HL Extension
Condensation Polymerisation Mechanisms
This is Higher Level (HL) content.
Key Principle
Bifunctional Monomers & Small Molecule Elimination
Condensation polymers form when monomers with two functional groups react with each other, releasing a small molecule (usually H₂O) at each linkage.
Two Types
| Polyester | Polyamide (Nylon) | |
|---|---|---|
| Monomers | Diol (-OH at each end) + Dicarboxylic acid (-COOH at each end) | Diamine (-NH₂ at each end) + Dicarboxylic acid (-COOH at each end) |
| Linkage formed | Ester bond (-COO-) | Amide bond (-CONH-) |
| Small molecule lost | H₂O | H₂O |
| Example | PET (polyethylene terephthalate) - plastic bottles | Nylon-6,6 - textiles, rope, parachutes |
Key Differences from Addition
Type 1
Addition Polymerisation (No Byproduct)
- One type of monomer (with C=C)
- No atoms lost
- Non-biodegradable
Type 2
Condensation Polymerisation (H2O or HCl Lost)
- Two types of monomer (bifunctional)
- H₂O lost per linkage
- Hydrolysable → potentially biodegradable
Exam Strategy
Identifying Ester and Amide Linkages on Mark Schemes
The small molecule lost is usually H₂O, but can also be HCl (e.g. When acyl chlorides are used). Always identify the specific small molecule in your answer - don't assume it's always water.
Examiner Trap
Differentiating Peptide Links from Amide Links
Each monomer must be bifunctional (functional group at BOTH ends). If a monomer only has one functional group, the chain terminates immediately. No polymer can form.
AQA GCSE & IB Chemistry
See our apps
Study this topic on the go
Get active recall flashcards, notes, and topic quizzes in ChemEasy, or build your revision schedule with ChemPlan IB.