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Class 12 Chemistry Notes Chapter 6 (Polymers) – Chemistry-II Book

Philoid

Philoid

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Chemistry-II
Detailed Notes with MCQs of Chapter 6: Polymers. This is an important chapter, not just for your board exams but also frequently features in various government competitive exams. Pay close attention to the definitions, classifications, examples, and specific reactions.

Chapter 6: Polymers - Detailed Notes for Competitive Exams

1. Introduction

  • Polymers: Very large molecules (macromolecules) having high molecular mass (10³ - 10⁷ u). They are formed by the joining of repeating structural units derived from smaller molecules.
  • Monomers: The simple, small reactive molecules from which the repeating structural units are derived.
  • Polymerization: The process by which monomers combine to form a polymer.
    • Example: Ethene (monomer) polymerizes to form Polythene (polymer).
      n CH₂=CH₂ → [−CH₂−CH₂−]n
      (Ethene) (Polythene)

2. Classification of Polymers

(a) Based on Source:

  • Natural Polymers: Found in plants and animals.
    • Examples: Starch, Cellulose, Proteins, Natural Rubber, Nucleic Acids.
  • Semi-synthetic Polymers: Derived from natural polymers by chemical modifications.
    • Examples: Cellulose acetate (Rayon), Cellulose nitrate, Vulcanized Rubber.
  • Synthetic Polymers: Man-made polymers synthesized in laboratories.
    • Examples: Polythene, Polyvinyl chloride (PVC), Nylon 6,6, Bakelite, Teflon.

(b) Based on Structure:

  • Linear Polymers: Long, straight chains of repeating units. Can be closely packed, leading to high density, high tensile strength, and high melting points.
    • Examples: High-density polythene (HDPE), PVC, Nylon, Polyesters.
  • Branched-chain Polymers: Linear chains with some branches. Irregular packing leads to lower density, tensile strength, and melting points compared to linear polymers.
    • Example: Low-density polythene (LDPE).
  • Cross-linked or Network Polymers: Formed from bi-functional or tri-functional monomers, containing strong covalent bonds between various linear polymer chains (cross-links). They are hard, rigid, and brittle.
    • Examples: Bakelite, Melamine-formaldehyde resin, Vulcanized Rubber.

(c) Based on Mode of Polymerization:

  • Addition Polymers (Chain Growth Polymers): Formed by the repeated addition of monomer molecules possessing double or triple bonds, without the elimination of any byproduct molecules. The empirical formula of the polymer is the same as that of the monomer.
    • Mechanism: Usually involves free radical, cationic, or anionic intermediates. Free radical mechanism is common (Initiation, Propagation, Termination steps).
    • Homopolymers: Formed from a single type of monomer (e.g., Polythene from ethene).
    • Copolymers: Formed from two different types of monomers (e.g., Buna-S from 1,3-butadiene and styrene).
    • Examples: Polythene (LDPE, HDPE), Teflon (Polytetrafluoroethene), Polyacrylonitrile (PAN), PVC, Polystyrene.
  • Condensation Polymers (Step Growth Polymers): Formed by the repeated condensation reaction between two different bi-functional or tri-functional monomeric units, usually with the elimination of small molecules like water (H₂O), alcohol (ROH), ammonia (NH₃), or HCl. The empirical formula of the polymer is different from that of the monomers.
    • Examples:
      • Polyamides: Contain amide linkages (−CONH−). E.g., Nylon 6,6 (from hexamethylenediamine and adipic acid), Nylon 6 (from caprolactam).
      • Polyesters: Contain ester linkages (−COO−). E.g., Terylene/Dacron (from ethylene glycol and terephthalic acid), Glyptal (from ethylene glycol and phthalic acid).
      • Phenol-formaldehyde Polymers: E.g., Bakelite (from phenol and formaldehyde). Novolac is a linear polymer formed initially, which on heating with more formaldehyde forms cross-linked Bakelite.
      • Melamine-formaldehyde Polymer: (from melamine and formaldehyde).

