NEET Chemistry: Polymers – Detailed Notes

Chapter 15: Polymers

1. Introduction to Polymers

• Polymers are high molecular mass substances (103 to 107u) consisting of a very large number of repeating structural units, derived from small simple molecules called monomers.
• The process of formation of polymers from monomers is called polymerization.
• The repeating structural units are linked to each other by strong covalent bonds.

2. Classification of Polymers

Polymers can be classified in various ways:

A. Based on Source:

1. Natural Polymers: Found in plants and animals.
   • Examples: Starch, cellulose, proteins, nucleic acids, natural rubber.
2. Semi-synthetic Polymers: Derived from naturally occurring polymers by chemical modifications.
   • Examples: Cellulose acetate (rayon), cellulose nitrate, vulcanized rubber.
3. Synthetic Polymers: Man-made polymers prepared in the laboratory.
   • Examples: Polythene, Nylon 6,6, Buna-S, PVC, Teflon, Bakelite.

B. Based on Structure:

1. Linear Polymers: Consist of long, straight chains.
   • Examples: High density polythene (HDPE), PVC, Nylon, Polyesters.
   • Characteristics: Close packing, high density, high tensile strength, high melting point.
2. Branched-Chain Polymers: Possess branched chains originating from the main chain.
   • Examples: Low density polythene (LDPE), Amylopectin, Glycogen.
   • Characteristics: Cannot pack closely, lower density, lower tensile strength, lower melting point.
3. Cross-Linked or Network Polymers: Formed from bifunctional and trifunctional monomers and contain strong covalent bonds between various linear polymer chains, forming a 3D network structure.
   • Examples: Bakelite, Melamine-formaldehyde resin, Vulcanized rubber.
   • Characteristics: Hard, rigid, brittle, insoluble, infusible.

C. Based on Mode of Polymerization:

1. Addition Polymers: Formed by the repeated addition of monomer molecules containing double or triple bonds without the elimination of any small molecules.
   • Monomers are unsaturated compounds (alkenes, alkadienes, alkynes).
   • Example: Polythene from ethene, PVC from vinyl chloride.
   • Homopolymers: Formed from a single type of monomer unit. (e.g., Polythene, PVC).
   • Copolymers (Heteropolymers): Formed by the addition of two or more different types of monomer units. (e.g., Buna-S, Buna-N).
   • Mechanism: Can occur via free radical, cationic, or anionic polymerization.
2. Condensation Polymers: Formed by the repeated condensation reaction between two different bifunctional or trifunctional monomeric units with the elimination of small molecules like water (H2​O), alcohol (CH3​OH), hydrogen chloride (HCl), etc.
   • Example: Nylon 6,6, Dacron (Polyester), Bakelite.

D. Based on Molecular Forces (Intermolecular Forces):

1. Elastomers: Possess elastic properties. Polymer chains are held together by weak intermolecular forces, allowing them to stretch and recoil.
   • Examples: Natural rubber, Buna-S, Buna-N, Neoprene.
   • Characteristics: Amorphous, significant elasticity.
2. Fibers: Thread-like polymers with high tensile strength and high modulus. Strong intermolecular forces (like H-bonding or dipole-dipole interactions) lead to close packing.
   • Examples: Nylon 6,6, Polyesters (Dacron), Silk, Wool.
   • Characteristics: Crystalline, high tensile strength, sharp melting points.
3. Thermoplastics: Linear or slightly branched polymers that can be softened on heating and hardened on cooling repeatedly. Intermolecular forces are intermediate between elastomers and fibers.
   • Examples: Polythene, PVC, Polystyrene, Nylon.
   • Characteristics: Can be reshaped and recycled.
4. Thermosetting Polymers: Cross-linked or heavily branched polymers which become hard and infusible on heating. They cannot be softened or reshaped once molded.
   • Examples: Bakelite, Urea-formaldehyde resins, Melamine-formaldehyde resins.
   • Characteristics: Cannot be recycled.

3. Types of Polymerization

A. Addition Polymerization (Chain Growth Polymerization):

• Free Radical Mechanism: Initiated by free radicals (e.g., peroxides, AIBN).
  • Initiation: Peroxide decomposes to form free radicals.
  • Propagation: Free radical adds to monomer, forming a larger radical.
  • Termination: Two radicals combine or disproportionate.
  • Example: Formation of Polythene from Ethene.
• Cationic Polymerization: Initiated by Lewis acids (BF3​, AlCl3​) and protonic acids (H2​SO4​). Requires monomers with electron-donating groups.
• Anionic Polymerization: Initiated by strong bases (e.g., organometallic compounds like Butyl lithium). Requires monomers with electron-withdrawing groups.
• Coordination Polymerization (Ziegler-Natta Polymerization): Uses catalysts (e.g., Ziegler-Natta catalyst, a mixture of TiCl4​ and (C2​H5​)3​Al) to control stereochemistry and produce linear polymers like HDPE.

B. Condensation Polymerization (Step Growth Polymerization):

• Monomers usually have two or more functional groups capable of condensation reactions (e.g., -OH, -COOH, -NH$_2$).
• Proceeds in a step-wise manner with the elimination of small molecules.

