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Unit I: Polymers:
Introduction:
Polymers are materials made of long, repeating chains of monomers. These chains of monomers are also called
macromolecules. Most polymer chains have a string of carbon atoms as a backbone. A single macromolecule can
consist of hundreds of thousands of monomers. The monomers, into a chain are held together by covalent
bonds. A process of reacting monomer molecules together in a chemical reaction to form polymer chains or
three-dimensional networks is called polymerization. A polymer is composed of many simple molecules that are
repeating structural units called monomers. A single polymer molecule may consist of hundreds to a million
monomers and may have a linear, branched or network structure.
The term polymer is often used to describe plastics, which are synthetic polymers. However, natural polymers
also exist. Rubber and wood, for example, are natural polymers that consist of a simple hydrocarbon, isoprene.
Proteins are natural polymers made up of amino acids and nucleic acids (DNA and RNA) are polymers of
nucleotides.
Polymers are abundant in nature, found in all living systems and materials such as wood, paper, leather, natural
fibers have found extensive use. While natural polymers retain their intrinsic importance, today synthetic
materials are mostly used. They have unique properties, depending on the type of molecules being bonded and
how they are bonded. Some polymers bend and stretch, like rubber and polyester. Others are hard and tough, like
epoxies and glass. The physical properties of a polymer such as its strength and flexibility depend on:
(i) Chain Length:
The strength of polymers depends on the chain length. In general, longer the chains stronger will be the polymer.
(ii) Side Groups:
Polar side groups (including those that lead to hydrogen bonding) give stronger attraction between polymer
chains, making the polymer stronger.
(iii) Branching:
Straight, un-branched chains can pack together more closely than highly branched chains, giving polymers that
have higher density, are more crystalline and therefore stronger.
(iv) Cross-linking:
If polymer chains are linked together extensively by covalent bonds, the polymer is harder and more difficult to
melt.
History Polymeric Materials:
Polymeric materials have been used from prehistoric times. The term polymer was coined in 1833 by Jöns Jakob
Berzelius. The first man-made polymer, formed by chemical modification of natural materials, was produced in
the second half of the nineteenth century. Friedrich Ludersdorf and Nathaneil Haward discovered that adding
sulfur to raw natural rubber (poly-isoprene) helped prevent the material from becoming sticky. In 1844 Charles
Goodyear received a U.S. patent for vulcanizing natural rubber with sulfur and heat. Thomas Hancock had
received a patent for the same process in the UK the year before. This process strengthened natural rubber and
prevented it from melting with heat without losing flexibility. This made practical products such as waterproofed
articles possible. Vulcanized rubber represents the first commercially successful product of polymer research.
In 1884 Hilaire de Chardonnet started the first artificial fiber plant as a substitute for silk, but it was very
flammable. In 1907 Leo Baekeland invented the first synthetic plastic, a thermosetting phenol-formaldehyde resin
called Bakelite. Fully synthetic polymers were developed in the twentieth century, most in the period 1950-1970s
(so-called plastics of modern society). Henri, along with Christian Schönbein and others, developed derivatives of
the natural polymer cellulose, producing new semi-synthetic materials, such as celluloid and cellulose acetate.
Classification of Polymers:
1. On the Basis of Thermal Response:
On the basis of thermal response, polymers can be classified into two groups:
(a) Thermoplastic Polymers:
Thermoplastic polymers can be re-melted back into a liquid
plasticized by heating and then solidified by cooling
properties. Think of thermoplastics as butter which can be melted and cooled multiple times to form various
shapes. When you heat thermoplastics
thermoplastic chains are held together by
speaking, thermoplastic chains are clumped together like a ball of tangled yarn. These are 100% recyclable.
Examples of such polymers are polyolefins, nylons, linear polyesters and polyethers, PVC, s
Most linear and slightly branched polymers are thermoplastic.
Major thermoplastics are produced by
are held together by relatively weak intermolecular forces
when exposed to heat and then returns to its original condition when cooled.
wide range of applications because they can be formed and reformed in so many shapes.
Some examples are food packaging, insulation, automobile bumpers and credit cards.
(b) Thermosetting Polymers or Thermosets
Thermosetting polymers or thermosets
Therefore they cannot be reshaped by heating
permanent solid state because they undergo certain chemical changes on heating and
convert themselves into an infusible mass
process involves chemical reaction resulting
linking (permanent chemical bonds) between polymer chains
the chains and leads to a rigid material.
rubbers, polyester resins, silicon resins, polyurethanes
and are used in automobiles and construction
boat hulls and glues, insulation parts, car parts, etc
2. On the Basis of Structure:
Bases on the structure, polymers are classified as:
(a) Linear Polymers:
Linear polymers are simplest polymer i
units are linked together to form long straight chains.
These are closely packed and hence have high densities, high melting point and high tensile strength.
nylon, polyethene, etc. When formed, these line
be very strong and hard to break through.
1. On the Basis of Thermal Response:
On the basis of thermal response, polymers can be classified into two groups:
melted back into a liquid. Therefore they can be repeatedly softened
by heating and then solidified by cooling on application of thermal energy, without much change in
as butter which can be melted and cooled multiple times to form various
thermoplastics, the molecules do not chemically bond with each other. Instead,
thermoplastic chains are held together by Vander Waal forces, weak attractions between molecules. Visually
speaking, thermoplastic chains are clumped together like a ball of tangled yarn. These are 100% recyclable.
