Class 12 Chemistry Notes Chapter 15 (Polymers) – Examplar Problems Book

Alright class, let's dive into Chapter 15: Polymers. This is a crucial chapter, not just for your board exams but also frequently tested in various government competitive exams. Polymers are everywhere around us, so understanding their chemistry is quite important.

We'll break down the key concepts systematically. Pay close attention to the classifications, examples, monomers, and specific reactions like vulcanization.

Chapter 15: Polymers - Detailed Notes for Competitive Exams

1. Introduction

• Polymers: High molecular mass (10³ - 10⁷ u) macromolecules formed by the joining of repeating structural units. These repeating units are derived from simple molecules.
• Monomers: The simple molecules from which the repeating structural units of a polymer 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 (Polythene)

2. Classification of Polymers

(A) Based on Source:

• Natural Polymers: Found in plants and animals.
  • Examples: Proteins (polymers of α-amino acids), Cellulose & Starch (polymers of glucose), Natural Rubber (polymer of isoprene), Nucleic acids (polymers of nucleotides).
• 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, Buna-S.

(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. Contain strong covalent bonds between various linear polymer chains (cross-links). Leads to hard, rigid, and brittle materials.
  • Examples: Bakelite, Melamine-formaldehyde resin, Vulcanized rubber (has some cross-links).

(C) Based on Mode of Polymerization:

• Addition Polymers (Chain Growth Polymers):
  • Formed by the repeated addition of monomer molecules possessing double or triple bonds.
  • No elimination of small molecules (like H₂O, NH₃).
  • The empirical formula of the polymer is the same as the monomer.
  • Usually follows free radical, cationic, or anionic mechanisms.
  • Homopolymers: Formed from a single type of monomer. Example: Polythene from ethene.
  • Copolymers: Formed from two or more different types of monomers. Example: Buna-S from 1,3-butadiene and styrene.
  • Further Examples: Polypropene, Polystyrene, PVC, Teflon (PTFE), Polyacrylonitrile (PAN), Orlon.
• Condensation Polymers (Step Growth Polymers):
  • Formed by the repeated condensation reaction between two different bi-functional or tri-functional monomeric units.
  • Involves the elimination of small molecules like water (H₂O), alcohol (ROH), ammonia (NH₃), or HCl.
  • The empirical formula of the polymer is different from its monomers.
  • Examples:
    • Polyamides: Contain amide linkages (–CONH–). Examples: Nylon 6,6 (from hexamethylenediamine and adipic acid), Nylon 6 (from caprolactam).
    • Polyesters: Contain ester linkages (–COO–). Examples: Terylene or Dacron (from ethylene glycol and terephthalic acid).
    • Phenol-formaldehyde resins: Example: Bakelite (from phenol and formaldehyde).
    • Melamine-formaldehyde resin: (from melamine and formaldehyde).

(D) Based on Molecular Forces:

• Elastomers:
  • Rubber-like solids with elastic properties.
  • Polymer chains are held by weak intermolecular forces (van der Waals forces), allowing them to be stretched.
  • Few cross-links are introduced (e.g., during vulcanization) which help the polymer retract to its original position after the force is removed.
  • 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 or dipole-dipole interactions exist between chains.
  • Chains are closely packed, imparting crystalline nature.
  • Examples: Polyamides (Nylon 6,6; Nylon 6), Polyesters (Terylene).
• Thermoplastics:
  • Linear or slightly branched long-chain polymers.
  • Soften on heating and harden on cooling; can be remoulded repeatedly.
  • Intermolecular forces are intermediate between elastomers and fibres.
  • Examples: Polythene, Polystyrene, PVC, Teflon.
• Thermosetting Plastics:
  • Cross-linked or heavily branched polymers.
  • Undergo extensive cross-linking in moulds upon heating, becoming infusible and insoluble.
  • Cannot be remoulded or reshaped once set.
  • Examples: Bakelite, Urea-formaldehyde resins, Melamine-formaldehyde resin.

3. Types of Polymerization Reactions

• Addition Polymerization:
  • Free Radical Mechanism: Most common for vinyl monomers (like ethene, propene, styrene, vinyl chloride). Involves three steps:
    1. Chain Initiation: A free radical initiator (e.g., benzoyl peroxide, AIBN) generates a free radical, which adds to the monomer's double bond.
    2. Chain Propagation: The new radical adds to another monomer molecule, repeating the process and growing the chain.
    3. Chain Termination: The active radical chains are deactivated by combination or disproportionation.
  • Examples: Polythene (LDPE via free radical, HDPE via Ziegler-Natta catalyst), Teflon, PAN.
• Condensation Polymerization:
  • Step-wise reaction between functional groups of monomers.
  • Elimination of small molecules occurs at each step.
  • Examples: Nylon 6,6, Dacron, Bakelite.

