Part 1 of 6Addition Polymerisation
The polymer name is poly followed by the monomer name in brackets.
Monomer
- Alkene molecules with a C=C bond act as the monomers that join in a long chain.
Process
- The π bond in each monomer breaks and the molecules link by σ bonds to form a long saturated chain.
Repeat unit
- The repeat unit is the part of the polymer chain that repeats, drawn with open bonds on both sides.
Only one product
- All atoms of the monomer end up in the polymer, so addition polymerisation has a 100% atom economy.
Examples
- Ethene gives poly(ethene), propene gives poly(propene) and chloroethene gives poly(chloroethene), also called PVC.
Part 2 of 6Drawing Repeat Units
1
Draw the monomer so that the C=C is in the centre, with other groups above and below.
2
Replace the double bond with a single bond, keeping the groups as they were.
3
Add a bond that extends outward from each carbon, shown through the end brackets of the repeat unit.
4
For poly(propene) the repeat unit is –CH₂–CH(CH₃)–, from the propene CH₂=CHCH₃.
5
To find the monomer from a polymer, take one repeat unit and put the double bond back between the two chain carbons.
Part 3 of 6Properties and Uses
Properties and Uses
Chemically inert
- The long chains of strong, non-polar C–C and C–H bonds resist attack by most reagents.
Intermolecular forces
- Chains attract each other by London forces, with stronger polar forces in polymers like PVC that contain C–Cl bonds.
Uses
- Poly(ethene) makes bags and bottles, poly(propene) makes crates and ropes, and PVC makes pipes and window frames.
Persistence
- Because they are inert, addition polymers are not biodegradable and can persist in the environment for a long time.
| Monomer | Polymer | Repeat unit |
|---|---|---|
| Ethene, CH₂=CH₂ | Poly(ethene) | –CH₂–CH₂– |
| Propene, CH₂=CHCH₃ | Poly(propene) | –CH₂–CH(CH₃)– |
| Chloroethene, CH₂=CHCl | Poly(chloroethene) | –CH₂–CHCl– |
Part 4 of 6Disposal and Recycling
Landfill
- Burying polymer waste uses land and the polymers do not break down, so it is a poor long-term option.
Incineration
- Burning polymers can release energy for heating or electricity but produces carbon dioxide and, for PVC, toxic hydrogen chloride.
Recycling
- Polymers can be sorted, melted and remoulded, saving raw materials but requiring careful separation.
Feedstock recycling
- Heating polymers breaks them down into monomers or other small molecules which can be reused to make new plastics.
Biodegradable alternatives
- Polymers designed to break down reduce waste, though conditions for decomposition may not be met in all disposal routes.
Part 5 of 6Worked Example
Question
- A sample of poly(chloroethene) has a relative molecular mass of 62 500.
- How many repeat units does one chain contain?
- (Ar: H = 1.0, C = 12.0, Cl = 35.5)
- The monomer is chloroethene, C₂H₃Cl, and the repeat unit has the same formula.
- Mr of repeat unit = 2 × 12.0 + 3 × 1.0 + 35.5 = 62.5.
- Number of repeat units = 62 500 / 62.5.
- = 1000.
Answer 1000 repeat units
Part 6 of 6Exam Checklist & Tips
4.3.5 – Key Points for the Exam
- Addition polymer A long chain molecule formed when many alkene monomers join without losing any atoms.
- Monomer A small molecule that can be joined with others to form a polymer.
- Repeat unit The section of the polymer chain that repeats along its length.
- Feedstock recycling Breaking polymers into small molecules that can be used as raw materials.
Common Mistakes
- Leaving the C=C double bond in the repeat unit; it becomes a single bond.
- Forgetting the extension bonds through the brackets of the repeat unit.
- Stating that addition polymers are biodegradable because they are 'organic'.
- Drawing the repeat unit with only the two chain carbons and omitting side groups.