polymers

CHE1019 POLYMER TECHNOLOGY
Instructor: Dr. Mohammed Rehaan Chandan
School of Chemical Engineering (SCHEME)
Vellore Institute of Technology (VIT), Vellore.
E-mail: md.rehaanchandan@vit.ac.in
1

Text / Reference Books
• Text/Reference Books
• 1. V R Gowariker, NV Viswanathan. Jayadev Sreedhar “Polymer
Science” New age Publishers.
• 2. DC Miles, JH Briston, “Polymer Technology” Chemical
Publishing Co., Inc New York 1979.
• 3. Williams DJ, Polymer Science and Engg, Prentice Hall New York
1971.
3

Syllabus
• Module 1: Introduction to fundamentals of polymers
• Module 2: Methods of Polymerization
• Module 3: Structure and size of polymer
• Module 4: Processing additives
• Module 5: Processing Techniques
• Module 6: Polymeric materials
• Module 7: Special and bio polymers
• Module 8: Contemporary issues
4

Course Objectives:
1.To provide the fundamental knowledge on
the chemistry, physics & technology of
polymers.
2. To make them understand about polymer
characterization methods and principles.
3. To equip them with processing methods
and additives for polymer manufacturing.
5

Course Outcomes
• At the end of the course, you are able to:
6
1. Classify and characterize polymers and polymeric
reactions.
2. Explain the different methods of polymerization
3. Identify the processing technologies for different
polymer synthesis and their additives.
4. Identify suitable polymer for specific application.
5. Distinguish different type of polymers for various
applications.
6. Analyze various polymers for novel applications

Assessment Method
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Assessment Method Total Marks Weightage %
Continuous Assessment (60%)
CAT I 50 15%
CAT II 50 15%
Assignment - I 10 10%
Assignment – II 10 10%
Assignment – III 10 10%
Final Examination (40%)
100 40%
Final Assessment Test
Total 100%

Monomer and Polymer
• Monomer
• The individual small molecule from which a polymer is formed.
• Polymer (poly means many & mer means parts)
• Made up of many small molecules (monomer) which have
combined to form a single long or large molecule.
• Polymerization
• The process by which the monomer molecules are linked to form a
big polymer molecule.
• Degree of Polymerization
• Number of repeat units that decides the size of the polymer
molecule.
8

Structure of Monomer and Polymer
9

• The word “Polymer” is derived from two Greek words, ‘Poly’ that means
many (numerous) and ‘Mer’ which means units. In basic terms, a
polymer is a long-chain molecule that is composed of a large number of
repeating units of identical structure.
• Those monomers can be simple — just an atom or two or three — or they
might be complicated ring-shaped structures containing a dozen or more
atoms.
10

Classification of Polymers
• Based on the Origin
• Natural polymer (ex: Cotton,
Silk, Wool, rubber)
• Synthetic polymer (ex:
Polyethylene, Nylon, PVC)
• Based on backbone Chain
• Organic polymer (most of the
polymers)
• Inorganic polymer (ex: Glass,
Silicone Rubber)
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Based on the property on
heating
o Thermoplastic polymer
o Thermosetting polymer
Based on its form and usage
o Plastic
o Elastomer
o Fiber
o Liquid resin

• Natural polymers
• The easiest way to classify polymers is their source of origin. Natural
polymers are polymers which occur in nature and are existing in natural
sources like plants and animals. Some common examples are
Proteins (which are found in humans and animals alike), Cellulose
and Starch (which are found in plants) or Rubber (which we harvest
from the latex of a tropical plant ).
• Synthetic polymers
• Synthetic polymers are polymers which humans can artificially
create/synthesize in a lab. These are commercially produced by
industries for human necessities. Some commonly produced polymers
which we use day to day are Polyethylene (a mass-produced plastic
which we use in packaging) or Nylon Fibers (commonly used in our
clothes, fishing nets etc.)
13

• Semi-Synthetic polymers
• Semi-Synthetic polymers are polymers obtained by making modification
in natural polymers artificially in a lab. These polymers formed by
chemical reaction (in a controlled environment) and are of commercial
importance. Example: Vulcanized Rubber (Sulphur is used in cross
bonding the polymer chains found in natural rubber) Cellulose acetate
(rayon) etc.
• Linear polymers
• These polymers are similar in structure to a long straight chain which
identical links connected to each other. The monomers in these are
linked together to form a long chain. These polymers have high melting
points and are of higher density. A common example of this is PVC
(Poly-vinyl chloride). This polymer is largely used for making electric
cables and pipes.
14

