35.3 Degradable polymers
- Syllabus
- 9701–2028–2029
- Topic
- 35.3
- Level
- A2
Learning objectives
35.3.1Poly(alkenes) are chemically inert• Recognise that poly(alkenes) are chemically inert and can therefore be difficult to biodegrade35.3.2Some polymers can be degraded by the action• Recognise that some polymers can be degraded by the action of light35.3.3Biodegradable polyesters and polyamides• Recognise that polyesters and polyamides are biodegradable by acidic and alkaline hydrolysis
Poly(alkenes) are chemically inert and often persist in the environment
In a poly(alkene), the monomer C=C bonds have become strong C-C single bonds in a saturated, largely non-polar carbon backbone. With no ester or amide link to hydrolyse, the material is relatively chemically inert.
This stability gives useful durability but also makes many poly(alkenes) difficult for organisms and ordinary environmental conditions to biodegrade.
Poly(ethene) has no hydrolysable backbone functional group. A polyester or polyamide, by contrast, contains polar backbone links that can be cleaved under suitable conditions.
Chemically inert does not mean indestructible or unable to react under extreme conditions; it means resistant under the ordinary biological and environmental conditions relevant to biodegradation.
Light can break down some polymers by photodegradation
Some polymers absorb light energy, especially ultraviolet light, and undergo bond breaking that shortens or fragments their chains. This light-driven breakdown is photodegradation.
Whether it occurs depends on the polymer structure, exposure to light and additives such as stabilisers. A polymer buried away from light may not photodegrade effectively.
Chain scission can make a material weaker or brittle and produce smaller fragments. This is evidence of degradation by light, but it does not by itself prove complete biological conversion to simple products.
Do not equate visible fragmentation with complete biodegradation: the syllabus claim is that some polymers can be degraded by light, not that all polymers disappear fully in sunlight.
Acidic and alkaline hydrolysis cleave polyester and polyamide chains
Polyesters contain ester links and polyamides contain amide links in their backbones. Aqueous acid or aqueous alkali can hydrolyse these links, cutting long chains into smaller molecules, so these polymer classes are biodegradable by acidic and alkaline hydrolysis.
| Polymer link | Acidic hydrolysis gives | Alkaline hydrolysis gives |
|---|---|---|
| polyester, -CO-O- | carboxylic acid + alcohol fragments | carboxylate salt + alcohol fragments |
| polyamide, -CO-NH- | carboxylic acid + protonated amine/ammonium fragments | carboxylate salt + amine/ammonia fragments |
Hydrolysis is possible because the chain contains susceptible ester or amide links. Poly(alkenes) lack these links, so the same acid/alkali route does not readily cut their saturated C-C backbone.
A hydrolysable link makes breakdown chemically possible, not automatically rapid in every environment. Temperature, pH, water access and polymer structure affect the rate.