.
ot
sp
442 ENGINEERING CHEMISTRY
og
and halides of V, Zr, Cr, Mo etc.) along with organometallic compounds (triethyl/trimethyl
aluminium) polymerization of olefins leads to stereospecific polymerization.
bl
Mechanism of Ziegler-Natta polymerization:
y.
Initiation:
Cat R′ + CH2 = CHR → Cat CH2CH(R)R′
it
Propagation:
rs
Cat CH2CHR + nCH2 = CHR Cat CH2CHCH2CHR
ve
R R R n
Termination:
ni
lu
Cat CH2CHCH2CHR + HX Cat X + CH3CHCH2CHR
R R R R
ca
n n
polymer
gi
By earlier free radical polymerization of ethylene, propylene yielded polymers of low
density having highly branched chain polymer of low crystallinity and mechanically weaker
lo
structure like LDPE (low density polyethylene). Ziegler-Natta polymerization yielded
stereospecific structure of high mechanical strength, unbranched, high melting and highly
no
crystalline polymer like HDPE (high density polyethylene).
Stereochemistry of polymers
ch
(i) Isotactic polymers have all the groups in one side of the polymeric backbone and the
monomers are joined in a regular head to tail arrangement.
te
H H H
Y Y Y
a
C C C
al
C C C
H H H
er
H H H
(ii) Syndiotactic polymers have similar head to tail arrangements but here Y groups
/k
appear on opposite sides of polymer backbone alternately.
H Y H
:/
H H H H
C C C
C C C C
tp
Y H Y
H H H H
ht
(iii) Atactic polymers have Y groups arranged randomly along the polymeric backbone
and the material is soft, elastic, rubbery.
H Y Y
H H H H
C C C
C C C C
Y H H
H H H H
http://keralatechnologicaluniversity.blogspot.com
C-8\N-ENGCHE\ECH20-1.PM5 442
, .
ot
sp
POLYMERS 443
og
Spatial arrangement of an atactic polymer: It is a form of addition polymer, such
as polypropene in which the side groups along the polymer chain are randomly orientated.
bl
Atactic polypropene is an amorphous, rubbery polymer of little value unlike isotactic
polypropene (Fig. 20.1).
y.
CH3 H H CH3 H CH3
H CH3 CH3 H CH3 H
it
rs
Fig. 20.1 Part of a chain of atactic polypropene.
ve
Condensation polymerization
ni
It is the type of reaction occurring between monomers containing polar groups which
form polymer along with the elimination of small molecules like H2O, HCl etc.
lu
In this type of polymerization, as it takes place through the functional groups, when any
monomer may contain two or three functional groups cross-linked polymer structure will be
ca
formed.
gi
lo
no
ch
Co-polymerization
te
Two or more monomers undergoing joint polymerization is called copolymerization
reaction such as the production of SBR (Styrene butadiene rubber).
a
CH = CH2
al
Copolymerization
er
n(·CH2 = CH—CH = CH2) + n —CH2CH = CH—CH2—CH—CH
butadiene styrene
/k
:/
n
SBR (GRS-rubber)
tp
Influence of structure of polymer on its properties
ht
Strength of polymer
In cross-linked polymer, the units are linked by interchain covalent bonds forming giant,
three dimensional structure and they are strong and tough as the movement of inter molecular
chains are totally restricted.
Strength of straight chain polymers depends on the chain length. Polymers of low mol.
wt. are soft and gummy but brittle. But higher-chain length polymers are tougher and more
heat resistant. Presence of polar groups along the chain length also increases the intermolecular
forces and increases the strength of polymer.
http://keralatechnologicaluniversity.blogspot.com
C-8\N-ENGCHE\ECH20-1.PM5 443
, .
ot
sp
444 ENGINEERING CHEMISTRY
og
Highlights:
bl
Types of polymerization
• Addition polymerization:
y.
The product polymer is exact multiple of the original monomeric molecule e.g.,
polythene from ethene.
it
• Condensation polymerization:
Combination through different functional groups of monomers with elimination
rs
of small molecules like H2O.
• Copolymerization:
ve
Joint polymerization of two or more types of monomers. As for example SBR.
ni
• Mechanisms of addition polymerization are:
(a) Free radical mechanism.
lu
(b) Ionic mechanism.
