Polymer Chemistry: Theory and Fundamentals

NEET Chemistry: Polymers – Detailed Notes and Practice Questions

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).

NEET Chemistry: Polymers – Practice Questions

I. Multiple Choice Questions (MCQs)

1. Question: Which of the following is a condensation polymer? a) Polythene b) PVC c) Nylon 6,6 d) Teflon

2. Question: Which of the following is a synthetic rubber? a) Natural rubber b) Isoprene c) Buna-N d) Gutta Percha

3. Question: The monomer of natural rubber is: a) Neoprene b) Isoprene c) Acrylonitrile d) Chloroprene

4. Question: The process of heating natural rubber with sulfur is called: a) Polymerization b) Vulcanization c) Esterification d) Condensation

5. Question: Which of the following is a thermoplastic polymer? a) Bakelite b) Melamine-formaldehyde resin c) Polythene d) Urea-formaldehyde resin

6. Question: Dacron is also known as: a) Orlon b) Terylene c) Acrilan d) Saran

7. Question: The monomers of Nylon 6,6 are: a) Caprolactam b) Hexamethylenediamine and Adipic acid c) Ethylene glycol and Terephthalic acid d) Phenol and Formaldehyde

8. Question: Which type of polymerization involves the elimination of small molecules? a) Addition polymerization b) Condensation polymerization c) Free radical polymerization d) Co-polymerization

9. Question: Which of the following is a biodegradable polymer? a) PVC b) PHBV c) Teflon d) Polypropene

10. Question: Ziegler-Natta catalyst is used in the preparation of: a) LDPE b) HDPE c) Teflon d) Nylon 6,6

11. Question: Which of the following is a cross-linked polymer? a) LDPE b) PVC c) Bakelite d) Nylon 6

12. Question: What is the repeating unit in PVC? a) −CH2​−CH2​− b) −CH2​−CHCl− c) −CF2​−CF2​− d) −CH2​−CH(CN)−

13. Question: Which of the following statements is true about thermosetting polymers? a) They can be re-molded on heating. b) They are linear or slightly branched. c) They become hard and infusible on heating. d) They have weak intermolecular forces.

14. Question: The monomer of Teflon is: a) Vinyl chloride b) Tetrafluoroethene c) Acrylonitrile d) Ethylene

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

II. Assertion-Reason Type Questions

Directions: In the following questions, a statement of Assertion (A) is followed by a statement of Reason (R). Choose the correct option. a) Both A and R are true and R is the correct explanation of A. b) Both A and R are true but R is NOT the correct explanation of A. c) A is true but R is false. d) A is false but R is true.

16. Assertion (A): Bakelite is a thermosetting polymer. Reason (R): Thermosetting polymers become hard and infusible on heating and cannot be reused.

17. Assertion (A): Natural rubber is a linear polymer of isoprene. Reason (R): Natural rubber is cis-polyisoprene.

18. Assertion (A): HDPE has higher density and melting point than LDPE. Reason (R): HDPE has a more compact and linear structure due to Ziegler-Natta catalysis.

19. Assertion (A): Cellulose is a natural polymer. Reason (R): Cellulose is found in plants and is a polymer of α-D-glucose.

20. Assertion (A): Polymers have very high molecular masses. Reason (R): Polymers are formed by the repetition of a large number of monomer units.

III. Short Answer / Conceptual Questions

21. Question: Differentiate between homopolymers and copolymers with one example each.

22. Question: Give the full form and uses of PHBV.

23. Question: What are elastomers? Give two examples.

24. Question: Why are Bakelite and Melamine-formaldehyde polymers classified as thermosetting polymers?

25. Question: What is polydispersity index (PDI) and what does its value (PDI > 1 or PDI = 1) signify for synthetic and natural polymers?

