Poly(lactic acid), PLA, is a biodegradable condensation polymer made from lactic acid monomers. It is increasingly used as an alternative to poly(ethene) in disposable packaging. Explain why PLA is classified as a condensation polymer, and describe two advantages and one disadvantage of using PLA rather than poly(ethene) for packaging.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Poly(lactic acid), PLA, is formed from lactic acid (2-hydroxypropanoic acid), a monomer that contains both a hydroxyl group (–OH) and a carboxylic acid group (–COOH). PLA breaks down in the environment through hydrolysis of its ester linkages.
Model answer (5 marks)
PLA is a condensation polymer because each polymerisation step releases a small molecule – water – when the –OH of one lactic acid reacts with the –COOH of another, forming an ester linkage.
Advantages:
1. PLA is biodegradable; it breaks down in the environment, reducing long‑term pollution compared with non‑biodegradable PE.
2. PLA is produced from renewable plant‑based feedstocks (e.g. corn starch, sugar cane), lowering dependence on fossil fuels.
Disadvantage:
PLA has lower thermal stability; it melts or deforms at lower temperatures than PE, limiting its use in hot conditions and increasing production cost.
Advantages:
1. PLA is biodegradable; it breaks down in the environment, reducing long‑term pollution compared with non‑biodegradable PE.
2. PLA is produced from renewable plant‑based feedstocks (e.g. corn starch, sugar cane), lowering dependence on fossil fuels.
Disadvantage:
PLA has lower thermal stability; it melts or deforms at lower temperatures than PE, limiting its use in hot conditions and increasing production cost.
Examiner tips
- Show the water elimination step to justify condensation polymer; mention ester linkages explicitly. List each advantage and disadvantage clearly, using the exact wording from the mark scheme. Keep the answer concise – 5 marks, so avoid extra detail.
Common mistakes
- Confusing PLA with a polycondensation of lactide rather than lactic acid. Failing to mention the water elimination or ester linkage. Using vague terms like "green" instead of "biodegradable" or "renewable feedstock".
Mark scheme (5 marks)
- PLA is a condensation polymer because a small molecule (water) is eliminated/released during each polymerisation step
- The monomers join through ester linkages/bonds formed between the –OH and –COOH groups
- Advantage 1: PLA is biodegradable so it does not persist in landfill/oceans and causes less long-term environmental pollution
- Advantage 2: PLA is made from a renewable/plant-based feedstock (e.g. corn starch/sugar cane), reducing dependence on crude oil/fossil fuels
- Disadvantage: PLA has lower thermal stability / melts or deforms at lower temperatures than poly(ethene), limiting its use in hot conditions / or PLA is more expensive to produce than poly(ethene)
Key terms in this question
condensation polymer · biodegradable
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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