Cambridge AS & A Level9701

Condensation polymerisation

Chemistry 9701 Chapter Notes

What this chapter covers

Condensation polymerisationPredicting the type of polymerisationDegradable polymers
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In this chapter

  1. 1What is Condensation Polymerisation?
  2. 2Polyamides: The Amide Link
  3. 3Polyesters: The Ester Link
  4. 4Working Backwards: Identifying Monomers
  5. 5Polymer Properties and Degradability

1. What is Condensation Polymerisation?

Condensation polymerisation is a process where large numbers of small molecules, called monomers, join together to form a very large molecule, a polymer. The 'condensation' part is key: each time two monomers join, a small molecule (like water, H₂O, or hydrogen chloride, HCl) is eliminated or 'lost'. This is fundamentally different from addition polymerisation, where monomers simply add to each other with no loss of atoms. For condensation polymerisation to occur, the monomers must have two reactive functional groups. These can be two different functional groups on the same monomer molecule, or one functional group on each end of two different types of monomer.

General Reaction: n(Monomer 1) + n(Monomer 2) → -[Repeat Unit]-n + n(Small Molecule)

Example using functional groups: n HOOC-R-COOH + n HO-R'-OH → -[O-R'-O-CO-R-CO]-n + 2n H₂O

Key term

Condensation Polymerisation: A chemical reaction in which monomers join together to form a polymer with the elimination of a small molecule, such as water.

Examiner insight

Examiners reward clear definitions that correctly mention both the joining of monomers to form a polymer and the simultaneous elimination of a small molecule.

Common pitfall

Confusing condensation polymerisation with addition polymerisation. Remember, if the polymer backbone contains only carbon atoms, it's likely addition; if it has O or N atoms in the backbone (in an ester or amide link), it's condensation.

Worked example 13 marks

Explain the difference between a monomer used for addition polymerisation and the monomers used for condensation polymerisation. [3]

  1. 1

    Step 1: Define the monomer requirement for addition polymerisation. A monomer for addition polymerisation must contain a carbon-carbon double bond (C=C). [1 mark]

  2. 2

    Step 2: Define the monomer requirement for condensation polymerisation. Monomers for condensation polymerisation do not require a C=C bond. Instead, they must possess two reactive functional groups. [1 mark]

  3. 3

    Step 3: Give examples of these functional groups. These functional groups are typically carboxyl (-COOH), hydroxyl (-OH), or amine (-NH₂) groups. The monomers can either be one type with two different functional groups (e.g., an amino acid) or two different types of monomer, each with two identical functional groups (e.g., a dicarboxylic acid and a diol). [1 mark]

Recap

  • Condensation polymerisation involves joining monomers and eliminating a small molecule (usually H₂O).
  • Monomers must have two reactive functional groups.
  • This is different from addition polymerisation, which involves C=C double bonds and no loss of atoms.
  • The polymer backbone in condensation polymers contains atoms like oxygen or nitrogen.
  • Polyesters and polyamides are the two main types of condensation polymer.

Quick check

  1. What small molecule is most commonly eliminated during condensation polymerisation?1 mark
  2. What structural feature must a monomer have to undergo addition polymerisation?1 mark

4. Working Backwards: Identifying Monomers

A common exam task is to identify the monomers that formed a given section of a condensation polymer. This involves mentally 'hydrolysing' the polymer chain. Hydrolysis is the breakdown of a compound by reaction with water. To do this on paper, follow these steps: 1. Locate the linkage in the polymer backbone (either the amide link, -CO-NH-, or the ester link, -CO-O-). 2. Break the single bond between the carbonyl carbon and the nitrogen/oxygen atom (the C-N bond in an amide or the C-O bond in an ester). 3. 'Add water back' to the broken ends. Add an -OH group to the carbonyl carbon to re-form the carboxylic acid (-COOH). Add an -H atom to the nitrogen or oxygen atom to re-form the amine (-NH₂) or alcohol (-OH) group.

Hydrolysis of an Amide Link: -[...-CO-NH-...]- + H₂O → [...-COOH] + [H₂N-...]

Hydrolysis of an Ester Link: -[...-CO-O-...]- + H₂O → [...-COOH] + [HO-...]

Key term

Hydrolysis: The chemical breakdown of a compound by reaction with water, used conceptually to break down a condensation polymer into its constituent monomers.

Examiner insight

When identifying monomers from a polymer chain, marks are awarded for drawing the full, correct structures of both monomers, not just for naming them or drawing fragments.

Worked example 14 marks

A section of a polyamide polymer has the following structure: ...-NH-(CH₂)₆-NH-CO-(CH₂)₈-CO-... Identify the full structures of the two monomers used to make this polymer. [4]

  1. 1

    Step 1: Identify the amide link in the polymer chain. It is the -NH-CO- group.

  2. 2

    Step 2: Break the C-N bond within the amide link.

  3. 3

    Step 3: 'Add water back'. Add -OH to the C=O group to regenerate the carboxylic acid. This gives -CO-OH, or a -COOH group. The monomer is HOOC-(CH₂)₈-COOH. [2 marks]

  4. 4

    Step 4: Add -H to the -NH- group to regenerate the amine. This gives -NH-H, or an -NH₂ group. The monomer is H₂N-(CH₂)₆-NH₂. [2 marks]

Recap

  • To find monomers, first locate the ester or amide link in the polymer chain.
  • Break the bond between the carbonyl carbon and the adjacent O or N atom.
  • Add -OH to the carbonyl carbon to make a carboxylic acid.
  • Add -H to the O or N atom to make an alcohol or amine.
  • Draw the full structure of the two resulting monomer molecules.

