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POLYMER CHEMISTRY (Basic Concept)

Reference Books: 1. P. C. Jain & M. Jain 2. Sunita Rattan 3. Shashi Chawla

1. Introduction and Definitions

The word polymer is derived from two Greek words:

polus (meaning many or much) → poly and meros (meaning units or pairs) → mer

The poly & mer add-up to constitute the term polymer, which literally means many units. The
term polymer was coined by Swedish Scientist Joens Jacub Berzelius, in the year 1833.

So by definition, Polymers are "macromolecules" (molecules having high molecular weights)

which are built-up by linking together a large numbers of small molecules (called
monomers). Thus the small molecules which combine with each other to form the polymer
chain are called the monomers: and the "repeating units" in a polymer chain is called mer.

Examples:

 Polythene = Poly + Ethene

H H H H
n C C C
C n
H H H H
Ethene or Ethylene Polythylene (PE)
Monomer Polymer

 Polystyrene = Poly + Styrene (1-phenylethene)

H
H H
C H C
H C C n
H
n

Styrene Polystyrene (PS)

Monomer Polymer

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Difference between Polymers and Macromolecules
Polymers are also called macromolecules due to their large size but converse is not always
true.

A macromolecule may or may not contain monomer units or in other words it can simply
be a large molecule of higher molecular mass consisting of no repeating units. For example,
chlorophyll (C55H72O5N4Mg) is a macromolecule but not a polymer since there are no
monomer units present. Therefore, we can conclude that all polymers are macromolecules
while all macromolecules may not be polymers in nature.

Polymerization and Degree of polymerization (DP)

 The Process or reaction through which monomers are converted into the polymer
chains is called polymerization. In general, the process of polymerization may be
represented as:

polymerization
n M M n
monomer polymer

The process of polymerization is generally effected under the influence of heat,

pressure, catalyst etc.

The Degree of Polymerization (DP) or n is defined as the number of repeating units

present in a given polymer chain. This is also termed as the chain length of polymer.

DP = M / m where, M = Mol. Wt. of the polymer chain

m = Mol. Wt. of the repeating unit
Polymer vs Oligomer

An oligomer is a complex molecule that is made out of only few monomer units. The main
difference between oligomer and polymer is that oligomers are formed due to the
polymerization of a few monomers whereas polymers are giant molecules formed due to
the polymerization of a large number of monomers. As a result the molecular mass of the
oligomers are generally much lower compared to that of the polymers.

2. Types of Polymers
The polymers can be classified in a number of ways. Some of these are discussed below.

(a) Classification of Polymers on the Basis of source:

On the basis of origin, polymers are classified as:

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1. Natural Polymers: The polymers obtained from nature (plants and animals) are
called natural polymers. Starch, cellulose, natural rubber, proteins, etc. are some
examples.

2. Synthetic Polymers: The polymers which are prepared in the laboratories are called
synthetic polymers. These are also called man-made polymers. Polyethene, PVC, nylon,
Teflon, bakelite, terylene, synthetic rubber, etc. are common examples.

(b) Classification of Polymers on the Basis of structure:

On the basis of structure of polymers, these can be classified as:

1. Linear polymers: These are polymers in which monomeric units are linked together to
form linear chains. These linear polymers are well packed and therefore, have high
densities, high tensile (pulling) strength and high melting points. For example,
polyethelene, nylons and polyesters are examples of linear polymers.

2. Branched chain polymers: These are polymers in which the monomers are joined to
form long chain with side chains or branches of different lengths. These branched chains
polymers are irregularly packed and therefore, they have lower densities, lower tensile
strength and melting points than linear polymers. For example, low density polyethene,
glycogen, starch, etc.

3. Cross-linked polymers: These are polymers in which long polymer chains are cross
linked together to form a three dimensional network. These polymers are hard, rigid
and brittle because of the network structure. Bakelite, melamine and formaldehyde
resin are some examples of this type.

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(c) Classification of Polymers on the Basis of Method of Polymerization/Synthesis:

On the basis of method of polymerization the polymers are classified as:

1. Addition polymer: A polymer formed by direct addition of repeated monomers without

the elimination of any small molecule is called addition polymer. In this type, the
monomers are unsaturated compounds and are generally derivatives of ethene. The
addition polymers have the same empirical formula as their monomers. Examples are
polyethene, polypropylene and polyvinyl chloride, etc.
H
H H
C H C
H C C n
H
n

Styrene Polystyrene (PS)

Monomer Polymer

2. Condensation polymer: A polymer formed by the condensation of two or more than

two monomers with the elimination of simple molecules like water, ammonia, hydrogen
chloride, alcohol, etc. is called condensation polymer. In this type, each monomer
generally contains two functional groups. For example, nylon – 66 is obtained by the
condensation of two monomers; hexa methylenediamine and adipic acid with the loss
of water molecules. Examples of condensation polymers are Nylon 66, terylene,
bakelite, alkylresins, etc.

