Synthetic polymers
Addition polymerisation
In , many small molecules () join together to make one very long molecule, a . The double bond in each monomer opens up to link with its neighbours. The polymer is the only product.
Ethene makes poly(ethene):
The part in brackets is the , and n stands for a very large number. The repeating unit has the same atoms as the monomer, but with a single bond, and with bonds sticking out of the brackets to join the next unit. Propene makes poly(propene) in the same way.
Key terms
Condensation polymerisation (Higher tier only)
In , each monomer has . When monomers join, a , usually water, is given off at each link.
are made from two different monomers:
- a diol, with two –OH groups
- a , with two –COOH groups
They join through ester links, releasing water each time. For n of each monomer, 2n molecules of water are made.
| Addition | Condensation | |
|---|---|---|
| Monomers | Alkenes (C=C) | Two functional groups each |
| Monomer types | Usually one | Usually two different ones |
| Products | Polymer only | Polymer + water (or another small molecule) |
Key terms
From a polymer back to its monomer
You can work out the monomer from an addition polymer's repeating unit:
- Take the two carbon atoms in the repeating unit.
- Remove the bonds that stick out of the brackets.
- Put a between the two carbons.
For example, poly(chloroethene), or PVC, has the repeating unit –CH₂–CHCl–, so its monomer is chloroethene, CH₂=CHCl.
The polymer's name is the monomer's name in brackets after 'poly': propene makes poly(propene), and chloroethene makes poly(chloroethene).
Exam tip
When you draw a repeating unit, include the brackets, the n after them, and a bond reaching through each bracket. Every carbon atom still needs four bonds.