NCERT Solutions for Class 12 Chemistry Polymers [Old Book]

NCERT
SOLUTIONS
CLASS - 12th

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Class : 12th
Subject : Chemistry
Chapter : 15
Chapter Name : Polymers

Q15.1 What are polymers?

Answer. Polymers are high molecular mass macromolecules, which consist of repeating structural
units derived from monomers. Polymers have a high molecular mass (103 - 107 u). In a polymer,
various monomer units are joined by strong covalent bonds. These polymers can be natural as well
as synthetic. Polythene, rubber, and nylon 6, 6 are examples of polymers.

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Q15.2 How are polymers classi ed on the basis of structure?

Answer. Polymers are classi ed on the basis of structure as follows:
1. Linear polymers:
These polymers are formed Of long straight chains. They can be depicted as:

For e.g., high density polythene (HOP), polyvinyl chloride, etc.
2. Branched chain polymers:
These polymers are basically linear chain polymers with some branches. These polymers are
represented as:

For e.g., low density polythene (LOP), amylopectin, etc.
3. Cross-linked or Network polymers:
These polymers have many cross-linking bonds that give rise to a network-like structure. These
polymers contain bifunctional and trifunctional monomers and strong covalent bonds between
various linear polymer chains. Examples of such polymers include bakelite and melmac.

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Q15.3 Write the names of monomers of the following polymers:

Answer.

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Q15.4 Classify the following as addition and condensation polymers: Terylene, Bakelite, Polyvinyl
chloride, Polythene.

Answer. Addition polymers: Polyvinyl chloride, polythene
Condensation polymers: Terylene, bakelite

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Q15.5 Explain the difference between Buna-N and Buna-S.

Answer. Buna — N is a copolymer of 1, 3—butadiene and acrylonitrile.
Buna — S is a copolymer of 1, 3—butadiene and styrene.

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Q15.6 Arrange the following polymers in increasing order of their intermolecular forces.

(i) Nylon 6, 6, Buna-S, Polythene.
(ii) Nylon 6, Neoprene, Polyvinyl chloride.

Answer. Different types of polymers have different intermolecular forces of attraction. Elastomers
or rubbers have the weakest while bres have the strongest intermolecular forces of attraction.
Plastics have intermediate intermolecular forces of attraction. Hence, the increasing order of the
intermolecular forces of the given polymers is as follows:
(i) Buna — S < polythene < Nylon 6, 6
(ii) Neoprene < polyvinyl chloride < Nylon 6

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Q15.1 Explain the terms polymer and monomer.

Answer. Polymers are high molecular mass macromolecules composed of repeating structural
units derived from monomers. Polymers have a high molecular mass (103 - 107 u). In a polymer,
various monomer units are joined by strong covalent bonds. Polymers can be natural as well as
synthetic.
Polythene, rubber, and nylon 6, 6 are examples of polymers.
Monomers are simple, reactive molecules that combine with each other in large numbers through
covalent bonds to give rise to polymers. For example, ethene, propene, styrene, vinyl chloride.

Q15.2 What are natural and synthetic polymers? Give two examples of each type.

Answer. Natural polymers are polymers that are found in nature. They are formed by plants and
animals. Examples include protein, cellulose, starch, etc. Synthetic polymers are polymers made
by human beings. Examples include plastic (polythene), synthetic bres (nylon 6, 6), synthetic
rubbers (Buna -S)

Q15.3 Distinguish between the terms homopolymer and copolymer and give an example of each.

Answer.

Q15.4 How do you explain the functionality of a monomer?

Answer. The functionality of a monomer is the number of binding sites that islare present in that
monomer. For example, the functionality of monomers such as ethene and propene is one and
that of 1, 3- butadiene and adipic acid is two.

Q15.5 De ne the term polymerisation.

Answer. Polymerization is the process of forming high molecular mass (10^3 10^7 u)
macromolecules, which consist of repeating structural units derived from monomers. In a
polymer, various monomer units are joined by strong covalent bonds.

Q15.6 Is NH − CHR − COₙ -e a homopolymer or copolymer?

Answer.
(NH − CHR − CO)ₙ is a homopolymer because it is obtained from a single monomer unit,
NH2 − CHR − COOH

Q15.7 In which classes, the polymers are classi ed on the basis of molecular forces?

Answer. On the basis of magnitude of intermolecular forces present in polymers, they are
classi ed into the following groups:
(i) Elastomers
(ii) Fibres
(iii) Thermoplastic polymers
(iv) Thermosetting polymers

Q15.8 How can you differentiate between addition and condensation polymerisation?

Answer. Addition polymerization is the process of repeated addition of monomers, possessing
double or triple bonds to form polymers. For example, polythene is formed by addition
polymerization of ethene.

nCH2 = CH2 ⟶ +CH2 − CH2 +n

Ethene

Condensation polymerization is the process of formation of polymers by repeated
condensation reactions between two different bi-functional or tri-functional monomers. A small
molecule such as water or hydrochloric acid is eliminated in each condensation. For example,
nylon 6, 6 is formed by condensation polymerization of hexamethylenediamine and adipic acid.

Q15.9 Explain the term co polymerisation and give two examples.

Answer. mers from two or more different monomeric units is called copolymerization. Multiple
units of each monomer are present in a copolymer. The process of forming polymer Buna—S from
1, 3-butadiene and styrene is an example of copolymerization

Q15.10 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:

Q15.11 De ne 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 modi ed 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.

Q15.12 Write the monomers used for getting the following polymers.
(i) Polyvinyl chloride
(ii) Te on
(iii) Bakelite

Answer. (i) Vinyl chloride (CH₂=CHCl)

(ii) Tetra uoroethylene (CF₂ = CF₂)
(iii) Formaldehyde (HCHO) and phenol (C₆H₅OH)

Q15.13 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.

Q15.14 How does the presence of double bonds in rubber molecules in uence their structure and
reactivity?

Answer. Natural rubber is a linear cis-polyisoprene in which the double bonds are present between
C₂ and C₃ of the isoprene units.

Because of this cis-con guration, 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.

Q15.15 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 (> 335 K),
it becomes even softer. At low temperatures (< 283 K), it becomes brittle.
Thus, to maintain its elasticity, natural rubber is generally used in the temperature range of 283 K-
335 K.
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 373 K and 415 K.

Q15.16 What are the monomeric repeating units of Nylon-6 and Nylon-6, 6?

Answer.

Q15.17 Write the names and structures of the monomers of the following polymers:
(i) auna-s
(ii) Buna-N
(iii) Dacron
(iv) Neoprene

Answer.

Q15.18 Identify the monomer in the following polymeric structures.

Answer.

Q15.19 How is dacron obtained from ethylene glycol and terephthalic acid?

Answer. The condensation polymerisation of ethylene glycol and terephthalic acid leads to the
formation of dacron.

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Q15.20 What is a biodegradable polymer? Give an example of a biodegradable aliphatic polyester.

Answer. A polymer that can be decomposed by bacteria is called a biodegradable polymer. Poly-ß-
hydroxybutyrate-CO-ß- hydroxyvalerate (PHBV) is a biodegradable aliphatic polyester.

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