HO OH H H
n H N + n C 6 C N N
2 6 NH2 - 2n H2O 6
O O 6
O O
n
Hexamethylene diamine Adipic acid Polyhexamethylene adipate (Nylon 6,6)
(diamine) (diacid) (Polyamide)

(d) Classification of Polymers on the Basis of Molecular Forces

Depending upon the intermolecular forces between monomer molecules, the polymers have
been classified into four types.

1. Elastomers: In case of elastomers the polymer chains are held together by weak van der
waals forces. Due to weak forces, the polymers can be easily stretched on applying small
stress and they regain their original shape when the stress is removed. This is due to the
presence of few- ‘cross links’ between the chains, which help the polymer to retract to

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its original position after the force is removed, as in vulcanized rubber. The most
important example of elastomer is natural rubber.

2. Fibres: These are the polymers which have strong intermolecular forces between the
chains. These forces are either hydrogen bonds or dipole-dipole interactions. Because of
the strong forces, the chains are closely packed, giving them high tensile strength and
less elasticity. These polymers can be drawn into long, thin and threadlike fibres and
therefore can be woven into fabrics. The common examples are nylon-66, dacron, silk,
etc.

3. Thermoplastics: These are linear polymers with very few cross linkages or no cross
linkages at all. The polymeric chains are held by weak van der waals forces and slide
over one another. Due to lack of cross linkages these polymers soften on heating and
harden or become rigid on cooling. Thus they can be molded to any shape. Polythene,
PVC, polystyrene are addition type thermoplastics and Terylene, nylon are condensation
type thermoplastics.

Plasticizers: Certain plastics do not soften much on heating. These can be easily
softened by the addition of some organic compounds which are called plasticizers. For
example, polyvinyl chloride (PVC) is very stiff and hard but is made soft by adding di-n-
butylphthalate (a plasticizer). Some other common plasticizers are dialkyl phthalates
and cresyl phthalate.

4. Thermosetting polymers: Usually thermosetting polymer can be heated only once when
it permanently sets into a solid which cannot be remelted and remolded. Thermosetting
polymers are produced from relatively low molecular mass semi fluid polymers (called
polymers) which on heating develop extensive cross-linking by themselves or by adding
some cross-linking agents and become infusible and insoluble hard mass. The cross links
hold the molecules in place so that heating does not allow them to move freely.
Therefore, a thermosetting plastic is cross linked and is permanently rigid. The common
examples are bakelite, melamine, formaldehyde resin, etc.

Thermoplastic Polymers Thermosetting Polymers

These are formed by addition These are formed by condensation
polymers polymerization
These are generally linear structures These have three dimensional cross-
linked structures.
They soften and then melt on These do not soften on heating,
heating. On cooling down they rather become hard. That means the
harden again. That means the process is irreversible.
process is reversible.
These can be remolded, recast and These cannot be remolded or

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reshaped. reshaped.
These are less brittle and are soluble These are more brittle and insoluble
in some organic solvents. in organic solvents.
Examples: Polythene, PVC, Teflon, Examples: Bakelite, Terylene,
Nylon, Polypropylene, Polyacrylo- Melamine, Formaldehyde resins and
nitrile. epoxy polymers.

(e) Classification of Polymers based on the Monomers

Based on the types of monomers the polymers can be divided into two types:

1. Homopolymers: Homopolymer is made from only one type of monomer unit. In the
repeating structural unit of a homopolymer, only one type of monomer unit is present.
E.g., Polythene, Polystyrene, Teflon, Nylon-6, etc.
H H H H
n C C C
C n
H H H H
Ethene or Ethylene Polythylene (PE)
Monomer Polymer
2. Copolymers: On the other hand, copolymer is made from two (or more) types of
monomer units. In the repeating structural unit of a copolymer, two (or more) types of
monomer unit are present. E.g., Nylon-6,6, Styrene-butadiene polymer, etc.
H2 H2 H
C C C
H2C CH H2C CH CH m C C n
m + n H H2
Ph HC CH2
Ph
styrene butadiene polystyrene-co-butadiene
or Styrene-butadiene rubber

(f) Classification of polymers based on the stereochemistry

Depending on the tacticity (i.e. spatial distribution of the substituents attached to the
asymmetric carbon atoms in the polymer chain), polymers can be classified as:

1. Isotactic polymers: In these polymers the side chains/group are oriented on the same
side of the main polymer chain having regular and definite order as shown below.
R R R R

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2. Syndiotactic polymers: In these polymers the side chains/groups regularly alternate on both
side of the polymer chain as shown below.