Examples of such polymers are polyolefins, nylons, linear polyesters and polyethers, PVC, s
polymers are thermoplastic.
chain polymerization. Molecules in such polymers
intermolecular forces so that the material softens
when exposed to heat and then returns to its original condition when cooled. They have a
wide range of applications because they can be formed and reformed in so many shapes.
ng, insulation, automobile bumpers and credit cards.
or Thermosets:
thermosets cannot be repeatedly softened by heating.
Therefore they cannot be reshaped by heating. These polymers always remain in a
because they undergo certain chemical changes on heating and
convert themselves into an infusible mass (i.e., solidifies or sets irreversibly). The setting
resulting in three-dimensional networked by cross-
between polymer chains which restricts the motion of
the chains and leads to a rigid material. E.g., phenolic resins, urea epoxy resins, diene
polyester resins, silicon resins, polyurethanes, etc. These are strong and durable
in automobiles and construction and also are used to make toys, varnishes,
insulation parts, car parts, etc.
structure, polymers are classified as:
are simplest polymer in which monomeric
units are linked together to form long straight chains.
These are closely packed and hence have high densities, high melting point and high tensile strength.
When formed, these linear polymers can create strands of fibers or form a mesh that can
be very strong and hard to break through.
Linear Chain Branched Chain
. Therefore they can be repeatedly softened or
on application of thermal energy, without much change in
as butter which can be melted and cooled multiple times to form various
, the molecules do not chemically bond with each other. Instead,
aal forces, weak attractions between molecules. Visually
speaking, thermoplastic chains are clumped together like a ball of tangled yarn. These are 100% recyclable.
Examples of such polymers are polyolefins, nylons, linear polyesters and polyethers, PVC, sealing wax, etc.
so that the material softens
They have a
wide range of applications because they can be formed and reformed in so many shapes.
repeatedly softened by heating.
always remain in a
because they undergo certain chemical changes on heating and
setting
restricts the motion of
epoxy resins, diene
trong and durable
also are used to make toys, varnishes,
These are closely packed and hence have high densities, high melting point and high tensile strength. Examples:
ar polymers can create strands of fibers or form a mesh that can
Thermoplastics
Thermosets
Branched Chain Cross-linked Chain
(b) Branched Chain Polymers:
In these polymers, monomers are linked together to form a main chain and side chains of different lengths arises
from the main chain thus forming branches. These polymers are irregularly packed and have low tensile strength,
low density and low melting points. Examples: amylopectin, glycogen, low density polyethene, etc.
(c) Cross Linked Polymers or Network Polymers:
In these polymers, the monomer units are linked together to form a three dimensional network. The monomers
are formed from bi-functional and tri-functional monomers and contain strong covalent bonds between various
linear polymer chains. These polymers are hard, rigid and brittle in nature. Examples: Melamine, Bakelite, etc.
3. On the Basis of Polymerization Mechanism:
Polymerization mechanism is the sequence of elementary chemical reactions by which polymerization (process
of converting monomer molecules into a polymer) proceeds. Based on polymerization mechanism, polymers can
be classified into two groups:
(a) Addition or Chain-Growth Polymerization:
Addition polymerization is the type of polymerization in which the similar or different monomers repeatedly
added to form a polymer (called addition polymer) without the co-generation of other products. When we keep
adding monomers to obtain a large chain, such a process is also called as the chain growth polymerization. The
monomers are unsaturated compounds. For example: alkenes, alkynes, alkadienes and their derivatives. E.g.,
(vinyl chloride) (polyvinyl chloride)
The mode of polymerization reaction in this case is governed by: free radicals or ion species. There are four types
of addition polymerizations which are: Free adical polymerization, Cationic polymerization, Anionic
polymerization and Coordination polymerization.
(i) Free Radical Polymerization Reaction:
A polymerization where a free radical is formed causing a chain reaction. For example: Polymerisation of vinyl
chloride to give polyvinyl chloride:
325-350 K/13 atm
CH2=CHCl [-CH2-CHCl-]n
(vinyl chloride) (polyvinyl chloride)
Mechanism:
Initiation Step:
It can be initiated by using a radical initiator, such as peroxide or certain azo compounds. On heating the initiator
molecule decomposes into a free radical (I.). Some common initiators are presented below:
Propagation Step:
Free radical (I.) combines with the monomer and forms a new radical for the chain propagation. The formed
radical reacts with another monomer and generate a new radical which propagates the chain. The chain
propagation process determines the length of a polymer chain.
Termination Step:
The macro-free radicals are deactivated by recombination of free radicals. There are three ways to terminate the
propagating chain either by coupling (with another growing free radical) or by disproportionation reaction or by
reaction with an impurity (such as oxygen) or by solvent.
The coupling is caused as a result of reaction of the growing free radical chain with the other growing free radical.
The disproportionation is caused by the acceptance of one hydrogen free radical by one free radical from the
other which is converted to an alkene or alkene derivative.
