Polymers: Addition and Condensation
A polymer is a long molecule built from many small monomers. Two mechanisms, and the marks are mostly for drawing structures correctly.
1. The two types
| Addition | Condensation | |
|---|---|---|
| Monomers | alkenes (contain C=C) | two different monomers with two functional groups each |
| Other product | NONE | a small molecule — usually WATER |
| Examples | poly(ethene), poly(propene), PVC | polyesters, polyamides (nylon) |
Addition polymerisation makes ONLY the polymer. Condensation also releases a small molecule, usually water. This is the fastest way to tell them apart, and confusion between the two was recorded twice.
2. Addition polymerisation
Many alkene monomers join when the C=C double bond OPENS, forming a long saturated chain. No other product is formed.
Example — poly(ethene) from ethene:
n CH₂=CH₂ → –[CH₂–CH₂]ₙ–
Drawing the repeat unit — the rules that earn the marks:
1. Change the DOUBLE bond to a SINGLE bond. 2. Draw EXTENSION LINES (bonds) out of BOTH ends of the unit. 3. Enclose in brackets with an “n” outside. 4. Keep every other group exactly as it was.
Forgetting the double bond in the MONOMER was a recorded error — the monomer must show C=C, and the polymer must not.
Extension lines belong on the POLYMER, not the monomer. A recorded error added them to the monomer, which breaks its valency. Every carbon must have exactly four bonds — check this before moving on.
Working backwards — polymer to monomer:
Take one repeat unit, remove the extension lines, and put the double bond back between the two carbons.
Identifying the repeat unit was a recorded difficulty. Look for the smallest section that repeats, and check the carbons in the backbone.
Common addition polymers: poly(ethene), poly(propene), poly(chloroethene) (PVC), PTFE.
3. Condensation polymerisation
Two monomers, each with two functional groups, join with the loss of a small molecule (water).
Polyesters
Dicarboxylic acid + diol → polyester + water
The ester linkage forms between a –COOH and an –OH.
Example: PET (Terylene).
Polyamides
Dicarboxylic acid + diamine → polyamide + water
The amide linkage forms between a –COOH and an –NH₂.
Example: nylon.
NYLON is a POLYAMIDE, not a polyester. This exact error was recorded, and confusing the two was recorded again separately. The clue is in the name: polyamide contains the amide link from an amine; polyester contains the ester link from an alcohol.
| Polymer | Monomers | Linkage |
|---|---|---|
| Polyester | diacid + diol (alcohol) | ester |
| Polyamide | diacid + diamine | amide |
Both need TWO functional groups per monomer — otherwise the chain cannot keep growing. Uncertainty about the monomers involved was recorded for both.
Drawing them: represent the monomers as blocks with their functional groups at each end, join them, and show water being lost at each link.
4. Natural polymers
- Proteins — polyamides made from amino acids (amide/peptide links)
- Carbohydrates / starch — from glucose monomers
- DNA — from nucleotides
Proteins are natural POLYAMIDES, which is why they share the amide linkage with nylon.
5. Properties and problems
Plastics are useful because they are cheap, light, durable, waterproof and easily moulded.
The environmental problem:
Most addition polymers are non-biodegradable, because the C–C backbone is unreactive and micro-organisms cannot break it down.
Consequences: they persist in landfill, pollute oceans, and harm wildlife. Burning them produces CO₂ (a greenhouse gas) and, if they contain chlorine (PVC), toxic HCl gas.
Solutions: recycling, developing biodegradable polymers, and reducing use.
Condensation polymers are generally more biodegradable than addition polymers, because the ester and amide links can be hydrolysed.
6. Mistakes that cost marks
Confusing addition with condensation.
Saying addition polymerisation produces water.
Omitting the C=C from the monomer.
Leaving the double bond in the polymer.
Adding extension lines to the monomer.
Carbons with the wrong number of bonds.
Calling nylon a polyester.
Forgetting the brackets and “n”.
Saying plastics don’t burn to produce CO₂.
Frequently asked questions
What is a polymer? A long molecule made from many monomers joined together.
What is addition polymerisation? Alkene monomers join as the C=C opens, with no other product.
What is condensation polymerisation? Two monomers with two functional groups each join, losing a small molecule — usually water.
How do I draw a repeat unit? Change the double bond to a single bond, add extension lines at both ends, and enclose in brackets with n.
How do I find the monomer from a polymer? Take one repeat unit, remove the extension lines, and restore the double bond.
What is nylon? A polyamide — from a dicarboxylic acid and a diamine.
What makes a polyester? A dicarboxylic acid and a diol, forming an ester linkage.
Why are addition polymers non-biodegradable? The C–C backbone is unreactive, so micro-organisms cannot break it down.
What happens when PVC is burnt? It produces CO₂ and toxic HCl gas.
Are proteins polymers? Yes — natural polyamides made from amino acids.
Quick revision checklist
- I can define a polymer and a monomer
- I know the differences between addition and condensation
- I know addition gives no other product
- I can draw an addition repeat unit with brackets, n and extension lines
- I check every carbon has four bonds
- I can work backwards from polymer to monomer
- I know polyesters come from a diol
- I know polyamides come from a diamine
- I know nylon is a polyamide
- I know both monomers need two functional groups
- I know proteins are natural polyamides
- I can explain why addition polymers are non-biodegradable
- I know the problems of disposal and burning
These notes cover polymers in the Cambridge IGCSE Chemistry (0620) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Chemistry lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus for your own exam series.
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