NCERT Textbook - Polymers | NCERT Textbooks (Class 6 to Class 12) - CTET & State TET PDF Download

 Page 1


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. This process of
formation of polymers from respective monomers is
called polymerisation. The transformation of ethene to
polythene and interaction of hexamethylene diamine and
adipic acid leading to the formation of Nylon 6, 6 are
examples of two different types of polymerisation
reactions.
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”.
Unit Unit Unit Unit Unit
15
Polymers Polymers Polymers Polymers Polymers
15
Polymers Polymers Polymers Polymers Polymers
2015-16
Page 2


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. This process of
formation of polymers from respective monomers is
called polymerisation. The transformation of ethene to
polythene and interaction of hexamethylene diamine and
adipic acid leading to the formation of Nylon 6, 6 are
examples of two different types of polymerisation
reactions.
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”.
Unit Unit Unit Unit Unit
15
Polymers Polymers Polymers Polymers Polymers
15
Polymers Polymers Polymers Polymers Polymers
2015-16
426 Chemistry
C:\Chemistry-12\Unit-15.pmd    28.02.07
There are several ways of classification of polymers based on some
special considerations. The following are some of the common
classifications of polymers:
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.
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.
There are three  different types based on the structure of the polymers.
1. Linear polymers
These polymers consist of long and straight chains. The examples
are high density polythene, polyvinyl chloride, etc. These are
represented as:
2. Branched chain polymers
These polymers contain linear chains having some branches, e.g.,
low density polythene. These are depicted as follows:
3. Cross linked or Network polymers
These  are  usually  formed  from  bi-functional  and tri-functional
monomers  and  contain  strong covalent bonds between various
linear polymer chains, e.g. bakelite, melamine, etc. These polymers
are depicted as follows:
15.1 15.1 15.1 15.1 15.1 Classification Classification Classification Classification Classification
of Polymers of Polymers of Polymers of Polymers of Polymers
15.1.1 Classifica-
tion Based
on Source
15.1.2 Classifica-
tion Based
onStructure
of Polymers
2015-16
Page 3


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. This process of
formation of polymers from respective monomers is
called polymerisation. The transformation of ethene to
polythene and interaction of hexamethylene diamine and
adipic acid leading to the formation of Nylon 6, 6 are
examples of two different types of polymerisation
reactions.
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”.
Unit Unit Unit Unit Unit
15
Polymers Polymers Polymers Polymers Polymers
15
Polymers Polymers Polymers Polymers Polymers
2015-16
426 Chemistry
C:\Chemistry-12\Unit-15.pmd    28.02.07
There are several ways of classification of polymers based on some
special considerations. The following are some of the common
classifications of polymers:
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.
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.
There are three  different types based on the structure of the polymers.
1. Linear polymers
These polymers consist of long and straight chains. The examples
are high density polythene, polyvinyl chloride, etc. These are
represented as:
2. Branched chain polymers
These polymers contain linear chains having some branches, e.g.,
low density polythene. These are depicted as follows:
3. Cross linked or Network polymers
These  are  usually  formed  from  bi-functional  and tri-functional
monomers  and  contain  strong covalent bonds between various
linear polymer chains, e.g. bakelite, melamine, etc. These polymers
are depicted as follows:
15.1 15.1 15.1 15.1 15.1 Classification Classification Classification Classification Classification
of Polymers of Polymers of Polymers of Polymers of Polymers
15.1.1 Classifica-
tion Based
on Source
15.1.2 Classifica-
tion Based
onStructure
of Polymers
2015-16
427 Polymers
C:\Chemistry-12\Unit-15.pmd    28.02.07
Polymers can also be  classified on the basis of mode of polymerisation
into two sub groups.
1. Addition polymers
The addition polymers are formed by  the repeated addition of
monomer  molecules  possessing  double or triple bonds, e.g., the
formation of polythene from ethene and polypropene from propene.
However, the addition polymers formed by the polymerisation of  a
single monomeric species are known as homopolymers, e.g.,
polythene.
