Polymers (Addition and Condensation)
Polymers for IGCSE 0620: addition polymerisation and repeat units from alkenes, condensation polymers (nylon, PET), and plastic disposal problems.
The IGCSE Chemistry Specialist Team · founded by Rig
Written to the Cambridge IGCSE Chemistry (0620) syllabus and mark-scheme conventions. Last updated 2026-06-11.
Polymer questions are drawing questions, and the marking is binary: a repeat unit with the double bond removed and the brackets bonded through scores; anything else scores nothing. Examiner reports return to the same two failures every series: double bonds left inside repeat units, and condensation answers that forget the water. Both are fixable in one careful read.
Addition polymerisation (Core)
A polymer is a long-chain molecule made from joined repeating units called monomers. In addition polymerisation, alkene monomers join when their C=C double bonds open. The polymer is the only product.
n C2H4 → −(CH2−CH2)n−
Ethene makes poly(ethene); the polymer is named by bracketing the monomer: propene → poly(propene), chloroethene → poly(chloroethene). Drawing the repeat unit from any monomer follows one routine:
- Draw the monomer with the C=C horizontal and the four attached groups around it.
- Open the double bond to a single bond.
- Move nothing else. Every atom keeps its carbon.
- Draw brackets with the chain bonds passing through them, and an n subscript outside.
The reverse skill is Supplement: given a polymer chain, cut it into two-carbon slices and restore the double bond to recover the monomer.
Condensation polymerisation (Supplement)
In condensation polymerisation, two different monomers, each carrying a functional group at both ends, join, and a small molecule, water, is released at every linkage.
| Polymer type | Monomers | Linkage | Example |
|---|---|---|---|
| Polyamide | dicarboxylic acid + diamine | amide, −CONH− | nylon |
| Polyester | dicarboxylic acid + diol | ester, −COO− | PET |
The ester linkage is the same one made in esterification, repeated along a chain. In 0620 you draw these with block diagrams: a rectangle for each monomer’s carbon skeleton, the functional groups written out at the ends, and the linkage shown with the water eliminated.
The contrast table examiners love:
| Addition | Condensation | |
|---|---|---|
| Monomers | one type, contains C=C | two types, two functional groups each |
| Products | polymer only | polymer + water |
| Examples | poly(ethene) | nylon, PET |
Plastics and disposal
Plastic waste is the application strand. The problems: most plastics are non-biodegradable (microorganisms cannot break them down), so they accumulate in landfill sites and in the oceans, where they harm marine life. Burning plastics releases carbon dioxide and, in limited oxygen, carbon monoxide; certain plastics release other toxic gases. Recycling reduces waste and saves crude-oil feedstock, but plastics must be sorted by type first. The environmental chain from petroleum fraction to pollutant runs back through fuels and into chemistry of the environment.
Worked exam question
Chloroethene has the structure CH2=CHCl. (a) Draw the repeat unit of poly(chloroethene). [2] (b) Name the type of polymerisation. [1] (c) Nylon is made from two different monomers. State the other product of this polymerisation and name the linkage formed. [2]
Model answer, mark by mark:
- (a) M1: the carbon skeleton correct: two carbons, single C−C bond, two H on one carbon, one H and one Cl on the other. M2: brackets with the bonds extending through them and n outside. No C=C anywhere.
- (b) M3: addition polymerisation. (“Polymerisation” alone is not enough; the type is the mark.)
- (c) M4: water. M5: amide linkage (−CONH−). “Ester linkage” here confuses nylon with PET. The diamine is the clue.
The mistakes that cost marks
- Double bond left in the repeat unit. The defining act of addition polymerisation is the C=C opening. A repeat unit with C=C contradicts itself and scores zero.
- Brackets without through-bonds. The repeat unit must show bonds crossing the brackets, which is what says “this continues”. Sealed brackets read as a single molecule.
- Atoms migrating during drawing. The Cl in poly(chloroethene) stays on its own carbon. Redistributing substituents creates a different polymer.
- Condensation answers missing the water. Every linkage releases one small molecule. Naming the polymer but not the water is the most common half-answer in Extended scripts.
- Monomer recovered without restoring the C=C. Cutting the chain gives the repeat unit; the monomer needs its double bond back.
How examiners want it phrased
| Student wording | Mark-scheme wording |
|---|---|
| ”Lots of ethenes stick together" | "The C=C double bonds open and the monomers join; the polymer is the only product" |
| "Nylon is made from two chemicals" | "A dicarboxylic acid and a diamine join through amide linkages, with water released at each link" |
| "Plastic doesn’t rot" | "Plastics are non-biodegradable, so they persist in landfill and harm marine life" |
| "Burning plastic is toxic" | "Combustion of plastics can release toxic gases, including carbon monoxide in limited oxygen” |
The drawing routine is the whole game here: four steps, applied to whichever monomer the paper invents. Bring any past-paper polymer question to a free trial lesson and we will mark your repeat unit the way Cambridge does, bracket bonds and all. The full reaction context sits in the organic chemistry pillar.
Test yourself
Three questions, no peeking at the tables above. Click each answer to check.
Q1 (2 marks). Propene undergoes polymerisation. Name the polymer formed and the type of polymerisation.
Show answer
• poly(propene) [1] • addition polymerisation [1]
Q2 (2 marks). State two environmental problems caused by the disposal of plastics.
Show answer
Any two of: • plastics are non-biodegradable, so they accumulate in landfill [1] • plastic waste in the oceans harms marine life [1] • burning plastics releases toxic gases, e.g. carbon monoxide in limited oxygen [1]
Q3 (3 marks). (Supplement) PET is a polyester. Name the two types of monomer used to make it, name the linkage formed, and state the other product of the polymerisation.
Show answer
• a dicarboxylic acid and a diol [1] • ester linkage, −COO− [1] • water [1]
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Frequently asked questions
How do I draw a repeat unit from an alkene monomer?
Open the C=C into a C−C single bond, keep every other atom and group exactly where it was, add brackets with bonds extending through them, and write n outside. A repeat unit that still contains a double bond scores zero.
What is the difference between addition and condensation polymerisation?
Addition: one type of monomer with a C=C joins, and the polymer is the only product. Condensation (Supplement): two different monomers, each with two functional groups, join with a small molecule (water) released at each linkage. Polyamides like nylon and polyesters like PET form this way.
What disposal problems do plastics cause?
Most plastics are non-biodegradable, so they persist in landfill and in the oceans, harming wildlife. Burning them can release toxic gases (carbon monoxide always; others depending on the plastic). Recycling reduces both problems but requires sorting.