• Branch chain polymers
• As the title describes, the structure of these polymers is like branches
originating at random points from a single linear chain. Monomers join
together to form a long straight chain with some branched chains of
different lengths. As a result of these branches, the polymers are not
closely packed together. They are of low density having low melting
points. Low-density polyethene (LDPE) used in plastic bags and
general purpose containers is a common example.
• Crosslinked or Network polymers
• In this type of polymers, monomers are linked together to form a three-
dimensional network. The monomers contain strong covalent bonds as
they are composed of bi-functional and tri-functional in nature. These
polymers are brittle and hard. Ex:- Bakelite (used in electrical
insulators), Melamine etc.
15

• Addition polymers
• These type of polymers are formed by the repeated addition of
monomer molecules. The polymer is formed by polymerization of
monomers with double or triple bonds (unsaturated compounds). Note,
in this process, there is no elimination of small molecules like water or
alcohol etc (no by-product of the process). Addition polymers always
have their empirical formulas same as their monomers. Example:
ethene n(CH2=CH2) to polyethene -(CH2-CH2)n-.
• Condensation polymers
• These polymers are formed by the combination of monomers, with the
elimination of small molecules like water, alcohol etc. The monomers in
these types of condensation reactions are bi-functional or tri-functional
in nature. A common example is the polymerization of
Hexamethylenediamine and adipic acid to give Nylon – 66, where
molecules of water are eliminated in the process.
16

Classification based on molecular forces
• Intramolecular forces are the forces that hold atoms
together within a molecule. In Polymers, strong covalent
bonds join atoms to each other in individual polymer
molecules. Intermolecular forces (between the
molecules) attract polymer molecules towards each other.
• Note that the properties exhibited by solid materials
like polymers depend largely on the strength of the forces
between these molecules.
17

• Elastomers
• Elastomers are rubber-like solid polymers, that are elastic
in nature. When we say elastic, we basically mean that the
polymer can be easily stretched by applying a little force.
• The most common example of this can be seen in rubber
bands(or hair bands). Applying a little stress elongates
the band. The polymer chains are held by the weakest
intermolecular forces, hence allowing the polymer to be
stretched. But as you notice removing that stress also
results in the rubber band taking up its original form. This
happens as we introduce crosslinks between the polymer
chains which help it in retracting to its original position, and
taking its original form.
18

• Thermoplastics
• Thermoplastic polymers are long-chain polymers in which inter-
molecules forces (Van der Waal’s forces) hold the polymer chains
together. These polymers when heated are softened (thick fluid like)
and hardened when they are allowed to cool down, forming a hard
mass. They do not contain any cross bond and can easily be shaped
by heating and using moulds. A common example is Polystyrene or
PVC (which is used in making pipes).
• Thermosetting
• Thermosetting plastics are polymers which are semi-fluid in nature with
low molecular masses. When heated, they start cross-linking
between polymer chains, hence becoming hard and infusible. They
form a three-dimensional structure on the application of heat. This
reaction is irreversible in nature. The most common example of a
thermosetting polymer is that of Bakelite, which is used in making
electrical insulation.
19

• Fibres
• In the classification of polymers these are a class of polymers which
are a thread like in nature, and can easily be woven. They have
strong inter-molecules forces between the chains giving them less
elasticity and high tensile strength. The intermolecular forces may
be hydrogen bonds or dipole-dipole interaction. Fibers have sharp and
high melting points. A common example is that of Nylon-66, which is
used in carpets and apparels.
• The above was the general ways to classify polymers. Another
category of polymers is that of Biopolymers. Biopolymers are
polymers which are obtained from living organisms. They are
biodegradable and have a very well defined structure. Various
biomolecules like carbohydrates and proteins are a part of the category.
20

Reaction of olefin polymer
21
Monomer Polymer
Ethylene
H3C
CH3
n
Repeat unit
Polyethylene
CH3
CH3
n
CH3 CH3 CH3 CH3 CH3 CH3
CH3
Propylene
Polypropylene
Ph
CH3
n
Ph Ph Ph Ph Ph Ph
Ph
Styrene
Polystyrene
Cl
CH3
n
Cl Cl Cl Cl Cl Cl
Cl
Vinyl Chloride
Poly(vinyl chloride)
F2C CF2
Tetrafluoroethylene
F3C
F2
C
C
F2
F2
C
C
F2
F2
C
C
F2
F2
C
C
F2
F2
C
C
F2
F2
C
C
F2
CF3
n
Poly(tetrafluoroethylene): Teflon

