Addition & Condensation Polymers
Section: 11. Organic Chemistry | Syllabus: Cambridge IGCSE Chemistry (0620)
More Addition Polymers (Supplement)
Any alkene monomer can undergo addition polymerisation. The repeat unit of an addition polymer always has the same atoms as its monomer, joined by a C-C single bond in place of the monomer's C=C double bond.
| Monomer | Polymer | Repeat unit |
|---|---|---|
| Propene, CH₃CH=CH₂ | Poly(propene) | -[CH₂-CH(CH₃)]- |
| Chloroethene, CH₂=CHCl | Poly(chloroethene), PVC | -[CH₂-CHCl]- |
Given the structure of an addition polymer's repeat unit, the monomer can be deduced in reverse: replace the C-C single bond in the repeat unit with a C=C double bond, and remove the extending bonds.
Condensation Polymerisation (Supplement)
In condensation polymerisation, monomers with two functional groups (one at each end) join together, releasing a small molecule (water) each time a new bond forms.
| Feature | Addition polymerisation | Condensation polymerisation |
|---|---|---|
| Monomer | one monomer, containing C=C | usually two different monomers, each with two functional groups |
| Small molecule released? | no - only the polymer forms | yes - water is released each time a linkage forms |
| Linkage formed | C-C bonds only | ester (-COO-) or amide (-CONH-) linkages |
Polyesters: PET
A polyester is a condensation polymer formed from a dicarboxylic acid (two -COOH groups) and a diol (two -OH groups), joined by ester linkages (-COO-), releasing water.
PET is a polyester (the full name is not required). Its repeat unit contains the pattern:
-[CO-...-CO-O-...-O]-
PET can be converted back into its original monomers, which can then be re-polymerised - this makes chemical recycling of PET possible.
Polyamides: Nylon
A polyamide is a condensation polymer formed from a dicarboxylic acid (two -COOH groups) and a diamine (two -NH₂ groups), joined by amide linkages (-CONH-), releasing water.
Nylon is a polyamide. Its repeat unit contains the pattern:
-[CO-...-CO-NH-...-NH]-
Worked Example
- Question (based on a real exam question): three statements are given about PET: (1) it is a condensation polymer; (2) it is a polyamide; (3) it can be converted into monomers and re-polymerised. Which statements are correct?
- Answer: 1 and 3. PET is a condensation polymer (correct) that can be depolymerised and re-polymerised (correct), but it is a polyester, not a polyamide (statement 2 is wrong - polyamides like nylon contain amide, -CONH-, linkages, not ester linkages).
- Question (based on a real exam question): a diagram shows part of a synthetic polyamide, with the pattern [dicarboxylic acid residue]-[diamine residue] repeated twice. (i) How many repeat units are shown? (ii) Name the two types of organic compound needed to make this polymer.
- Answer: (i) 2; (ii) a dicarboxylic acid and a diamine.
Common Mistakes
- ❌ Assuming PET, because it is a plastic like polyethene, must also be an addition polymer, or calling it a polyamide
- ✓ PET is made by condensation polymerisation (a dicarboxylic acid + a diol, releasing water) and contains ester linkages, making it a polyester - not a polyamide (amide linkages, like nylon) and not an addition polymer (like polyethene)
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