Class 12 Chemistry Chapter 15 Notes: Polymers | Important Questions & PYQs

 Class 12 Chemistry Chapter 15 Polymers Notes

🔹 15.1 Polymers – Introduction

• Polymers are high molecular mass compounds (10³–10⁷ u)

• Made up of repeating structural units called monomers

• Units are joined by covalent bonds

• Also called macromolecules

Examples: Polythene, PVC, nylon-6,6, bakelite, rubber

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🔹 15.2 Classification of Polymers

(A) Based on Source

✔ Natural Polymers

• Occur in nature

• Examples: Starch, cellulose, proteins, natural rubber, nucleic acids

✔ Synthetic Polymers

• Prepared in laboratories

• Examples: PVC, nylon-66, terylene, bakelite

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(B) Based on Structure

✔ Linear Chain Polymers

• Monomers linked in straight chains

• Closely packed → high density & strength

• Examples: HDPE, nylon, PVC, polyester

✔ Branched Chain Polymers

• Main chain + side chains

• Less packing → low density

• Examples: LDPE, starch, glycogen

✔ Cross-linked (Network) Polymers

• 3-D network structure

• Hard, rigid, brittle

• Examples: Bakelite, melamine-formaldehyde

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(C) Based on Molecular Forces

Type	Intermolecular Forces	Properties	Examples
Elastomers	Weak	Elastic	Rubber, Buna-S
Thermoplastics	Moderate	Moldable	Polythene, PVC
Fibres	Strong	High tensile strength	Nylon-66
Thermosetting	Very strong	Hard, infusible	Bakelite

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🔹 15.3 Types of Polymerisation

✔ Addition Polymerisation

• No elimination of small molecules

• Monomers contain double bonds

• Example:

n CH₂=CH₂ → (–CH₂–CH₂–)n

• Examples: Polythene, PVC, Teflon

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✔ Condensation Polymerisation

• Elimination of small molecules (H₂O, NH₃)

• Monomers are bi-functional

• Example: Nylon-6,6

Hexamethylenediamine + Adipic acid → Nylon-6,6 + H₂O

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✔ Copolymerisation

• Two or more different monomers

• Examples:

  • Buna-S → butadiene + styrene

  • Buna-N → butadiene + acrylonitrile

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🔹 15.4 Functionality of a Monomer

• Functionality = number of bonding sites

• Functionality = 1 → addition polymerisation

• Functionality ≥ 2 → condensation polymerisation

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🔹 15.5 Molecular Mass of Polymers

• Polymers have very high molar mass

• Expressed as average molecular mass

• Depends on:

  • Degree of polymerisation

  • Chain length

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🔹 15.6 Biodegradable Polymers

• Decompose naturally by microorganisms

• Environment friendly

Example:

• PHBV (Poly-β-hydroxybutyrate-co-β-hydroxyvalerate)

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🔹 15.7 Important Commercial Polymers

✔ Polythene

• Monomer: Ethene

• Uses: Bags, pipes, insulation

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✔ PVC (Polyvinyl Chloride)

• Monomer: Vinyl chloride

• Uses: Pipes, cables, raincoats

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✔ Nylon-6,6

• Monomers:

  • Hexamethylenediamine

  • Adipic acid

• Fibre polymer

• Uses: Ropes, textiles

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✔ Nylon-6

• Monomer: Caprolactam

• Strong synthetic fibre

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✔ Teflon

• Monomer: Tetrafluoroethene

• Non-stick coating

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✔ Bakelite

• Monomers: Phenol + formaldehyde

• Thermosetting polymer

• Uses: Electrical switches

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✔ Buna-S

• Butadiene + styrene

• Synthetic rubber

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✔ Buna-N

• Butadiene + acrylonitrile

• Oil-resistant rubber

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🔹 15.8 Rubber and Vulcanisation

✔ Natural Rubber

• cis-polyisoprene

• Weak intermolecular forces

• Soft & elastic

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✔ Vulcanisation of Rubber

• Heating rubber with sulphur (3–5%)

• Temperature: 373–415 K

• Forms S-cross links

Advantages:

• Increased elasticity

• Higher tensile strength

• Resistant to heat & solvents

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🔹 15.9 Thermoplastics vs Thermosetting Polymers

