carry at least two reactive functional groups link repeatedly to form a polymer. As each new bond forms between monomer units, a small molecule is eliminated, most commonly water. For example, when a carboxylic acid group reacts with an alcohol group, an ester link forms and one water molecule is released per link. The word condensation signals this loss of a small molecule, distinguishing the process from addition polymerisation, where monomers join without losing anything. The small molecule is not a side product of the whole reaction only; it is expelled at every joining step, so the polymer repeat unit contains fewer atoms than the sum of the monomer fragments that formed it.
The simplest polymers are produced from two different monomers with two of the same functional groups on each monomer.
The simplest condensation polymers form when two different monomers each carry two identical functional groups. One monomer might be a diol with two alcohol groups, and the other a dicarboxylic acid with two carboxylic acid groups. Because each monomer has two reactive groups, it can bond at both ends, allowing a long chain to build up. If a monomer had only one functional group, chain growth would stop after one link. Using two different monomers with matching pairs of groups gives a regular alternating pattern in the polymer chain. This is why the simplest polyesters are made from pairs such as a diol and a dicarboxylic acid, reacting to form ester links and water.
For example: ethanediol and hexanedioic acid polymerise to produce a polyester: Students should be able to explain the basic principles of condensation polymerisation by reference to the functional groups in the monomers and the repeating units in the polymers.
Ethanediol, HO–CH₂–CH₂–OH, has two alcohol groups. Hexanedioic acid, HOOC–(CH₂)₄–COOH, has two carboxylic acid groups. When they react, an alcohol group on ethanediol reacts with a carboxylic acid group on hexanedioic acid to form an ester link, releasing water. Because both monomers have two functional groups, the process repeats and builds a polyester chain. The repeating unit shows the two monomer residues joined by ester links, with the atoms of the eliminated water molecules missing. To explain the principles, identify the functional groups in each monomer, state the link formed, and show how the repeating unit reflects the loss of water at each join.
Your focus
- State that condensation polymerisation involves monomers with two functional groups.
- Identify the functional groups in monomers that can undergo condensation polymerisation.
- Explain how the reaction of two functional groups leads to the formation of a polymer and a small molecule.
Show all 12 objectives
- Describe condensation polymerisation as joining of monomers with loss of a small molecule.
- Identify water as a typical small molecule eliminated.
- Explain why monomers need two functional groups for chain growth.
- Identify monomers with two identical functional groups.
- Explain how two functional groups per monomer enable polymer chain growth.
- Recognise diols and dicarboxylic acids as suitable monomer pairs.
- Explain how alcohol and carboxylic acid groups react to form ester links.
- Draw or recognise the repeating unit of a polyester.
- Account for the loss of water when forming the repeating unit.
Condensation polymerisation (HT only) exam tips
Marking Points
- Condensation polymerisation involves monomers that each have two functional groups.
- The two functional groups react to form bonds between monomers, extending the chain in two directions.
- A small molecule, often water, is eliminated for each new bond formed.
- Common examples include dicarboxylic acids with diols to form polyesters, and diamines with dicarboxylic acids to form polyamides.
- The repeating unit of a condensation polymer contains the atoms of the monomers minus the atoms lost as small molecules.
- Monomers must have at least two functional groups so that chains can grow in two directions.
- Each linking step forms a new covalent bond between monomer units.
- A small molecule, commonly water, is eliminated at each link formed.
- The reaction is called condensation because of this loss of a small molecule.
- The polymer repeat unit has fewer atoms than the combined monomer fragments because of the eliminated molecules.
- Polyester formation from a diol and a dicarboxylic acid is a valid example of this process.
- Two different monomers are used in the simplest condensation polymers.
- Each monomer has two of the same functional group.
- A diol has two alcohol groups and a dicarboxylic acid has two carboxylic acid groups.
- Two functional groups per monomer allow chain growth in two directions.
- A monomer with only one functional group would terminate chain growth.
- The resulting polymer has an alternating pattern of the two monomer units.
- Ethanediol contains two alcohol groups and hexanedioic acid contains two carboxylic acid groups.
- An alcohol group reacts with a carboxylic acid group to form an ester link.
- Water is eliminated at each ester link formed.
- The repeating unit shows both monomer residues joined by ester links.
- The repeating unit contains fewer atoms than the monomers because water has been lost.
- The two functional groups on each monomer allow the chain to continue growing.
Examiner Tips
- 💡When identifying monomers for condensation polymerisation, look for two functional groups on each molecule.
- 💡When drawing the repeating unit, show the ester or amide linkage and include the atoms that remain after elimination of the small molecule.
- 💡Remember that condensation polymerisation is higher tier only, so focus on understanding the process rather than memorising specific polymers.
- 💡State clearly which small molecule is eliminated and why it is eliminated.
- 💡Use the phrase 'loss of a small molecule' when explaining the name condensation.
- 💡Link the functional groups present to the type of link formed, such as ester links from alcohol and carboxylic acid groups.
- 💡Name the functional groups present on each monomer before predicting the polymer.
- 💡Check that each monomer has two reactive sites before describing chain growth.
- 💡Use 'diol' and 'dicarboxylic acid' accurately when giving examples.
- 💡Identify the functional groups in each monomer before drawing the repeating unit.
- 💡Show the ester link clearly and account for the eliminated water.
- 💡Use the monomer structures to check that the repeating unit has the correct number of carbon atoms.
Common Mistakes
- Thinking that monomers for condensation polymerisation need a C=C double bond; correct this by stating that they need two functional groups such as –COOH or –OH.
- Forgetting that a small molecule is eliminated; correct this by showing water or another small molecule as a product.
- Drawing a repeating unit that includes all atoms of the monomers without removing the small molecule; correct this by subtracting the atoms of the eliminated molecule.
- Thinking that water is lost only once for the whole polymer: correct this by stressing that one small molecule is lost per new link formed.
- Confusing condensation with addition polymerisation: correct this by noting that addition polymerisation loses no small molecule.
- Assuming any monomer can undergo condensation polymerisation: correct this by requiring two functional groups per monomer.
- Thinking the two functional groups on a monomer must be different: correct this by noting they are the same on each monomer in the simplest case.
- Believing one functional group per monomer is enough: correct this by explaining that chain growth needs two reactive sites.
- Assuming the two monomers must be identical: correct this by stating that two different monomers are used.
- Drawing the repeating unit with the water molecule still present: correct this by removing the atoms of water at each ester link.
- Showing only one functional group reacting: correct this by showing both ends of each monomer able to react.
- Labelling the link as an amide link: correct this by naming it an ester link when alcohol and carboxylic acid groups react.