Biodegradable and Non-Biodegradable Polymers

A polymer is called biodegradable if micro-organisms present in soil or water (bacteria, fungi) can break its long chains down into small, harmless molecules -- typically carbon dioxide, water and biomass -- within a reasonably short time under natural environmental conditions; a polymer that resists this microbial attack and instead persists largely unchanged for many years, decades or longer is called non-biodegradable.

Why linkage type matters. The key structural factor is the chemical nature of the bonds joining the repeat units. Condensation polymers whose backbone contains hydrolysable linkages -- ester ($-\text{CO--O}-$) or amide ($-\text{CO--NH}-$) groups, the same functional groups that living cells' own enzymes (esterases, proteases/amidases) routinely break in natural biomolecules such as fats and proteins -- are generally far more accessible to microbial enzymatic attack, because those enzymes can recognise and hydrolyse the SAME linkage type in the synthetic polymer. Addition polymers, by contrast, typically have a purely saturated carbon-carbon backbone ($-\text{CH}_2\text{--CH}_2-$ repeating in polythene, for instance) with no such hydrolysable linkage anywhere along the chain -- ordinary soil micro-organisms have no enzyme evolved to attack an inert $\text{C--C}$ bond buried inside a bulky, largely water-insoluble plastic, so these polymers persist essentially unchanged for a very long time (estimates for polythene commonly run to several hundred years under typical landfill/marine conditions). …