🌟 Organic Chemistry – Polymers: Addition Polymers
What are Addition Polymers?
Addition polymers are long chains made by linking together many copies of a simple molecule called an alkene. Think of it like a train where each car (the repeat unit) is identical and they’re all connected by the same type of couplings.
Key Idea: The Repeat Unit is the “Building Block”
When you see an alkene, you can usually guess the repeat unit of the polymer it will form. Conversely, if you know the repeat unit, you can work backwards to find the original alkene.
Step‑by‑Step Guide to Deduce the Repeat Unit
- Identify the alkene’s structure. Look for the double bond C=C and any substituents.
- Remove the double bond. Replace it with a single bond C–C – that’s the repeat unit.
- Check for symmetry. If the alkene is symmetrical, the repeat unit will be symmetrical too.
- Write the repeat unit. Use the same atoms and connectivity as the alkene but with a single bond.
Step‑by‑Step Guide to Deduce the Alkene from a Repeat Unit
- Look at the repeat unit. Identify where the single bonds are that could have come from a double bond.
- Convert single bonds back to double bonds. Replace the appropriate C–C bonds with C=C.
- Check for possible branching. If the repeat unit has branches, the alkene will have the same branches.
- Write the alkene. Ensure the double bond is correctly positioned.
Common Examples
- Ethylene (C₂H₄) → Polyethylene repeat unit: –CH₂–CH₂–
- Propylene (C₃H₆) → Polypropylene repeat unit: –CH₂–CH(CH₃)–
- Styrene (C₈H₈) → Polystyrene repeat unit: –CH₂–CH(Ph)–
- Vinyl chloride (C₂H₃Cl) → Polyvinyl chloride (PVC) repeat unit: –CH₂–CHCl–
Analogy: Building with LEGO® Blocks
Imagine each alkene is a special LEGO® block that has a “hinge” (the double bond). When you snap many of these blocks together, the hinges become ordinary joints (single bonds), creating a long chain. The shape of the block tells you exactly how the chain will look.
Exam Tips Box
Tip 1: Always start by locating the double bond in the alkene.
Tip 2: Remember that the repeat unit is just the alkene with the double bond replaced by a single bond.
Tip 3: Check for symmetry – it can help you spot the correct repeat unit quickly.
Tip 4: Practice converting between alkene and repeat unit until you can do it in 30 seconds!
Practice Problems
- Given the alkene CH₂=CH–CH₃, write the repeat unit of the addition polymer.
- Given the repeat unit –CH₂–CH(OH)–, deduce the alkene that would polymerise to give this polymer.
- Sketch the repeat unit for the polymer formed from the alkene CH₂=C(CH₃)₂.
Quick Reference Table
| Alkene | Repeat Unit | Polymer |
|---|
| CH₂=CH₂ | –CH₂–CH₂– | Polyethylene |
| CH₂=CH–CH₃ | –CH₂–CH(CH₃)– | Polypropylene |
| CH₂=CH–C₆H?? | –CH₂–CH(Ph)– | Polystyrene |
Final Thought
Remember, the double bond is the “magic hinge” that lets the polymer chain grow. Once you spot it, the rest is just a simple replacement. Keep practicing and you’ll master this skill in no time! 🚀