(c) Co-ordination polymerization (Ziegler-Natta polymerization).
ca
Plastic deformation
gi
Polymers consisting of linear-chain molecules are always soluble and thermoplastic,
even for very high molecular weight.
lo
Three-dimensional polymer molecules are insoluble in any conventional solvent and are
no
thermosetting. Hence, thermoplastic and thermosetting qualities of polymer depend on structure
of the polymers. Artificially creating crosslinking converts thermoplastic material into
thermosetting.
ch
Physical state
te
Random arrangement of molecules in the polymer leads to amorphous state whereas
regular arrangements of molecules or chains in a polymer lead to crystalline state.
a
The regular arrangement increases the intermolecular forces of attraction and leads to
al
higher softening point, greater rigidity, brittleness and strength of the polymer. The amorphous
nature of polymer provides flexibility.
er
Very long chain polymers having free rotating groups form irregularly coiled and
entangled snorts, which can be stretched and again returned back to its original state giving
/k
rise to elastic character to the polymer.
Solubility and chemical resistance
:/
Polymer containing polar groups are more soluble in polar solvents like water, alcohol
but are chemically resistant to non-polar solvents like benzene, toluene, CCl4 etc. Similarly,
tp
non-polar group containing polymers are chemically resistant to polar solvents but are soluble
in non-polar solvents, particularly the greater the degree of cross-linking, less is the solubility
ht
of the polymer in a solvent.
Shapes and formsmechanical properties
The internal arrangement of the long-chain molecules provides the forms in which the
polymer may form i.e., fibres, plastics and rubbers. If the internal forces between the molecules
are low, molecules become bulky, form random arrangement and show rubbery character. If
the internal forces are high they ultimately give rise to orderly arrangement and form fibrous
nature. Intermediate forces lead to plastic nature.
http://keralatechnologicaluniversity.blogspot.com
C-8\N-ENGCHE\ECH20-1.PM5 444
, .
ot
sp
POLYMERS 445
og
Stress
Hard, brittle
bl
polymers
Rigid and high impact
y.
thermoplastics
it
Polymer fibres
rs
Rubbery polymers
ve
ni
Strain
Fig. 20.2 Four types of stress-strain curves illustrating how polymers of different types
lu
behave (the lines end where the sample breaks).
The strength of the polymer is controlled by the length of the polymer chains and its
ca
cross-linking. Strength of a polymer is estimated from typical stress-strain curves for different
polymers as shown in Fig. 20.2. On increasing strain continuously ultimately the polymer
gi
chains are uncoiled and fully stretched called necking and after this point the polymer reaches
its break point and yields.
lo
Effect of heat
Behaviour of polymer is controlled by the temperature. Amorphous polymers, which do
no
not have melting point have softening points. At very low temperature both the crystalline
and amorphous polymers behave like glass and on heating, the glass transition temperature is
ch
reached (Tg) after which they soften. Amorphous polymer becomes rubbery and then gummy
and on further heating it liquefies. But the crystalline polymer on heating above Tg shows
thermoplastic behaviour and finally liquefies.
te
PLASTICS (RESINS)
a
Plastics are a class of high polymers which can be moulded into any desired form by
al
heat and pressure. Resins are actually the binders used for plastics and these two terms are
used synonymously.
er
There are two classes of plastics or resins.
1. Thermoplastic resins soften on heating and harden on cooling and this change is not
/k
chemical but physical in nature, hence repeated heating and cooling also does not alter its
nature.
:/
2. Thermosetting resins are those which are heated during moulding and heating is
continued until is set and hardened. This hardened material cannot be softened again, hence
tp
the setting is permanent and irreversible.
Compounding
ht
Plastics for manufacturing of finished articles are always mixed with 4-10% of other
materials, which impart some durable properties to the moulded material. These materials
are known as additives which not only impart some properties to plastics but also make the
processing easy. The process of mixing these additives to virgin plastics is called Compounding.
Additives and their functions are discussed below:
(i) Resin is the binding constituent, which binds all the additives together. Thermoset-
ting resins are added as linear low molecular weight polymers, which actually form
crosslinking during the moulding process in presence of catalyst.
http://keralatechnologicaluniversity.blogspot.com
C-8\N-ENGCHE\ECH20-1.PM5 445
ot
sp
442 ENGINEERING CHEMISTRY
og
and halides of V, Zr, Cr, Mo etc.) along with organometallic compounds (triethyl/trimethyl
aluminium) polymerization of olefins leads to stereospecific polymerization.
bl
Mechanism of Ziegler-Natta polymerization:
y.
