Biodegradable polyesters
4 marks · NormalThe table shows the time taken for different types of polymers to degrade in soil:
| Polymer | Degradation time (months) |
|---|---|
| PLA (polylactic acid) | 12 |
| Poly(ethene) | 400 |
| Poly(ester) | 18 |
| Poly(propene) | 350 |
Use the data to answer the questions.
Which polymer is the most biodegradable? Give the evidence from the table.
Explain, using your knowledge of chemical structure, why PLA degrades faster than poly(ethene) or poly(propene).
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) PLA (polylactic acid), because it has the shortest degradation time (12 months).
(b) PLA degrades faster than poly(ethene) or poly(propene) because PLA contains ester linkages in its structure. These ester bonds can be broken down by hydrolysis or by enzymes, making the polymer biodegradable. Poly(ethene) and poly(propene) do not contain ester linkages and are less reactive, so they are much less biodegradable.
Common mistakes: Quoting the wrong degradation time; Not mentioning ester linkage
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Identifies PLA as the most biodegradable; uses table evidence (shortest time).
M1 (b) · 1 mark · Mentions that PLA contains ester linkages while others do not.
A1 (b) · 1 mark · Explains that ester links allow hydrolysis or breakdown by enzymes.
A2 (b) · 1 mark · Compares to poly(ethene)/poly(propene) as addition polymers lacking reactive (ester) linkages.
Worked solution
a) PLA is the most biodegradable as it degrades in just 12 months, which is much quicker than the other polymers (evidence from table).
b) PLA has ester linkages in its structure, making it able to be broken down by water or enzymes. Poly(ethene) and poly(propene) are addition polymers with only strong carbon-carbon bonds, which do not easily degrade.