Revise Degradable polymers for Cambridge 9701 (35.3)

Polymers formed by addition polymerisation of alkenes, such as poly(ethene) and poly(propene), are remarkably resistant to environmental degradation. Their structure consists of a long backbone of carbon atoms joined by strong, non-polar covalent sigma bonds. The C-H bonds are also strong and non-polar. This lack of polarity and absence of any susceptible functional groups means they are not attacked by nucleophiles like water, nor are they recognised by the enzymes of microorganisms. Consequently, they are non-biodegradable and accumulate in the environment.

Biodegradable polymers are designed with 'weak links' in their chains that can be broken by natural processes. This is typically achieved by forming polymers via condensation polymerisation to include functional groups like esters or amides. These groups contain polar bonds ($C^{\delta+}=O^{\delta-}$) which are susceptible to nucleophilic attack by water in a hydrolysis reaction. This process is often catalysed by acids, alkalis, or, crucially in the environment, by enzymes produced by microorganisms.

A prime example is poly(lactic acid), or PLA. Lactic acid (2-hydroxypropanoic acid) is a hydroxycarboxylic acid, meaning it has both a hydroxyl (-OH) and a carboxylic acid (-COOH) group. It can polymerise to form a polyester. The resulting ester links can be hydrolysed back to the monomer.

$\text{Hydrolysis of PLA:} \\ n(\text{HOCH(CH}_3\text{)COOH}) \xrightarrow{\text{Polymerisation}} \underbrace{[-OCH(CH_3)CO-]_n}_{\text{Poly(lactic acid)}} + nH_2O \\ \underbrace{[-OCH(CH_3)CO-]_n}_{\text{Poly(lactic acid)}} + nH_2O \xrightarrow{\text{Hydrolysis (H}^+ \text{/OH}^- \text{/enzymes)}} n\underbrace{(\text{HOCH(CH}_3\text{)COOH})}_{\text{Lactic acid}}$

An alternative approach is to make polymers that degrade upon exposure to light. Photodegradation does not rely on microbial action but on photochemical reactions. To achieve this, carbonyl groups (C=O) are deliberately incorporated into the polymer backbone at intervals. The pi-bond in the carbonyl group can absorb photons of ultraviolet (UV) light from sunlight. This absorption of energy excites the molecule and is sufficient to cause the cleavage of the adjacent, weaker C-C sigma bonds in the polymer chain. This fragments the long polymer chain into shorter pieces, causing the plastic to become brittle and break apart.