Biodegradability

A material is biodegradable if it can be broken down by microorganisms, such as bacteria and fungi, into small molecules like carbon dioxide and water.Poly(alkenes) are non-biodegradable. Their chains are saturated and non-polar: they contain only C–C and C–H bonds, which are strong and have no partial charges. There is no polar bond or functional group for water, acids or the enzymes of microorganisms to attack. The same chemical inertness that makes poly(ethene) and poly(propene) useful means they last for hundreds of years in landfill or in the sea.In the environment these plastics do not break down chemically, but sunlight and physical wear break them into smaller and smaller pieces. Tiny fragments, less than 5 mm across, are called microplastics. They spread through water and soil, and are eaten by marine animals and enter food chains.Condensation polymers are different. Polyesters and polyamides have polar ester or amide links in their chains. These links can be hydrolysed (broken by reaction with water, helped by acid, alkali or enzymes), so these polymers can be broken down. For example poly(lactic acid), PLA, a polyester made from plant sugars, is biodegradable under industrial composting conditions, whereas poly(propene) is not.Summary: whether a polymer biodegrades depends on its bonds. A backbone of only C–C and C–H bonds resists attack; polar links in the backbone give a point where it can be broken.