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.
What a polymer is
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 ; each monomer becomes one 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
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
Reading the monomer from a polymer's repeat unit
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
Exam tip
Drawing the repeat unit of an addition polymer
What comes up: draw the displayed formula of the repeat unit from a given alkene monomer (or name the monomer from a given repeat unit).
Write (two marks): (1) The correct structural unit: every carbon now carries a single bond, with its attached atoms (H, Cl, CH₃, etc.) shown explicitly. (2) Two extension bonds reaching outward through (or to the edge of) the brackets, the brackets themselves, and a subscript n placed outside — all three elements must appear for M2.
Watch out: the mark scheme rejects a repeat unit that has no extension bonds, so do not leave the carbons at the ends as bare stubs. Extension bonds do not have to cross the bracket line, but they must be present; n can be placed anywhere adjacent to the closing bracket.