Reaction of Natural Polymers
22
Monomer Polymer
Isoprene
n
Polyisoprene:
Natural rubber
O
H
HO
H
HO
H
H
OH
H
OH
OH
Poly(ß-D-glycoside):
cellulose
O
H
O
H
HO
H
H
OH
H
OH
OH
H
n
ß-D-glucose
H3N
O
O
R
Polyamino acid:
protein
H3N
O
H
N
R1
O
H
N
Rn+1
O
OH
Rn+2
n
Amino Acid
Base
O
OH
O
P
O
O
O
oligonucleic acid
DNA
Nucleotide
Base = C, G, T, A
Base
O
O
O
P
O
O
O
DNA
DNA

Reaction of Polyesters, Amides, and Urethanes
23
Monomer Polymer
CO2H
HO2C
HO
OH
O O
HO O
H2
C
H2
C O
n
Terephthalic
acid
Ethylene
glycol
Poly(ethylene terephthalate
H
Ester
HO OH
O O
4
H2N NH2
4
Adipic Acid 1,6-Diaminohexane Nylon 6,6
HO N
H
N
H
H
O O
4 4
n
CO2H
HO2C
Terephthalic
acid
NH2
H2N
1,4-Diamino
benzene
Kevlar
O
HO
O
H
N
H
N H
n
Amide
HO
OH
Ethylene
glycol
H2
C
OCN NCO
4,4-diisocyantophenylmethane
Spandex
H2
C
H
N
H
N
O
HO
O
O
H2
C
H2
C O H
n
Urethane linkage

Summary
• Polymer are made up of many small molecules
(monomer) which have combined to form a single long or
large molecule.
• Polymerization is a process by which the monomer
molecules are linked to form a big polymer molecule.
• Number of repeat units that decides the size of the
polymer molecule is denoted as the degree of
Polymerization.
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Step versus chain polymerization
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Step-growth polymerization Chain-growth polymerization
Growth throughout matrix Growth by addition of monomer only at
one end of chain
Rapid loss of monomer early in the
reaction
Some monomer remains even at long
reaction times
Same mechanism throughout Different mechanisms operate at
different stages of reaction (i.e.
Initiation, propagation and termination)
Average molecular weight increases
slowly at low conversion and high
extents of reaction are required to
obtain high chain length
Molar mass of backbone chain
increases rapidly at early stage and
remains approximately the same
throughout the polymerization
Ends remain active (no termination) Chains not active after termination
No initiator necessary Initiator required

Contents
• Chain polymerization reaction
• Free Radical Polymerization reaction
• Initiation step
• Propagation step
• Termination step
27

Chain polymerization
• Chain polymerization is characterized by a self-addition of the
monomer molecules, to each other, very rapidly through a
chain reaction.
• The initiator is a source of any chemical species that reacts
with a monomer (single molecule that can form chemical
bonds) to form an intermediate compound capable of linking
successively with a large number of monomers into a polymeric
compound.
• The functionality of initiators depends on the presence of
functional end groups such as hydroxyl and carbonyl, or azo
and perester bonds which undergo dissociation to alkyl, alkoxy
or acyloxy radicals under the influence of temperature or
irradiation.
28

Monomers capable of undergoing chain polymerization
reaction
29

reaction
30

reaction
31
• Chain polymerization consists of 3 major steps.
 Initiation
 Propagation
 Termination.

Free radical chain polymerization reaction
• The initiation of polymer chain growth is brought about
free radicals produced by the decomposition of
compounds called ‘initiators’.
• The term ‘chain growth’ represents a process involving a
continuous and very rapid addition of the monomer units
to form molecules or polymer chains.
• As more and more units are added, the length of the
polymer chains increases continuously and the chains
grow rapidly.
• Initiators are thermally unstable compounds and
decompose in to products called ‘free-radicals’.
32

Initiators
• If R- -R is an initiator, and the pair of electrons forming the
bond between the two R’s, can be represented by the dots, the
initiator can be written as
• When energy is supplied to this compound, say, in the form of
heat, the molecule is split into two symmetrical components.
• Each component caries with it one of the electrons from the
electron pair.
• This type of decomposition, where the molecules are split into
two identical fragments, is called ‘homolytic decomposition’.
33

