Degradable polymers

Addition polymers such as poly(ethene), poly(propene) and PVC have a backbone made entirely of strong, non-polar C–C bonds (with C–H bonds along the chain).

Because these bonds are non-polar, there is no δ+ carbon for a nucleophile (like water) to attack, and no functional group for an enzyme to recognise. The chain is therefore chemically unreactive (inert) and not biodegradable — it persists in the environment for a very long time. This chemical inertness is exactly what makes these plastics so useful (durable packaging), yet also what makes them a waste problem.

Condensation polymers (polyesters and polyamides) contain polar ester (–CO–O–) or amide (–CO–NH–) linkages along the chain. The carbonyl carbon is δ+, so it is open to attack by water (a nucleophile).

These linkages can therefore be hydrolysed — by acid, alkali, or enzymes — breaking the chain back toward its monomers: $\text{...–CO–O–...} + \text{H}_2\text{O} \rightarrow \text{–COOH} + \text{HO–...}$

Because the links can be cleaved, condensation polymers are (at least partly) biodegradable. Natural polyamides (proteins) and polyesters are readily broken down by enzymes for exactly this reason — the same hydrolysable linkage that forms them allows them to be recycled by nature.

Photodegradable polymers contain carbonyl (C=O) groups built into the chain. These absorb UV light, which provides enough energy to break bonds and fragment the polymer — useful for litter that is exposed to sunlight (though it does not help buried waste).

Disposal of plastics — the trade-offs:

• Landfill — cheap, but takes up space and inert plastics persist for decades.
• Incineration — recovers energy and reduces volume, but can release toxic gases (e.g. HCl from PVC, or CO/dioxins) and CO₂.
• Recycling — conserves resources and crude oil, but needs sorting/cleaning and the polymer quality can degrade.

A balanced answer weighs cost, energy, pollution and resource conservation rather than declaring one method "best".