The polymers made by addition polymerisation from two  different
monomers are termed as copolymers, e.g., Buna-S, Buna-N, etc.
2. Condensation polymers
The condensation polymers are formed by repeated condensation
reaction between two different bi-functional or tri-functional
monomeric units. In these polymerisation reactions, the elimination
of small  molecules such as water, alcohol, hydrogen chloride, etc.
take place.  The examples are terylene (dacron), nylon 6, 6, nylon 6,
etc. For example, nylon 6, 6 is formed by the condensation of
hexamethylene diamine with adipic acid.
15.1.3 Classifica-
tion Based
on Mode of
Polymerisa-
tion
A large number of polymer applications in different fields depend on
their  unique  mechanical  properties  like  tensile  strength, elasticity,
toughness, etc. These mechanical properties are governed by
intermolecular forces, e.g., van der Waals forces and hydrogen bonds,
present in the polymer. These forces also bind the polymer chains.
Under  this  category, the polymers are classified into the following four
sub  groups on the basis of magnitude of intermolecular forces present
in them.
1. Elastomers
These are rubber – like solids with elastic properties. In these
15.1.4 Classification
Based on
Molecular
Forces
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 15.1 Example 15.1 Example 15.1 Example 15.1 Example 15.1
Solution Solution Solution Solution Solution
2015-16
Page 4


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. This process of
formation of polymers from respective monomers is
called polymerisation. The transformation of ethene to
polythene and interaction of hexamethylene diamine and
adipic acid leading to the formation of Nylon 6, 6 are
examples of two different types of polymerisation
reactions.
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”.
Unit Unit Unit Unit Unit
15
Polymers Polymers Polymers Polymers Polymers
15
Polymers Polymers Polymers Polymers Polymers
2015-16
426 Chemistry
C:\Chemistry-12\Unit-15.pmd    28.02.07
There are several ways of classification of polymers based on some
special considerations. The following are some of the common
classifications of polymers:
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.
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.
There are three  different types based on the structure of the polymers.
1. Linear polymers
These polymers consist of long and straight chains. The examples
are high density polythene, polyvinyl chloride, etc. These are
represented as:
2. Branched chain polymers
These polymers contain linear chains having some branches, e.g.,
low density polythene. These are depicted as follows:
3. Cross linked or Network polymers
These  are  usually  formed  from  bi-functional  and tri-functional
monomers  and  contain  strong covalent bonds between various
linear polymer chains, e.g. bakelite, melamine, etc. These polymers
are depicted as follows:
15.1 15.1 15.1 15.1 15.1 Classification Classification Classification Classification Classification
of Polymers of Polymers of Polymers of Polymers of Polymers
15.1.1 Classifica-
tion Based
on Source
15.1.2 Classifica-
tion Based
onStructure
of Polymers
2015-16
427 Polymers
C:\Chemistry-12\Unit-15.pmd    28.02.07
Polymers can also be  classified on the basis of mode of polymerisation
into two sub groups.
1. Addition polymers
The addition polymers are formed by  the repeated addition of
monomer  molecules  possessing  double or triple bonds, e.g., the
formation of polythene from ethene and polypropene from propene.
However, the addition polymers formed by the polymerisation of  a
single monomeric species are known as homopolymers, e.g.,
polythene.
The polymers made by addition polymerisation from two  different
monomers are termed as copolymers, e.g., Buna-S, Buna-N, etc.
2. Condensation polymers
The condensation polymers are formed by repeated condensation
reaction between two different bi-functional or tri-functional
monomeric units. In these polymerisation reactions, the elimination
of small  molecules such as water, alcohol, hydrogen chloride, etc.
take place.  The examples are terylene (dacron), nylon 6, 6, nylon 6,
etc. For example, nylon 6, 6 is formed by the condensation of
hexamethylene diamine with adipic acid.