NEET Chemistry: Polymers – Additional NCERT-Based Practice Questions (Class 12)

I. Multiple Choice Questions (MCQs)

26. Question: Which of the following is a homopolymer? a) Buna-S b) Nylon 6,6 c) Polythene d) Dacron

27. Question: Which of the following is used for making non-stick cookware? a) Polyacrylonitrile b) Polythene c) Polytetrafluoroethene d) Polypropene

28. Question: Which of the following is an example of a fiber? a) Natural rubber b) Polythene c) Nylon 6,6 d) Buna-N

29. Question: The monomer unit of Nylon 6 is: a) Caprolactam b) Hexamethylenediamine c) Adipic acid d) Both (b) and (c)

30. Question: Which of the following polymerization methods is suitable for synthesizing linear HDPE? a) Free radical polymerization b) Ziegler-Natta polymerization c) Cationic polymerization d) Anionic polymerization

31. Question: Glyptal is a polymer of: a) Phthalic acid and ethylene glycol b) Adipic acid and hexamethylenediamine c) Terephthalic acid and ethylene glycol d) Phenol and formaldehyde

32. Question: Which of the following biodegradable polymers is obtained from 3-hydroxybutanoic acid and 3-hydroxypentanoic acid? a) Nylon 2-Nylon 6 b) PHBV c) PLA d) Dextron

33. Question: The correct statement among the following regarding Buna-N is: a) It is a homopolymer. b) It is resistant to oils and organic solvents. c) Its monomer units are styrene and butadiene. d) It is used in tire manufacturing.

34. Question: Which of the following is a characteristic of elastomers? a) They are highly crystalline. b) They have strong intermolecular forces. c) They possess elasticity. d) They cannot be stretched.

35. Question: Melamine-formaldehyde resin is used for making: a) Electrical switches b) Non-stick cookware c) Unbreakable crockery d) Paints and lacquers

II. Assertion-Reason Type Questions

Directions: In the following questions, a statement of Assertion (A) is followed by a statement of Reason (R). Choose the correct option. a) Both A and R are true and R is the correct explanation of A. b) Both A and R are true but R is NOT the correct explanation of A. c) A is true but R is false. d) A is false but R is true.

36. Assertion (A): Vulcanization improves the elasticity of natural rubber. Reason (R): Sulphur forms cross-links between polyisoprene chains.

37. Assertion (A): LDPE is used in the insulation of electric wires. Reason (R): LDPE is a poor conductor of electricity and is flexible.

38. Assertion (A): Proteins are natural polymers. Reason (R): Proteins are polymers of amino acids.

39. Assertion (A): Condensation polymerization is a step-growth polymerization. Reason (R): It involves the elimination of small molecules like water or alcohol.

40. Assertion (A): The polydispersity index (PDI) for natural polymers is generally 1. Reason (R): Natural polymers are usually monodisperse, meaning all polymer chains in the sample have almost the same molecular mass.

III. Short Answer / Conceptual Questions

41. Question: What is the significance of the term ‘6,6’ in Nylon 6,6? Name its monomers.

42. Question: Give one example each of a natural and a semi-synthetic polymer.

43. Question: Write the general reactions for addition and condensation polymerization processes.

44. Question: Differentiate between thermoplastics and thermosetting polymers based on their properties and reusability.

45. Question: What are biodegradable polymers? Why is their development considered important? Give one example of a synthetic biodegradable polymer.

46. Question: How is HDPE prepared? List two of its properties and uses.

47. Question: Give the structures of the monomers used in the preparation of Dacron.

48. Question: Explain the function of sulfur in the vulcanization of natural rubber.

49. Question: Identify the monomers of Buna-S and Buna-N.

50. Question: What is Novolac? How is it different from Bakelite in terms of structure and use?

Additional Answers and Explanations (for new questions)

I. Multiple Choice Questions (MCQs) – Answers

26. Answer: c) Polythene Explanation: Homopolymers are formed from a single type of monomer unit. Polythene is formed only from ethene. Buna-S and Dacron are copolymers, while Nylon 6,6 is formed from two different monomers via condensation.

27. Answer: c) Polytetrafluoroethene Explanation: Polytetrafluoroethene (PTFE), commonly known as Teflon, is known for its non-stick properties and is used in non-stick cookware due to its chemical inertness and high thermal stability.

28. Answer: c) Nylon 6,6 Explanation: Fibers are polymers with strong intermolecular forces leading to high tensile strength and used in textile applications. Nylon 6,6 is a synthetic fiber. Natural rubber and Buna-N are elastomers, while polythene is a thermoplastic.

