Notes-Polymers.pdf

Chemistry Notes

1. UNIT 15
POLYMERS
✓ The word ‘polymer’ is coined from two Greek words: poly means many and mer means unit or part.
✓ Polymers are high molecular mass substances, which are formed by joining of repeating structural units
on a large scale. They are also referred to as macromolecules.
✓ The repeating structural units of polymer are derived from some simple and reactive molecules known
as monomers and are linked to each other by covalent bonds.
✓ The process of formation of polymers from respective monomers is called polymerisation.
✓ Polymers formed by the polymerisation of a single monomeric species are known as homopolymers.
e.g.: Polythene, Polyvinyl chloride (PVC), Polystyrene, Nylon-6, Neoprene etc.
CH
2
CH
2
CH
2
CH
2
[ ]
n
n
polymerisation
Ethene Partial structure of Polythene CH
2
CH
Cl
CH
Cl
CH
2
[ ]
n
n
polymerisation
Partial structure of Polyvinyl chloride (PVC)vinyl chloride
CH
2
CH
C
6
H
5
CH
C
6
H
5
CH
2
[ ]
n
n
polymerisation
Partial structure of PolystyreneStyrene

✓ Polymers formed by the polymerisation of two or more different monomeric species are known as
copolymers. e.g: Nylon-6,6 , Terylene, Bakelite, Buna-S, Buna-N etc.
CH
2
N NH
H
H
H
( )
6 +
OH CCH
2
C OH
O O
( )
4
polymerisation
CH
2
N N
H H
CCH
2
C
O O
( )
6
( )
4
[ ]
n
Hexamethylene diamine Adipic acid Partial structure of Nylon-6,6
n n
Classification of Polymers:
There are several ways of classification of polymers based on some special considerations. The following
are some of the common classifications of polymers:
A. Classification Based on Source:
Under this type of classification, there are three sub categories.
(i) Natural polymers: These polymers are found in plants and animals. Examples are proteins, nucleic
acids, cellulose, starch, some resins and natural rubber.
(ii) Semi-synthetic polymers: These polymers are obtained by chemical modification of natural
polymers. e.g: Cellulose derivatives such as cellulose acetate (rayon) and cellulose nitrate.
(iii) Synthetic polymers: These are man-made polymers extensively used in daily life as well as in
industry. A variety of synthetic polymers as plastic (polythene), synthetic fibres (nylon 6,6) and
synthetic rubbers (Buna - S) are examples of synthetic polymers.

B. Classification Based on Structure of Polymers:
There are three different types based on the structure of the polymers.
(i) Linear polymers: These polymers consist of long and straight chains which are closely packed. Hence
possess high density and high tensile strength. The examples are high density polythene, polyvinyl
chloride, etc. These are represented as:

2. (ii) Branched chain polymers: These polymers contain linear chains having some branches and are less
tightly packed. Hence possess low density. e.g., low density polythene. These are depicted as follows:

(iii) Cross linked or Network polymers: These are usually formed from bi-functional and tri-functional
monomers and contain strong covalent bonds between various linear polymer chains. e.g. bakelite,
melamine-formaldehyde polymer, etc. These polymers are depicted as follows:

C. Classification Based on Mode of Polymerisation:
(i) Addition polymers: The addition polymers are formed by the repeated addition of monomer molecules
possessing double or triple bonds on a large scale. e.g: the formation of polythene from ethene and
polypropene from propene. The addition polymers formed by the polymerisation of a single
monomeric species are known as homopolymers. e.g: polythene. CH
2
CH
2
CH
2
CH
2
[ ]
n
n
polymerisation
Ethene
Partial structure of Polythene(Homopolymer)

The polymers made by addition polymerisation of more than one monomeric species are termed as
copolymers, e.g: Buna-S, Buna-N, etc. CH
2
CHCHCH
2+
CH
2
CH
C
6
H
5
CH
2
CHCH CH
2
CH
2
CH
C
6
H
5
[ ]
n
n n
Polymerisation
1,3-Butadiene Styrene Partial structure of Buna-S (Copolymer)


(ii) Condensation polymers: The condensation polymers are formed by repeated condensation reaction
between bi-functional or tri-functional monomeric units on a large scale with the elimination of small
molecules such as water, alcohol, hydrogen chloride, etc. e.g: terylene (dacron), nylon-6, 6, nylon-6,
etc. For example, nylon 6, 6 is formed by the condensation of hexamethylene diamine with adipic acid.
CH
2
N NH
H
H
H
( )
6 +
OH CCH
2
C OH
O O
( )
4
polymerisation
CH
2
N N
H H
CCH
2
C
O O
( )
6
( )
4
[ ]
n
Hexamethylene diamine Adipic acid Partial structure of Nylon-6,6
n n OH
2
n +
D. Classification Based on Molecular Forces:
The mechanical properties like tensile strength, elasticity, toughness, etc. of polymers are governed by
intermolecular forces, e.g., van der Waals forces and hydrogen bonds, present in the polymer. These
forces also bind the polymer chains. Under this category, the polymers are classified into the following
four sub groups on the basis of magnitude of intermolecular forces present in them.
(i) Elastomers: These are rubber – like solids with elastic properties in which the polymer chains
are held together by the weakest intermolecular forces. These weak binding forces permit the
polymer to be stretched. A few ‘crosslinks’ are introduced in between the chains, which help
the polymer to retract to its original position after the force is released as in vulcanised rubber.
The examples are buna-S, buna-N, neoprene, etc.

