Polyalkenes are chemically inert and non-biodegradable.

The idea

An addition polymer made from alkenes has a backbone built only from carbon and hydrogen. Carbon and hydrogen have almost the same electronegativity, so the C-C and C-H bonds are essentially non-polar and no carbon atom in the chain carries a δ⁺ charge. A nucleophile such as water or a hydroxide ion has nothing to attack, and the C-C bonds are strong, so acids, alkalis, water and the enzymes that micro-organisms use simply leave the chain alone. That inertness is exactly why polyalkenes are useful for packaging and pipes, and exactly why they are a problem afterwards: buried in landfill, a polyalkene article stays essentially unchanged for hundreds of years.

What earns the marks

• The backbone containing only carbon and hydrogen atoms.
• Those bonds being non-polar, so there is no δ⁺ carbon atom.
• Stating that no nucleophile can attack the chain, so it is not hydrolysed.
• Linking that inertness to persistence in landfill.

Common mistakes and how to put them right

• Saying polyalkenes are unreactive because they still contain C=C double bonds.

• Explaining inertness by chain length rather than by the polarity of the bonds.

• Confusing chemical inertness with being insoluble in water.

• Claiming that because some plastics break up in sunlight they are therefore biodegradable.

Exam tips

• The word the examiner wants is 'non-polar'. Write it explicitly rather than implying it.
• Break the argument into two halves: no polar site, therefore no nucleophilic attack, therefore no hydrolysis.