Polymers: addition and condensation polymerisation
Polymers: Addition and Condensation Polymerisation
Addition Polymerisation 🧱
In addition polymerisation, the double bonds of monomers open up and link together to form a long chain. Think of it like a line of Lego bricks snapping together without losing any pieces.
- Monomer double bonds open to form radicals or cations.
- Propagation continues until termination.
- Resulting polymer retains all atoms of the monomer.
| Step | What Happens |
|---|---|
| Initiation | A radical or cation starts the chain by opening a double bond. |
| Propagation | The active centre keeps adding monomers. |
| Termination | Chains stop growing by combining or by abstraction. |
Example: Polyethylene from ethylene: $$\ce{n CH2=CH2 ->[-\text{Initiator}] (CH2-CH2)_n}$$
Exam Tip:
- Addition polymers retain all atoms of the monomer.
- Use the term “chain‑growth” for addition.
Condensation Polymerisation 🔗
Condensation polymerisation involves two different monomers reacting, with a small molecule (often water or methanol) leaving as a by‑product. It’s like two Lego sets that need a connector piece that gets removed during assembly.
- Two functional groups react to form a new bond.
- Each step releases a small molecule (H₂O, CH₃OH, etc.).
- Resulting polymer is a step‑growth product.
| Step | What Happens |
|---|---|
| Activation | Functional groups become more reactive (e.g., carboxylic acid to acid chloride). |
| Chain Growth | Monomers add to the growing chain, forming new bonds. |
| Elimination | A small molecule (H₂O, CH₃OH, etc.) is released. |
Example: Nylon‑6,6 from hexamethylenediamine and adipic acid: $$\ce{n (H2N-(CH2)6-NH2 + HOOC-(CH2)4-COOH) ->[-\text{Condensation}] (–NH-(CH2)6-NH–CO-(CH2)4-CO–)_n + 2n H2O}$$
Exam Tip:
- Condensation polymers lose a small molecule.
- Use the term “step‑growth” and remember the general formula: $$\ce{(A-B)_n + nX}$$ where X is the small molecule.
Comparison Summary 📊
| Feature | Addition | Condensation |
|---|---|---|
| Mechanism | Chain‑growth | Step‑growth |
| Small molecule loss | None | Yes (e.g., H₂O) |
| Typical monomers | Alkenes, alkynes, anilines | Di‑functional compounds (diols, diamines, dicarboxylic acids) |
| Common examples | Polyethylene, polystyrene | Nylon‑6,6, PET, polyesters |
Exam Tip:
- When drawing a polymer, show the repeating unit and label the small molecule lost (if any).
- Use the correct terminology: “chain‑growth” for addition, “step‑growth” for condensation.
Revision
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