Chemistry Class 12 NCERT Solutions: Chapter 15 Polymers Part 3

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Q: 10. Write the free radical mechanism for the polymerisation of ethene.

Answer:

Polymerization of ethene to polythene consists of heating or exposing to light a mixture of ethene with a small amount of benzoyl peroxide as the initiator.

The reaction involved in this process is given below:

Chain Initiation Step:

Q 10 1 Chain Initiation Step

Q 10 1 Chain Initiation Step

Q 10 1 Chain Initiation Step

Chain Propagating step:

Q 10 2 Chain Propagating Step

Q 10 2 Chain Propagating Step

Q 10 2 Chain Propagating Step

Chain Terminating step:

Q 10 3 Chain Terminating Step

Q 10 3 Chain Terminating Step

Q 10 3 Chain Terminating Step

Q: 11. Define thermoplastics and thermosetting polymers with two examples of each.

Answer:

Thermoplastic polymers are linear (slightly branched) long chain polymers, which can be repeatedly softened and hardened on heating. Hence, they can be modified again and again. Examples include polythene, polystyrene.

Thermosetting polymers are cross-linked or heavily branched polymers which get hardened during the molding process. These plastics cannot be softened again on heating. Examples of thermosetting plastics include bakelite, urea-formaldehyde resins.

Q: 12. Write the monomers used for getting the following polymers.

(i) Polyvinyl Chloride

(ii) Teflon

(iii) Bakelite

Answer:

(i) Vinyl chloride

(ii) Tetrafluoroethylene

(iii) Formaldehyde and phenol

Q: 13. Write the name and structure of one of the common initiators used in free radical addition polymerisation.

Answer:

One common initiator used in free radical addition polymerization is benzoyl peroxide. Its structure is given below.

Q 13 Structure of Benzoyl Peroxide

Q 13 Structure of Benzoyl Peroxide

Q 13 Structure of Benzoyl Peroxide

Q: 14. How does the presence of double bonds in rubber molecules influence their structure and reactivity?

Answer:

Natural rubber is a linear cis-polyisoprene in which the double bonds are present between and of the isoprene units.

Q 14 Structure of Natural Tubber

Q 14 Structure of Natural Tubber

Q 14 Structure of Natural Tubber

Because of this cis-configuration, intermolecular interactions between the various strands of isoprene are quite weak. As a result, various strands in natural rubber are arranged randomly. Hence, it shows elasticity.

Q: 15. Discuss the main purpose of vulcanisation of rubber.

Answer:

Natural rubber though useful has some problems associated with its use. These limitations are discussed below:

1. Natural rubber is quite soft and sticky at room temperature. At elevated temperatures , it becomes even softer. At low temperatures , it becomes brittle. Thus, to maintain its elasticity, natural rubber is generally used in the temperature range of .

2. It has the capacity to absorb large amounts of water.

3. It has low tensile strength and low resistance to abrasion.

4. It is soluble in non-polar solvents.

5. It is easily attacked by oxidizing agents.

Vulcanization of natural rubber is done to improve upon all these properties. In this process, a mixture of raw rubber with sulphur and appropriate additive is heated at a temperature range between and .

Q: 16. What are the monomeric repeating units of Nylon-6 and Nylon-6, 6?

Answer:

The monomeric repeating unit of nylon 6 is , which is derived from caprolactam.

The monomeric repeating unit of nylon 6, 6 is , which is derived from hexamethylene diamine and adipic acid.