Thermoplastics	Thermosetting
Linear polymers	Cross-linked polymers
Can be remoulded	Cannot be remoulded
Soften on heating	Become permanently hard
Polythene, PVC	Bakelite

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🔥 EXAM HOTSPOTS (VERY IMPORTANT)

✔ Addition vs condensation polymerisation
✔ Linear / branched / cross-linked polymers
✔ Nylon-6 vs Nylon-6,6
✔ Buna-S vs Buna-N
✔ Vulcanisation
✔ Thermoplastics vs thermosetting
✔ Biodegradable polymers

✍️ Top 10 Short Question–Answers (Board Exam Oriented)

1. What are polymers?
   Polymers are high molecular mass compounds made up of repeating structural units called monomers.

2. What are monomers?
   Small molecules that join together to form polymers.

3. What is addition polymerisation?
   Polymerisation without elimination of small molecules, involving unsaturated monomers.

4. What is condensation polymerisation?
   Polymerisation involving elimination of small molecules like H₂O or NH₃.

5. What are natural polymers?
   Polymers occurring in nature such as starch, cellulose and proteins.

6. What are thermoplastics?
   Polymers that soften on heating and can be remoulded.

7. Name a thermosetting polymer.
   Bakelite.

8. What is vulcanisation of rubber?
   Process of heating rubber with sulphur to improve its properties.

9. What is a biodegradable polymer?
   A polymer that decomposes naturally by microorganisms.

10. Give one example of a synthetic rubber.
    Buna-S.

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📝 Long Answer Questions

1. Classify polymers on the basis of structure and intermolecular forces.

(A) Based on Structure

• Linear polymers:
  Straight chain polymers with close packing and high density.
  Examples: HDPE, nylon, PVC

• Branched polymers:
  Main chain with side chains, low density.
  Examples: LDPE, starch

• Cross-linked polymers:
  Three-dimensional network, hard and rigid.
  Examples: Bakelite, melamine-formaldehyde

(B) Based on Intermolecular Forces

• Elastomers: Weak forces, elastic (rubber, Buna-S)

• Thermoplastics: Moderate forces, mouldable (polythene, PVC)

• Fibres: Strong forces, high tensile strength (nylon-66)

• Thermosetting polymers: Very strong forces, hard (bakelite)

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2. Explain addition and condensation polymerisation with examples.

Addition Polymerisation

• Monomers contain double bonds

• No elimination of small molecules

• Example:
  nCH₂=CH₂ → (–CH₂–CH₂–)ₙ (polythene)

Condensation Polymerisation

• Bi-functional monomers

• Elimination of small molecules

• Example:
  Hexamethylenediamine + adipic acid → Nylon-6,6 + H₂O

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3. Explain vulcanisation of rubber and its advantages.

Vulcanisation

• Natural rubber is heated with 3–5% sulphur

• Temperature: 373–415 K

• Sulphur forms cross-links between polymer chains

Advantages:

• Increased elasticity

• Higher tensile strength

• More resistant to heat, wear and solvents

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❓ FAQs (Concept Clarity)

1. Why are polymers called macromolecules?
   Because they have very high molecular mass.

2. Why is nylon-6,6 stronger than nylon-6?
   Due to more hydrogen bonding between polymer chains.

3. Why are thermosetting polymers hard?
   Because of extensive cross-linking between chains.

4. Why is LDPE less dense than HDPE?
   Because branched chains prevent close packing.

5. Why are biodegradable polymers important?
   Because they reduce environmental pollution.

Chapter No.	Chapter Name	Visit
1	The Solid State	Visit
2	Solutions	Visit
3	Electrochemistry	Visit
4	Chemical Kinetics	Visit
5	Surface Chemistry	Visit
6	General Principles and Processes of Isolation of Elements	Visit
7	The p-Block Elements	Visit
8	The d and f Block Elements	Visit
9	Coordination Compounds	Visit
10	Haloalkanes and Haloarenes	Visit
11	Alcohols, Phenols and Ethers	Visit
12	Aldehydes, Ketones and Carboxylic Acids	Visit
13	Amines	Visit
14	Biomolecules	Visit
15	Polymers	Visit
16	Chemistry in Everyday Life	Visit

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