29. Answer: a) Caprolactam Explanation: Nylon 6 is a homopolymer formed by the heating of caprolactam with water at high temperature.

30. Answer: b) Ziegler-Natta polymerization Explanation: Ziegler-Natta catalysts are specifically used for coordination polymerization, which facilitates the formation of linear, high-density polymers like HDPE with controlled stereochemistry, unlike free radical polymerization which yields branched structures (LDPE).

31. Answer: a) Phthalic acid and ethylene glycol Explanation: Glyptal is an example of an alkyd resin, which is a polyester formed by the condensation polymerization of phthalic acid (or phthalic anhydride) and ethylene glycol.

32. Answer: b) PHBV Explanation: PHBV (Poly-β-hydroxybutyrate-co-β-hydroxyvalerate) is a biodegradable polymer formed by the copolymerization of 3-hydroxybutanoic acid and 3-hydroxypentanoic acid.

33. Answer: b) It is resistant to oils and organic solvents. Explanation: Buna-N (Acrylonitrile Butadiene Rubber) is known for its resistance to the action of petrol, lubricating oil, and organic solvents. It is a copolymer (not homopolymer) of 1,3-butadiene and acrylonitrile (not styrene). While used in rubber products, its primary advantage is chemical resistance, not just general tire manufacturing (where Buna-S is more common).

34. Answer: c) They possess elasticity. Explanation: Elastomers are characterized by their ability to stretch and return to their original shape, i.e., possessing elasticity. They are typically amorphous and have weak intermolecular forces, not strong ones.

35. Answer: c) Unbreakable crockery Explanation: Melamine-formaldehyde resin is extensively used in the manufacturing of unbreakable crockery and laminated sheets. Bakelite is used for electrical switches and handles, and Novolac is used in paints.

II. Assertion-Reason Type Questions – Answers

36. Answer: a) Both A and R are true and R is the correct explanation of A. Explanation: Vulcanization indeed improves the elasticity of natural rubber. This improvement occurs because sulfur forms cross-links (sulfur bridges) between the individual polyisoprene chains, which restricts their movement and provides structural integrity, allowing the rubber to retain its shape after stretching.

37. Answer: a) Both A and R are true and R is the correct explanation of A. Explanation: LDPE (Low-Density Polythene) is flexible and a poor conductor of electricity, making it suitable for insulation of electric wires. Its branched structure prevents close packing, contributing to its flexibility.

38. Answer: a) Both A and R are true and R is the correct explanation of A. Explanation: Proteins are natural polymers because they are biological macromolecules found in living organisms. They are fundamentally polymers formed by the condensation polymerization of many amino acid units linked by peptide bonds.

39. Answer: a) Both A and R are true and R is the correct explanation of A. Explanation: Condensation polymerization is also known as step-growth polymerization because the polymer chain grows in a step-wise manner by the reaction between functional groups, with the characteristic elimination of small molecules like water, alcohol, or HCl.

40. Answer: a) Both A and R are true and R is the correct explanation of A. Explanation: Natural polymers often have a very narrow range of molecular weights for their chains, meaning their chains are almost uniform in length. This results in their number average and weight average molecular masses being very close, hence a PDI value close to 1 (monodisperse).

III. Short Answer / Conceptual Questions – Answers

41. Answer: The ‘6,6’ in Nylon 6,6 indicates that both of its monomer units, hexamethylenediamine and adipic acid, contain six carbon atoms each.

  • Monomers: Hexamethylenediamine (H2​N−(CH2​)6​−NH2​) and Adipic acid (HOOC−(CH2​)4​−COOH).

