Addition polymers from alkenes. Condensation polymers, including polyesters and polyamides, from two monomers or one monomer with two functional groups. Identifying the repeating unit and the monomers. Hydrolysis of condensation polymers. Biodegradability and disposal of polymers.

What this dot point is asking

AQA wants you to draw addition polymers from alkene monomers, draw condensation polyesters and polyamides from their monomers, identify repeat units and deduce monomers, write equations for hydrolysis of condensation polymers, and discuss the disposal and biodegradability of polymers.

Addition polymers

Alkene monomers open their $C=C$ double bond and join end to end.

The repeat unit is drawn in brackets with bonds extending through the brackets and the subscript $n$. The backbone is a saturated, non-polar carbon chain, so addition polymers are chemically inert and not biodegradable. Different monomers give different properties: ethene gives poly(ethene) for bags and bottles, chloroethene gives poly(chloroethene) (PVC) for pipes, and tetrafluoroethene gives PTFE for non-stick coatings, but all share the inert saturated backbone.

Condensation polymers

Two monomers join with the loss of a small molecule (usually water).

Polyesters form from a diol and a dicarboxylic acid, joined by ester linkages ($-COO-$). Example: Terylene from benzene-1,4-dicarboxylic acid and ethane-1,2-diol.

Polyamides form from a diamine and a dicarboxylic acid (or a dicarbonyl chloride), joined by amide linkages ($-CONH-$). Examples: nylon-6,6 and Kevlar.

Hydrolysis

Condensation polymers can be hydrolysed by acid or alkali, breaking the ester or amide links and regenerating the monomers (or their salts). This is why polyesters and polyamides are more biodegradable than addition polymers, which have no such bonds to hydrolyse. The key structural difference is that a condensation polymer's backbone contains polar $C-O$ and $C-N$ bonds within the ester or amide links, which water (and enzymes in the environment) can attack, whereas an addition polymer's backbone is an unbroken chain of non-polar $C-C$ bonds with nothing for a nucleophile to attack. This single difference explains the contrast in durability and disposal: addition polymers persist for centuries, while condensation polymers can be broken down chemically back to their monomers, which also makes feedstock recycling possible.

Disposal of polymers

• Recycling: sorting and reusing reduces crude-oil use, but mixed plastics are hard to separate.
• Combustion for energy: releases $CO_2$ and can release toxic gases (e.g. HCl from PVC), so flue gases must be treated.
• Biodegradable and photodegradable polymers: break down naturally, easing landfill pressure.

Try this

Q1. What small molecule is lost when a polyamide forms from a diamine and a dicarboxylic acid? [1 mark]

• Cue. Water.

Q2. Why is a polyester more biodegradable than poly(ethene)? [2 marks]

• Cue. Polyester has ester bonds that can be hydrolysed; poly(ethene) has an unreactive saturated C-C backbone.