Quick check

  1. When hydrolysing a polymer to find its monomers, what do you add to the carbonyl carbon after breaking the linkage?1 mark
  2. What do you add to the nitrogen atom of an amide link during hydrolysis?1 mark

5. Polymer Properties and Degradability

The properties of polymers are determined by their structure and the forces between their chains. Addition polymers like poly(ethene) have non-polar chains with only weak van der Waals forces between them, making them flexible and relatively weak. In contrast, polyamides like Nylon and Kevlar have polar C=O and N-H bonds that allow for strong hydrogen bonds to form between adjacent polymer chains. These numerous, strong intermolecular forces hold the chains rigidly together, giving materials like Kevlar immense tensile strength. Polyesters are also polar due to the ester link, but the hydrogen bonding is weaker than in polyamides. A key difference in properties is biodegradability. The C-C backbone of addition polymers is very strong and non-polar, making it inert to chemical or biological attack. They are non-biodegradable. However, the ester and amide links in condensation polymers can be broken down by hydrolysis. This means polyesters and polyamides are biodegradable, as water (often catalysed by acids, alkalis or enzymes) can break them back down into their monomers.

Key term

Hydrogen Bond: A strong type of intermolecular force between a hydrogen atom covalently bonded to a very electronegative atom (N or O) and another nearby electronegative atom.

Examiner insight

Top-level answers on polymer properties explicitly link the specific structural features (e.g., amide groups) to the type of intermolecular forces (hydrogen bonds) and then to the resulting macroscopic property (e.g., high tensile strength).

Common pitfall

Stating that condensation polymers are 'weak'. While the links can be hydrolysed over time, strong intermolecular forces can make the bulk materials incredibly strong (e.g., Kevlar).

Worked example 13 marks

Explain why Kevlar is an exceptionally strong material, making reference to its structure and bonding. [3]

  1. 1

    Step 1: Identify the key structural feature. Kevlar is a polyamide, containing polar -CO-NH- amide links. [1 mark]

  2. 2

    Step 2: Identify the intermolecular force. The N-H group in one chain and the C=O group in an adjacent chain are aligned. This allows for strong intermolecular hydrogen bonds to form between the polymer chains. [1 mark]

  3. 3

    Step 3: Link the forces to the property. These hydrogen bonds are numerous and strong, holding the rigid, linear polymer chains closely together in a sheet-like structure. A large amount of energy is required to overcome these forces, giving Kevlar its very high tensile strength. [1 mark]

Worked example 23 marks

A shopping bag made of poly(ethene) is discarded, while a disposable cup made of the polyester PLA (polylactic acid) is composted. Explain the difference in their long-term environmental fate. [3]

  1. 1

    Step 1: Describe poly(ethene). Poly(ethene) is an addition polymer with a non-polar backbone of strong C-C single bonds. [1 mark]

  2. 2

    Step 2: Explain its lack of degradation. This C-C backbone is chemically inert and cannot be broken down by microorganisms or hydrolysis. Therefore, the poly(ethene) bag is non-biodegradable and will persist in the environment for hundreds of years. [1 mark]

  3. 3

    Step 3: Explain the degradation of the polyester. PLA is a polyester containing ester links (-COO-). These links can be hydrolysed (broken down by water, aided by enzymes from microorganisms in the compost). This breaks the polymer down into its small monomer molecules (lactic acid), so it is biodegradable. [1 mark]

Recap

  • Polyamides are very strong due to hydrogen bonding between chains.
  • Addition polymers have weaker van der Waals forces and are generally less strong.
  • Condensation polymers (polyesters, polyamides) are biodegradable because the ester/amide links can be hydrolysed.
  • Addition polymers (polyalkenes) are non-biodegradable because their C-C backbone is chemically inert.
  • The properties of a polymer are directly related to its chemical structure and intermolecular forces.

Quick check

  1. What type of intermolecular force is responsible for the high strength of Nylon and Kevlar?1 mark
  2. Why is a polyester biodegradable while poly(propene) is not?2 marks

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. Define the term condensation polymerisation and state one key difference from addition polymerisation. [2]2 marks
  2. Terylene is a polyester formed from ethane-1,2-diol and benzene-1,4-dicarboxylic acid. Draw the repeat unit of Terylene. [2]2 marks
  3. Nylon 6,6 is a polyamide. Name the two general types of functional group that react to form nylon, and name the linkage formed. [3]3 marks
  4. The polymer below is a section of a condensation polymer. Draw the full structures of the two monomers from which it was made. Structure: ...-O-CH₂-CH₂-O-CO-(CH₂)₄-CO-... [4]4 marks
  5. Explain, with reference to intermolecular forces, why polyamides are typically stronger materials than polyalkenes. [3]3 marks
  6. What is meant by the term 'biodegradable'? Explain why polyesters are biodegradable but poly(ethene) is not. [4]4 marks
  7. A polymer is made from the monomer 5-aminopentanoic acid, H₂N(CH₂)₄COOH. i) State the type of polymerisation that occurs. ii) Draw the repeat unit of the polymer formed. [3]3 marks
  8. Acyl chlorides (R-COCl) can be used instead of carboxylic acids to make polymers. They react with alcohols to form a polyester. What small molecule would be eliminated in this reaction instead of water? [1]1 mark
  9. Kevlar is made from 1,4-diaminobenzene (molar mass 108.1 g/mol) and benzene-1,4-dicarboxylic acid (molar mass 166.1 g/mol). i) Calculate the molar mass of the repeat unit of Kevlar. ii) Explain your calculation. [4]4 marks
  10. Compare and contrast the formation of a polyamide and a polyester, mentioning the monomers required, the linkage formed, and the small molecule eliminated in both cases. [5]5 marks

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