R R R R

3. Atactic polymers: In these polymers the groups / substituents are arranged in an irregular,
random fashion as shown below.

R R R R R R

The physical properties of a polymer chain depend upon its configuration.

Polymers having isotactic and syndiotactic configuration are more likely to be
crystalline solids because the regular arrangement of substituents in them allows
for a more regular packing arrangement. Polymers having atactic configuration
are more disordered and cannot pack together as well, so these polymers are
softer, less rigid and does not have any substantial commercial usage.

(g) Classification of polymers based on the types of atoms present in the polymer chain

Based on the type of atoms in the polymer, we can divide the polymers into two types:

1. Organic polymers: These are obtained from organic molecules. E.g., polyethylene,
natural rubber.
2. Inorganic rubbers: Polymers that are obtained from inorganic monomers are called
inorganic polymers. E.g., silicates, silicones, polyphosphazene, etc.

3. Types of Polymerization
Depending of the type of reaction (mode of polymerization) involved, polymerization reactions
can be divided mainly into two broader types: (a) Addition or chain-reaction polymerization
and (b) Condensation or step-reaction polymerization. Apart from these two there is another
type (third one) of polymerization reaction which is more of a specialized type of addition
polymerization and is called (c) Copolymerization.

Let us start from Condensation Polymerization. Then we will talk about co-polymerization.
Finally we discuss the addition polymerization in detail.

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Condensation or step growth-polymerization
Condensation polymerization is brought about by monomers containing two or more reactive
sites with the elimination of small molecules like H2O, HCl, ROH etc. The reaction is called (step
growth) condensation polymerization and the product formed is called condensation polymer.
The process involves the elimination of by product molecules; therefore, the molecular mass of
the polymer is not the integral multiple of the monomer units.
In this type polymerization the byproduct of the reaction is called condensate. Examples of
some polymer linkages formed by condensation pathway are given below in the table.

Functional Linkage Polymer Type Examples

Groups
-OH + -COOH -C(=O)-O- Polyester Polyethylene
terephthalate
(terylene)
-NH2 + -COOH -NH-C(=O)- Polyamide Nylon-6,6
-OH + -NCO -CO-NH- Polyurethane Spandex fibre

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(c) Copolymerization: Copolymerization is a polymerization of two or more monomeric
species together. It is very common in the nature, e.g., in the polypeptides which may
contain as many as 20 different amino acids.
H2 H2
C C
H2C CH H2C CH CH m CH n
m + n
CN Cl Cl
CN
acrylnitril vinylchloride Polyacrylonitrile-co-vinyl chloride
H2 H2 H
C C C
H2C CH H2C CH CH m C C n
m + n H H2
Ph HC CH2
Ph
styrene butadiene polystyrene-co-butadiene
or Styrene-butadiene rubber

The resultant copolymers possess some advantageous properties which are required for
end use. For examples, styrene-butadiene rubber (SBR) has both rigidity and toughness. In
addition, it has very good oil and petrol resistance.
The properties of copolymers depend on the type of monomers and how they link together
in copolymers. For any two random monomers A and B, depending on their linking, the
following four types of co-polymers are possible:

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Addition or Chain growth Polymerization

Addition polymerization is characterized by self-addition of the monomer molecules to each

other, without the loss of any small molecules (or material). The final product or the polymer is
exact multiple of the monomer.
Some examples of everyday used addition polymers are given in the table below.

