Contents: 6 sections
Syllabus points
- Describe addition polymerisation and deduce the repeat unit from a monomer, and the monomer from a section of polymer.
- Describe condensation polymerisation to form polyesters and polyamides.
- Deduce the repeat unit and the monomers of a condensation polymer.
- Compare the properties of addition and condensation polymers, including their resistance to hydrolysis.
- Discuss the environmental problems of polymer disposal and the ways they are addressed.
Addition polymerisation
An addition polymer forms when many alkene monomers join by the opening of their C=C double bonds. Nothing else is produced, so the polymer has the same empirical formula as the monomer.
n CH₂=CH₂ → -(CH₂-CH₂)ₙ-
| Monomer | Polymer |
|---|---|
| Ethene | Poly(ethene) |
| Propene | Poly(propene) |
| Chloroethene | Poly(chloroethene), PVC |
| Phenylethene | Poly(phenylethene), polystyrene |
| Tetrafluoroethene | PTFE |
Deducing the repeat unit
From the monomer: draw the C=C, open the double bond, and put a bond out from each carbon. Keep all the side groups exactly where they were.
The repeat unit must be drawn with the two bonds sticking out and the square brackets with the n. A common way to lose the mark is to draw the repeat unit with a double bond still in it, which would mean the polymerisation had not happened.
Deducing the monomer
Working backwards from a section of polymer is the harder direction and appears often.
- Find the repeating pattern in the chain. The backbone of an addition polymer is all carbon, and the repeat unit is two carbon atoms long.
- Take one repeat unit of two carbons.
- Put the double bond back between them and remove the two spare bonds.
So a chain drawn as -CH₂-CH(CH₃)-CH₂-CH(CH₃)- has the repeat unit -CH₂-CH(CH₃)-, and the monomer is CH₂=CH(CH₃), which is propene.
The step people get wrong is taking a repeat unit of the wrong length. Count carefully until the pattern genuinely repeats.
Condensation polymerisation
A condensation polymer forms when monomers join with the loss of a small molecule, usually water or HCl. Each monomer needs two functional groups, one at each end, so that the chain can grow in both directions.
Polyesters
Made from a diol and a dicarboxylic acid, joined by ester links with the loss of water. Terylene, also called PET, is the standard example.
The ester link is -COO-.
Polyamides
Made from a diamine and a dicarboxylic acid, joined by amide links with the loss of water. Nylon is the example, and proteins are natural polyamides.
The amide link is -CONH-.
An acyl chloride can be used in place of the acid, in which case HCl is lost rather than water, and the reaction is faster.
Deducing the monomers
Find the linkage in the chain and break it, then add back what was lost:
- Break an ester link between the carbonyl carbon and the single-bonded oxygen. Add OH to the carbon side to make the acid, and H to the oxygen side to make the alcohol.
- Break an amide link between the carbonyl carbon and the nitrogen. Add OH to the carbon side, and H to the nitrogen side.
Adding water back is the step that gets forgotten, and it leaves the answer one OH and one H short.
Comparing the two
| Addition | Condensation | |
|---|---|---|
| Monomer | One, an alkene | Two, each with two functional groups |
| Small molecule lost | None | Water or HCl |
| Backbone | Carbon only | Contains C-O or C-N links |
| Hydrolysed? | No | Yes |
The last row is the most important, and it explains the environmental difference.
An addition polymer has a backbone of nothing but strong, non-polar carbon-carbon bonds. There is no δ+ carbon for a nucleophile to attack, so it is not hydrolysed and is chemically inert. That is exactly what makes poly(ethene) useful and also what makes it persist in the environment for centuries.
A condensation polymer has polar ester or amide links in its backbone, which can be hydrolysed by acid or alkali, and slowly by water. So polyesters and polyamides are biodegradable, at least in principle.
Disposal
Several routes, each with a real drawback:
- Landfill is cheap but takes space, and addition polymers do not break down.
- Incineration recovers energy but releases carbon dioxide, and burning PVC releases HCl, which is toxic and acidic, so the flue gases must be scrubbed.
- Recycling is the best option in principle but requires sorting by polymer type, and mixed or contaminated plastic is difficult and expensive to process.
- Biodegradable and photodegradable polymers are being developed, and condensation polymers are inherently better suited to this.
- Cracking waste polymers back to monomers or to fuel is a form of chemical recycling.
The judgement the syllabus expects is that no single route solves it, and that the properties which make polymers useful, their durability and inertness, are the same properties that make disposal difficult.
Common mistakes
- Drawing a repeat unit that still contains a C=C double bond.
- Omitting the two bonds sticking out of the repeat unit, or the brackets and the n.
- Taking a repeat unit of the wrong length when deducing a monomer from a polymer chain.
- Forgetting to add water back when breaking an ester or amide link to find the monomers.
- Saying addition polymers are biodegradable. Their carbon backbone cannot be hydrolysed.
- Forgetting that burning PVC produces HCl, which is the reason it needs special treatment.