Degradable polymers

· Degradable polymers = polymers that can be broken down into smaller molecules by environmental or chemical processes.
· CIE focus: compare poly(alkenes) with polyesters and polyamides.
· Key exam idea: degradability depends on the bonds in the polymer backbone.
· C–C bonds in poly(alkenes) are very resistant to attack.
· Ester links and amide links can be broken by hydrolysis.

Why poly(alkenes) are difficult to biodegrade

· Poly(alkenes) are made by addition polymerisation of alkene monomers.
· Their chains contain mainly strong, non-polar C–C and C–H bonds.
· They are chemically inert, so they resist attack by water, acids, alkalis and enzymes.
· Therefore, poly(alkenes) are usually non-biodegradable and persist in the environment.
· Examples: poly(ethene) and poly(propene).
· Exam phrase: poly(alkenes) are chemically inert and therefore difficult to biodegrade.

This image helps visualise polymer chains as long repeating structures. For poly(alkenes), the chain backbone contains strong C–C bonds, explaining why they are chemically inert and hard to biodegrade. Source

Degradation by light

· Some polymers can be degraded by the action of light, especially UV radiation.
· This is called photodegradation.
· UV light can provide energy to break bonds in polymer chains.
· Breaking long polymer chains forms shorter-chain molecules.
· Photodegradation is useful for reducing persistence of some plastics, but it may require exposure to sunlight.
· Exam phrase: some polymers can be degraded by the action of light.

The diagram shows how light energy can break bonds between repeating units in a polymer chain. This represents photodegradation, where long chains are converted into smaller molecules. Source

Biodegradable polyesters

· Polyesters contain ester linkages, written as –COO–.
· The ester bond is a reactive link in the polymer chain.
· Polyesters can be broken down by hydrolysis.
· Acidic hydrolysis breaks ester links to form carboxylic acids and alcohols.
· Alkaline hydrolysis breaks ester links to form carboxylate salts and alcohols.
· This makes polyesters more biodegradable than poly(alkenes).
· Exam phrase: polyesters are biodegradable by acidic and alkaline hydrolysis.

This diagram shows that a polyester chain contains ester links between repeating units. These ester links can be hydrolysed, which explains why polyesters are more degradable than poly(alkenes). Source

Biodegradable polyamides

· Polyamides contain amide linkages, written as –CONH–.
· The amide bond can be broken by hydrolysis.
· Acidic hydrolysis of polyamides produces carboxylic acid groups and ammonium salts.
· Alkaline hydrolysis of polyamides produces carboxylate salts and amines.
· Polyamides are therefore more biodegradable than poly(alkenes).
· Exam phrase: polyamides are biodegradable by acidic and alkaline hydrolysis.

This image shows the key reaction behind polyamide breakdown. In a polyamide, many amide links can be hydrolysed, causing the polymer chain to split into smaller molecules. Source

Poly(alkenes) vs polyesters and polyamides

· Poly(alkenes): mostly C–C backbone, chemically inert, difficult to biodegrade.
· Polyesters: contain ester links, broken by acidic or alkaline hydrolysis.
· Polyamides: contain amide links, broken by acidic or alkaline hydrolysis.
· Key comparison: polyesters and polyamides contain hydrolysable functional groups, but poly(alkenes) do not.
· In exam answers, always link structure → reactivity → degradability.

Common exam wording

· “Explain why poly(ethene) is difficult to biodegrade.”
· Answer: poly(ethene) has a chemically inert, non-polar C–C backbone that resists hydrolysis and enzyme attack.

· “Explain why polyesters are biodegradable.”
· Answer: polyesters contain ester links that can be broken by acidic or alkaline hydrolysis.

· “Explain why polyamides are biodegradable.”
· Answer: polyamides contain amide links that can be broken by acidic or alkaline hydrolysis.

· “State one way polymers can degrade.”
· Answer: action of light / photodegradation or acidic/alkaline hydrolysis.