Monomer Polymer Polymer Name Applications

CH2=CH2 H H Polyethylene or Plastic bags, bottles, toys,
C polythene electrical insulation
C n
H H
CH2=CHCH3 H H Polypropylene Carpeting, bottles, luggage,
C exercise clothing
C n
H Me
CH2=CHCl H H Polyvinyl chloride Bags for intravenous solutions,
C Or PVC pipes, tubing, floor coverings
C n
H Cl
CF2=CF2 F F Polytetrafluoroethylene Nonstick coatings, electrical
C Or teflon insulations
C n
F F

Addition polymerization can also take place by the occurrence of ring opening, for example:

H H2H
O H3C C C OH
CH3OH + n+1H C O C n C
2 CH2
H2
H H
In this case the methanol attacks the epoxide ring and opens it easily by taking the advantage of
the high ring strain in the epoxide. The product of the ring opening / addition is again an
primary alcohol with nucleophilicity to attack and open another epoxide ring. The reaction goes
on like this in an addition / chain reaction pathway, until there is epoxide left in the reaction
mixture.

Any types of Vinyl compounds (CH2=CHR), allyl compounds (CH2=CHCH2R), olifins (CH2=CHR)
and dienes (CH2=CH-CH=CH2) typically undergo addition polymerization.

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This process involves the addition of monomer units to themselves to form a growing chain by a
chain reaction mechanism. It is for this reason that the process is also known as chain growth
polymerization. Addition polymerization is achieved by adding a catalyst (known as initiator),
which provides some reactive species like free radicals, cation or anions.

Generalized mechanism: Addition polymerization pathway has three distinct stages, in general.

(i) Chain initiation step: Interaction of the monomer (M) with the initiator (In) for its
activation and generation of the reactive particle (M*).
In
M M*
(ii) Chain propagation step: In this step the reactive particle interacts and reacts with
other monomer molecules in a series of consequetive reactive reactions carry the
chain-growth process.
M* + M M*1
M*1 + M M*2
M*2 + M M*3

M*(n-1) + M M*n
(iii) Chain termination step: The polymer chain growth is terminated in this step.
M*n Mn
Here the M*n is activated growing polymer chain and Mn is inactive polymer molecule.
The reactive particle / species (M*) can be a free radical, cation or anion and accordingly
addition polymerization can be classified as:
(a) Free radical polymerization
(b) Cationic Polymerization
(c) Anionic Polymerization.

There is another type of addition polymerization which is called the Coordination

polymerization. This will be discussed separately.

(a) Free radical polymerization: Here the monomer is activated by its transformation into a
free radical by the action of light (photochemically), heat (thermally), radiatively (α,β or γ
radiations) or by adding chemicals, known as initiators.
Initiators are compounds which readily decompose into free radicals and form the reactive
species (which is also free radical) by interacting with a monomer. Some well known
initiators and their way of generating radicals are shown below:

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O O
O Ph 2 2Ph + 2 CO2
Ph O Ph O
O
Benzoylperoxide Bezoyl radical phenyl radical

CH3
NC CH3
CH3
60 oC H3C
N N 2 C + N2
CH3
CH3 CN
NC
Azobis iso butyronirile (AIBN)
CH3 CH3 O
H3C H3C
O CH3 2 2 + 2 CH3
H3C O H3C O H3C CH3
CH3
CH3

ditert-butylperoxide

Here the in general reaction can be written as:

In 2R

Where In is the initiator and R• is the radical. So the three steps of the mechanism in this case
are as follows:

(i) Chain Initiation:

In 2R
R
R + H2C CH H2C CH
SG SG
reactive species
SG (Substituent group) = H, CH3, Cl, CN, Ph, etc.
(ii) Chain propagation:

R H2 H H2
H2C CH + n H2C CH R C C C CH
n
SG SG SG SG

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The chain propagation steps determine,
a. The rate of polymerization
b. The molecular weight of the polymer
c. The structure of the polymer.

(iii) Chain termination: In this step the reactive polymer chain is converted into stable
polymer by virtue of chain termination.
The chain can be termination in several ways, as follows:
• Recombination, where two radical merge together.

H H2 H
C CH + C CH C C C C
H2 H2 H2
SG SG SG SG

H
C CH + C C R
R H2
H2 SG
SG
• Reaction with solvent molecules like CCl4.

H
C CH + CCl4 C C Cl + CCl3
H2 H2
SG SG

2 CCl3 Cl3C CCl3

• Disproportination, where there is transfer of a H atom from one chain to another.

C CH + C CH C CH2 + H2C C
H2 H2 H2
SG SG SG SG
• Chain inhibitors like hydroquinone, trinitrobenzene or oxygen can also terminate the chain.