(d) Based on Molecular Forces:

  • Elastomers: Rubber-like solids with elastic properties. Polymer chains are held by weak intermolecular forces, allowing them to be stretched. Cross-links are introduced between chains to help the polymer retract to its original position.
    • Examples: Natural rubber, Buna-S, Buna-N, Neoprene.
  • Fibres: Thread-forming solids possessing high tensile strength and high modulus. Strong intermolecular forces (like hydrogen bonding) exist between chains, leading to close packing.
    • Examples: Nylon 6,6, Terylene (Polyester), Silk, Wool.
  • Thermoplastics: Linear or slightly branched polymers capable of repeatedly softening on heating and hardening on cooling. Intermolecular forces are intermediate between elastomers and fibres. Can be reshaped.
    • Examples: Polythene, Polystyrene, PVC, Teflon.
  • Thermosetting Plastics: Cross-linked or heavily branched polymers which undergo extensive cross-linking in moulds upon heating, becoming infusible and insoluble. They cannot be reshaped by heating once set.
    • Examples: Bakelite, Urea-formaldehyde resins, Melamine-formaldehyde resins.

3. Important Addition Polymers

  • Polythene:
    • Low-Density Polythene (LDPE): Monomer: Ethene. Formed under high pressure (1000-2000 atm) and temperature (350-570 K) using traces of O₂ or peroxide initiator (free radical mechanism). Branched structure. Chemically inert, tough but flexible, poor conductor. Uses: Insulation for wires, squeeze bottles, toys, flexible pipes.
    • High-Density Polythene (HDPE): Monomer: Ethene. Formed at low pressure (6-7 atm) and temperature (333-343 K) using Ziegler-Natta catalyst (e.g., TiCl₄ + Al(C₂H₅)₃). Linear structure, closely packed. More tough and hard. Uses: Buckets, dustbins, bottles, pipes.
  • Polytetrafluoroethene (PTFE) / Teflon: Monomer: Tetrafluoroethene (CF₂=CF₂). Formed by free radical polymerization under pressure with persulphate catalyst. Chemically inert, resistant to heat and attack by corrosives. Uses: Non-stick coating for utensils, seals, gaskets.
  • Polyacrylonitrile (PAN) / Orlon / Acrilan: Monomer: Acrylonitrile (CH₂=CH−CN). Addition polymerization using peroxide catalyst. Substitute for wool. Uses: Making commercial fibres like Orlon or Acrilan, blankets, carpets.

4. Important Condensation Polymers

  • Polyamides:
    • Nylon 6,6: Monomers: Hexamethylenediamine [H₂N−(CH₂)₆−NH₂] and Adipic acid [HOOC−(CH₂)₄−COOH]. High tensile strength. Uses: Sheets, bristles for brushes, textile fabrics, ropes.
    • Nylon 6: Monomer: Caprolactam. Formed by heating caprolactam with water at high temperature. Uses: Tyre cords, fabrics, ropes.
  • Polyesters:
    • Terylene / Dacron: Monomers: Ethylene glycol [HO−CH₂−CH₂−OH] and Terephthalic acid [HOOC−C₆H₄−COOH]. Crease resistant. Uses: Blending with cotton/wool, glass reinforcing materials, safety helmets.
    • Glyptal: Monomers: Ethylene glycol and Phthalic acid. Uses: Manufacturing paints and lacquers.
  • Phenol-Formaldehyde Polymer (Bakelite): Monomers: Phenol [C₆H₅OH] and Formaldehyde [HCHO].
    • Novolac: Linear polymer formed with acid/base catalyst (used in paints).
    • Bakelite: Cross-linked thermosetting polymer formed by heating Novolac with more formaldehyde. Uses: Combs, electrical switches, handles of utensils, phonograph records.
  • Melamine-Formaldehyde Polymer: Monomers: Melamine and Formaldehyde. Thermosetting. Uses: Manufacturing unbreakable crockery.

5. Rubber

  • Natural Rubber:
    • Natural polymer obtained from rubber latex (colloidal dispersion of rubber in water).
    • Monomer: Isoprene (2-methyl-1,3-butadiene).
    • Structure: Linear cis-1,4-polyisoprene. The cis-configuration gives it elasticity.
    • Vulcanization: Process of heating natural rubber with sulfur (and additives) at 373-415 K. Sulfur forms cross-links (at reactive allylic positions) between polyisoprene chains. This makes the rubber harder, stronger, more elastic, and resistant to temperature changes and solvents. The extent of hardness depends on the amount of sulfur used (e.g., ~5% for tyre rubber).
  • Synthetic Rubbers: Polymers capable of getting stretched to twice their length and returning to original shape when force is released.
    • Neoprene (Polychloroprene): Monomer: Chloroprene (2-chloro-1,3-butadiene). Formed by free radical polymerization. Superior resistance to vegetable/mineral oils. Uses: Conveyor belts, gaskets, hoses.
    • Buna-S: Copolymer of 1,3-Butadiene and Styrene (approx. 3:1 ratio). Formed by copolymerization. Good substitute for natural rubber. Uses: Autotyres, footwear components, cable insulation. (Bu = Butadiene, Na = Sodium catalyst, S = Styrene).
    • Buna-N: Copolymer of 1,3-Butadiene and Acrylonitrile. Formed by copolymerization. Resistant to petrol, lubricating oil, organic solvents. Uses: Oil seals, tank linings. (Bu = Butadiene, Na = Sodium catalyst, N = Acrylonitrile).

