NCERT Solutions for Class 12 Chemistry Chapter 15 - Polymers

1. Explain the terms polymer and monomer.

Ans. Polymer: Polymers are the high molecular mass (10³ – 10⁷u) molecules formed by the joining of smaller subunets, called monomers and the process is called polymerization.

Monomer: Monomers are simple moleculers capable of undergoing polymerisation and leading the formation of the corresponding polymer.

Example: Polythene is a polymer of ethene monomer.

2. What are natural and synthetic polymers? Give two examples of each type.

Ans. Natural polymers: The polymers which are found naturally in plants and animals are called natural polymers.

Example: Cellulose, Proteins

Synthetic polymers: The polymers, which are synthesized in laboratory, are called synthetic polymers.

Example: Polythene, Nylon-6, 6.

3. Distinguish between the terms homopolymer and copolymer and give an example of each.

Ans. Homopolymer: Polymers whose repeating structural units are derived from only one type of monomer units are called homopolymers, e.g., PVC polyethene, PAN, teflon, polystyrene, nylon- 6 etc.

Heteropolymer: Polymers whose repeating structural units are derived from two or more types of monomer molecules are copolymers, e.g., Buna-S, Buna-N, nylon-6, 6, polyester, bakelite.

4. How do you explain the functionality of a monomer?

Ans. Functionality of a monomer implies the number of bonding sites present in it. For example, monomers like propene, styrene, acrylonitrile have functionality of one which means that have one bonding site.

Monomers such as ethylene glycol, hexamethylenediamine, adipic acid have functionality of two which means that they have two bonding sites.

5. Define the term polymerisation?

Ans. Polymerisation is a process of formation of polymers by joining of several monomers by covalent bonds.

6. Is (NH—CHR—CO–)_n , a homopolymer or copolymer?

Ans. It is a homopolymer because the repeating structural unit has only one type of monomer, i.e., NH₂—CHR—COOH.

7. In which classes, are the polymers classified on the basis of molecular forces?

Ans. Polymers are classified into four classes on the basis of molecular forces. These are:

(i) Elastomers
(ii) Fibres
(iii) Thermoplastic polymers
(iv) Thermosetting polymers.

8. How can you differentiate between addition and condensation polymerisation?

Ans.

Addition Polymerisation	Condensation Polymerisation
Monomers must have either a double or triple bond.	Monomers must have two similar or different functional groups.
No byproducts formed.	By products such as NH₃, H₂O and HCl formed.
The addition of monomers results in the formation of the polymer.	The condensation reaction between monomers results in the formation of the polymer.
Common examples of addition polymerisation are PVC, polythene, teflon etc.	Common example of condensation polymerisation are nylon, bakelite, silicon etc.

9. Explain the term copolymerisation and give two examples.

Ans. When two or more different monomers are allowed to polymerise together the product formed is called a copolymer, and the process in called copolymerisation. Example, Buna-S and Buna-N. Buna-S is a copolymer of 1, 3-butadiene and styrene while Buna-N is a copolymer of 1, 3-butadiene and acrylonitrile.

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

Ans. It takes place in 3 steps:

(a) Chain initiation step:

(b) Chain propagating step:

(c) Chain termination step:

(i) By combination of free radicals (II):

11. Define thermoplastics and thermo setting polymers with two examples of each.

Ans. Thermoplastic polymers: Polymers, which are capable of softening on heating and hardening on cooling are called thermoplastics polymers.

Example: Polythene, PVC.

Thermosetting polymers: Polymers, which on heating do not soften and cannot be remoulded are thermosetting polymers.

Example: Bakelite, Urea etc.

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

(i) Polyvinyl chloride

(ii) Teflon

(iii) Bakelite

Ans.

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

Ans.

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

Ans. Natural rubber is cis-poly isoprene and is obtained by 1, 4-polymerisation of isoprene units. In this polymer, double bonds are located between C₂ and C₃ of each isoprene unit. These cis-double bonds do not allow the polymer chains to come closer for effective interactions and hence intermolecular forces are quite weak. As a result, natural rubber, i.e., cis-poly isoprene has a randomly coiled structure not the linear one and hence show elasticity.

15. Discuss the main purpose of vulcanisation of rubber.

Ans. The main purpose of vulcanisation of rubber are as follows:

(i) It makes rubber quite resistant to the action of organic acids.

(ii) It makes rubber resistant to wear and tear.

(iii) The elasticity of rubber increases and it remain unaffected over a wide range of temperature.

(iv) It makes rubber resistant to the attack of air.

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

Ans.

which is derived from caprolactam.

The monomeric repeating unit of Nylon 6, 6 is derived from two monomers, hexamethylenediamine and adipic acid and has the following structure:

17. Write the names and structures of the monomers of the following polymers:

(i) Buna-S

(ii) Buna-N

(iii) Dacron

(iv) Neoprene

$$\textbf{Ans.}\space\text{(i)}\space\text{Buna-S:}\underset{\text{1,3-Butadiene}}{\text{CH}_{2}=\text{CH}-\text{CH}=\text{CH}_{2}}\space\text{and}\space\underset{\text{Styrene}}{\text{C}_{6}\text{H}_{5}-\text{CH}=\text{CH}_{2}}\\\text{(ii)}\space\text{Buna-N:}\underset{\text{1,3-Butadiene}}{\text{CH}_{2}=\text{CH}-\text{CH}=\text{CH}_{2}}\space\text{and}\space\underset{\text{Acrylonitrile}}{\text{CH}_{2}=\text{CH}-\text{CN}}$$

18. Identify the monomer in the following polymeric structures:

19. How is dacron obtained from ethylene glycol and terephthalic acid?

Ans. Dacron is obtained by condensation polymerisation of ethylene glycol and terephthalic acid with the elimination of water molecules. The reaction is carried out at 420 – 460 K in presence of a catalyst consisting of a mixture of zinc acetate and antimony trioxide.

20. What is a biodegradable polymer? Give an example of a biodegradable aliphatic polyester.

Ans. A polymer, that is decomposed over a long period of time either by itself or by the action of microorganisms is called biodegradable polymer.

Example: PHBV, i.e., Poly-b-Hydroxybutyrate-co-b-Hydroxyvalerate.