3.4.3Organic Chemistry
Addition Polymers
5 min read 6 parts Lesson 17 of 55
Section 1 of 6Forming an Addition Polymer
Addition polymerisation turns many small unsaturated molecules into one very long saturated chain.
1
The monomer must contain a C=C double bond, as in ethene, propene or chloroethene.
2
Under suitable conditions the π bond in each monomer breaks and each carbon forms a new single bond to a neighbouring monomer.
3
Thousands of monomers add on in turn, so the chain grows without losing any atoms.
4
Because nothing is lost, the polymer is the only product and the reaction has 100% atom economy.
5
The polymer is named by putting the monomer name in brackets after poly, for example poly(chloroethene).
Section 2 of 6Repeating Units and Monomers
The repeating unit is the smallest section that, repeated end to end, builds the whole chain.
Repeating unit
- Draw the monomer with the double bond replaced by a single bond and open bonds extending at each end, enclosed in brackets with n outside.
Ethene
- The monomer CH₂=CH₂ gives the repeating unit –CH₂–CH₂– and the polymer poly(ethene).
Propene
- The monomer CH₃CH=CH₂ gives –CH(CH₃)–CH₂– because the methyl group stays as a side group on the chain.
Finding a monomer
- Take two adjacent carbon atoms of the main chain, convert the single bond between them back to C=C and put the side groups back on.
Chain backbone
- The backbone of an addition polymer contains only carbon atoms, so any oxygen or halogen in the polymer is part of a side group.
Section 3 of 6Properties and Uses
Properties and Uses
Intermolecular forces
- Chains attract each other by van der Waals forces, which are weak individually but very strong overall because chains are long.
PVC rigidity
- The polar C–Cl bonds add permanent dipole attractions between chains, so PVC is harder and more rigid than poly(ethene).
Plasticisers
- Small molecules added to PVC sit between chains, push them apart and weaken attractions, making the material flexible.
Density of poly(ethene)
- Branched chains pack loosely to give low density,
- whereas straight chains made with a catalyst pack closely to give high density.
| Polymer | Monomer | Typical property | Use |
|---|---|---|---|
| Poly(ethene) | Ethene | Flexible (low density) or rigid (high density) | Bags, bottles |
| Poly(propene) | Propene | Tough, resists fatigue | Ropes, crates |
| PVC | Chloroethene | Rigid unless plasticised | Pipes, window frames |
| PTFE | Tetrafluoroethene | Very unreactive, slippery | Non-stick coatings |
Section 4 of 6Disposal and Environment
Disposal and Environment
Unreactive
- The non-polar C–C and C–H bonds are hard for water or microbes to attack, so addition polymers do not biodegrade.
Landfill
- Burying waste polymer uses land and the material persists for many decades.
Incineration
- Burning releases energy but produces CO₂, and burning PVC also gives toxic hydrogen chloride that must be removed from the gases.
Recycling
- Sorting and remelting saves crude oil, while feedstock recycling cracks waste polymer back into small molecules.
Section 5 of 6Worked Example
Question
- A section of an addition polymer has the repeating unit –CH₂–CH(CN)–.
- Deduce the monomer and name the polymer.
- The backbone has two carbon atoms per repeating unit, so the monomer has one C=C bond between them.
- Change the single bond between these two carbons back into a double bond: CH₂=CH(CN), written CH₂=CHCN.
- The CN group (a nitrile) is a side group, so the monomer is propenenitrile.
- Put poly in front of the bracketed monomer name.
Answer
- Monomer CH₂=CHCN (propenenitrile);
- the polymer is poly(propenenitrile).
Section 6 of 6Exam Checklist & Tips
3.4.3 – Key Points for the Exam
- Monomer A small molecule, here an alkene, that joins with many others to form a polymer.
- Polymer A very long molecule built from many repeating units.
- Repeating unit The smallest group of atoms that repeated along the chain gives the whole polymer.
- Plasticiser An additive that sits between polymer chains and makes the material more flexible.
- Biodegradable Able to be broken down by microorganisms into simpler substances.
Common Mistakes
- Leaving the double bond in the repeating unit;
- in the polymer all the carbon–carbon bonds in the backbone are single.
- Writing side groups such as CH₃ or Cl as part of the chain instead of hanging off it.
- Giving a repeating unit that has more than the two backbone carbons derived from the original C=C.
- Saying addition polymers are biodegradable because they are made from alkenes;
- the polymer itself has no reactive double bonds.