15.1.3 Classifica-
tion Based
on Mode of
Polymerisa-
tion
A large number of polymer applications in different fields depend on
their  unique  mechanical  properties  like  tensile  strength, elasticity,
toughness, etc. These mechanical properties are governed by
intermolecular forces, e.g., van der Waals forces and hydrogen bonds,
present in the polymer. These forces also bind the polymer chains.
Under  this  category, the polymers are classified into the following four
sub  groups on the basis of magnitude of intermolecular forces present
in them.
1. Elastomers
These are rubber – like solids with elastic properties. In these
15.1.4 Classification
Based on
Molecular
Forces
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 15.1 Example 15.1 Example 15.1 Example 15.1 Example 15.1
Solution Solution Solution Solution Solution
2015-16
428 Chemistry
C:\Chemistry-12\Unit-15.pmd    28.02.07
elastomeric polymers, the polymer chains are held together by the
weakest intermolecular forces. These weak binding forces permit the
polymer to be stretched.  A  few ‘crosslinks’ are introduced in between
the chains, which help the polymer to retract to its original  position
after  the  force is released as in vulcanised rubber. The examples
are buna-S, buna-N, neoprene, etc.
2. Fibres
Fibres are the thread forming solids which possess high tensile
strength   and   high   modulus.   These   characteristics   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. The examples are polyamides
(nylon 6, 6), polyesters (terylene), etc.
3. Thermoplastic polymers
These  are  the  linear  or  slightly  branched long chain molecules
capable of repeatedly softening on heating and hardening on cooling.
These polymers possess intermolecular forces of attraction
intermediate between elastomers and fibres. Some common
thermoplastics are polythene, polystyrene, polyvinyls, etc.
4 Thermosetting polymers
These  polymers  are  cross  linked  or heavily branched molecules,
which on heating undergo extensive cross linking in moulds and
again become infusible. These cannot be reused. Some common
examples are bakelite, urea-formaldelyde resins, etc.
The addition and condensation polymers are nowadays also referred as
chain  growth polymers and step growth polymers depending on  the
type of polymerisation mechanism they undergo during their formation.
15.1.5 Classifica-
tion Based
on Growth
Polymerisa-
tion
15.1 What are polymers ?
15.2 How are polymers classified on the basis of structure?
Intext Questions Intext Questions Intext Questions Intext Questions Intext Questions
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 leading to an increase
in chain length or 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.
15.2 15.2 15.2 15.2 15.2 Types of Types of Types of Types of Types of
Polymerisation Polymerisation Polymerisation Polymerisation Polymerisation
Reactions Reactions Reactions Reactions Reactions
15.2.1 Addition
Polymerisa-
tion or
ChainGrowth
Polymerisa-
tion
2015-16
Page 5


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. This process of
formation of polymers from respective monomers is
called polymerisation. The transformation of ethene to
polythene and interaction of hexamethylene diamine and
adipic acid leading to the formation of Nylon 6, 6 are
examples of two different types of polymerisation
reactions.
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”.
Unit Unit Unit Unit Unit
15
Polymers Polymers Polymers Polymers Polymers
15
Polymers Polymers Polymers Polymers Polymers
2015-16
426 Chemistry
C:\Chemistry-12\Unit-15.pmd    28.02.07
There are several ways of classification of polymers based on some
special considerations. The following are some of the common
classifications of polymers:
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.
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.
There are three  different types based on the structure of the polymers.
1. Linear polymers
These polymers consist of long and straight chains. The examples
are high density polythene, polyvinyl chloride, etc. These are
represented as:
2. Branched chain polymers
These polymers contain linear chains having some branches, e.g.,
low density polythene. These are depicted as follows:
3. Cross linked or Network polymers
These  are  usually  formed  from  bi-functional  and tri-functional
monomers  and  contain  strong covalent bonds between various
linear polymer chains, e.g. bakelite, melamine, etc. These polymers
are depicted as follows:
15.1 15.1 15.1 15.1 15.1 Classification Classification Classification Classification Classification
of Polymers of Polymers of Polymers of Polymers of Polymers
15.1.1 Classifica-
tion Based
on Source
15.1.2 Classifica-
tion Based
onStructure
of Polymers
2015-16
427 Polymers
C:\Chemistry-12\Unit-15.pmd    28.02.07
Polymers can also be  classified on the basis of mode of polymerisation
into two sub groups.