 Branching and cross linking during free radical polymerization takes place via back-biting,
through which the radical migration takes place from the end to the middle of the marcoradical chain.
This leads to transfer of the active center from the growing end to the middle of the chain. Then from
the middle active point the branching takes place. For cross linking takes place via recombination of two
macroradical chain having active center in the middle. Therefore free radical polymerization produces
more branching. It makes LDPE (Low density Polyethylene) at high pressure, as branching reduces the
density.

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back-biting

+
H2C CH
SG
SG branching

cross linking

(b) Cationic polymerization: Monomers with electron donating substituents (like OMe, OEt,
Ph, etc) Undergo cationic polymerization in the presence of Lewis acids (like BF3, AICI3, SnCI4,
etc.) and protonic acids such as H2SO4, HF, etc. The reaction goes through the formation of a
carbocation. This takes place at a very high rate, even at very low temperature. It takes place in
the following steps (polymerization of isobutene is taken as an example here):
(i) Initiation:

(ii) Propagation:

(iii) Termination:

(c) Anionic polymerization: Monomers with electron withdrawing substituents (CN, COOR,
Ph) prefer to polymerize via anionic pathway in the presence of alkali metals (Na, K) or
Grignard reagents or any strong bases. Thus strong bases are typical catalysts.
The mechanism is explained by considering the anionic polymerization of acrylonitrile in the
presence of KNH2.

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(i) Initiation:

(ii) Propagation:

(iii) Termination:

(d) Coordination addition polymerization: These polymerizations are catalyzed by Ziegler

Natta Catalysts formed by mixing AlEt3 and TiCl4. A coordination complex is formed between these
two, in which the ethyl group is coordinated to Titanium. This is the active catalyst:

Mechanism:

Activation of Ziegler-Natta catalyst

It is necessary to understand the catalyst’s structure before understanding how this catalyst
system works. Herein, TiCl4+AlEt3 catalyst system is taken as an example. The titanium chloride
compound has a crystal structure in which each Ti atom is coordinated to 6 chlorine atoms. On
the crystal surface, a Ti atom is surrounded by 5 chlorine atoms with one empty orbital to be
filled. When Et3Al comes in, it donates an ethyl group to Ti atom and the Al atom is coordinated
to one of the chlorine atoms. Meanwhile, one chlorine atom from titanium is kicked out during

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Addition polymerization vs condensation polymerization

Addition/Chain polymerization Condensation/ Step polymerization

1 It requires the presence of double It requires two reactive functional groups to
bond in the monomer, e.g., ethylene, be present at both ends of the monomer.
vinyl chloride, styrene etc. There should be at least two different bi- or
poly functional monomers having functional
groups with affinity for each other.
2 No by product is formed during the Generally a bi-product is formed here. E.g.
course this type of polymerization. H2O, HCl, ROH etc.
3 The growth of the chain is at one The growth of chain occurs at minimum of
active center. two active centers.
4 Empirical formula of the polymer Empirical formula of the polymer chain is
chain is same as that of the monomer. different than that of the monomer. This is
due to the fact that some small molecules
are eliminated during the course of the
polymerization.
5 At any instance only the monomer No monomer. Mixtures of dimers, trimers
and the polymer is present. and tetramers etc. are present.
6 Examples of polymers made: Examples of polymers made: Nylon, terylene,
Polyethylene (PE), Polyvinylchloride silicones.
(PVC), Polystyrene (PS).

3. Thermodynamics of Polymerization:

For any reaction to be spontaneous, the free energy of the reaction mixture (ΔG) of the
reaction should be negative.

ΔG = ΔH - T ΔS
Where the ΔG is the enthalpy of the polymerization reaction
And ΔG is the entropy of the polymerization (= entropy of a polymer – entropy of a monomer)

And T is the temperature of polymerization.

Here generally the ΔH is negative since the polymerization reactions are exothermic. But the
ΔS is also negative for polymerization. ΔS = SP – SM = Negative.

This makes the TΔS term negative. As a result the ΔG is negative only below a certain temperature.
If the T rises beyond that temperature then the ΔG is positive or in other others the TΔS becomes
higher in values than the ΔH term.

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At this particular temperature the ΔH = TΔS, or the ΔG = 0. Here at this temperature the
polymerization reaction is at a state of dynamic equilibrium. This temperature is called the Ceiling
Temperature (Tc). Beyond this temperature there is no polymerization happening.

Similarly, there is a lower temperature limit referred to as the floor temperature (Tf) below which
no appreciable reaction occurs. The optimum temperature for polymerization reactions fall
between the Tf and Tc.

