Polymers — JEE Main Questions

47 JEE Main practice questions on Polymers, part of Chemistry. Below are 12 of them in full, each with the answer and a written explanation.

Questions & explanations

1. Which of the following monomers undergoes cationic addition polymerisation?

  1. Vinyl chloride
  2. Acrylonitrile
  3. Isobutylene
  4. Styrene

Answer: Isobutylene

Cationic polymerisation requires electron-rich monomers with electron-donating substituents. Isobutylene (CH2=C(CH3)2) has two methyl groups that donate electrons, stabilising the carbocation intermediate. Acrylonitrile, vinyl chloride, and styrene are electron-poor or neutral and typically undergo anionic or free-radical polymerisation.

2. Given the repeat unit [-NH-(CH2)6-NH-CO-(CH2)4-CO-]n, identify the monomer(s) and the by-product formed during polymerisation.

  1. Caprolactam; no by-product
  2. Caprolactam; 1 H2O per repeat unit
  3. Hexamethylenediamine and adipic acid; 1 H2O per repeat unit
  4. Hexamethylenediamine and adipic acid; 2 H2O per repeat unit

Answer: Hexamethylenediamine and adipic acid; 2 H2O per repeat unit

The repeat unit is from nylon-6,6, formed by condensation of hexamethylenediamine and adipic acid. Each monomer contributes two functional groups; the diamine has two -NH2 and the diacid has two -COOH. Each amide bond formation eliminates one H2O, so two H2O molecules are released per repeat unit.

3. Which of the following is the correct sequence of steps in free-radical addition polymerisation?

  1. Propagation → Initiation → Termination
  2. Initiation → Propagation → Termination
  3. Termination → Initiation → Propagation
  4. Initiation → Termination → Propagation

Answer: Initiation → Propagation → Termination

Free-radical addition polymerisation proceeds via three steps: initiation (radical formation and addition to monomer), propagation (chain growth), and termination (combination or disproportionation). This sequence is standard in NCERT Class 12 Chemistry Ch 15 §15.2.1.

4. A polymer has a glass-transition temperature (Tg) of -100°C and a melting temperature (Tm) of 130°C. Which of the following is most likely true about its structure?

  1. It has a rigid backbone with strong intermolecular forces.
  2. It is completely amorphous with no crystalline regions.
  3. It has a flexible backbone with weak intermolecular forces.
  4. It is a cross-linked polymer with high Tg.

Answer: It has a flexible backbone with weak intermolecular forces.

A very low Tg (like -100°C) indicates a flexible backbone and weak intermolecular forces, allowing chain segmental motion even at low temperatures. The presence of a Tm (130°C) shows it has crystalline regions, which is consistent with a linear polymer like polythene.

5. Which of the following statements about the Ziegler-Natta catalyst is correct?

  1. It consists of K2S2O8 and is used to prepare LDPE.
  2. It consists of TiCl4 and Al(C2H5)3 and is used to prepare LDPE.
  3. It consists of benzoyl peroxide and is used to prepare HDPE.
  4. It consists of TiCl4 and Al(C2H5)3 and is used to prepare HDPE.

Answer: It consists of TiCl4 and Al(C2H5)3 and is used to prepare HDPE.

The Ziegler-Natta catalyst is a combination of TiCl4 and triethylaluminium (Al(C2H5)3). It is used to produce high-density polyethylene (HDPE) via coordination polymerisation at moderate temperature and pressure, giving a linear polymer with high density.

6. A polythene sample has M_n = 1.4 × 10^5 g/mol and M_w = 2.8 × 10^5 g/mol. What is the polydispersity index (PDI) and what does it indicate about the synthesis route?

  1. PDI = 2.0; indicates free-radical polymerisation
  2. PDI = 1.0; indicates living anionic polymerisation
  3. PDI = 2.0; indicates living anionic polymerisation
  4. PDI = 1.0; indicates free-radical polymerisation

Answer: PDI = 2.0; indicates free-radical polymerisation

PDI = M_w / M_n = (2.8 × 10^5) / (1.4 × 10^5) = 2.0. A PDI of 2.0 is characteristic of free-radical chain-growth polymerisation, where random termination leads to a broad molecular weight distribution. Living anionic polymerisation gives PDI close to 1.0.

7. A student heats a sample of a plastic. It softens, can be remoulded, and re-solidifies on cooling. Which of the following best describes this plastic?

  1. Thermosetting plastic with linear chains held by intermolecular forces
  2. Thermosetting plastic with cross-linked network
  3. Thermoplastic with cross-linked network
  4. Thermoplastic with linear chains held by intermolecular forces

Answer: Thermoplastic with linear chains held by intermolecular forces

The plastic softens on heating and can be remoulded, which is the operational test for a thermoplastic. Structurally, thermoplastics have linear or branched chains held by weak intermolecular forces, allowing chains to slide past each other when heated.

8. Which of the following is a condensation polymer?

  1. Nylon-6,6
  2. Polythene
  3. PVC
  4. Teflon

Answer: Nylon-6,6

Nylon-6,6 is formed by condensation polymerisation of hexamethylenediamine and adipic acid with elimination of water. Its backbone contains amide linkages (heteroatoms), and the empirical formula of the repeat unit differs from that of the monomers.

9. Which of the following polymers is thermosetting and cannot be recycled by re-melting?

  1. Polythene
  2. Bakelite
  3. Vulcanised rubber
  4. Raw natural rubber

Answer: Bakelite

Bakelite is a thermosetting polymer with extensive cross-links; on heating it chars and does not soften, so it cannot be recycled by re-melting. Polythene and raw rubber are thermoplastics; vulcanised rubber is an elastomer but not thermosetting.

10. Which of the following correctly describes the density range and branching of LDPE?

  1. 0.94-0.97 g/cm³, linear
  2. 0.94-0.97 g/cm³, branched
  3. 0.92-0.94 g/cm³, linear
  4. 0.92-0.94 g/cm³, branched

Answer: 0.92-0.94 g/cm³, branched

LDPE is prepared by free-radical polymerisation at high temperature and pressure, leading to chain branching. The branching reduces packing efficiency, giving a low density of 0.92-0.94 g/cm³. Hence, LDPE is branched with density in that range.

11. Why is a plasticiser like dioctyl phthalate added to PVC?

  1. To make PVC flexible by reducing intermolecular forces
  2. To increase the density of PVC
  3. To initiate the polymerisation of vinyl chloride
  4. To cross-link PVC chains for rigidity

Answer: To make PVC flexible by reducing intermolecular forces

Pure PVC is rigid due to strong dipole-dipole interactions between C-Cl bonds. A plasticiser like dioctyl phthalate inserts between polymer chains, reducing intermolecular forces and allowing chain movement, thus making PVC flexible.

12. Which of the following is the correct four-axis classification of Bakelite?

  1. synthetic, linear, addition, thermoplastic
  2. synthetic, cross-linked, condensation, thermosetting
  3. natural, cross-linked, condensation, thermosetting
  4. semi-synthetic, linear, addition, elastomer

Answer: synthetic, cross-linked, condensation, thermosetting

Bakelite is a synthetic polymer (made from phenol and formaldehyde), has a cross-linked structure, is formed by condensation polymerisation (elimination of water), and is thermosetting (cannot be remelted). This matches option (b).

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