1. Addition polymers
The addition polymers are formed by  the repeated addition of
monomer  molecules  possessing  double or triple bonds, e.g., the
formation of polythene from ethene and polypropene from propene.
However, the addition polymers formed by the polymerisation of  a
single monomeric species are known as homopolymers, e.g.,
polythene.
The polymers made by addition polymerisation from two  different
monomers are termed as copolymers, e.g., Buna-S, Buna-N, etc.
2. Condensation polymers
The condensation polymers are formed by repeated condensation
reaction between two different bi-functional or tri-functional
monomeric units. In these polymerisation reactions, the elimination
of small  molecules such as water, alcohol, hydrogen chloride, etc.
take place.  The examples are terylene (dacron), nylon 6, 6, nylon 6,
etc. For example, nylon 6, 6 is formed by the condensation of
hexamethylene diamine with adipic acid.
15.1.3 Classifica-
tion Based
on Mode of
Polymerisa-
tion
A large number of polymer applications in different fields depend on
their  unique  mechanical  properties  like  tensile  strength, elasticity,
toughness, etc. These mechanical properties are governed by
intermolecular forces, e.g., van der Waals forces and hydrogen bonds,
present in the polymer. These forces also bind the polymer chains.
Under  this  category, the polymers are classified into the following four
sub  groups on the basis of magnitude of intermolecular forces present
in them.
1. Elastomers
These are rubber – like solids with elastic properties. In these
15.1.4 Classification
Based on
Molecular
Forces
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 15.1 Example 15.1 Example 15.1 Example 15.1 Example 15.1
Solution Solution Solution Solution Solution
2015-16
428 Chemistry
C:\Chemistry-12\Unit-15.pmd    28.02.07
elastomeric polymers, the polymer chains are held together by the
weakest intermolecular forces. These weak binding forces permit the
polymer to be stretched.  A  few ‘crosslinks’ are introduced in between
the chains, which help the polymer to retract to its original  position
after  the  force is released as in vulcanised rubber. The examples
are buna-S, buna-N, neoprene, etc.
2. Fibres
Fibres are the thread forming solids which possess high tensile
strength   and   high   modulus.   These   characteristics   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. The examples are polyamides
(nylon 6, 6), polyesters (terylene), etc.
3. Thermoplastic polymers
These  are  the  linear  or  slightly  branched long chain molecules
capable of repeatedly softening on heating and hardening on cooling.
These polymers possess intermolecular forces of attraction
intermediate between elastomers and fibres. Some common
thermoplastics are polythene, polystyrene, polyvinyls, etc.
4 Thermosetting polymers
These  polymers  are  cross  linked  or heavily branched molecules,
which on heating undergo extensive cross linking in moulds and
again become infusible. These cannot be reused. Some common
examples are bakelite, urea-formaldelyde resins, etc.
The addition and condensation polymers are nowadays also referred as
chain  growth polymers and step growth polymers depending on  the
type of polymerisation mechanism they undergo during their formation.
15.1.5 Classifica-
tion Based
on Growth
Polymerisa-
tion
15.1 What are polymers ?
15.2 How are polymers classified on the basis of structure?
Intext Questions Intext Questions Intext Questions Intext Questions Intext Questions
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 leading to an increase
in chain length or 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.
15.2 15.2 15.2 15.2 15.2 Types of Types of Types of Types of Types of
Polymerisation Polymerisation Polymerisation Polymerisation Polymerisation
Reactions Reactions Reactions Reactions Reactions
15.2.1 Addition
Polymerisa-
tion or
ChainGrowth
Polymerisa-
tion
2015-16
429 Polymers
C:\Chemistry-12\Unit-15.pmd    28.02.07
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 may be
depicted as follows:
Chain initiation steps
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:
2 Preparation of some important addition polymers
(a)Polythene
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
2015-16