Polymers – IIT‑JEE Chemistry Study Notes
(Main + Advanced)
Polymers are large molecules (macromolecules) made of repeating structural units (monomers) linked by covalent bonds.
They form the basis of most of the materials we use every day – plastics, rubbers, fibers, adhesives, etc.
1. Simple Explanation
| Concept | What it means | Everyday example |
|---|---|---|
| Monomer | Small molecule that repeats in the chain | Ethylene (C₂H₄) → polyethylene |
| Polymer | Long chain of monomers | Nylon, PVC, Teflon |
| Polymerization | Chemical reaction that joins monomers | Free‑radical polymerization of styrene |
| Chain‑growth vs Step‑growth | Different mechanisms of adding units | Free‑radical = chain‑growth; Polycondensation = step‑growth |
| Cross‑linking | Inter‑chain bonds that give 3‑D network | Vulcanized rubber |
2. Key Points
| Category | Key Details |
|---|---|
| Classification of Polymers | • Natural – e.g., cellulose, starch, rubber, silk. • Synthetic – e.g., polyethylene, PVC, nylon. • Biopolymers – e.g., polylactic acid (PLA). |
| Polymerization Types | Chain‑growth (free radical, anionic, cationic). Step‑growth (polycondensation, polyaddition). |
| Chain‑Growth Mechanism | Initiation → Propagation → Termination (or chain transfer). |
| Step‑Growth Mechanism | All functional groups react → network grows gradually; high molar mass only after most monomers reacted. |
| Thermal Properties | Glass Transition (T<sub>g</sub>) – softening of amorphous regions. Melting Temperature (T<sub>m</sub>) – crystalline phase disappearance. |
| Mechanical Properties | Young’s modulus, tensile strength, elongation at break – depend on chain length, branching, cross‑linking. |
| Solubility | Depends on polarity, branching, crystallinity. |
| Copolymers | Acrylics, styrene–butadiene, polyethylene‑propylene blends. |
| Terminology | • Linear – no branches. • Branched – side chains. • Cross‑linked – 3‑D network. • Random – monomers inserted randomly. • Block – long sequences of one monomer then another. |
3. Worked Example
Free‑Radical Polymerization of Styrene → Polystyrene
1. Reaction Scheme
Initiation:
1. (CH3)3C–C6H5 (benzoyl peroxide, BPO)
2. BPO → 2·O–C6H5 (radical cleavage)
3. ·O–C6H5 + C8H8 (styrene) → ·Ph–C8H7–O–C6H5 (propagating radical)
Propagation:
4. ·Ph–C8H7–O–C6H5 + C8H8 → ·Ph–C8H7–O–C6H5–C8H7–O–C6H5
(Repeat step 4 until chain is long)
Termination (coupling):
5. 2·Ph–C8H7–O–C6H5 → (Ph–C8H7–O–C6H5)2
2. Balanced Overall Reaction
(For simplicity, consider n units of styrene)
n C8H8 + (CH3)3C–C6H5 → (CH3)3C–[C8H7]n–C6H5 + n H2
3. Molar Mass (Approximate)
- Monomer: 104 g/mol
- For 10,000 units:
MW ≈ 10,000 × 104 = 1.04 × 10⁶ g/mol
4. Properties of Polystyrene
- T<sub>g</sub> ≈ 100 °C
- Density ≈ 1.05 g cm⁻³
- Color – transparent white (unless pigment added)
- Uses – CD cases, disposable cutlery, insulation.
5. Reaction Conditions
- Initiator: benzoyl peroxide (temperature 60–90 °C).
- Solvent: none (bulk polymerization) or toluene for solution polymerization.
- Atmosphere: nitrogen or argon (avoid oxygen).
4. Quick Revision Summary
| Topic | Quick Recall |
|---|---|
| Monomer | Small repeating unit |
| Polymer | Macromolecule |
| Chain‑growth | Initiation → Propagation → Termination |
| Step‑growth | All groups react; high M<sub>w</sub> only after most monomers used |
| Cross‑linking | 3‑D network → vulcanized rubber |
| Glass transition (T<sub>g</sub>) | Amorphous softening |
| Melting temperature (T<sub>m</sub>) | Crystalline phase disappearance |
| Copolymers | Random, block, graft |
| Common polymers | PE, PP, PVC, PET, nylon, polystyrene |
| Key equations | R• + M → RM• (Propagation); RM• + RM• → R–M–R (Termination) |
| Free‑radical initiators | Benz |