Initiators
• The two fragments, each carrying one unpaired (lone) electron
with it, are called ‘free radicals’.
• The decomposition of the initiator to form free-radicals can be
induced by heat, light energy or catalysts.
• A host of low molecular weight compounds used as useful
initiators are comprising mainly
• azo compounds
• peroxides
• hydroperoxides
• peracids
• peresters
34

Initiator formation
• Thermal decomposition of benzoyl peroxide and azobis
isobutyronitrile to form free-radicals is shown here.
41

Decomposition behavior of initiators
• The rate of decomposition of these initiators depends,
apart from their chemical nature, on the reaction
temperature and the solvents used.
42

Decomposition behavior of initiators
• This means the same initiator in a given solvent decomposes at
different rates at different temperatures; and the same initiator at a
given temperature will have different decomposition rates in
different solvents.
43

Initiator decomposition using UV
• Initiators can also be decomposed by using ultraviolet (UV) light.
• The free-radicals formed by the photodecomposition of an initiator
are the same as those formed by its thermal decomposition.
44
hυ represents light energy
• The rate of decomposition depends mainly on the intensity and wavelength of
radiation and not on the temperature.
• The solvent controls the intensity of the radiation incident on the initiator
molecule – influence the rate of decomposition.

Free-radicals by direct excitation of
monomers
• In addition to the decomposition of the initiator, ultraviolet
light can also produce free-radicals by the direct excitation of
the monomer molecules.
• It should be noted that polymerization reactions initiated by
ultraviolet light fall under the class of ‘photoinitiated
polymerization’.
45

Decomposition of initiators by catalysts
• Initiators can also be induced to decompose into free-radicals
by using suitable catalysts.
46
• An electron transfer
mechanism resulting in a
reduction-oxidation
reaction is involved in all
these decompositions.
• Peroxides and
hydroperoxides are more
susceptible to this kind of
decomposition at room
temperature with an
aromatic tertiary amine.

Decomposition using redox initiators
• The decomposition of hydrogen peroxide by a ferrous ion
and that of a hydroperoxide by a cobaltous ion are other
examples.
• Polymerization reactions utilizing such redox initiators are
termed as ‘redox polymerization’.
47

Initiation step
• We know that a free-radical contains a lone (unpaired)
electron. A free-radical is, therefore, highly reactive and can
attack any molecule which either has a lone electron or is
prepared to part with one of its electrons.
• This is what happens in the process of initiation. The free-
radical R attacks the double bond in the monomer molecule,
resulting in the following chemical change.
• This process is explained as follows. In a double formed
between two carbon atoms, one pair of electrons exists as
sigma electrons.
48

Initiation step
• while the other pair, occupying certain orbitals in the
molecule not so close to the nucleus, exists as π
electrons.
• The π electrons are not very close to the nucleus and
protrude a little away from the axis of the molecule. They
are therefore susceptible to attack by other reactive
species.
• When free radicals are produced by a homolytic
decomposition of the initiator, what happens is that the
free-radical interferes with one of the πelectrons and
forms a normal pair of electrons at the sigma level.
49

Initiation step
• The other electron of the π pair is transferred to the other
end of the molecule.
• The jump of the dot in the direction of the arrows should
be noted.
• It is as if the two π electrons of the monomer molecule
‘divorce’ each other and one of the ‘separated’ partners
couples with the lone electron present in the free-radical
to form a sigma bond.
• Now, the monomer unit is linked to the free-radical unit
through a sigma bond forming a single molecule.
50

Initiation step
• The other electron of the original of the π pair, now
deprived of the partner, becomes ‘unpaired’.
• That is the free-radical site is now shifted from the initiator
fragment to the monomer unit.
• This process of the electron pair coming down from its π
energy level to a sigma level is associated with an energy
release of some 20 kilocalories, as π electrons are at a
higher energy level than the sigma electrons.
• Hence, the free-radical attacks a monomer initiating
polymerization is an exothermic process, whereas free-
radical formation by initiator decomposition is an
endothermic process.
51

Propagation step
• After ‘initiation’, in the propagation step, the radical site at the
first monomer unit attacks the double bond of a fresh monomer
molecule.
• This results in the linking up of the second monomer unit to the
first and the transfer of the radical site from the first monomer
unit to the second, by the unpaired electron transfer process.
• It should be noted that this chain still contains a radical site
(indicated by a dot) at its end carbon atom and can, therefore,
attack yet another monomer molecule with a simultaneous
transfer of the radical site to the new monomer unit added.
52