(ii) Cationic Polymerization Reaction: It is a polymerization where a cation intermediate is is generated by the addition of a Lewis acid such as BF3, AlCl3,
etc. with the monomer typically alkene causing a chain reaction. It results in forming a long chain of repeating
monomers. The cation formed in the initiation step reacts with another monomer and generate a new cation, this
process is repeated until chain termination occurs.
Chain propagation can be affected by three ways either by loss of a proton or by addition of a nucleophile or by
reacting with the solvent molecule.
(iii) Anionic Polymerization Reaction
Anionic polymerization occurs by the addition of a nucleophile
new anions, this process occurs only when the nucleophilicity of the initiator nucleophiles are strong enough to
attack the electron rich olefins. Similarly, if electron withdrawing substituents are attach
increases the rate of addition. Here the chain propagation step was terminated by reaction of generated
nucleophiles with impurity or solvent molecules.
(iv) Coordination Polymerization Reaction
This method was invented by two scientists Ziegler and Natta who won a Nobel Prize for their work. They
developed a catalyst which let us control the free radical polymerization. It produces a polymer which has more
density and strength.
(b) Condensation or Step-Growth P
This type of reaction involves the repetitive condensation between two bi
some simple molecules such as water and alcohol due to the condensation mechanism. Condensation mechanism
refers to combining of smaller molecules to obtain larger molecules.
Reaction:
Anionic polymerization occurs by the addition of a nucleophile initiator with the monomer alkene and generates a
new anions, this process occurs only when the nucleophilicity of the initiator nucleophiles are strong enough to
attack the electron rich olefins. Similarly, if electron withdrawing substituents are attach
increases the rate of addition. Here the chain propagation step was terminated by reaction of generated
nucleophiles with impurity or solvent molecules.
Reaction:
two scientists Ziegler and Natta who won a Nobel Prize for their work. They
developed a catalyst which let us control the free radical polymerization. It produces a polymer which has more
Polymerization:
This type of reaction involves the repetitive condensation between two bi-functional monomers. There is a loss of
some simple molecules such as water and alcohol due to the condensation mechanism. Condensation mechanism
ules to obtain larger molecules. Examples: Nylon 66, Dacron, etc.
Terylene
initiator with the monomer alkene and generates a
new anions, this process occurs only when the nucleophilicity of the initiator nucleophiles are strong enough to
attack the electron rich olefins. Similarly, if electron withdrawing substituents are attached to the olefin bond
increases the rate of addition. Here the chain propagation step was terminated by reaction of generated
two scientists Ziegler and Natta who won a Nobel Prize for their work. They
developed a catalyst which let us control the free radical polymerization. It produces a polymer which has more
functional monomers. There is a loss of
some simple molecules such as water and alcohol due to the condensation mechanism. Condensation mechanism
Nylon 66, Dacron, etc.
Terylene
In these types of polymerization reaction, the product that is formed after every step is again a
species and so these species undergo a sequence of condensation process. At each step we have distinct
functional species and every species is independent of the other species and so we call this process as the step
growth polymerization. Example: ethylene glycol and terephthalic acid.
types of polymerization reaction, how the polymerization of monomers take place and the addition and
condensation mechanism.
Polymer Structure:
The properties of polymers are strongly influen
the sequence of monomer units in the case of copolymers, the stereochem
geometric isomerization in the case of diene
(a) Copolymers:
Polymers are composed of many simple molecules that are repeating
bonds hold the atoms in the polymer
of polymer chains together to form the polymeric material.
different types of monomer species. The
polymers, ter-polymers and quarter-polymers, respectively.
There are many commercially relevant copolymers. Some examples include
styrene (ABS), styrene/butadiene co-polymer
(SIS) and ethylene-vinyl acetate (formed by
polyester family (formed by step-growth polymerization
the glass transition temperature (Tg), which is i
be classified based on how these units a
copolymers, random or statistical copolymers
(i) Block Copolymers:
Block copolymers comprise two or more
homopolymer subunits may require an intermediate non
copolymers with two or three distinct blocks are called
Technically, a block is a portion of a macromole
feature which is not present in the adjacent portions.
copolymer might be ~A-A-A-A-A-A-A-B-B
Homopolymer
Block copolymers are made up of blocks of different
poly(methyl methacrylate) or PS-b-PMMA (where b = block) is usually made by first polymerizing
subsequently polymerizing methyl methacrylate
polymer is a diblock copolymer because
multiblocks, etc. can also be made.
(ii) Alternating Copolymers:
Alternating copolymers have regular alternating A and B units and is
B-)n-. Molar ratio of each monomer in
of styrene maleic anhydride copolymer, most chains
chains ending in maleic anhydride add a styrene unit. This leads to a predominantly alternating structure.
In these types of polymerization reaction, the product that is formed after every step is again a
species and so these species undergo a sequence of condensation process. At each step we have distinct
functional species and every species is independent of the other species and so we call this process as the step
ethylene glycol and terephthalic acid. So far we have seen that there are two
types of polymerization reaction, how the polymerization of monomers take place and the addition and
trongly influenced by the chain structure, the overall chemical composition and
the sequence of monomer units in the case of copolymers, the stereochemistry or tacticity of the chain
geometric isomerization in the case of diene-type polymers for which several synthesis routes may be possible.
composed of many simple molecules that are repeating structural units called monomers. Covalent
polymer molecules together and secondary bonds th
chains together to form the polymeric material. Copolymers are obtained by
e copolymers obtained from two, three and four monomers are called
polymers, respectively.