42. Answer:

  • Natural Polymer: Starch (or Cellulose, Proteins, Natural Rubber)
  • Semi-synthetic Polymer: Cellulose acetate (Rayon) (or Cellulose nitrate, Vulcanized rubber)

43. Answer:

  • Addition Polymerization (General Reaction): n(CH2​=CHX)Initiator​−(CH2​−CHX)n​− (Where X is an atom or group, and CH2​=CHX represents an unsaturated monomer like ethene, propene, vinyl chloride, etc.)
  • Condensation Polymerization (General Example with -OH and -COOH): n(HO−R1​−OH)+n(HOOC−R2​−COOH)Condensation​−(O−R1​−O−CO−R2​−CO)n​−+2nH2​O (Where HO−R1​−OH is a diol and HOOC−R2​−COOH is a dicarboxylic acid, forming a polyester)

44. Answer: | Feature | Thermoplastics | Thermosetting Polymers | | :————– | :——————————————- | :—————————————————- | | Structure | Linear or slightly branched chains | Heavily branched or cross-linked (3D network) | | Reheating | Soften on heating | Do not soften on heating; become hard and infusible | | Reusability | Can be remolded and reused (recycled) | Cannot be remolded or reused (non-recyclable) | | Forces | Intermediate intermolecular forces | Strong covalent bonds (cross-links) | | Examples | Polythene, PVC, Nylon, Polystyrene | Bakelite, Urea-formaldehyde resins, Melamine-formaldehyde resins |

45. Answer:

  • Biodegradable polymers are polymers that can be degraded or decomposed by microorganisms (like bacteria, fungi, algae) in the environment into simpler, non-toxic substances over a reasonable period.
  • Importance: Their development is crucial to address the severe environmental pollution caused by the accumulation of non-biodegradable synthetic plastics. Biodegradable polymers offer a sustainable alternative, reducing waste and mitigating ecological impact.
  • Synthetic Biodegradable Polymer Example: PHBV (Poly(β-hydroxybutyrate-co-β-hydroxyvalerate)) or Nylon 2-Nylon 6.

46. Answer:

  • Preparation of HDPE: High-Density Polythene (HDPE) is prepared by the polymerization of ethene at low pressure (6−7atm) and relatively low temperature (333−343K) in the presence of a Ziegler-Natta catalyst (a mixture of triethylaluminium and titanium tetrachloride, TiCl4​/(C2​H5​)3​Al). This process results in a linear polymer chain with minimal branching.
  • Properties:
    1. Hard and rigid.
    2. High tensile strength.
    3. Higher melting point.
  • Uses:
    1. Manufacturing buckets and dustbins.
    2. Making pipes and bottles.

47. Answer: Dacron (also known as Terylene) is a polyester. Its monomers are:

  1. Ethylene glycol: HO−CH2​−CH2​−OH
  2. Terephthalic acid: HOOC−C6​H4​−COOH (benzene-1,4-dicarboxylic acid)

48. Answer: In the vulcanization of natural rubber, sulfur acts as a cross-linking agent. When natural rubber (cis-polyisoprene) is heated with sulfur (usually 3-10%), sulfur atoms form covalent bonds (sulfur bridges) between adjacent polyisoprene chains at the sites of residual double bonds. These cross-links reduce the slippage of polymer chains, thereby significantly improving the rubber’s:

  • Elasticity
  • Tensile strength
  • Resistance to abrasion (wear and tear)
  • Resistance to organic solvents
  • Hardness and stiffness

49. Answer:

  • Buna-S: It is a copolymer of:
    1. 1,3-Butadiene: CH2​=CH−CH=CH2​
    2. Styrene: C6​H5​−CH=CH2​
  • Buna-N: It is a copolymer of:
    1. 1,3-Butadiene: CH2​=CH−CH=CH2​
    2. Acrylonitrile: CH2​=CH−CN

50. Answer:

  • Novolac is a linear phenol-formaldehyde polymer. It is formed when phenol and formaldehyde react in an acidic medium with a limited amount of formaldehyde relative to phenol. The −CH2​− linkages are formed mainly at ortho and para positions, resulting in a linear chain.
  • Difference from Bakelite:
    • Structure: Novolac is a linear polymer. Bakelite is a highly cross-linked (three-dimensional network) polymer.
    • Formation: Bakelite is formed by further heating Novolac in the presence of more formaldehyde or by reacting phenol and formaldehyde in an alkaline medium, which leads to extensive cross-linking.
    • Properties: Novolac is a thermoplastic (can be softened on heating) and is used in paints and lacquers. Bakelite is a thermosetting polymer (hard, infusible, cannot be softened on heating) and is used for electrical switches, handles of utensils, etc.

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