CHEMCA
EXAM MASTER REVIEW SHEET
Organic Chemistry: Polymers
1 Classification of Polymers
By Structure
- Linear: Long straight chains. High density, high melting point (e.g., HDPE, PVC).
- Branched Chain: Linear chains with branches. Lower density (e.g., LDPE).
- Cross-linked (Network): Bifunctional/trifunctional monomers with strong covalent bonds between chains (e.g., Bakelite, Melamine).
By Intermolecular Forces
- Elastomers: Weakest forces (van der Waals). Highly elastic (e.g., Buna-S, Buna-N).
- Fibres: Strongest forces (H-bonding). High tensile strength (e.g., Nylon 6,6, Terylene).
- Thermoplastics: Intermediate forces. Soften on heating, moldable (e.g., Polythene, Polystyrene).
- Thermosetting: Extensively cross-linked. Permanently harden on heating (e.g., Bakelite).
2 Addition Polymers (Chain Growth)
Formed by repeated addition of monomer molecules possessing double or triple bonds. Usually proceeds via Free Radical Mechanism (Initiation $\rightarrow$ Propagation $\rightarrow$ Termination).
| Polymer | Monomer & Formula | Key Properties & Uses |
|---|---|---|
| LDPE (Low Density) | Ethene ($CH_2=CH_2$) High T/P, Peroxide initiator |
Highly branched. Used for squeeze bottles, flexible pipes. |
| HDPE (High Density) | Ethene ($CH_2=CH_2$) Ziegler-Natta Catalyst (Low T/P) |
Linear, high density. Used for buckets, dustbins, tough pipes. |
| Teflon (PTFE) | Tetrafluoroethene ($CF_2=CF_2$) | Chemically inert, heat resistant. Non-stick cookware, gaskets. |
| PAN (Orlon) | Acrylonitrile ($CH_2=CH-CN$) | Substitute for wool in making commercial fibres (blankets, sweaters). |
| PVC | Vinyl Chloride ($CH_2=CH-Cl$) | Water pipes, raincoats, vinyl flooring. |
| Polystyrene | Styrene ($C_6H_5-CH=CH_2$) | Insulator, wrapping material, TV cabinets, toys. |
3 Condensation Polymers (Step Growth)
Formed by repeated condensation reaction between two different bi-functional or tri-functional monomeric units. Involves elimination of small molecules like $H_2O, NH_3, HCl$.
Monomers: Adipic Acid ($HOOC-(CH_2)_4-COOH$) + Hexamethylenediamine ($H_2N-(CH_2)_6-NH_2$).
Uses: Making sheets, bristles for brushes, textile.
Monomer: Caprolactam (Heated with water at high temp).
Uses: Tyre cords, fabrics, ropes.
Monomers: Ethylene glycol ($HO-CH_2-CH_2-OH$) + Terephthalic acid (Benzene-1,4-dicarboxylic acid).
Uses: Crease-resistant fibres, blended with cotton (poly-cot).
Monomers: Ethylene glycol + Phthalic acid (Benzene-1,2-dicarboxylic acid).
Uses: Manufacture of paints and lacquers.
Initial linear product is Novolac (used in paints). Heating with HCHO forms extensively cross-linked Bakelite.
Uses: Combs, electrical switches, handles of utensils.
Monomers: Melamine (heterocyclic triamine) + Formaldehyde.
Uses: Unbreakable crockery.
Monomers: Urea ($NH_2CONH_2$) + Formaldehyde ($HCHO$).
Uses: Unbreakable cups, laminated sheets.
4 Natural & Synthetic Rubbers
Natural Rubber
Linear polymer of Isoprene (2-methyl-1,3-butadiene).
Configuration: Cis-1,4-polyisoprene
VulcanizationHeating raw rubber with Sulfur (3-5%) at 373-415 K. Sulfur forms cross-links at the reactive sites of double bonds, making rubber stiff, highly elastic, and resistant to abrasion.
Synthetic Rubbers
-
Neoprene
Polymer of Chloroprene (2-chloro-1,3-butadiene).
Resistant to vegetable and mineral oils. Used for conveyor belts, gaskets, hoses. -
Buna-N (Nitrile Rubber)
Copolymer of 1,3-Butadiene + Acrylonitrile.
Resistant to action of petrol, lubricating oil, and organic solvents. Used in tank linings. -
Buna-S (SBR)
Copolymer of 1,3-Butadiene + Styrene.
Tough and good substitute for natural rubber. Used in auto tyres, footwear.
5. Biodegradable Polymers
Monomers: 3-Hydroxybutanoic acid + 3-Hydroxypentanoic acid.
Uses: Specialty packaging, orthopedic devices, controlled drug release.
Monomers: Glycine + Amino caproic acid.
An alternating polyamide.
6. Molecular Mass of Polymers
Polymers consist of chains of varying lengths, so their molecular mass is expressed as an average.
$\bar{M_n} = \frac{\sum N_i M_i}{\sum N_i}$
$\bar{M_w} = \frac{\sum N_i M_i^2}{\sum N_i M_i}$
Ratio of weight average to number average mass: PDI = $\bar{M_w} / \bar{M_n}$
- For natural polymers (monodisperse), PDI = 1.
- For synthetic polymers, PDI > 1 ($\bar{M_w} > \bar{M_n}$).
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