4. Polymer and crystallinity

The structure of a polymer is described in terms of crystallinity. Usually most of the polymers
are in amorphous in nature or semicrystalline. And crystalline solids have a sharp melting
point while amorphous ones do not have a sharp melting point.

Amorphous vs Crystalline Polymer:

• Amorphous polymer do not have uniformly packed molecules, crystalline polymers have
uniformly packed molecules.
• Amorphous polymer do not have sharp melting point, but crystalline polymers have sharp
melting pt.
• Amorphous polymers are transparent, whereas, crystalline polymers are opaque or
translucent.
• Amorphous polymers have poor chemical resistance, but crystalline polymers have good
chemical resistance.
• Amorphous are soft but crystalline polymers are hard in nature.

A. Glass Transition Temperature (Tg):

Polymers with amorphous structure exhibit a transition from hard, brittle, glassy state to
rubbery/viscous state upon heating. This happens due to the fact that upon heating the
polymer chains can move easily and can slide over each other. This enables them to show
rubber like properties. The temperature at which this transition from glassy, hard, brittle

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state to the rubbery state takes place is called the Glass transition temperature of that
polymer (Tg). The glass transition phenomenon is a reversible and gradual phenonmenon.
Upon cooling the viscous/rubbery state again turns back into the hard, brittle and glassy
state.
In the rubbery state the polymer chains adopts random and coiled conformation as a
result of free rotation along single covalent bonds in the polymer chain. The molecules
can stretch and expand upon application of stress in the rubbery state. Upon release of
the stress they again coil-up.
Whether a given polymer behaves like a glass or rubber depends on whether the room
temperature is higher or lower than its Glass Transition Temperature. For example,
polystyrene (Tg= 100 ˚C) behaves like a glass at room temperature, but most rubbers
(polyisoprene, polyisobutylene, etc.) (Tg ‹-20˚C) shows rubbery properties at room
temperature.
Factors affecting the Glass Transition Temperature (Tg):
1. Chain Flexibility: Greater the intrinsic chain flexibility, smaller will be the Tg.
For example,
(i) Tg(PP) › Tg(PE)
Since substitution of H (in PE) by CH3 groups (in PP) hinders the rotation about a C-C
single bond, the Tg is affected by the nature of substitution.
(ii) Tg(PE) › Tg(cis-1,4-polybutadiene)
The inclusion of double bonds stiffen the polymer chain at that particular point of
inclusion but at the same time increase the flexibility of the adjacent bonds across the
chain. This is the way the Tg is affected by the Configuration.
(iii) Tg(PVC) or Tg(Nylon) › Tg(PE)
Secondary interaction among the polymer chains due to some dipole-dipole
interaction or H-bonding reduces the mobility and the flexibility of the polymer chain,
which in turn increases the Tg.
2. Molecular weight: Tg is directly proportional to the molecular weight of the polymer
chain.
3. Cross-Links: The more the degree of cross-linking between the chains, the higher the
Tg, since cross-linking reduces the chain flexibility.
4. Plasticizer: Addition of any plasticizer to the polymer reduces the Tg of the polymer.
This is due to the fact that the added plasticizer causes separation of the chains and
increases their chain mobility.
5. Copolymerization: Random copolymers have lower Tg’s since random
copolymerization tends to promote disorder, reduce molecular packing and also
reduce the interchain forces of attraction.

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B. Crystalline melting point temperature (Tm):
Even a polymer with regularity of molecular structure does not exist entirely in the crystalline
form. This is because of their large size. In fact they have regions of crystallinity, called crystallities,
embedded in amorphous material. The density of a crystalline polymer is much greater than that
of an amorphous material.
Crystallinity of a polymer is characterized by the Crystalline Melting Point Temperature (Tm),
which is defined as the temperature at which sufficient thermal energy is available to disrupt the
ordered structure of the crystalline polymers.