There are many commercially relevant copolymers. Some examples include: acrylonitrile butadiene
polymer (SBR), nitrile rubber, styrene-acrylonitrile, styrene
formed by chain-growth polymerization) and nylon 66
growth polymerization). Copolymerization is particularly useful in tuning
), which is important in the operating conditions of polymers.
be classified based on how these units are arranged along the chain, as: block copolymers,
statistical copolymers and graft copolymers.
comprise two or more homo-polymer subunits linked by covalent bonds. The union of the
subunits may require an intermediate non-repeating subunit, known as a
with two or three distinct blocks are called diblock copolymers and triblock copolymers
Technically, a block is a portion of a macromolecule, comprising many constitutional units, that has at least one
feature which is not present in the adjacent portions. A possible sequence of repeat units A and B in a triblock
B-B-B-B-B-B-A-A-A-A-A~.
Block copolymer
Block copolymers are made up of blocks of different polymerized monomers. For example, polystyrene
PMMA (where b = block) is usually made by first polymerizing
methyl methacrylate (MMA) from the reactive end of the polysty
because it contains two different chemical blocks. Triblocks, tetrablocks,
gular alternating A and B units and is described by formula:
ar ratio of each monomer in polymer is normally close to one. E.g., in free-radical copolymerization
copolymer, most chains ending in styrene add a maleic anhydride unit
chains ending in maleic anhydride add a styrene unit. This leads to a predominantly alternating structure.
Alternating copolymer
In these types of polymerization reaction, the product that is formed after every step is again a bi-functional
species and so these species undergo a sequence of condensation process. At each step we have distinct
functional species and every species is independent of the other species and so we call this process as the step
So far we have seen that there are two
types of polymerization reaction, how the polymerization of monomers take place and the addition and
the overall chemical composition and
istry or tacticity of the chain and the
for which several synthesis routes may be possible.
units called monomers. Covalent
molecules together and secondary bonds then hold groups
obtained by polymerization of
three and four monomers are called bi-
acrylonitrile butadiene
, styrene-isoprene-styrene
nylon 66, and the co-
is particularly useful in tuning
mportant in the operating conditions of polymers. Copolymers can
block copolymers, alternating
subunits linked by covalent bonds. The union of the
repeating subunit, known as a junction block. Block
triblock copolymers, respectively.
cule, comprising many constitutional units, that has at least one
A possible sequence of repeat units A and B in a triblock
. For example, polystyrene-b-
PMMA (where b = block) is usually made by first polymerizing styrene and then
(MMA) from the reactive end of the polystyrene chains. This
it contains two different chemical blocks. Triblocks, tetrablocks,
y formula: -A-B-A-B-A-B- or -(-A-
radical copolymerization
ene add a maleic anhydride unit and almost all
chains ending in maleic anhydride add a styrene unit. This leads to a predominantly alternating structure.
A step-growth copolymer -(-A-A-B-B-)n-
is in principle a perfectly alternating copolymer of these two monomers, but is usually considered as
a homopolymer of the dimeric repeat unit A
(CH2)6-NH-, formed from a dicarboxylic acid
(iii) Random or Statistical Copolymers
Statistical copolymers are the simplest type of copolymer
copolymerized. In such copolymers the sequence of monomer residues follows a statistical rule.
commonly used to modify and/or improve the mechanical and physical properties of many polymers.
Ran
Examples of commercially relevant random copolymers include
copolymers and resins from styrene-acrylic or
(iv) Graft Copolymers:
Graft copolymers consist of a main polymer chain or backbone (A) covalently bonded to one or more side chains
(B). These are a special type of branched copolymer in which the side chains are structurally distinct from the
main chain. The individual chains of a graft copolymer may be homopolymers or copolymers. For
example, polystyrene chains may be grafted onto
C=C double bond per repeat unit. The polybutadiene is dissolved in styrene, which is then subjected to
radical polymerization. The growing chains can add across the double bonds of rubber molecules forming
polystyrene branches. The graft copolymer is formed in a mixture with un
molecules.
As with block copolymers, the quasi-composite
polystyrene chains grafted onto polybutadiene
is much less brittle than ordinary polystyrene. The product is called
(b) Tacticity (Configuration) of Polymers
Tacticity (relating to arrangement or order
a macromolecule. It is particularly significant in
unit with a substituent R on one side of the polymer backbone is followed by th
substituent on the same side as the previous one, the other side as the previous one or positioned randomly with
respect to the previous one. In a hydrocarbon macromolecule with all carbon atoms making up the backbone in
a tetrahedral molecular geometry, the zigzag backbone is in the paper plane with the substituents either sticking
out of the paper or retreating into
Natta. Monotactic macromolecules have one stereoisomeric atom per repeat unit,
macromolecules have more than one stereoisomeric atom per unit.
The practical significance of tacticity rests on the effects on the physical properties of the
of the macromolecular structure influences the degree to which it has rigid,
flexible, amorphous long range disorder.
geometric arrangement of the functional (side) groups.
polymer (stereoregular) and Atactic polymer (
(i) Isotactic Polymers:
In isotactic polymers all the substituents are located on the same side of the polymer chain (macromolecular
backbone). They consist of 100% meso diads.
formed by the condensation of two bifunctional monomers A
perfectly alternating copolymer of these two monomers, but is usually considered as
of the dimeric repeat unit A-A-B-B. An example is nylon 66 with repeat unit
dicarboxylic acid monomer and a diamine monomer.