Initiation:
Cat R′ + CH2 = CHR → Cat CH2CH(R)R′
it
Propagation:
rs
Cat CH2CHR + nCH2 = CHR Cat CH2CHCH2CHR
ve
R R R n
Termination:
ni
lu
Cat CH2CHCH2CHR + HX Cat X + CH3CHCH2CHR
R R R R
ca
n n
polymer
gi
By earlier free radical polymerization of ethylene, propylene yielded polymers of low
density having highly branched chain polymer of low crystallinity and mechanically weaker
lo
structure like LDPE (low density polyethylene). Ziegler-Natta polymerization yielded
stereospecific structure of high mechanical strength, unbranched, high melting and highly
no
crystalline polymer like HDPE (high density polyethylene).
Stereochemistry of polymers
ch
(i) Isotactic polymers have all the groups in one side of the polymeric backbone and the
monomers are joined in a regular head to tail arrangement.
te
H H H
Y Y Y
a
C C C
al
C C C
H H H
er
H H H
(ii) Syndiotactic polymers have similar head to tail arrangements but here Y groups
/k
appear on opposite sides of polymer backbone alternately.
H Y H
:/
H H H H
C C C
C C C C
tp
Y H Y
H H H H
ht
(iii) Atactic polymers have Y groups arranged randomly along the polymeric backbone
and the material is soft, elastic, rubbery.
H Y Y
H H H H
C C C
C C C C
Y H H
H H H H
http://keralatechnologicaluniversity.blogspot.com
C-8\N-ENGCHE\ECH20-1.PM5 442
, .
ot
sp
POLYMERS 443
og
Spatial arrangement of an atactic polymer: It is a form of addition polymer, such
as polypropene in which the side groups along the polymer chain are randomly orientated.
bl
Atactic polypropene is an amorphous, rubbery polymer of little value unlike isotactic
polypropene (Fig. 20.1).
y.
CH3 H H CH3 H CH3
H CH3 CH3 H CH3 H
it
rs
Fig. 20.1 Part of a chain of atactic polypropene.
ve
Condensation polymerization
ni
It is the type of reaction occurring between monomers containing polar groups which
form polymer along with the elimination of small molecules like H2O, HCl etc.
lu
In this type of polymerization, as it takes place through the functional groups, when any
monomer may contain two or three functional groups cross-linked polymer structure will be
ca
formed.
gi
lo
no
ch
Co-polymerization
te
Two or more monomers undergoing joint polymerization is called copolymerization
reaction such as the production of SBR (Styrene butadiene rubber).
a
CH = CH2
al
Copolymerization
er
n(·CH2 = CH—CH = CH2) + n —CH2CH = CH—CH2—CH—CH
butadiene styrene
/k
:/
n
SBR (GRS-rubber)
tp
Influence of structure of polymer on its properties
ht
Strength of polymer
In cross-linked polymer, the units are linked by interchain covalent bonds forming giant,
three dimensional structure and they are strong and tough as the movement of inter molecular
chains are totally restricted.
Strength of straight chain polymers depends on the chain length. Polymers of low mol.
wt. are soft and gummy but brittle. But higher-chain length polymers are tougher and more
heat resistant. Presence of polar groups along the chain length also increases the intermolecular
forces and increases the strength of polymer.
http://keralatechnologicaluniversity.blogspot.com
C-8\N-ENGCHE\ECH20-1.PM5 443
, .
ot
sp
444 ENGINEERING CHEMISTRY
og
Highlights:
bl
Types of polymerization
• Addition polymerization:
y.
The product polymer is exact multiple of the original monomeric molecule e.g.,
polythene from ethene.
it
• Condensation polymerization:
Combination through different functional groups of monomers with elimination
rs
of small molecules like H2O.
• Copolymerization:
ve
Joint polymerization of two or more types of monomers. As for example SBR.
ni
• Mechanisms of addition polymerization are:
(a) Free radical mechanism.
lu
(b) Ionic mechanism.