 Factors affecting the Crystalline Melting Point Temperature (Tm):

1. Molecular mass: Polymers are composed of long macromolecular chains. For a specific polymer,
longer chains and larger Molecular mass results in higher Tm.
2. Chemical structure: Tm highly depends on the chemical structure of polymer. For instance,
presence of H-bonding (for instance, in cellulose, Nylons and PET), polar elements (like, —Cl in PVC) and
polar groups (like —CN in polyacrylonitril PAN) dramatically increase Tm.
3. Back bone / Main chain elements: The back bone / main chain of a polymer contain methylene
groups, bulky benzene rings, other elements like oxygen, nitrogen depending on the polymer. The
polymer with methylene groups have very low Tm as methylene groups are very light and flexible
elements so very less thermal energy is required to make the chain move but polymers with bulky
benzene groups require very high energy to make the chain move. Melting point of LDPE (Low Density
Polyethylene) is ~ 118'C whereas melting point of PEEK (Polyether ether ketone) is ~343'C.
4. Primary Bonding: Polymers are made by primary bonds (covalent bond) between the individual
monomer units. If two polymer chains are connected with a primary bond it is known as cross linking.
Cross linking restricts the mobility of a chain as it is attached to another chain by means of a primary
bond. When a polymer is highly cross linked then it becomes non-melting thermoset.
5. Secondary Bonding: Secondary bonding in case of polymers is hydrogen bonding and polar
bonding. Hydrogen bonding is seen in nylons and polyesters polar bonding is seen in polymers like PVC.
Hence to break these secondary bonds it requires more thermal energy, thus the melting point of
Polyamides and Polyesters are very high compared to hydrocarbon polymers like PE.
6. Additives: Addition of certain additives like nucleating induces crystallization in polymers thereby
increasing the melting point of the polymer and certain additives like plasticizer breaks the interaction

Polymer Chemistry Dept. of Chem., NIT Agartala

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between the chains and creates more free volume such that the polymer on very less thermal energy
flows easily without any restriction.

 Effects of crystallinity on the properties of a polymer:

With increased % of crystallinity,

1. Strength and stiffness of a polymer increases alongwith increment in the brittleness.
2. Solubility and permeability of the polymer decreases.
3. Density and melting point of polymer increases.
4. Opacity of the polymer also increases.

5. Molecular weights of Polymers

All the molecules of the polymer sample do not have identical molecular weight, hence polymers
are polydisperse. This is a consequence of the fact that in a polymerization reaction, the length of
the chain formed is determined entirely by random events. In a condensation reaction, it depends
on the availability of a suitable reactive functional group and in addition polymerization reaction, it
depends on the life-time of the chain carriers. So, the product of the polymerization process is a
mixture of chains of different lengths.
As there is a spread in the molecular weight of the product there is no fixed molecular weight. This
is why need a average molecular weight. There are two types of average molecular weights, (1)
Number average molecular weight and (2) Wight average molecular weight.

(1) Number average molecular weight: It is denoted as 𝑴𝑴n. The number average molecular
weight (𝑴𝑴n) is defined as the total weight (w) of all molecules in a polymer sample divided
by the total number of moles present.

A polymer solution of known concentration is made by dissolving a weighed amount of the

polymer in its solvent. The colligative properties of this solution are then determined,
which counts the number of molecules in a given volume or mass. Each molecule makes
equal contribution to the colligative property regardless of their molecular weight or size.
Hence, this method depends on the numbers of molecules present. Hence the molecular
weight average determined by colligative property measurement is called the 𝑴𝑴n. It can be
done by methods like osmometry, cryoscopy (freezing point depression measurement).

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(2) Weight average molecular weight: Weight average molecular weight (𝑴𝑴w) cane be defined
by the following relation,

In the cases of monodisperse systems (Natural polymers and synthetic polymers made by
anionic polymerization), PDI = 1 (unity).
In most of the cases PDI › 1.

𝑴𝑴w is much better indicator of the properties to be expected in a polymer since most
polymer properties such as strength and melt viscosity are determined by the size of the
polymer molecules that make up the bulk of the sample by weight. The utility of 𝑴𝑴n lies
primarily in its use to obtain a indication of polydispersity in a polymer sample by
measuring PDI = 𝑴𝑴w/𝑴𝑴n.

Some commercially useful polymers and their preparation, properties,

uses etc.

Polyethylene:
Polyethylene or polyethene is formed by polymerization of ethylene (CH2 = CH2). It is
manufactured in large quantities and is the most common polymer which you find almost
everywhere.
Polyethene is of two types Low Density Polyethene (LDPE) and High Density Polyethene
(HDPE) depending upon the nature of has branching in polymer chain and is not compact in
polymer molecules. Low density polyethene has branching in polymer chains and is not
compact in packing. While high density polyethere has linear chain of molecules which are
packed in a more compact fashion.

Polymer Chemistry Dept. of Chem., NIT Agartala