(iii) Random or Statistical Copolymers:
he simplest type of copolymers where two or more co-monomers are simultaneously
the sequence of monomer residues follows a statistical rule.
commonly used to modify and/or improve the mechanical and physical properties of many polymers.
Random copolymer
Examples of commercially relevant random copolymers include: rubbers made from styrene
acrylic or methacrylic acid derivatives.
of a main polymer chain or backbone (A) covalently bonded to one or more side chains
are a special type of branched copolymer in which the side chains are structurally distinct from the
ain. The individual chains of a graft copolymer may be homopolymers or copolymers. For
chains may be grafted onto polybutadiene, a synthetic rubber which retains one reactive
. The polybutadiene is dissolved in styrene, which is then subjected to
. The growing chains can add across the double bonds of rubber molecules forming
polystyrene branches. The graft copolymer is formed in a mixture with un-grafted polystyrene chains and rubber
Graft copolymer
composite product has properties of both components
polybutadiene, the rubbery chains absorb energy when the substance is hit, s
is much less brittle than ordinary polystyrene. The product is called high-impact polystyrene
(Configuration) of Polymers:
ting to arrangement or order) is the relative stereochemistry of adjacent
is particularly significant in vinyl polymers of the type -H2C-CH(R)-
R on one side of the polymer backbone is followed by the next repeating unit with the
substituent on the same side as the previous one, the other side as the previous one or positioned randomly with
respect to the previous one. In a hydrocarbon macromolecule with all carbon atoms making up the backbone in
, the zigzag backbone is in the paper plane with the substituents either sticking
the paper. This projection is called the Natta projection
ecules have one stereoisomeric atom per repeat unit,
have more than one stereoisomeric atom per unit.
rests on the effects on the physical properties of the polymer
of the macromolecular structure influences the degree to which it has rigid, crystalline
long range disorder. There are three different types of polymers depending upon the relative
of the functional (side) groups. These are: Isotactic polymer (stereoregular),
Atactic polymer (stereo irregular).
all the substituents are located on the same side of the polymer chain (macromolecular
of 100% meso diads. and orient in very closely dense fashion. This permits effective
monomers A–A and B–B
perfectly alternating copolymer of these two monomers, but is usually considered as
with repeat unit -OC-(CH2)4-CO-NH-
monomers are simultaneously
the sequence of monomer residues follows a statistical rule. This technique is
commonly used to modify and/or improve the mechanical and physical properties of many polymers.
made from styrene-butadiene
of a main polymer chain or backbone (A) covalently bonded to one or more side chains
are a special type of branched copolymer in which the side chains are structurally distinct from the
ain. The individual chains of a graft copolymer may be homopolymers or copolymers. For
which retains one reactive
. The polybutadiene is dissolved in styrene, which is then subjected to free-
. The growing chains can add across the double bonds of rubber molecules forming
grafted polystyrene chains and rubber
properties of both components. In the example of
the rubbery chains absorb energy when the substance is hit, so it
impact polystyrene or HIPS.
of adjacent chiral centers within
where each repeating
e next repeating unit with the
substituent on the same side as the previous one, the other side as the previous one or positioned randomly with
respect to the previous one. In a hydrocarbon macromolecule with all carbon atoms making up the backbone in
, the zigzag backbone is in the paper plane with the substituents either sticking
Natta projection after Giulio
ecules have one stereoisomeric atom per repeat unit, ditactic to n-tactic
polymer. The regularity
crystalline long range order or
There are three different types of polymers depending upon the relative
(stereoregular), Syndiotactic
all the substituents are located on the same side of the polymer chain (macromolecular
orient in very closely dense fashion. This permits effective
formation of inter-polymer forces of attraction to give a cohesive polymer system and thus a useful fiber. These
are usually semi-crystalline and often form a helix configuration. Example: Polypropylene formed by Ziegler–Natta
catalysis. polypropylene and pure acrylonitrile.
Isotactic poly(vinyl chloride)
(ii) Syndiotactic Polymers:
In syndiotactic polymers or syntactic macromolecules the substituents have regular on alternate positions along
the polymeric chain. This regular polymer structure allows close enough alignment or orientation to form enough
inter-polymer forces of attraction, giving a cohesive enough system to form a useful fiber. They consist 100% of
racemo diads. Syndiotactic polystyrene, made by metallocene catalysis polymerization, is crystalline with
a melting point of 161 °C. Example: The polymers of cellulose and some chloro-fibers, Gutta percha, etc.
Syndiotactic polystyrene
(iii) Atactic Polymers:
In atactic polymers the substituents are placed randomly along the chain. The percentage of meso diads is
between 1 and 99%. These are formed by free-radical mechanisms such as polyvinyl chloride. Due to their random
nature these are usually amorphous. In hemi-isotactic polymers every other repeat unit has a random
substituent. These are technologically very important. A good example is polystyrene (PS). If a special catalyst is
used in its synthesis it is possible to obtain the syndiotactic version of this polymer, but most industrial
polystyrene produced are atactic.