4. Important Polymers: Preparation, Properties, and Uses

• Polythene:
  • Monomer: Ethene (CH2​=CH2​)
  • LDPE (Low Density Polythene):
    • Preparation: Polymerization of ethene at high pressure (1000−2000atm) and temperature (373−573K) in presence of peroxide initiator. Results in branched structure.
    • Properties: Soft, flexible, poor conductor of electricity, low tensile strength.
    • Uses: Squeeze bottles, toys, flexible pipes, insulation of electric wires.
  • HDPE (High Density Polythene):
    • Preparation: Polymerization of ethene at low pressure (6−7atm) and temperature (333−343K) in presence of Ziegler-Natta catalyst. Results in linear structure.
    • Properties: Hard, rigid, higher tensile strength.
    • Uses: Buckets, dustbins, pipes, bottles.
• Polypropene:
  • Monomer: Propene (CH2​=CHCH3​)
  • Uses: Manufacture of ropes, pipes, toys, fibers.
• Polytetrafluoroethene (PTFE) or Teflon:
  • Monomer: Tetrafluoroethene (CF2​=CF2​)
  • Preparation: Polymerization of tetrafluoroethene at high pressure in the presence of persulphate initiator.
  • Properties: Chemically inert, resistant to heat and chemicals, non-stick surface.
  • Uses: Non-stick coating on cookware, oil seals, gaskets, insulation.
• Polyacrylonitrile (PAN) or Orlon/Acrilan:
  • Monomer: Acrylonitrile (CH2​=CH−CN)
  • Preparation: Addition polymerization of acrylonitrile in presence of peroxide catalyst.
  • Uses: Orlon (synthetic wool), blankets, carpets.
• Polyamides: Polymers containing amide linkages (-$CONH$- ). Examples: Nylon 6,6 and Nylon 6.
  • Nylon 6,6:
    • Monomers: Hexamethylenediamine (H2​N−(CH2​)6​−NH2​) and Adipic acid (HOOC−(CH2​)4​−COOH).
    • Preparation: Condensation polymerization at high temperature and pressure.
    • Properties: Strong, high tensile strength, elastic, lustrous, resistant to abrasion.
    • Uses: Sheets, bristles for brushes, textile fibers, ropes.
  • Nylon 6:
    • Monomer: Caprolactam.
    • Preparation: Heating Caprolactam with water at high temperature.
    • Uses: Tire cords, fabrics, ropes.
• Polyesters: Polymers containing ester linkages (-$COOR$- ). Example: Dacron or Terylene.
  • Dacron (Terylene):
    • Monomers: Ethylene glycol (HO−CH2​−CH2​−OH) and Terephthalic acid (HOOC−C6​H4​−COOH).
    • Preparation: Condensation polymerization.
    • Properties: Wrinkle resistant, blendable with cotton/wool, low moisture absorption.
    • Uses: Fabric blends (terrycot, terrywool), safety belts, sails, packaging materials.
• Phenol-Formaldehyde Polymers (Bakelite and Novolac):
  • Monomers: Phenol and Formaldehyde (HCHO).
  • Novolac: Linear polymer formed when phenol and formaldehyde react in acidic medium (less formaldehyde than phenol). Used in paints.
  • Bakelite: Cross-linked polymer formed when phenol and formaldehyde react in alkaline medium (more formaldehyde) or with heating of Novolac.
    • Properties: Hard, rigid, infusible, heat resistant, excellent electrical insulator.
    • Uses: Electrical switches, handles of utensils, telephone casings, computer discs.
• Melamine-Formaldehyde Polymer:
  • Monomers: Melamine and Formaldehyde.
  • Uses: Unbreakable crockery.
• Natural Rubber:
  • Monomer: Isoprene (2-methyl-1,3-butadiene).
  • Structure: Cis-polyisoprene.
  • Properties: Soft, sticky, low tensile strength, large water absorption capacity.
  • Vulcanization of Rubber: Process of heating natural rubber with sulfur (3-10%) at 373−415K. Sulfur forms cross-links, improving elasticity, tensile strength, and resistance to wear and tear.
• Synthetic Rubbers:
  • Buna-S (Styrene Butadiene Rubber – SBR):
    • Monomers: 1,3-Butadiene and Styrene.
    • Uses: Auto tires, floor tiles, footwear, cable insulation.
  • Buna-N (Acrylonitrile Butadiene Rubber – NBR):
    • Monomers: 1,3-Butadiene and Acrylonitrile.
    • Properties: Resistant to action of petrol, lubricating oil, and organic solvents.
    • Uses: Oil seals, tank linings.
  • Neoprene (Polychloroprene):
    • Monomer: Chloroprene (2-chloro-1,3-butadiene).
    • Properties: Superior resistance to vegetable and mineral oils.
    • Uses: Conveyor belts, gaskets, hoses.

5. Biodegradable Polymers

• Polymers that can be decomposed by microorganisms (bacteria, fungi) into environmentally friendly products.
• Necessary due to environmental concerns of non-biodegradable plastics.
• Examples:
  • PHBV (Poly-β-hydroxybutyrate-co-β-hydroxyvalerate):
    • Monomers: 3-hydroxybutanoic acid and 3-hydroxypentanoic acid.
    • Uses: Packaging, orthopedic devices, specialty chemicals.
  • Nylon 2-Nylon 6:
    • Monomers: Glycine (H2​N−CH2​−COOH) and Aminocaproic acid (H2​N−(CH2​)5​−COOH).
    • Biodegradable polyamide.
  • Poly(Lactic acid) (PLA): Another common biodegradable polymer.

6. Molecular Mass of Polymers

• Polymers are macromolecules and do not have a sharp molecular weight. They are polydisperse.
• Number Average Molecular Mass (Mˉn​): Mˉn​=∑Ni​∑Ni​Mi​​
  • Ni​ = number of molecules with molecular mass Mi​.
• Weight Average Molecular Mass (Mˉw​): Mˉw​=∑Ni​Mi​∑Ni​Mi2​​
• Polydispersity Index (PDI): PDI=Mˉn​Mˉw​​
  • For natural polymers, PDI is usually 1 (monodisperse).
  • For synthetic polymers, PDI > 1 (polydisperse).