Condensation polymers (A-level only) — AQA A-Level Chemistry
Test yourself on Condensation polymers (A-level only) with AQA A-Level practice questions.
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Condensation polymers (A-level only) explained
In a condensation polymerisation each new link is made by a reaction that also throws out a small molecule, usually water, or hydrogen chloride if a diacyl dichloride is used instead of the diacid.
Read the full explanation
For a chain to grow at both ends, every monomer needs two reacting groups. A diol and a dicarboxylic acid give an ester link, -COO-, repeated all the way along, so the product is a polyester. A diamine and a dicarboxylic acid give an amide link, -CONH-, so the product is a polyamide. An amino acid needs no partner, because it carries an -NH₂ at one end and a -COOH at the other, so it condenses with itself over and over to give a polyamide, which is what a polypeptide is.
The repeating units in polyesters (eg Terylene) and polyamides (eg nylon 6,6 and Kevlar) and the linkages between these repeating units.
A repeating unit is the smallest section of chain that repeats, drawn inside brackets with the two bonds crossing the brackets and n written outside. Terylene is a polyester made from benzene-1,4-dicarboxylic acid and ethane-1,2-diol, so its repeating unit contains one aromatic ring, one -CH₂CH₂- unit and two ester links. Nylon 6,6 is a polyamide from hexanedioic acid and hexane-1,6-diamine, each of which contributes six carbon atoms, which is where the name comes from; its repeating unit contains two amide links. Kevlar is also a polyamide, but both monomers are aromatic, benzene-1,4-dicarboxylic acid and benzene-1,4-diamine, which is what makes its chains rigid and flat.
Typical uses of these polymers.
Properties follow from the links and the forces between chains, and uses follow from the properties. Terylene can be drawn into a fine fibre that resists stretching and does not crease easily, so it is used in clothing, and the same polymer moulded rather than spun becomes the drinks bottle. Nylon 6,6 has an N-H on every amide link, so its chains hydrogen bond to one another strongly; it is tough and resists abrasion, which suits ropes, carpets, parachutes and clothing. Kevlar's chains are aromatic and rigid, and they hydrogen bond into flat sheets that stack, giving a tensile strength greater than steel for the same mass, hence body armour and protective gloves. All three hydrolyse slowly, so they eventually degrade.
Students should be able to: draw the repeating unit from monomer structure(s) draw the repeating unit from a section of the polymer chain draw the structure(s) of the monomer(s) from a section of the polymer explain the nature of the intermolecular forces between molecules of condensation polymers.
Work in whichever direction the question asks. From monomers: join them, take out a molecule of water at each new link, then bracket the result with both bonds passing through the brackets. From a chain: find where the pattern repeats and cut through two equivalent points, usually the middle of two identical links. From polymer back to monomers: break each link and put an -OH on the carbonyl carbon and an -H on the oxygen or nitrogen, which is adding the water back. For the forces, the polar C=O gives permanent dipole-dipole attractions in both families, and van der Waals forces act along the chains, but only a polyamide has an N-H, so only a polyamide hydrogen bonds between chains.
Your focus
- Identify from a pair of monomer structures whether a polyester or a polyamide would form, and name the small molecule lost.
- Draw the link formed between two named monomers and show where the water molecule came from.
- Explain why each monomer in a condensation polymerisation must carry two functional groups.
Show all 12 objectives
- Draw the repeating unit of Terylene, nylon 6,6 or Kevlar from the structures of its monomers.
- Decide from a drawn section of chain whether the link is an ester or an amide, and state the one atom that tells them apart.
- Explain what the numbers in the name nylon 6,6 refer to.
- Match Terylene, nylon 6,6 and Kevlar to a typical use and justify each match by a named property.
- Explain the strength of a named polyamide in terms of hydrogen bonding between its chains.
- Suggest why a polyester is chosen over a polyamide for a given application.
- Write the polymerisation equation using n molecules of each monomer, giving the number of water molecules released alongside the polymer.
- Break a drawn chain at its links and add back H and OH to the right atoms, so the monomers recovered are complete acids, diols or diamines and not fragments.
- Explain the difference in intermolecular forces between a polyester and a polyamide and its consequence for strength.
Condensation polymers (A-level only) exam tips
Marking Points
- one mark for water, or hydrogen chloride from a diacyl dichloride, named as the small molecule lost
- one mark for each monomer needing two functional groups so the chain can grow at both ends
- one mark for the ester link formed from a diol and a dicarboxylic acid
- one mark for the amide link formed from a diamine and a dicarboxylic acid, or from an amino acid alone
- one mark for the repeating unit drawn with open bonds passing through the brackets and n outside
- one mark for the correct ester linkage in the polyester
- one mark for the correct amide linkage, with the nitrogen bonded to the carbonyl carbon, in the polyamide
- one mark for naming or drawing both monomers of nylon 6,6 or of Kevlar
- one mark for a named use matched to the named polymer
- one mark for linking that use to a property such as tensile strength, flexibility or resistance to abrasion
- one mark for tracing that property back to hydrogen bonding between the chains
- one mark for naming a drawback, such as slow degradation in landfill
- one mark for a repeating unit with the bonds drawn through the brackets
- one mark for adding water back across each link when the monomers are deduced from the polymer
- one mark for hydrogen bonding named between the N-H of one chain and the C=O of another in a polyamide
- one mark for a polyester having no N-H, so only permanent dipole-dipole and van der Waals forces act between its chains
Examiner Tips
- 💡Count the functional groups on every monomer first. Two groups means condensation is possible; one means it is not.
- 💡For each link you draw, cross out an -OH and an -H and write the water molecule you removed.
- 💡The bonds must be drawn through the brackets. A repeating unit with tidy closed ends is the commonest way to lose this mark.
- 💡Check your repeat unit by imagining two copies side by side: the ends must join without adding or removing atoms.
- 💡Answer 'suggest a use' questions in three steps: structure, then property, then use.
- 💡Hydrogen bonding needs an N-H or an O-H. Polyamides have one; polyesters do not, which is why they differ in strength.
- 💡When deducing monomers, count the links you cut: each one needs a water molecule added back, split as -OH and -H.
- 💡A force question wants the force named and located: say which atom on which chain is attracted to which.
Common Mistakes
- using a monomer with only one functional group, which caps the chain instead of extending it
- calling the product of two C=C monomers a condensation polymer
- leaving the water molecules out of the equation for the polymerisation
- calling the amide link an ester because it contains a C=O group
- drawing the repeating unit with the water still present, so the monomers are simply placed side by side
- ending the repeating unit with an -OH and an -H, so the bonds do not continue to the next unit
- reversing the amide link, putting the nitrogen where the oxygen should be
- drawing Kevlar with aliphatic chains, which makes it nylon rather than Kevlar
- giving a use with no property to justify it, so there is nothing for the explanation mark
- attributing Kevlar's strength to covalent cross-links between separate chains
- saying the chains in a polymer are joined to each other by covalent bonds
- claiming polyesters and polyamides do not degrade at all
- returning monomers with the link still joined, so the two 'monomers' are really a dimer
- drawing hydrogen bonds between polyester chains, which have no N-H or O-H to donate
- showing the hydrogen bonds inside one chain when the question asks about forces between chains
- saying that hydrolysis breaks the intermolecular forces, when it breaks covalent bonds in the backbone