Atactic polypropylene
Effect of Tacticity on the Properties of Polymers:
The tacticity of a polymer chain can have a major influence on its properties. Isotactic and syndiotactic polymers
are considered as stereospecific or stereoregular while atactic polymers are viewed as random or stereo–
irregular. The overall molecular symmetry and crystallinity are in the order isotactic > syndiotactic >> atactic.
Isotactic polymers are generally characterized by high rigidity substance, high melting temperature (Tm) and high
mechanical properties with relatively high resistance to solvents and chemicals and excellent resistance to
mechanical stress. Atactic polymers being more disordered cannot crystallize. For example atactic polypropylene
is a soft, rubbery material with no commercial value.
(iii) Geometric Isomerism:
When there are unsaturated sites along a polymer chain, several different isomeric forms are possible. The 1,3-
butadiene can be polymerized to give 1,2-poly(1,3-butadiene) or either of two geometric isomers of 1,4-poly(1,3-
butadiene).
The numbers preceding the poly prefix designate the first and last carbon atoms of the backbone repeating unit.
1,2-poly(1,3-butadiene) has a vinyl-type structure, where the substituent group (ethene) contains an unsaturated
site. Therefore, this geometric isomer can be atactic, syndiotactic or isotactic. In the case of the commercially
more important 1,4-poly(1,3-butadiene), all four carbons in the repeating unit lie along the chain. Carbons 1 and 4
can lie either on the same side of the central double bond (i.e., cis-configuration) or on the opposite side
(i.e., trans-configuration). The structure of polybutadiene used in SBR rubber (i.e., a copolymer of styrene and
butadiene) is principally the trans-1,4 isomer with some cis-1,4- and 1,2-poly(1,3-butadiene) content.
(iv) Nomenclature of Polymers:
(i) Naming of Vinyl Polymers:
When the monomer name consists of one word vinyl then the polymers are designated by attaching the
prefix poly to the monomer name (e.g., polystyrene, polyethylene and polypropylene). However, when the
monomer name consists of more than one word or is preceded by a letter or number, the monomer name is
enclosed by parentheses preceded by the prefix poly. For example, the polymer obtained from the polymerization
of 4-chlorostyrene is poly(4-chlorostyrene) and that from vinyl acetate is poly(vinyl acetate). Tacticity may be
noted by prefixing the letter i (isotactic) or s (syndiotactic) before poly as in i-polystyrene. Geometric and
structural isomers may be indicated by using the appropriate prefixes, cis or trans and 1,2- or 1,4-, before poly, as
in trans-1,4-poly(1,3-butadiene).
(ii) Naming of Non-vinyl Polymers:
Non-vinyl polymers such as condensation polymers usually named according to the initial monomer or the
functional group of the repeating unit. For example, the most important commercial nylon, commonly called
nylon-6,6 (66 or 6/6), is more descriptively called poly(hexamethylene adipamide) denoting the polyamidation of
hexamethylenediamine (alternatively called 1,6-hexane diamine) with adipic acid. Similarly, the aliphatic nylon
obtained by the polyamidation of hexamethylenediamine with a 10-carbon dicarboxylic acid, sebacic acid, is
nylon-6,10 or poly(hexamethylene sebacamide).
In some cases, common names are used almost exclusively in place of the more chemically correct nomenclature.
For example, the polycondensation of phosgene and bisphenol-A—the common name for 2,2-bis(4-
hydroxyphenyl)propane produces the thermoplastic, polycarbonate. Often, the prefix bisphenol-A is placed
before polycarbonate to distinguish it from other polycarbonates that can be polymerized by using bisphenol
monomers other than bis-phenol-A, such as tetramethylbisphenol-A.
The IUPAC name for polystyrene is poly(1-phenylethylene) and that for polytetrafluoroethylene (tradename,
Teflon) is poly(difluoromethylene). The IUPAC name for the polycarbonate of bisphenol-A mentioned earlier is
poly(oxycarbonyloxy-1,4-phenyleneisopropylidene-1,4-phenylene).
A very useful set of two-, three and four-letter abbreviations for the names of many common thermoplastics,
thermosets, fibers, elastomers and additives have been developed. Sometimes, abbreviations for the same
polymer may vary, but there is widespread agreement on the abbreviations for a large number of important
polymers. These abbreviations are convenient and widely used. Examples: PS abbreviation for polystyrene, PVC
for poly(vinyl chloride), PMMA for poly(methyl methacrylate), PTFE for polytetrafluoroethylene and PC for bis-
phenol-A polycarbonate.
Following IUPAC recommendations, copolymers are named by incorporating an italicized connective term
between the names of monomers contained within parentheses or brackets or between two or more polymer
names. The connective term designates the type of copolymer as indicated for six important classes of
copolymers in following Table.