Propagation step
• This process involving a continuing attack on fresh monomer
molecules which, inturn, keep successively adding to the
growing chain one after another is termed as ‘propagation’.
• The propagation lasts till the chain growth is stopped by the
free-radical site being ‘killed’ by some impurities or by a sheer
termination process, or till there is no further monomer left for
attack.
53

Propagation step
• The structure of the growing chain can be represented by
• where ‘n’ denotes the number of monomer units added up
in the chain growth, and a wavy line indicates the polymer
chain made of ‘n’ number of monomeric units.
• The mode of addition of the incoming monomer to the
growing chain can be of the head-to-tail, tail-to-tail, head-
to-head or tail-to-head type.
54

Propagation step
• If we call the –CH2– and the –CHX– parts of a monomeric unit its head
and tail respectively, the four modes of addition can be represented as
55

Propagation step
• Each time a new monomer unit is added to the growing
chain, the π electron pair cascades down to the sigma level.
• Whenever this happens, there is an energy release of about
20 kcal and the process of chain propagation is brought
about without adding any external energy to the system.
• That is a very small amount of energy is supplied to
decompose the initiator so as to form its free-radical
fragments, and the polymer chain starts growing with large
amounts of energy release.
56

Termination step
• Any further addition of monomers to the growing chain is
stopped and the growth of the polymer chain is arrested.
• Since the decomposition of the initiator produces many
free-radical at the same time, each one of them can
initiate and propagate the chain simultaneously.
• Hence, at any time there may be quite a few growing
chain present in the system.
• Depending on the factors such as temperature, time and
monomer and initiator concentrations, there exists a
statistical probability of the two growing chains coming
close to and colliding with each other.
57

Termination step
• When such collision take place, the following two reactions
occur, resulting in the arrest of the chain growth.
58

Termination step
• In the first case, the two growing chains unite by the
coupling of the lone electron present in each chain to form
an electron pair and thus nullify the reactiveness.
• Since this process involves the coupling of the two lone
electrons, this kind of termination is known as ‘termination
by coupling’.
• In the second case, one H from one growing chain is
abstracted by the other growing chain and utilized by the
lone electron for getting stabilized.
• While the chain, which had donated the H, gets stabilized
by the formation of a double bond.
59

Termination step
• In this case, the termination process results in the
formation of two polymer molecules of shorter chain
length as against a single molecule of a longer chain
length obtainable by the first method. This type of
termination is called ‘termination by disproportionation’.
• From the termination reactions, it may be noted that the
product molecules formed do not contain any free-radical
site, hence cannot grow any further.
• The process of termination thus results in the deactivation
of the growing chain.
• The deactivation chain, in fact, forms the polymer.
60

Termination step
• In order to contrast it with an active growing chain, the
polymer molecule formed can be referred to as a ‘dead’
polymer chain.
• In addition to the several repeat units, the polymer
molecule contains certain other groups as well at the two
chain ends. This groups are called the ‘end groups’.
61

Termination by chain transfer
• In the case of ‘transfer reaction’, however, the growth of
one polymer chain is stopped or arrested, forming a dead
polymer, there is a simultaneous generation of a new free-
radical capable of initiating a fresh polymer chain growth.
• This reaction takes place by the abstraction of a hydrogen
atom or some other atom from the initiator, monomer or
polymer or from any other species present in the system,
including the solvent or any inadvertent impurity.
62

Termination by chain transfer
• The growing chain is now terminated, but a new free-
radical is formed.
• will now initiate the polymer chain growth afresh,
which will be followed by the chain propagation.
• Here, the termination of one chain growth and the
initiation of a new one takes place simultaneously.
• It is as if the chain growth is transferred from one site to
another and hence this phenomenon is called ‘chain
transfer’.
63

Termination using inhibitors
• Inhibitors are chemical substances capable of inhibiting or killing
the chain growth by combining with the active free-radicals and
forming either stable products or inacitve free-radicals.
• Hydroquinone, nitrobenzene, dinitrobenzene and benzothiazine
are some of the inhibitors customarily used in the polymer industry.
64
• The inhibiting
action of these
compounds can
be adopted by
the given
example.

