Addition Polymers — Synthetic Polymers | IGCSE Chemistry

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 5% of your exam marks.

stable

Rare

Stable5%

Addition polymerisation (drawing repeat units, identifying monomers, common polymers + uses) and disposal / environmental impact regularly tested.

A polymer is a very long molecule built by joining together a large number of small repeating units
The small repeating starting molecules are called monomers; each monomer becomes one repeat unit in the polymer chain
Polymerisation reactions join hundreds to thousands of monomers end-to-end through covalent bonds, giving a single chain molecule of high relative molecular mass
Synthetic polymers are manufactured by industry (plastics, nylon, polyester fabrics, polystyrene cups); natural polymers occur in living things (starch, cellulose, proteins, DNA)

Addition polymerisation joins monomer molecules that each have a C=C double bond
One of the two bonds in the C=C of each monomer opens up; the spare bonds from neighbouring monomers link together to form a long single-bond chain
No other product is made — the polymer is the only outcome (compare with condensation polymerisation in section 3)
The molecular formula of the polymer is just n times the molecular formula of the monomer
General equation:

n CH₂=CHR → −(CH₂−CHR)−_n

where R is whatever atom or group is attached to the second carbon of the monomer

Monomer	Polymer	Repeat unit	Typical use
Ethene, CH₂=CH₂	Poly(ethene)	−(CH₂−CH₂)−_n	Plastic bags, bottles, food wrap
Propene, CH₂=CHCH₃	Poly(propene)	−(CH₂−CH(CH₃))−_n	Crates, ropes, food containers
Chloroethene, CH₂=CHCl (vinyl chloride)	Poly(chloroethene) — PVC	−(CH₂−CHCl)−_n	Drainpipes, window frames, electrical cable insulation
Tetrafluoroethene, CF₂=CF₂	Poly(tetrafluoroethene) — PTFE / Teflon	−(CF₂−CF₂)−_n	Non-stick coatings, plumber's tape, low-friction bearings

A four-step recipe:
- Write out the displayed formula of the monomer
- Change the C=C double bond to a single C−C bond
- Add a "stub" bond going outward from each carbon, sticking out of the brackets to show where the chain continues
- Enclose the unit in brackets and write a subscript n outside the bracket
Worked example: poly(chloroethene) from chloroethene
- Monomer: CH₂=CHCl
- Open the double bond to a single bond and add extension bonds: −(CH₂−CHCl)−
- Add the subscript: −(CH₂−CHCl)−_n

Reverse the recipe: take the repeat unit, remove the extension bonds and the subscript n, and turn the C−C single bond between the two carbons back into a C=C double bond
Worked example: poly(propene) repeat unit −(CH₂−CH(CH₃))−_n
- Remove the brackets and the subscript: CH₂−CH(CH₃)
- Put the double bond back: CH₂=CH(CH₃) — propene
This skill is exam-mark assessable in both directions: monomer → polymer and polymer → monomer