Scheme for Naming Copolymers
Type Connective Example Type Connective Example
Unspecified -co- Poly[styrene-co-(methyl
methacrylate)]
Alternating -alt- Poly[styrene-alt-(maleic
anhydride)]
Statistical -stat- Poly(styrene-stat-butadiene) Block -block- Polystyrene-block-polybutadiene
Random -ran- Poly[ethylene-ran-(vinylacetate)] Graft -graft- Polybutadiene-graft-polystyrene
Molecular Weight of Polymers:
Introduction:
Polymers are long chain molecules produced by linking small repeat units (monomers) together. There are many
ways to link different types of monomer to form polymers. They exhibit very different physical properties
compared to the monomers, dependent on the length of the polymer chains. The presence of small amounts of
very long or very short chains can have drastic effects on properties of the material. Therefore knowledge of the
molecular weight of polymers is very important because the physical properties of polymer molecules are
affected by their molecular weight. The interrelation between molecular weight and strength for a typical polymer
is given as:
Unlike simpler pure compounds, most polymers (mainly synthetic) are not composed of identical molecules. The
HDPE molecules, for example, are all long carbon chains, but the lengths may vary by thousands of monomer
units. Because of this, polymer molecular weights are usually given as averages. Molecular weight of a polymer is
defined as sum of the atomic weight of each of the atoms in the molecules, which is present in the polymer. Two
experimentally determined values are common:
(i) The number average molecular weight (Mn) (ii) The weight average molecular weight (Mw)
(i) Number Average of Molecular Weight (Mn):
Number average molecular weight is measured to determine number of molecules in given sample of the
polymer. It is the ratio of total weight (W) of all the molecules with the total number of polymer molecules (N)
present in a polymer sample. It is calculated as:
Numberaveragemolecularweight���� =��������� ��!���� �"���#$��%
�����&$"'�(�!���� �"���#$��%=
)
*+ =
*+,�,
*+,
Where, W is the total mass, N is the the total no. of molecules, Ni is the no. of molecules of mass Mi. If N1, N2,
N3,…… etc. be the molecules with molecular masses M1, M2, M3,……etc. respectively, then:
Total mass of N1 molecules = N1M1
Total mass of N2 molecules = N2M2
Total mass of N3 molecules = N3M3
ΣNiMi = N1M1 + N2M2 + N3M3 + ......
ΣNi = N1 + N2 + N3 + ......
Therefore,
�� = +-�-.+/�/.+0�0.⋯
+-.+/.+0.⋯
Characteristics of Number Average Molecular Weight:
• It is determined by measurement of colligative properties.
• It is good index of physical properties such as impact and tensile strength but not for other properties such
as flow.
(ii) Weight Average of Molecular Weight (Mw):
Weight average of molecular weight (Mw) is measured to determine the mass of polymer molecule. It is the ratio
of product of the total weight fraction (Wi) and mass of all the molecules (Mi) with the total weight fraction of
polymer molecules (Wi) present in a polymer sample. It is calculated as:
Weightaveragemolecularweight��)� =��������� �3!(�#���&�!���� �"���#$��%4"���#$��("�%%�!���� �"���#$��%
�����&$"'�(�!���� �"���#$��%
�) =*),�,
*), =
*�+,�,�5�,
*+,�, =
*+,�,/
*+,�,
If N1, N2, N3,……etc. be the molecules with molecular masses M1, M2, M3,……etc. respectively, then:
�) =+-�-
/ .+/�//.+0�0
/.⋯…
+-�-.+/�/.+0�0.⋯
Characteristics of Weight Average Molecular Weight:
• It is determined by measurement of light scattering and ultra-centrifugation techniques which measure
molecular size.
• Mw is always greater than Mn (except in mono-disperse system in which all molecules have identical
molecular mass).
Chemical Structures and Thermal
Transitions of Polymers:
Semi-crystalline polymers have both
amorphous and crystalline regions. The
amorphous regions can be either in the glassy
or rubbery state. Individual chains in
amorphous region are randomly coiled and
interwined with no molecular order or
structure. Commercial grade (atactic)
polystyrene and poly(methylmethacrylate) is
an examples of amorphous in solid state.
At a low temperature the amorphous regions of a polymer are in the glassy
state. In this state the molecules are frozen on place. They may be able to
vibrate slightly, but do not have any segmental motion in which portions of
the molecule wiggles around. When the amorphous regions of a polymer
are in the glassy state, it generally will be hard, rigid and brittle. The glassy
state is similar to a super cooled liquid where the molecular motion is in the
frozen state. This glassy state is also analogous to a crystalline solid with
molecular disorder as a liquid.
When the polymer is heated, the polymer chains are able to wiggle (to move up) around each other. This state is
called the rubbery state. At this state the polymer becomes soft and flexible similar to rubber. The temperature at
which the glassy state changes to rubbery state is called the glass transition temperature (Tg). The glass
transition is a property of only the amorphous portion of a semi-crystalline solid. The crystalline portion remains
crystalline during the glass transition, which is not the same as melting. For example the chewing gum at body
temperature is soft and pliable, which is characteristic of an amorphous solid in the rubbery state. If you put a cold
drink in your mouth or hold an ice cube on the gum, it becomes hard and rigid. The Tg of the gum is somewhere
between 0oC and 37
oC.
Below Tg, long range cooperative motions of individual chains cannot occur. However short-range motion
involving several contiguous groups along the chains backbone or substituent group is possible. Such motions are
called secondary-relaxation processes and can occur at temperatures as low as 70K. By comparison, Tg vary from
150K for polymers with very flexible chains such as polydmethylsiloxane [-Si(CH3)2-o-] so well over 600K for those
with highly rigid aromatic backbones such as the high-modulus fiber, poly[2,2’-(m-phenylene)-5,5’-
bibenzimidazole], PBI, with a Tg in the range from 700-773K.
