Do you think that daily life would have been easier and
colourful without the discovery and varied applications
of polymers? The use of polymers in the manufacture
of plastic buckets, cups and saucers, children’s toys,
packaging bags, synthetic clothing materials, automobile
tyres, gears and seals, electrical insulating materials and
machine parts has completely revolutionised the daily
life as well as the industrial scenario. Indeed, the
polymers are the backbone of four major industries viz.
plastics, elastomers, fibres and paints and varnishes.
The word ‘polymer’ is coined from two Greek words:
poly means many and mer means unit or part. The
term polymer is defined as very large molecules having
high molecular mass (10
3
-10
7
u). These are also referred
to as macromolecules, which are formed by joining of
repeating structural units on a large scale. The repeating
structural units are derived from some simple and
reactive molecules known as monomers and are linked
to each other by covalent bonds. The process of
formation of polymers from respective monomers is
called polymerisation
.
After studying this Unit, you will be
able to
explain the terms - monomer,
polymer and polymerisation and
appreciate their importance;
distinguish between various
classes of polymers and different
types of polymerisation processes;
appreciate the formation of
polymers from mono- and bi-
functional monomer molecules;
describe the preparation of some
important synthetic polymers and
their properties;
appreciate the importance of
polymers in daily life.
Objectives
“Copolymerisation has been used by nature in polypeptides which
may contain as many as 20 different amino acids. Chemists are still
far behind”.
UnitUnit
UnitUnit
Unit
15
PolymersPolymers
Polymers
Polymers
Polymers
15
PolymersPolymers
PolymersPolymers
Polymers
15.115.1
15.115.1
15.1
ClassificationClassification
ClassificationClassification
Classification
of Polymersof Polymers
of Polymersof Polymers
of Polymers
There are several ways of classification of polymers based
on some special considerations. One of the common
classifications of polymers is based on source from which
polymer is derived.
Under this type of classification, there are three sub
categories.
1. Natural polymers
These polymers are found in plants and animals.
Examples are proteins, cellulose, starch, some resins
and rubber.
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2. Semi-synthetic polymers
Cellulose derivatives as cellulose acetate (rayon) and cellulose
nitrate, etc. are the usual examples of this sub category.
3. Synthetic polymers
A variety of synthetic polymers as plastic (polythene), synthetic
fibres (nylon 6,6) and synthetic rubbers (Buna - S) are examples
of man-made polymers extensively used in daily life as well as
in industry.
Polymers can also be classified on the basis of their structure, molecular
forces or modes of polymerisation.
There are two broad types of polymerisation reactions, i.e., the addition
or chain growth polymerisation and condensation or step growth
polymerisation.
In this type of polymerisation, the molecules of the same monomer or
diferent monomers add together on a large scale to form a polymer. The
monomers used are unsaturated compounds, e.g.
, alkenes, alkadienes
and their derivatives. This mode of polymerisation leads to an increase in
chain length and chain growth can take place through the formation of
either free radicals or ionic species. However, the free radical governed
addition or chain growth polymerisation is the most common mode.
1. Free radical mechanism
A variety of alkenes or dienes and their derivatives are polymerised in
the presence of a free radical generating initiator (catalyst) like
benzoyl peroxide, acetyl peroxide, tert-butyl peroxide, etc. For example,
the polymerisation of ethene to polythene consists of heating or
exposing to light a mixture of ethene with a small amount of benzoyl
peroxide initiator. The process starts with the addition of phenyl free
radical formed by the peroxide to the ethene double bond thus
generating a new and larger free radical. This step is called chain
initiating step. As this radical reacts with another molecule of ethene,
another bigger sized radical is formed. The repetition of this sequence
with new and bigger radicals carries the reaction forward and the step
is termed as chain propagating step. Ultimately, at some stage the
product radical thus formed reacts with another radical to form the
polymerised product. This step is called the chain terminating step.
The sequence of steps involved in the formation of polythene are
depicted as follows:
Chain initiation steps
15.2.1.1
Mechanism of
Addition
Polymerisation
15.215.2
15.215.2
15.2
Types ofTypes of
Types of
Types of
Types of
PolymerisationPolymerisation
PolymerisationPolymerisation
Polymerisation
ReactionsReactions
ReactionsReactions
Reactions
15.2.1
Addition
Polymerisation
or Chain Growth
Polymerisation
15.1 What are polymers ?
Intext QuestionsIntext Questions
Intext QuestionsIntext Questions
Intext Questions
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435
Polymers
Chain propagating step
Chain terminating step
For termination of the long chain, these free radicals can combine
in different ways to form polythene. One mode of termination of
chain is shown as under:
The addition polymers formed by the polymerisation of a single
monomeric species are known as homopolymers, for example
polythene discussed above is a homopolymer.
The polymers made by addition polymerisation from two different
monomers are termed as copolymers. Buna-S, which is formed by
polymerisation of buta–1, 3–diene and styrene is an example of
copolymer formed by addition polymerisation.
15.2.1.2
Some Important
Addition Polymers
(a)Polythene
Polythenes are linear or slightly branched long chain molecules.
These are capable of repeatedly softening on heating and
hardening on cooling and are thus thermoplastic polymers.
There are two types of polythene as given below:
(i) Low density polythene: It is obtained by the polymerisation
of ethene under high pressure of 1000 to 2000
atmospheres at a temperature of 350 K to 570 K in the
presence of traces of dioxygen or a peroxide initiator
(catalyst). The low density polythene (LDP) is obtained
through the free radical addition and H-atom abstraction.
It has highly branched structure. These polymers have
straight chain structure with some branches as
shown below.
Low density polythene is chemically inert and tough but
flexible and a poor conductor of electricity. Hence, it is used
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436Chemistry
in the insulation of electricity carrying wires and manufacture
of squeeze bottles, toys and flexible pipes.
