AS Chemistry: Addition Polymerisation — Revision Notes

Condensed for the final weeks. For the full explanation, use the Addition Polymerisation study guide.

The reaction

Many alkene monomers join by opening their C=C double bonds to form a single long chain.

n CH2=CH2  ->  -[CH2-CH2]n-

Only one product is formed — the polymer. Nothing is eliminated, which is why addition polymerisation has 100% atom economy: every atom present in the monomers ends up in the polymer chain. That is the standard comparison point against condensation polymerisation, where a small molecule such as water is lost at every linkage.

Drawing the repeat unit is the most commonly examined skill:

1. Open the C=C to a single bond.
2. Draw two carbons of the backbone.
3. Extend a bond from each end through the brackets.
4. Write n outside the bracket.

The most frequent errors are keeping the double bond and forgetting the extending bonds through the brackets — both lose the mark even when everything else is correct.

Worked examples

Repeat unit from a monomer. Deduce the repeat unit of the polymer formed from propene, CH₂=CHCH₃. Open the double bond into two single bonds, keeping every atom and the CH₃ side-group exactly where it was: –[CH₂–CH(CH₃)]ₙ–.

Monomer from a polymer section. A section of an addition polymer is drawn as …–CH₂–CCl₂–CH₂–CCl₂–CH₂–CCl₂–… Deduce the monomer. The repeating unit is –[CH₂–CCl₂]–; closing the two dangling bonds back into a double bond gives the monomer CH₂=CCl₂ (1,1-dichloroethene).

Common addition polymers

Monomer	Polymer	Use
Ethene	Poly(ethene)	Bags, bottles
Propene	Poly(propene)	Crates, ropes
Chloroethene	Poly(chloroethene), PVC	Pipes, cables
Phenylethene	Poly(phenylethene), polystyrene	Packaging, insulation
Tetrafluoroethene	PTFE	Non-stick coatings

Properties

The polymer chains are held together by weak induced-dipole (van der Waals) forces. These are individually weak but there are very many of them along a long chain, so the total attraction is substantial.

Chain structure determines properties:

• Longer chains → more points of contact → stronger forces → higher melting point and greater strength.
• Branched chains cannot pack closely, so there is less contact between them, giving lower density and a lower melting point — this is the difference between low-density and high-density poly(ethene).

PVC is naturally rigid because the C–Cl dipoles give permanent dipole–dipole attractions between chains. Plasticisers are added to force the chains apart, weakening those attractions and making the material flexible — which is why the same polymer serves for both rigid drainpipes and flexible cable insulation.

Disposal

The C–C backbone is unreactive and non-polar, with no bonds that enzymes or water can attack, so addition polymers are non-biodegradable. This is the root of the environmental problem and should be stated as the reason rather than just the fact.

Method	Advantage	Disadvantage
Landfill	Cheap, simple	Takes up space; polymers persist for centuries
Incineration	Releases energy; reduces volume	Produces CO₂; PVC releases toxic HCl
Recycling	Conserves crude oil; reduces waste	Sorting is costly; quality degrades on reprocessing
Feedstock recycling	Recovers monomers for reuse	Energy-intensive

Feedstock recycling breaks the polymer back down chemically into its monomers (or other useful small molecules), which can then be used again as a genuine chemical feedstock, rather than simply reshaping the existing plastic as ordinary mechanical recycling does.

Burning a plain poly(alkene) like poly(ethene) produces the same products as burning any hydrocarbon (CO₂, CO and soot depending on how complete the combustion is). Burning PVC is the specific hazard worth naming — the chlorine produces hydrogen chloride, which is corrosive and toxic, so incinerators handling PVC require scrubbers. Incomplete or poorly controlled combustion of chlorinated plastics can also produce other harmful chlorinated by-products beyond HCl itself, which is why proper flue-gas treatment matters.

Exam traps

• Leaving the C=C in the repeat unit.
• Omitting the bonds extending through the brackets.
• Forgetting the n outside the bracket.
• Saying addition polymers are biodegradable.
• Explaining branching without linking it to packing and contact area.
• Not naming HCl as the hazard from burning PVC.

Self-test

1. Why does addition polymerisation have 100% atom economy?
2. Give the four steps for drawing a repeat unit.
3. Why does branched poly(ethene) have a lower melting point than the unbranched form?
4. Why is PVC rigid, and how is it made flexible?
5. Why are addition polymers non-biodegradable?
6. Deduce the repeat unit of the polymer formed from propene, CH₂=CHCH₃.
7. A polymer section is drawn as …–CH₂–CCl₂–CH₂–CCl₂–… Deduce the monomer.

Answers: 1. All the atoms of the monomers end up in the polymer — there is only one product and nothing is eliminated. 2. Open the double bond to a single bond, draw the two backbone carbons, extend a bond from each end through the brackets, and write n outside. 3. Branches prevent the chains from packing closely, so there is less contact area and weaker total induced-dipole attraction between chains. 4. The C–Cl dipoles create permanent dipole–dipole attractions between chains; plasticisers force the chains apart and weaken these attractions. 5. The carbon–carbon backbone is unreactive and non-polar, with no bonds that enzymes or water can attack. 6. –[CH₂–CH(CH₃)]ₙ–, opening the double bond into two single bonds and keeping the CH₃ side-group in place. 7. CH₂=CCl₂ (1,1-dichloroethene), closing the two dangling bonds of the repeating unit –[CH₂–CCl₂]– back into a double bond.