• Here, the inhibitor, nitrobenzene, adds on the growing
chain P , forming a polymer chain with a nitrobenzene end
group carrying a radical site.
• The nitrocompound end of the chain is, however,
resonance stabilized.
• This resonance-stabilized free-radical end is not active
enough to attack a fresh monomer molecule and add it on
the chain.
• No further propagation can, therefore, take place.
• The free-radical nature of the end group is, however,
powerful enough to recombine with the radical of another
growing chain and terminate the growth of the latter.
65

• In this case, we can see that a single molecule od the
inhibitor has killed two growing chains.
66

• Some of the inhibitors such as diphenyl picryl hydrazide
(DPPH) exist in the form of stable free-radicals which can
easily stop the chain growth by direct coupling.
67

• Atmospheric oxygen is a good inhibitor. The inhibiting
action of oxygen is due to its biradical nature, as depicted
by
• This is why the radical polymerization is generally carried
out under a nitrogen atmosphere, i.e. to avoid contact with
atmospheric oxygen.
• A major use of inhibitors, is in the preservation of
monomers, during production and storage.
• Without inhibitors, we cannot think of so many monomers
being transported from one place to another and stored
before actual use.
68

• Before use, monomers have to be freed from the inhibitors. This is
done either by distilling the monomer, when we can get the
inhibitor-free monomer.
• otherwise by washing the monomer with an aqueous solution of
sodium or potassium hydroxide, when the inhibitor is destroyed
and removed by the aqueous phase.
• In industrial practice, inhibitors are not removed prior to
polymerization.
• They are killed by adding further quantities of initiators to the
monomer.
• Inhibitors are also used in the polymer industry for the purpose of
arresting the polymerization beyond a certain conversion (short
stops) so as to achieve a uniform product and avoid cross-linking.
69

• They are normally added towards the end of
polymerization, when they kill all the active radicals and
arrest polymerization.
70

Anionic polymerization
• So we have again, as in cationic polymerization, an
electron pair displacement process, where the strong
negatively charged anion pushes the π electron pair of
the monomer double bond down to sigma level.
• The difference between cationic and anionic
polymerization process
• In cationic polymerization, the movement of the electron pair is in
the opposite direction to that of the chain growth.
• In anionic polymerization, the movement of the electron pair is in
the same direction to that of the chain growth.
71

Contents
• Coordination Polymerization
• Ziegler-Natta Catalyst
72

Chain polymerization reaction –Coordination
Polymerization
• Polymerization reactions especially of olefins and dienes,
catalysed by organo-metallic compounds, fall under the
category of coordination polymerization.
• First step in the polymerization process is the formation of
monomer-catalyst complex between the organo-metallic
compound and the monomer.
73
• where, Mt represents
the transition metals
such as Ti, Mo, Cr, V,
Ni or Rh.

Coordination Polymerization
• In the formation of the monomer-catalyst complex, a
coordination bond is involved between a carbon atom of
the monomer and the metal of the catalyst.
• The coordinated metal-carbon bond formed in the
monomer-catalyst complex acts as the active center from
where the propagation starts.
• The incoming monomer is incorporated in the active
center of the metal-carbon bond, from where the chain
growth starts.
• In coordination polymerization, the catalyst-monomer
complex is a heterogeneous system, with the metal ion in
the solid phase and the carbanion of the alkyl group in the
solvent phase.
74

• The monomer is
inserted in
between the
metal ion and the
carbanion.
• The polymer
chain formed is
pushed out from
the solid catalyst
surface.
75

• Coordination polymerization is also known as insertion polymerization.
76
Here, M1, M2, M3
are first, second,
third monomer
units added to
the polymer
chain.

• Another interesting feature of the coordination polymerization is
that, depending on the polarity of the metal-carbon bond and
on that od the solvent medium, the metal counter-ion is placed
in a particular spatial arrangement with respect to the anion.
• In certain catalyst system, the spatial arrangement as a
tremendous effect on the spatial orientation of the incoming
monomer and also on the manner in which the monomer is
inserted in to the growing chain.
• The specific spatial arrangement of the monomeric unit
inserted in the growing chain imparts stereo-regularity to the
polymer formed.
• By choosing a proper catalyst/solvent system, the coordination
mechanism can be used to formulate highly stereo-regular
polymers.
77

Zieglar-Natta catalysts
• These are special type of coordination catalysts, comprising two
components as against single-component organo-metallic
compounds.
• The two components are generally referred to as the catalyst and
the co-catalyst.
• The catalyst component consists of halides of IV-VIII group
elements having transition valence.
• The co-catalysts are organo-metallic compounds such as alkyls,
aryls and hydrides of group I-IV metals.
• Although there are hundreds of such catalyst-co-catalyst systems,
those based on organo-aluminium compounds such as triethyl
aluminium (AlEt3) or diethyl aluminium chloride (AlEt2Cl) in
combination with titanium chlorides are the most commonly used.
80

