Chapter 15 polymers

Class XII

Chapter 15 – Polymers

Chemistry

Question 15.1:
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.

Question 15.2:
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 fibres (nylon 6, 6), synthetic rubbers (Buna − S).

Question 15.3:
Distinguish between the terms homopolymer and copolymer and give an example of
each.
Answer
Homopolymer
The

polymers

Copolymer
that

are

formed

by

the

The polymers whose repeating units

polymerization of a single monomer are known as

are

homopolymers. In other words, the repeating

monomers

units of homopolymers are derived only from one

copolymers. For example, Buna−S

monomer.

is a copolymer of 1, 3-butadiene

For

example,

polythene

is

a

homopolymer of ethane.

derived

from

two

are

types

known

of
as

and styrene.

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Question 15.4:
How do you explain the functionality of a monomer?
Answer
The functionality of a monomer is the number of binding sites that is/are 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.

Question 15.5:
Define the term polymerisation.
Answer
Polymerization is the process of forming high molecular mass (103 − 107 u)
macromolecules, which consist of repeating structural units derived from monomers. In
a polymer, various monomer units are joined by strong covalent bonds.

Question 15.6:
Is

, a homopolymer or copolymer?

Answer
is a homopolymer because it is obtained from a single monomer unit,
NH2−CHR−COOH.

Question 15.7:
In which classes, the polymers are classified on the basis of molecular forces?
Answer
On the basis of magnitude of intermolecular forces present in polymers, they are
classified into the following groups:
(i) Elastomers
(ii) Fibres
(iii) Thermoplastic polymers
(iv) Thermosetting polymers

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Question 15.8:
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.

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.

Question 15.9:
Explain the term copolymerisation 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

Nylon 6, 6 is also a copolymer formed by hexamethylenediamine and adipic acid.

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Question 15.10:
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 benzoyal peroxide as the initiator.
The reaction involved in this process is given below:

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Question 15.11:
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.

Question 15.12:
Write the monomers used for getting the following polymers.
(i) Polyvinyl chloride (ii) Teflon (iii) Bakelite
Answer
(i) Vinyl chloride (CH2=CHCl)
(ii) Tetrafluoroethylene (CF2 = CF2)
(iii) Formaldehyde (HCHO) and phenol (C6H5OH)

Question 15.13:
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.

Question 15.14:
How does the presence of double bonds in rubber molecules influence their
structure and reactivity?
Answer

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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-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.

Question 15.15:
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.

Question 15.16:
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.
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The monomeric repeating unit of nylon 6, 6 is

,

which is derived from hexamethylene diamine and adipic acid.

Question 15.17:
Write the names and structures of the monomers of the following polymers:
(i) Buna-S (ii) Buna-N
(iii) Dacron (iv) Neoprene
Answer
Polymer

Monomer

Structure of monomer

1, 3-butadiene
i

Buna-S
Styrene
1, 3-butadiene

ii

Buna-N
Acrylonitrile

iii

Neoprene

Chloroprene

Ethylene glycol
iv

Dacron
Terephthalic acid

Question 15.18:
Identify the monomer in the following polymeric structures.
(i)

(ii)

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Answer
(i)

The

monomers

of

the

given

polymeric

structure

are

decanoic

and hexamethylene diamine

acid

.

(ii) The monomers of the given polymeric structure are

Question 15.19:
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.

Question 15.20:
What is a biodegradable polymer? Give an example of a biodegradable aliphatic
polyester.
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Answer
A polymer that can be decomposed by bacteria is called a biodegradable polymer.
Poly-β-hydroxybutyrate-CO-β- hydroxyvalerate (PHBV) is a biodegradable aliphatic
polyester.

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Question 15.1:
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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Question 15.2:
How are polymers classified on the basis of structure?
Answer
Polymers are classified 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 (HDP), 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 (LDP), 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 bi-functional and tri-functional monomers and strong covalent
bonds between various linear polymer chains. Examples of such polymers include
bakelite and melmac.

Question 15.3:
Write the names of monomers of the following polymers:

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Answer
(i)

Hexamethylenediamine

and

adipic

acid

(ii)

(iii) Tetrafluoroethene

Question 15.4:
Classify the following as addition and condensation polymers: Terylene, Bakelite,
Polyvinyl chloride, Polythene.
Answer
Addition polymers:
Polyvinyl chloride, polythene
Condensation polymers:
Terylene, bakelite

Question 15.5:
Explain the difference between Buna-N and Buna-S.
Answer
Buna − N is a copolymer of 1, 3−butadiene and acrylonitrile.

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Buna − S is a copolymer of 1, 3−butadiene and styrene.

Question 15.6:
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 fibres 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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Chapter 15 polymers

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Chapter 15 polymers