4. Important Polymers: Monomers and Uses

5. Natural Rubber and Vulcanization

• Natural Rubber: A linear polymer of isoprene (2-methyl-1,3-butadiene), specifically cis-1,4-polyisoprene.
• It is soft, tacky, has low tensile strength, and low resistance to abrasion. Becomes brittle at low temperatures and soft at high temperatures (>335 K). Soluble in non-polar solvents.
• Vulcanization: Process discovered by Charles Goodyear. Heating raw rubber with sulphur (and appropriate additives) at 373-415 K.
  • Sulphur forms cross-links (–S–S– bridges) between the polymer chains at the reactive allylic positions (adjacent to double bonds).
  • Result: Vulcanized rubber is harder, stronger, more elastic, has higher tensile strength, better resistance to abrasion, heat, and chemicals. The extent of hardness depends on the amount of sulphur used (e.g., ~5% for tyre rubber).

6. Biodegradable Polymers

• Polymers that can be decomposed by microbial action (bacteria) in the environment.
• Developed to address environmental problems caused by non-biodegradable polymeric waste.
• Contain functional groups (like ester, amide) similar to those found in biopolymers, which are susceptible to enzymatic hydrolysis.
• Examples:
  • Poly β-hydroxybutyrate-co-β-hydroxy valerate (PHBV): A copolymer of 3-hydroxybutanoic acid and 3-hydroxypentanoic acid. It's a biodegradable polyester.
  • Nylon 2-Nylon 6: An alternating polyamide copolymer of glycine and aminocaproic acid.

7. 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 polymer molecules divided by the total number of polymer molecules. Determined by methods sensitive to the number of molecules (e.g., colligative properties).
• Weight-Average Molecular Mass (M̄w): Based on the weight fraction of molecules of each particular mass. Determined by methods sensitive to molecular size (e.g., light scattering).
• Polydispersity Index (PDI): Ratio of weight-average molecular mass to number-average molecular mass (PDI = M̄w / M̄n).
  • For natural polymers or perfectly uniform synthetic polymers, PDI = 1.
  • For most synthetic polymers, PDI > 1, indicating a distribution of chain lengths.

Multiple Choice Questions (MCQs)

Here are 10 MCQs based on the concepts we just discussed. Try to answer them.

1. Which of the following is a natural polymer?
   (a) Polythene
   (b) Cellulose
   (c) PVC
   (d) Teflon

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

4. Which process is used to improve the properties of natural rubber by heating it with sulphur?
   (a) Polymerization
   (b) Vulcanization
   (c) Condensation
   (d) Hydrolysis

5. 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 Acrylonitrile

6. Which of the following is a biodegradable polymer?
   (a) PVC
   (b) Polythene
   (c) PHBV
   (d) Bakelite

7. Teflon (PTFE) is chemically inert and resistant to heat due to the presence of strong:
   (a) C–H bonds
   (b) C–C bonds
   (c) C–F bonds
   (d) C=C bonds

8. Which type of polymerization involves the elimination of small molecules like water?
   (a) Addition polymerization
   (b) Free radical polymerization
   (c) Condensation polymerization
   (d) Cationic polymerization

9. Low-density polythene (LDPE) is characterized by:
   (a) A linear structure and high density
   (b) A branched structure and low density
   (c) Cross-linking between chains
   (d) High tensile strength and hardness

10. Orlon is a polymer of:
    (a) Styrene
    (b) Acrylonitrile
    (c) Vinyl chloride
    (d) Tetrafluoroethene

Answers to MCQs:

1. (b) Cellulose
2. (b) Hexamethylenediamine and Adipic acid
3. (d) Thermosetting polymer
4. (b) Vulcanization
5. (a) 1,3-Butadiene and Styrene
6. (c) PHBV
7. (c) C–F bonds
8. (c) Condensation polymerization
9. (b) A branched structure and low density
10. (b) Acrylonitrile

Make sure you revise these notes thoroughly. Focus on the monomers, polymer types, specific reactions, and uses, as these are the areas most frequently targeted in exams. Good luck with your preparation!