(ii) High density polythene: It is formed when addition
polymerisation of ethene takes place in a hydrocarbon solvent
in the presence of a catalyst such as triethylaluminium and
titanium tetrachloride (Ziegler-Natta catalyst) at a temperature
of 333 K to 343 K and under a pressure of 6-7 atmospheres.
High density polythene (HDP) thus produced, consists of linear
molecules as shown below and has a high density due to
close packing. Such polymers are also called linear polymers.
High density polymers are also chemically inert and more
tough and hard. It is used for manufacturing buckets,
dustbins, bottles, pipes, etc.
(b)Polytetrafluoroethene (Teflon)
Teflon is manufactured by heating tetrafluoroethene with a free
radical or persulphate catalyst at high pressures. It is chemically
inert and resistant to attack by corrosive reagents. It is used in
making oil seals and gaskets and also used for non – stick surface
coated utensils.
(c) Polyacrylonitrile
The addition polymerisation of acrylonitrile in presence of a
peroxide catalyst leads to the formation of polyacrylonitrile.
Polyacrylonitrile is used as a substitute for wool in making
commercial fibres as orlon or acrilan.
Is a homopolymer or a copolymer?
It is a homopolymer and the monomer from which it is obtained
is styrene C
6
H
5
CH = CH
2
.
Example 15.1Example 15.1
Example 15.1Example 15.1
Example 15.1
SolutionSolution
SolutionSolution
Solution
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Polymers
This type of polymerisation generally involves a repetitive
condensation reaction between two bi-functional or trifunctional
mono-meric units. These polycondensation reactions may result in
the loss of some simple molecules as water, alcohol, hydrogen
chloride, etc., and lead to the formation of high molecular mass
condensation polymers.
In these reactions, the product of each step is again a bi-functional
species and the sequence of condensation goes on. Since, each step
produces a distinct functionalised species and is independent of each
other, this process is also called as step growth polymerisation.
The formation of terylene or dacron by the interaction of ethylene
glycol and terephthalic acid is an example of this type
of polymerisation.
(a) Polyamides
These polymers possessing amide linkages are important
examples of synthetic fibres and are termed as nylons. The
general method of preparation consists of the condensation
polymerisation of diamines with dicarboxylic acids or
condensation of amino acids or their lactams.
Nylons
(i) Nylon 6,6: It is prepared by the condensation
polymerisation of hexamethylenediamine with adipic acid
under high pressure and at high temperature.
Nylon 6, 6 is fibre forming solid. It possess high tensile
strength. This characteristic can be attributed to the strong
intermolecular forces like hydrogen bonding. These strong
forces also lead to close packing of chains and thus impart
crystalline nature.
Nylon 6, 6 is used in making sheets, bristles for brushes
and in textile industry.
(ii) Nylon 6: It is obtained by heating caprolactum with water
at a high temperature.
15.2.2
Condensation
Polymerisation
or Step Growth
Polymerisation
n CH + n COOH
2
HOH C OH HOOC
2
Ethylene glycol
(Ethane-1, 2 - diol)
Terephthalic acid
(Benzene-1,4 - di
carboxylic acid)
Terylene or dacron
C
C
n
O
O
OCH CH
2 2
–O
15.2.2.1
Some Important
Condensation
Polymers
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Nylon 6 is used for the manufacture of tyre cords, fabrics
and ropes.
(b) Polyesters
These are the polycondensation products of dicarboxylic
acids and diols. Dacron or terylene is the best known example
of polyesters. It is manufactured by heating a mixture of ethylene
glycol and terephthalic acid at 420 to 460 K in the presence of
zinc acetate-antimony trioxide catalyst as per the reaction given
earlier. Dacron fibre (terylene) is crease resistant and is used
in blending with cotton and wool fibres and also as glass
reinforcing materials in safety helmets, etc.
Novolac on heating with formaldehyde undergoes cross linking
to form an infusible solid mass called bakelite. It is
thermosetting polymer which cannot be reused or remoulded.
Thus, bakelite is formed by cross linking of linear chains of the
polymer novolac. Bakelite is used for making combs, phonograph
records, electrical switches and handles of various utensils.
(c) Phenol – formaldehyde polymer (Bakelite and related
polymers)
Phenol – formaldehyde polymers are the oldest synthetic
polymers. These are obtained by the condensation reaction of
phenol with formaldehyde in the presence of either an acid or a
base catalyst. The reaction starts with the initial formation of
o-and/or p-hydroxymethylphenol derivatives, which further
react with phenol to form compounds having rings joined to
each other through–CH
2
groups. The initial product could be a
linear product – Novolac used in paints.
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Polymers
(d) Melamine — formaldehyde polymer
Melamine formaldehyde polymer is formed by the condensation
polymerisation of melamine and formaldehyde.
It is used in the manufacture of unbreakable crockery.
Bakelite
CH
2
CH
2
OH
OH
H C
2
CH
2
CH
2
CH
2
~
~
~
~
~
~
~
~
H C
2
CH
2
CH
2
CH
2
OH
OH
OH
~
~
~
~
OH
CH
2
~
~
~
~
15.2 Write the names of monomers of the following polymers:
15.3 Classify the following as addition and condensation polymers: Terylene, Bakelite,
Polythene, Teflon.
Intext QuestionsIntext Questions
Intext QuestionsIntext Questions
Intext Questions
Copolymerisation is a polymerisation reaction in which a mixture of
more than one monomeric species is allowed to polymerise and form
a copolymer. The copolymer can be made not only by chain growth
polymerisation but by step growth polymerisation also. It contains
multiple units of each monomer used in the same polymeric chain.
15.2.3
Copolymerisation
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