Biodegradability of Polymers

A biodegradable polymer is one that can be broken down in the environment, for example by microorganisms, into smaller molecules.Polyalkenes such as poly(ethene) and poly(propene) contain only strong, non-polar C-C and C-H bonds. They are chemically inert: nothing in the environment attacks these bonds easily, so polyalkenes are non-biodegradable and can last for hundreds of years in landfill or in the sea.Polyesters and polyamides are different. Their ester and amide links contain a polar C=O group with a δ+ carbon atom, which can be attacked by nucleophiles such as water. The links can therefore be broken by hydrolysis, so these polymers are biodegradable, although the process may still be slow.Hydrolysis is faster with hot acid or alkali:
• a polyester hydrolysed by aqueous NaOH gives the diol and the sodium salt of the dicarboxylic acid
• a polyamide hydrolysed by aqueous HCl gives the dicarboxylic acid and the ammonium salt of the diamineWorked example. What are the products of hydrolysing nylon-6,6 with hot hydrochloric acid? Each amide link, -CO-NH-, is split: the C gains -OH and the N gains -H. This gives hexanedioic acid, HOOC(CH₂)₄COOH, and hexane-1,6-diamine, which in acid is protonated to its salt, ⁺H₃N(CH₂)₆NH₃⁺ 2Cl⁻.This difference comes straight from the type of polymerisation: addition polymers have a saturated carbon backbone, while condensation polymers have polar links that can be hydrolysed.