• Auminium alkyls acts as electron acceptor
• Titanium halides and the combination acts as electron donor
81

• The active centers, from where the polymer chain growth
propagates, are formed at the surface of the solid phase
of the catalyst complex, and the monomer is complexed
with the metal ion of the active center before its insertion
into the growing chain.
• The actual mechanism by which the propagation takes
place and the factors governing the formation of the
stereo-regular polymers formed a topic of great debate at
one time.
• Among the several mechanism, more attention is received
for
• Bimetallic mechanism of Natta.
• Monometallic mechanism of Cossee.
82

Natta’s Bimetallic mechanism
• When the catalyst and the co-catalyst compounds are mixed,
there occurs a chemisorption of the aluminium alkyl (electro
positive in nature) on the titanium chloride solid surface,
resulting in the formation of an electron-deficient bridge
complex of the structure.
• This complex now acts as the active center. The monomer is
then attracted towards the Ti-C bond (C from the alkyl group
R) in the active center, when it forms a π complex with the Ti
ion.
83

• The bond between R and Ti
opens up, producing an
electron-deficient Ti and a
carbanion at R:
84

• The Ti ion attacks the π electron pair of the monomer and
forms a σ bond, while the counter ion attacks the electron-
deficient center of the monomer.
• The monomer is inserted in to the transition state ring
structure in this way.
85

• The transition state now
gives rise to the chain growth
at the metal-carbon bond,
regenerating the active
center.
• Repeating the whole
sequence, with the addition
of a second monomer
molecule, we will get the
structure of the resultant
chain growth as shown here.
86

• The monomer insertion in this manner and the orientation
of the substituent group of the monomer is always taken
from the metal-ion end, resulting in a stereo-regulated
polymer.
87

Contents
• Average molecular weight
• Number averaged molecular weight
• Weight averaged molecular weight
88

Polymer molecular weight
A polymer comprises molecules of different molecular
weights and hence its molecular weight is expressed in
terms of an average value.
When ethylene is polymerized to form polyethylene, a
number of polymer chain start growing at any instant, but all
of them do not get terminated after growing to the same
size.
The chain termination is a random process and hence each
polymer molecule formed can have a different number of
monomer units and thus different molecular weight.
 A polymer thus be thought of as a mixture of molecules of
the same chemical type, but of different molecular weights.
89

• We can see that three molecules are have different sizes,
their molecular weights are different, and yet, they all are very
much ‘polyethylenes’.
• In this situation, the molecular weight of the polymer sample
can only be viewed statistically and expressed as some
average of the molecular weights contributed by the
individual molecules that make the sample.
90



2 2 500
2 2 550
2 2 600
R [ CH -CH ] R (molecular weight 14,000)

• The two most common and experimentally verifiable
methods of averaging is
• number average method
• weight average method
• In computing the molecular weight of a polymer we can
also use either number fraction or the weight fraction of
the molecules present in the polymer to get either the
number-average molecular weight (designated as Mn) or
the weight-average molecular weight (designated as Mw).
• The method of working out Mn or Mw can now be easily
generated using simple mathematics.
91

• Assume that there are ‘n’ numbers of molecules in a
polymer sample and
92
n1 of them have M1
molecular weights
n2 of them have M2
molecular weights
n3 of them have M3
molecular weights
and so on

• Now, we have a total number of molecules (n) given by
• Similarly, molecular weight contribution by other fraction
will be as follows
93

• Number-average molecular weight of the whole polymer
will then be given by
• Similarly, total weight of the polymer = W = ΣniMi and
Weight of fraction 1 = W1 = n1M1.
94

• Similarly, molecular weight contribution by the other
fractions will be
• The weight-average molecular weight of the whole
polymer will then be
• For all synthetic polymers, Mw is greater than Mn. If they
were to be equal, the polymer sample may be considered
as perfectly homogeneous (i.e., each molecule has the
same molecular weight). But this does not happen.
95

Sedimentation and viscosity-average molecular weight
• Apart from Mn and Mw, there are two other ways of
expressing the molecular weight based on the sedimentation
and flow behavior of the polymer solution.
• They are z-average molecular weight (Ms) and the viscosity-
average molecular weight (Mv), expressed as
96
• In these two equations ni is the number of molecules having the
molecular weight of M1 present in the sample.
• ‘a’ is the variable (its value ranging from 0.5 to 1) in the mark-Houwink
equation which relates intrinsic viscosity (η) with the viscosity average
molecular weight (Mv).