6. Molecular Mass of Polymers

  • Polymers consist of chains of varying lengths, so their molecular mass is expressed as an average.
  • Number-Average Molecular Mass (M̄n): Total mass of all molecules divided by the total number of molecules. Determined by methods depending on colligative properties (e.g., osmotic pressure).
  • Weight-Average Molecular Mass (M̄w): Takes into account the mass contribution of each chain length. Determined by methods like light scattering and ultracentrifugation.
  • Polydispersity Index (PDI): Ratio of M̄w / M̄n. For natural polymers, PDI is usually 1 (monodispersed). For synthetic polymers, PDI > 1 (polydispersed).

7. Biodegradable Polymers

  • Polymers that can be degraded by microbial action over time. Developed to combat environmental pollution caused by persistent synthetic polymer waste.
  • Poly β-hydroxybutyrate-co-β-hydroxy valerate (PHBV):
    • Copolymer of 3-hydroxybutanoic acid and 3-hydroxypentanoic acid.
    • Polyester class. Biodegradable.
    • Uses: Specialty packaging, orthopaedic devices, controlled drug release.
  • Nylon 2−Nylon 6:
    • Alternating polyamide copolymer of Glycine (H₂N−CH₂−COOH) and Aminocaproic acid [H₂N−(CH₂)₅−COOH].
    • Biodegradable.

Key Takeaways for Exams:

  • Memorize monomers for common polymers (Polythene, PVC, Teflon, PAN, Nylon 6,6, Nylon 6, Terylene, Bakelite, Natural Rubber, Neoprene, Buna-S, Buna-N, PHBV, Nylon 2-Nylon 6).
  • Understand the difference between Addition and Condensation polymerization and classify polymers accordingly.
  • Know the classification based on structure and molecular forces, with examples.
  • Understand the process and significance of Vulcanization.
  • Be aware of the properties and uses of commercially important polymers.
  • Know the examples of biodegradable polymers and their monomers.

Multiple Choice Questions (MCQs)

  1. Which of the following is a natural polymer?
    (a) Bakelite
    (b) Cellulose
    (c) PVC
    (d) Nylon 6

  2. The monomer units of Nylon 6,6 are:
    (a) Caprolactam
    (b) Hexamethylenediamine and Adipic acid
    (c) Ethylene glycol and Terephthalic acid
    (d) Phenol and Formaldehyde

  3. Which polymer is formed by condensation polymerization?
    (a) Polythene
    (b) Teflon
    (c) Terylene (Dacron)
    (d) Polyacrylonitrile

  4. Bakelite is an example of a:
    (a) Thermoplastic polymer
    (b) Elastomer
    (c) Fibre
    (d) Thermosetting polymer

  5. The process of heating natural rubber with sulfur to improve its properties is called:
    (a) Polymerization
    (b) Vulcanization
    (c) Galvanization
    (d) Sulphonation

  6. Which of the following is a biodegradable polymer?
    (a) Buna-S
    (b) PVC
    (c) PHBV
    (d) Polystyrene

  7. Teflon (PTFE) is chemically inert due to the presence of strong:
    (a) C-H bonds
    (b) C-C bonds
    (c) C-F bonds
    (d) Intermolecular forces

  8. Which catalyst is used in the preparation of High-Density Polythene (HDPE)?
    (a) Peroxide initiator
    (b) Ziegler-Natta catalyst
    (c) Sodium metal
    (d) Acid catalyst

  9. Buna-S is a copolymer of:
    (a) 1,3-Butadiene and Styrene
    (b) 1,3-Butadiene and Acrylonitrile
    (c) Isoprene and Styrene
    (d) Chloroprene and Styrene

  10. Which of the following polymers contains ester linkages?
    (a) Nylon 6
    (b) Bakelite
    (c) PVC
    (d) Glyptal

Answer Key for MCQs:

  1. (b)
  2. (b)
  3. (c)
  4. (d)
  5. (b)
  6. (c)
  7. (c)
  8. (b)
  9. (a)
  10. (d)

Make sure you revise these concepts thoroughly. Understanding the structure-property relationships and the specific examples is key to scoring well. Good luck with your preparation!

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