Describe the differences between addition and condensation polymerisation
Organic Chemistry – Polymers
What is Polymerisation?
Polymerisation is the process of linking many small molecules (monomers) together to form a large chain called a polymer. Think of it like building a long train from many carriages – each carriage is a monomer, and the whole train is the polymer.
Addition Polymerisation
In addition polymerisation, monomers with double bonds (like $C=C$) add together without losing any atoms. The reaction is usually a chain‑growth process and no small molecules are released.
- 🔗 Key Feature: Chain‑growth mechanism.
- 🧪 Typical Monomers: Ethylene ($C_2H_4$), Styrene ($C_8H_8$).
- ⚙️ Example Polymers: Polyethylene ($(-CH_2-CH_2-)_n$), Polystyrene ($(-CH_2-CH(Ph)-)_n$).
- 💧 By‑product: None.
🧩 Analogy: Imagine Lego bricks snapping together – each brick (monomer) attaches to the next without any glue or extra pieces.
Condensation Polymerisation
Condensation polymerisation (also called step‑growth) involves monomers with two different functional groups. When they react, a small molecule such as water or methanol is released.
- 🔗 Key Feature: Step‑growth mechanism.
- 🧪 Typical Monomers: Diamines & diacids (e.g., hexamethylenediamine & adipic acid).
- ⚙️ Example Polymers: Nylon‑6,6 ($(-NH-(CH_2)_6-NH-CO-(CH_2)_4-CO-)_n$), PET ($(-O-CH_2-CH_2-O-CO-Ph-CO-)_n$).
- 💧 By‑product: Water, methanol.
🧪 Analogy: Think of building a chain with glue – each link (monomer) is joined by glue (reaction), and a drop of water is left behind.
Comparison Table
| Feature | Addition Polymerisation | Condensation Polymerisation |
|---|---|---|
| Mechanism | Chain‑growth | Step‑growth |
| Monomer type | Unsaturated (e.g., $C=C$) | Two functional groups (e.g., –NH₂ & –COOH) |
| By‑product | None | Small molecule (H₂O, MeOH) |
| Typical Polymers | Polyethylene, Polystyrene | Nylon‑6,6, PET |
Exam Tips
Key Points to Remember:
- Define addition polymerisation and condensation polymerisation clearly.
- List the main differences: mechanism, monomer type, by‑product, and examples.
- Use the comparison table to structure your answer.
- Show a simple reaction scheme for each type (you can draw it with text).
- Remember that condensation always releases a small molecule – a good hint for identification.
Good luck! 🚀
Revision
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