Molecular weight and degree of
polymerization
• The size of polymer molecule depends on the number of
repeat units it contains and that this number represents
the ‘degree of polymerization’.
• For instance, if there are 1000 repeat units in a polymer
molecule, the degree of polymerization (DP) is 1000.
• Both DP and molecular weight are related to the
molecular size. Hence, like the molecular weight, DP can
also be averaged over the size of the sample.
97
where, M is the molecular weight of the polymer
Dp is the degree of polymerization
m is the molecular weight of the monomer
M = DP × m

Molecular weight and degree of
polymerization
• The ‘number-average’ and ‘weight-average’ degree of
polymerization can then be defined in a manner similar to
that in which Mw, Mn are defined earlier.
• Each of these averages can be related to the
corresponding molecular weight averages by the following
two equations.
98
and

Polydispersity and molecular weight distribution
in polymers
• Simple chemical compounds contains molecules, each of
which has the same molecular weight – a
‘monodispersed’ system.
• Polymer contains molecules, each of which can have
different molecular weights – a ‘polydispersed’ system.
103

Polydispersity and molecular weight distribution
in polymers
• While the polydisperse nature of the polymer is the basis of the
concept of ‘average’ molecular weight, this ‘average’ by itself
conveys nothing on the dispersity pattern in a given polymer
sample.
• Take a polymer sample of, say, 40,000 average molecular
weight.
• Now, this may mean that the molecules have molecular weight
ranging from 20,000 to 80,000 or from 500 to 100,000-the
figure is uncertain.
• This is the reason why two polymer samples of the same
40000 Mn can display similar properties in some aspects but
not in some others.
• To know a polymer property, we must have a knowledge of
both the average molecular weight as well as the dispersion
pattern.
104

Polydispersity and molecular weight
distribution in polymers
• The dispersity with respect to the lowest to the highest
molecular weight homologues is expressed by a simple
molecular weight distribution curve.
• Such a curve for a polymer sample is computed by
plotting the number fraction (ni) of molecules having a
particular molecular weight (Mi) against the corresponding
molecular weight.
• Such a curve reveals the pattern of the different molecular
species present in a polymer sample.
• It may be noted that Mw is greater than Mn and that Mv is
closer to Mw than Mn.
105

106

• Molecular weight distribution in two samples having the same
number-average molecular weight but different polydispersities.
107
• Sample 1 obviously
has a narrower
dispersion pattern and
hence lower
polydispersity than
sample 2.

• Polydispersity is a very important parameter that gives an
idea of the lowest and the highest molecular weight
species as well as the distribution pattern of the
intermediate molecular weight species.
• To quickly ascertain the degree of polydispersity, we can
determine Mw and Mn by two different experimental
methods and then compute Mw/Mn.
• This ratio is indicative of the extent of polydispersity - for
all synthetic polymers, the ratio is higher than 1.
• As the molecular weight distribution becomes broader the
value of Mw/Mn increases.
108

• The 3 curves show the dispersity ratio increasing for the 3 samples with the same
Mn, but a different molecular weight distribution.
109
• Polydispersity arises
owing to a variation in the
degree of polymerization
attained by different
molecules during the
polymerization process.
• One would, therefore,
except polymers obtained
by different polymerization
techniques to show
different polydispersities.

Mw/Mn values for synthetic polymers obtained by different
polymerization techniques
110

Practical significance of polymer
molecular weight
• Many commercially useful polymers are selected on the basis
of their properties such as
• Melt viscosity
• Impact strength
• Tensile strength
111
• These properties are directly
dependent on the molecular
weight of the polymer or its
degree of polymerization.
 Example: Tensile strength and
impact strengths increases
with molecular weight.

molecular weight
• The melt viscosity of the polymers, however, shows a different trend.
112
• At very high molecular
weights, the melt viscosity
rises more steeply than at
lower molecular weights.
• A commercial useful polymer
should have a low melt
viscosity to permit ease of
processing, but at the same
time, should exhibit good
strength.

molecular weight
• Effect of molecular weight on physical properties –
mechanical strength and degree of polymerization
113
• Every polymer has a threshold
value (TV) for its DP below
which the polymer does not
posses any strength and exists
as a friable powder or as liquid
resin.

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