Biodegradability of Polymers Revision Notes | A Level AQA Chemistry

1. Why Biodegradability Matters for Polymers

Plastic waste is one of the most persistent environmental problems on the planet — and the reason comes down to chemistry. Whether a polymer breaks down in the environment depends entirely on the bonds within its backbone.

At A-level, you need to be able to explain — not merely state — why some polymers resist degradation while others do not. The answer lies in comparing the bond types found in polyalkenes versus those in polyesters and polyamides.

2. Polyalkenes: Chemically Inert and Non-Biodegradable

Polyalkenes such as poly(ethene) and poly(propene) are formed by addition polymerisation of alkene monomers. The resulting polymer backbone consists entirely of carbon-carbon (C-C) single bonds and carbon-hydrogen (C-H) bonds.

These bonds are non-polar and have no sites that water molecules or biological enzymes can attack. There are no functional groups — no ester linkages, no amide linkages, no carbonyl groups — anywhere along the chain.

Because there are no hydrolysable bonds present, water cannot break the polymer chain. Microorganisms in soil or water cannot produce enzymes capable of cleaving C-C bonds under ambient conditions either. The result: polyalkenes persist in the environment for hundreds of years, which is why they are classified as non-biodegradable.

The structure above represents a section of a polyalkene chain. Notice the unbroken sequence of C-C bonds with no heteroatoms (oxygen or nitrogen) present. This uniform, non-polar backbone is precisely what makes polyalkenes so resistant to chemical attack.

3. Polyesters and Polyamides: Hydrolysable Condensation Polymers

Now contrast polyalkenes with condensation polymers. Polyesters and polyamides are formed when monomers join together with the elimination of a small molecule. When a dicarboxylic acid reacts, that small molecule is often water; when a diacyl chloride reacts, the small molecule is HCl. This process creates characteristic functional group linkages within the polymer backbone.

In a polyester, the repeating unit contains an ester linkage: $-\text{COO}-$. In a polyamide, the repeating unit contains an amide linkage: $-\text{CONH}-$. Both of these linkages contain a carbonyl group ($\text{C=O}$) bonded to a heteroatom (oxygen or nitrogen), making them polar and susceptible to nucleophilic attack by water.

The carbonyl carbon in an ester or amide linkage carries a partial positive charge ($\delta+$) due to the electronegative oxygen or nitrogen attached to it. Water molecules — acting as nucleophiles — can attack this carbon, breaking the bond and regenerating the original monomer functional groups.

The same logic applies to polyamides. The amide linkage $-\text{CONH}-$ is also susceptible to hydrolysis, though it is generally somewhat less reactive than an ester linkage under mild conditions. Under acidic or alkaline conditions, or in the presence of appropriate enzymes, the amide bond is cleaved to regenerate a carboxylic acid and an amine.

Worked example: Writing the Hydrolysis Products of a Polyester

A polyester is formed from benzene-1,4-dicarboxylic acid (terephthalic acid) and ethane-1,2-diol. Write the equation for the acid hydrolysis of this polyester and identify the products.

1. Repeating unit: $-\text{OC}-\text{C}_6\text{H}_4-\text{CO}-\text{O}-\text{CH}_2\text{CH}_2-\text{O}-$ The $-\text{COO}-$ groups are the ester linkages. — In a polyester, the ester linkage $-\text{COO}-$ is the hydrolysable bond. Locate it in the repeating unit before writing the equation.
2. $(-\text{OC}-\text{C}_6\text{H}_4-\text{CO}-\text{O}-\text{CH}_2\text{CH}_2-\text{O}-)_n + n\,\text{H}_2\text{O} \rightarrow n\,\text{HOOC}-\text{C}_6\text{H}_4-\text{COOH} + n\,\text{HOCH}_2\text{CH}_2\text{OH}$ — Each ester linkage reacts with one molecule of water. The polymer chain is broken down into its two original monomers.
3. Product 1: benzene-1,4-dicarboxylic acid (terephthalic acid) Product 2: ethane-1,2-diol — Acid hydrolysis of a polyester regenerates the diol and the dicarboxylic acid monomers. Under alkaline hydrolysis, the carboxylic acid would instead form its carboxylate salt.
4. The ester linkages are cleaved by water (and by microbial enzymes in the environment), so the polymer chain is broken into small molecules that microorganisms can metabolise. — This is why polyesters are biodegradable: the hydrolysable ester linkages allow the polymer to be broken down under environmental conditions.

Answer: Acid hydrolysis of the polyester produces benzene-1,4-dicarboxylic acid and ethane-1,2-diol. The ester linkages are cleaved by water, regenerating the original monomers and making the polymer biodegradable.

4. Comparing Polyalkenes, Polyesters, and Polyamides

Now that the mechanism is clear, it helps to place all three polymer types side by side. The key variable is always the same: what bonds are present in the backbone, and can water react with them?

The table makes the pattern clear: biodegradability tracks directly with the presence of hydrolysable linkages. Polyalkenes have none; polyesters and polyamides have them built into every repeating unit.

In the environment, microorganisms produce enzymes (hydrolases) that catalyse the same hydrolysis reaction you have seen above. These enzymes can attack ester and amide bonds, breaking the polymer into fragments small enough to be absorbed and metabolised. Polyalkene chains offer no such target, so they accumulate as persistent waste.

5. Exam Technique and Common Mistakes

Questions on biodegradability of polymers are frequently 'explain' questions, meaning you must give the reason — not just the observation. Stating 'polyalkenes are non-biodegradable' earns no marks on its own; you must link this to the absence of hydrolysable bonds.

When asked to compare polyalkenes with polyesters or polyamides, structure your answer around three elements: the bond type present, whether hydrolysis can occur, and the consequence for biodegradability. This mirrors the mark scheme logic for AQA 'explain' and 'compare' questions.

The model answer above demonstrates the mark-winning structure for a 6-mark 'explain' question: it addresses polyalkenes first (stating the bond type and why hydrolysis cannot occur), then contrasts with polyesters and polyamides (naming the linkage, explaining the mechanism, and identifying the products). Every sentence earns its place by directly addressing a mark point.

To consolidate the full picture: the biodegradability of a polymer is determined entirely by its backbone bond types. Polyalkenes — non-polar, all-carbon backbones — are chemically inert and non-biodegradable. Polyesters and polyamides — with polar, hydrolysable ester or amide linkages — can be broken down by hydrolysis and are therefore biodegradable. This single structural distinction underpins every exam question on this topic.