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9701 · 35.3

Degradable polymers — practice questions

Practice and worked examples for 9701 Degradable polymers. Short previews only — attempt the full question in MarkScheme against the official scheme.

Worked example 1

A biodegradable polymer Terylene, poly(ethylene terephthalate) or PET, has the repeating unit shown below. Draw the structures of the two monomers that would be formed upon complete acid hydrolysis of this polymer. \n [-O-CH₂-CH₂-O-CO-C₆H₄-CO-]n

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  1. Identify the functional group: The repeating unit contains an ester linkage (-COO-). Hydrolysis breaks this linkage.
  2. Identify the break point: The bond that breaks during hydrolysis is the acyl-oxygen bond (C-O) within the ester group. The C=O carbon gains an -OH group and the single-bonded oxygen gains an H atom.
  3. Visualise the break: [-O-CH₂-CH₂-O- || -CO-C₆H₄-CO-]n where '||' is the hydrolysis point.
  4. Form the products:
    • The alcohol part becomes HO-CH₂-CH₂-OH (ethane-1,2-diol).
    • The carboxylic acid part becomes HOOC-C₆H₄-COOH (benzene-1,4-dicarboxylic acid or terephthalic acid).
  5. Final Answer: The two monomers are ethane-1,2-diol and benzene-1,4-dicarboxylic acid.

Worked example 2

A sample of 14.4 g of poly(lactic acid), PLA, was completely hydrolysed. The molar mass of the repeating unit in PLA, -OCH(CH₃)CO-, is 72.0 g mol⁻¹. Calculate the mass of lactic acid (Mᵣ = 90.0) produced.

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  1. Write the balanced equation for hydrolysis: \n [-OCH(CH₃)CO-]n + nH₂O → n(HOCH(CH₃)COOH) \n This shows that 1 mole of repeating units produces 1 mole of lactic acid.
  2. Calculate moles of repeating units: \n Moles = Mass / Molar mass \n Moles of PLA repeating units = 14.4 g / 72.0 g mol⁻¹ = 0.200 mol.
  3. Use stoichiometry to find moles of product: \n From the equation, the mole ratio of (PLA repeating unit) : (lactic acid) is 1:1. \n Therefore, moles of lactic acid produced = 0.200 mol.
  4. Calculate mass of product: \n Mass = Moles × Molar mass \n Mass of lactic acid = 0.200 mol × 90.0 g mol⁻¹ = 18.0 g.
  5. Final Answer: 18.0 g of lactic acid is produced.
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