(c) Co-ordination polymerization (Ziegler-Natta polymerization).
ca
Plastic deformation
gi
Polymers consisting of linear-chain molecules are always soluble and thermoplastic,
even for very high molecular weight.
lo
Three-dimensional polymer molecules are insoluble in any conventional solvent and are
no
thermosetting. Hence, thermoplastic and thermosetting qualities of polymer depend on structure
of the polymers. Artificially creating crosslinking converts thermoplastic material into
thermosetting.
ch
Physical state
te
Random arrangement of molecules in the polymer leads to amorphous state whereas
regular arrangements of molecules or chains in a polymer lead to crystalline state.
a
The regular arrangement increases the intermolecular forces of attraction and leads to
al
higher softening point, greater rigidity, brittleness and strength of the polymer. The amorphous
nature of polymer provides flexibility.
er
Very long chain polymers having free rotating groups form irregularly coiled and
entangled snorts, which can be stretched and again returned back to its original state giving
/k
rise to elastic character to the polymer.
Solubility and chemical resistance
:/
Polymer containing polar groups are more soluble in polar solvents like water, alcohol
but are chemically resistant to non-polar solvents like benzene, toluene, CCl4 etc. Similarly,
tp
non-polar group containing polymers are chemically resistant to polar solvents but are soluble
in non-polar solvents, particularly the greater the degree of cross-linking, less is the solubility
ht
of the polymer in a solvent.
Shapes and formsmechanical properties
The internal arrangement of the long-chain molecules provides the forms in which the
polymer may form i.e., fibres, plastics and rubbers. If the internal forces between the molecules
are low, molecules become bulky, form random arrangement and show rubbery character. If
the internal forces are high they ultimately give rise to orderly arrangement and form fibrous
nature. Intermediate forces lead to plastic nature.
http://keralatechnologicaluniversity.blogspot.com
C-8\N-ENGCHE\ECH20-1.PM5 444
, .
ot
sp
POLYMERS 445
og
Stress
Hard, brittle
bl
polymers
Rigid and high impact
y.
thermoplastics
it
Polymer fibres
rs
Rubbery polymers
ve
ni
Strain
Fig. 20.2 Four types of stress-strain curves illustrating how polymers of different types
lu
behave (the lines end where the sample breaks).
The strength of the polymer is controlled by the length of the polymer chains and its
ca
cross-linking. Strength of a polymer is estimated from typical stress-strain curves for different
polymers as shown in Fig. 20.2. On increasing strain continuously ultimately the polymer
gi
chains are uncoiled and fully stretched called necking and after this point the polymer reaches
its break point and yields.
lo
Effect of heat
Behaviour of polymer is controlled by the temperature. Amorphous polymers, which do
no
not have melting point have softening points. At very low temperature both the crystalline
and amorphous polymers behave like glass and on heating, the glass transition temperature is
ch
reached (Tg) after which they soften. Amorphous polymer becomes rubbery and then gummy
and on further heating it liquefies. But the crystalline polymer on heating above Tg shows
thermoplastic behaviour and finally liquefies.
te
PLASTICS (RESINS)
a
Plastics are a class of high polymers which can be moulded into any desired form by
al
heat and pressure. Resins are actually the binders used for plastics and these two terms are
used synonymously.
er
There are two classes of plastics or resins.
1. Thermoplastic resins soften on heating and harden on cooling and this change is not
/k
chemical but physical in nature, hence repeated heating and cooling also does not alter its
nature.
:/
2. Thermosetting resins are those which are heated during moulding and heating is
continued until is set and hardened. This hardened material cannot be softened again, hence
tp
the setting is permanent and irreversible.
Compounding
ht
Plastics for manufacturing of finished articles are always mixed with 4-10% of other
materials, which impart some durable properties to the moulded material. These materials
are known as additives which not only impart some properties to plastics but also make the
processing easy. The process of mixing these additives to virgin plastics is called Compounding.
Additives and their functions are discussed below:
(i) Resin is the binding constituent, which binds all the additives together. Thermoset-
ting resins are added as linear low molecular weight polymers, which actually form
crosslinking during the moulding process in presence of catalyst.
http://keralatechnologicaluniversity.blogspot.com
C-8\N-ENGCHE\ECH20-1.PM5 445
