Biodegradability and Disposal of Polymers

Specification Reference Organic chemistry, Polymers 3.3.12.2

Quick Notes

Polyalkenes (e.g., polyethene, polypropene) are chemically inert and non-biodegradable.
Polyesters and polyamides are biodegradable because they can undergo hydrolysis at their ester (–COO–) and amide (–CONH–) links.
Disposal methods:
- Landfill – simple storage but uses space.
- Incineration – releases energy but produces CO₂ and possible toxic gases.
- Recycling – reduces waste but requires sorting and processing.

Full Notes

Why Are Polyalkenes Non-Biodegradable?

Polyalkenes (e.g., polyethene, polypropene) are chemically unreactive.

AQA A-Level Chemistry diagram showing polyalkene chains with strong non-polar C–C bonds that resist hydrolysis

The C–C bonds that hold repeating units together are non-polar and strong, so they resist hydrolysis. Microorganisms lack enzymes to break these bonds, making polyalkenes non-biodegradable. These plastics can remain in the environment for hundreds of years.

Why Are Polyesters and Polyamides Biodegradable?

Polyesters contain ester (–COO–) bonds and polyamides contain amide (–CONH–) bonds. These functional groups can be hydrolysed by water or an acid/alkali, allowing them to be broken down.

AQA A-Level Chemistry schematic showing hydrolysis breaking ester and amide links in condensation polymers

Microorganisms produce enzymes (e.g., esterases, proteases) that catalyse these hydrolysis reactions, so polyesters and polyamides are biodegradable.

AQA A-Level Chemistry example of polyester hydrolysis forming a diol and a dicarboxylic acid

AQA A-Level Chemistry example of polyamide hydrolysis forming a diamine and a dicarboxylic acid

Methods of Polymer Disposal

Landfill

Advantages: Easy, cheap disposal method.

Disadvantages: Takes up space; non-biodegradable waste remains for a long time.

Incineration

Advantages: Reduces waste volume and releases energy that can be used (e.g., electricity production).

Disadvantages: Produces CO₂ and toxic gases (e.g., HCl from PVC combustion).

Recycling

Advantages: Reduces waste and conserves raw materials.

Disadvantages: Sorting is difficult and time-consuming; only certain polymers can be recycled; polymers degrade after multiple recycling cycles.

Biodegradable Plastics

Advantages: Can break down naturally in suitable conditions.

Disadvantages: Expensive to produce and requires specific environmental conditions.

Recycling Polymers

Mechanical recycling: Plastics are sorted, washed, melted, and remoulded.
Feedstock recycling: Plastics are broken down into monomers and repolymerised.
Chemical recycling: Converts polymers into useful chemicals or fuels.

Challenges: Sorting is difficult; polymers can degrade after multiple cycles; not all plastics are recyclable.

Biodegradability of Polymers

Polymer	Bond between repeating units	Biodegradability	Reason
Polyalkenes	C–C	Non-biodegradable	Strong, non-polar bonds cannot undergo hydrolysis.
Polyesters	–COO–	Biodegradable	Hydrolysis breaks ester bonds.
Polyamides	–CONH–	Biodegradable	Hydrolysis breaks amide bonds.

Summary

Polyalkenes are non-biodegradable because their strong, non-polar C–C backbones resist hydrolysis.
Polyesters and polyamides are biodegradable: ester and amide links can be hydrolysed (often enzyme-catalysed).
Disposal options include landfill, incineration, and recycling — each with advantages and drawbacks.
Recycling can be mechanical, feedstock, or chemical; sorting and polymer degradation are major challenges.
Understanding polymer structure explains environmental persistence and informs sustainable disposal choices.