4.8 Synthetic PolymersEdexcel IGCSE Chemistry: Revision notes
Section 1
What is an addition polymer?
An addition polymer is a very long molecule formed by joining together many small molecules, called monomers, in an addition polymerisation reaction. The monomers are usually alkenes, and their C=C double bonds open up so that the monomer units link directly together with no other product formed — this is why it is an addition reaction, not a condensation reaction.
many monomer molecules → one long polymer chain
Section 2
How do we draw the repeat unit of an addition polymer?
The repeat unit shows the smallest repeating section of the polymer chain, drawn in square brackets with a bond extending through each bracket to show that the pattern continues, and a subscript 'n' outside to show it repeats many times.
Common addition polymers required for this course:
| Monomer | Polymer | Repeat unit backbone |
|---|---|---|
| Ethene (CH2=CH2) | Poly(ethene) | -CH2-CH2- |
| Propene (CH3CH=CH2) | Poly(propene) | -CH2-CH(CH3)- |
| Chloroethene (CH2=CHCl) | Poly(chloroethene), i.e. PVC | -CH2-CHCl- |
| Tetrafluoroethene (CF2=CF2) | Poly(tetrafluoroethene), i.e. PTFE | -CF2-CF2- |
To go from monomer to repeat unit: open the C=C double bond into a single bond, and add a bond through each end of the unit.
To go the other way — from repeat unit back to monomer — simply reform the C=C double bond between the two carbons and remove the extending bonds.
Section 3
What problems does disposing of addition polymers cause?
Addition polymers cause disposal problems because they are:
- Inert: the strong C-C covalent bonds along the backbone mean microorganisms cannot break them down
- Non-biodegradable: they do not decompose naturally, so they persist in landfill for a very long time
- A source of toxic gases if burned, especially polymers containing chlorine (like PVC), which release toxic hydrogen chloride gas on combustion
Section 4
How does condensation polymerisation form a polyester?
Condensation polymerisation of a dicarboxylic acid (a molecule with a -COOH group at each end) with a diol (a molecule with an -OH group at each end) produces a polyester and water, released at each link formed.
dicarboxylic acid + diol → polyester + water
Each new ester (-COO-) link releases one water molecule, which is why this is a condensation reaction rather than an addition reaction — unlike addition polymerisation, a small molecule (water) is lost.
Section 5
How do we draw a polyester repeat unit, and what are biopolyesters?
Given the formulae of a dicarboxylic acid and a diol, the polyester repeat unit is drawn by joining the -COOH end of the acid to the -OH end of the diol as an ester (-COO-) link, releasing water, and repeating this on the other end of each monomer, showing the repeat unit in brackets with bonds extending through them and a subscript n.
Some polyesters, called biopolyesters, are biodegradable — unlike addition polymers, they can be broken down naturally by microorganisms, making them a more environmentally friendly option for some applications.
Must Know
- Addition polymers form when many alkene monomers join together, with no other product
- Repeat units are drawn in brackets with extending bonds and a subscript n
- Poly(ethene), poly(propene), poly(chloroethene) and poly(tetrafluoroethene) are the required addition polymers
- Addition polymers are inert and non-biodegradable, and can release toxic gases (e.g. HCl from PVC) when burned
- Condensation polymerisation of a dicarboxylic acid with a diol forms a polyester and releases water at each link
- Biopolyesters are polyesters that are biodegradable
That's the notes covered.
Carry on to the next subtopic.