Thermodynamic transitions are classified as being first or second-order. In a first-order transition there is a
transfer of heat between system and surroundings and the system undergoes an abrupt volume change. In a
second-order transition, there is no transfer of heat, but the heat capacity does change. The volume changes to
accommodate the increased motion of the wiggling chains, but it does not change discontinuously.
Comparison between Glass Transition and Melting
S. No. Glass Transition Melting
1.
2.
3.
4.
Property of the amorphous region
Below Tg, disordered amorphous solid with immobile molecules
Above Tg, disordered amorphous solid in which portions of
molecules can wiggle around
A second order transition
Property of the crystalline region
Below Tg, ordered crystalline solid
Above Tm, disordered melt
A first-order transition
Problems for Practice:
1. Calculate the Mn and MW for a polymer sample comprising of 5 moles of polymer molecules having
molecular weight of 45000 g/mol, 10 moles of polymer molecules having molecular weight of 55000 g/mol
and 2.5 moles of polymer molecules having molecular weight of 38000 g/mol.
2. In a polymer sample comprising of 5 moles of polymer molecules has molecular weight of 40000 g/mol and
15 moles of polymer molecules having molecular weight of 30 000 g/mol. Calculate the Mn and MW for a
polymer sample.
3. Calculate the MW for a polymer sample comprising of 9 moles of polymer molecules having molecular
weight of 30000 g/mol and 5 moles of polymer molecules having molecular weight of 50000 g/mol.
4. Calculate Mn and MW of an equimolecular (i.e. equimolar) mixture of samples of molecular masses of
50000 and 100000.
Ans:
5. How are polymers classified on the basis of polymerization process?
Ans: On the basis of mode of polymerisation, polymers are classified into the following groups:
Addition polymers or chain growth polymers: 1)A polymerformed by direct addition of repeated monomers
without the elimination of byproduct molecules is called addition polymer.
6. What is polymer Tacticity?
Tacticity (from Greek τακτικός taktikos "of or relating to arrangement or order") is the relative
stereochemistry of adjacent chiral centers within a macromolecule. The practical significance
of tacticity rests on the effects on the physical properties of the polymer.
7. What is the difference between Atactic isotactic and syndiotactic polymers?
Ans: Isotactic and syndiotactic polymers provide long-range order, which leads to higher crystallinity in
the polymer chain. Polypropylene is a great example of howtacticity has a dramatic effect on the physical
properties of the polymer. Atactic polypropylene has little order in the polymer backbone and is
amorphous.
8. What are syndiotactic polymers?
Ans: In chemistry of industrial polymers: Organometallic catalysis. Isotactic and syndiotactic polymers are
referred to as stereoregular—that is, polymers having an ordered arrangement of pendant groups along the
chain. A polymer with a random orientation of groups is said to be atactic.
9. Are all polymers amorphous?
Ans: The structure of a polymer is defined in terms of crystallinity. ... A well-ordered polymer is considered
crystalline. The opposite is an amorphous polymer. Molecular arrangements Polymers – the materials often
referred to as plastics, elastomers or rubber – are made up of long chains of molecules.
10. Why do polymers have average molecular weight?
Ans: Unlike simpler pure compounds, most polymers are not composed of identical molecules. ... Since
larger molecules in a sample weigh more than smaller molecules, the weight average Mw is necessarily
skewed to higher values.
11. What is addition polymerization reaction?
Ans: An addition polymer is a polymer that forms by simple linking of monomers without the co-generation
of other products. Addition polymerization differs from condensation polymerization, which does co-
generate a product, usually water. ...Addition polymers are formed by the addition of some simple
monomer units repeatedly.
12. How does addition polymerisation differ from Condensation polymerization types of polymerisation?
Ans: These are mostly radical based polymerization, for those monomers with double bonds.
Condensation polymerization, as a contrast, normally involves the generation of small molecule products,
like water. ... Meanwhile, water is generated. It looks like two molecules "condense" with each other to
form this polymer.
13. Why is this polymerization reaction known as addition polymerization?
Ans: Addition polymerization occurs by a chain reaction in which one carbon-carbon double bond adds to
another. Monomers continue to react with the end of the growing polymer chain in an addition
polymerization reaction until the reactive intermediate is destroyed in a termination reaction.
14. What is the mechanism of polymerization?
Ans: A polymerization mechanism is the sequence of elementary chemical reactions by which
polymerization (the process of converting monomer molecules into a polymer) proceeds.
15. What is average molecular weight of polymer?
Ans: It is important to be able to characterize the polymer structure. Determining the weight-average
molecular weight or the number-average molecular weight is a part of any polymer characterization. As an
example, a polyvinyl chloride molecule may have a molecular weight of 35,000 amu (or 35,000 g/mol).
16. What is number average molecular weight?
Ans: The number average molecular weight is not too difficult to understand. It is just the total weight of
all the polymer molecules in a sample, divided by the total number of polymer molecules in a sample.
17. Why isotactic and syndiotactic polymers are referred to as stereoregular while atactic stereoirregular?
18. What is number average molecular weight? How is it measured?