Addition polymersAQA A-Level Chemistry: Revision notes

Addition polymers are formed when many alkene or substituted alkene molecules, called monomers, join together. The C=C double bond in each monomer opens and the molecules link into a long saturated chain. Nothing is lost, so the polymer is the only product.

For example, n CH₂=CHCl → (–CH₂–CHCl–)ₙ. The part in brackets is the repeating unit, and n is a large number.

To draw a repeating unit, put the two carbon atoms of the C=C bond in the chain and place all other groups on side branches. For propene, CH₃CH=CH₂, the unit is (–CH₂–CH(CH₃)–).

To find the monomer from a section of chain, take a two-carbon backbone unit and put the C=C bond back. To get the repeating unit from a longer section, pick any two adjacent backbone carbon atoms that repeat.

The IUPAC name is poly(monomer name), for example poly(propene) and poly(chloroethene).

Addition polymers are unreactive because the C=C bonds of the monomers have been used up. The chain contains only saturated, strong, non-polar C–C and C–H bonds, so it is not attacked by nucleophiles, electrophiles or other common reagents.

This is useful for containers and pipes, but it means addition polymers are not biodegradable and persist in the environment.

Poly(ethene) and poly(propene) are non-polar, so the chains are held together by London forces (induced dipole–dipole). Because the chains are very long, there are many points of contact, so the total force between chains is large and the polymers are solids.

In poly(chloroethene), the polar C–Cl bonds also give permanent dipole–dipole forces between chains, so PVC is more rigid than poly(ethene).

PVC is rigid when unmodified, and is used for window frames and drainpipes. Adding a plasticiser places small molecules between the chains. This increases the spacing, weakens the forces between chains and lets them slide, so the PVC becomes flexible and can be used for electrical cable insulation and flooring.

Knowledge of how polymers are produced and how their properties can be controlled has developed over time, allowing chemists to design polymers for specific uses.