(ii) Fibres: Fibres are the thread forming solids with high tensile strength and high modulus, in
which polymeric chains are held together by the strong intermolecular forces like hydrogen
bonding. These strong forces also lead to close packing of chains and thus impart crystalline
nature. The examples are polyamides (nylon 6, 6), polyesters (terylene), etc.

3. (iii) Thermoplastic polymers: These are the linear or slightly branched long chain molecules which
can be repeatedly softened on heating and hardened on cooling, hence can be reused. These
polymers possess intermolecular forces of attraction intermediate between elastomers and
fibres. Some common thermoplastics are polythene, polystyrene, PVC etc.
(iv) Thermosetting polymers: These polymers are cross linked or heavily branched molecules,
which on heating undergo extensive cross linking in moulds and become infusible. These
cannot be reused. In these polymers, the polymeric chains are held together by strong
covalent bonds. Some common examples are bakelite, urea-formaldelyde resins, etc.

E. Classification Based on Growth Polymerisation:
The addition and condensation polymers are nowadays also referred as chain growth polymers and
step growth polymers depending on the type of polymerisation mechanism they undergo during their
formation.
Types of Polymerisation Reactions:
There are two broad types of polymerisation reactions, i.e., the addition or chain growth polymerisation
and condensation or step growth polymerisation.
I. Addition Polymerisation or Chain Growth Polymerisation:
✓ In this type of polymerisation, the molecules of the same monomer or different monomers
containing double or triple bonds undergo repeated addition on a large scale to form a polymer.
✓ This mode of polymerisation leading to an increase in chain length or chain growth can take place
through the formation of either free radicals or ionic species. However, the free radical governed
addition or chain growth polymerisation is the most common mode.
✓ A variety of alkenes or dienes and their derivatives are polymerised in the presence of a free radical
generating initiator (catalyst) like benzoyl peroxide, acetyl peroxide, tert-butyl peroxide, etc. For
example, the polymerisation of ethene to polythene consists of heating or exposing to light a mixture
of ethene with a small amount of benzoyl peroxide initiator.
CH
2
CH
2
CH
2
CH
2
[ ]
n
n
peroxide catalyst
Ethene Partial structure of Polythene
The sequence of steps may be depicted as follows:
Step 1: Chain initiation steps: The process starts with the addition of phenyl free radical formed by the
peroxide to the ethene double bond thus generating a new and larger free radical.

Step 2: Chain propagating steps: The new radical generated reacts with another molecule of ethene to
give bigger sized radical. The repetition of this sequence with new and bigger radicals carries the
reaction forward.

Step 3: Chain terminating steps: For termination of the long chain, these free radicals can combine in
different ways to form polythene. One mode of termination of chain is shown as under:

4. Preparation of some important addition polymers:
a) Polythene: There are two types of polythene as given below:
(i) Low density polythene (LDP):
✓ It is obtained by the polymerisation of ethene under high pressure of 1000 to 2000 atmospheres at a
temperature of 350 K to 570 K in the presence of traces of dioxygen or a peroxide initiator (catalyst).
✓ The low density polythene (LDP) obtained through the free radical addition and H-atom abstraction has
highly branched structure.
✓ Low density polythene is chemically inert and tough but flexible and a poor conductor of electricity.
Hence, it is used in the insulation of electricity carrying wires and manufacture of squeeze bottles, toys
and flexible pipes.
(ii) High density polythene (HDP):
✓ It is formed when addition polymerisation of ethene takes place in a hydrocarbon solvent in the
presence of a catalyst such as triethylaluminium and titanium tetrachloride (Ziegler-Natta catalyst) at a
temperature of 333 K to 343 K and under a pressure of 6-7 atmospheres.
✓ High density polythene (HDP) thus produced, consists of linear molecules and has a high density due to
close packing.
✓ It is also chemically inert and more tough and hard. It is used for manufacturing buckets, dustbins,
bottles, pipes, etc.

b) Polytetrafluoroethene (Teflon or PTFE):
✓ Teflon is manufactured by heating tetrafluoroethene with a peroxide or persulphate catalyst at high
pressure. CF
2
CF
2
[ ]
n
n
peroxide catalyst
Tetrafluorothene Partial structure of Teflon
CF
2
F
2
C
D , High pressure

✓ It is chemically inert and resistant to attack by corrosive reagents.
✓ It is used in making oil seals and gaskets and also used for non – stick surface coated utensils.
✓ Teflon coatings undergo decomposition at temperatures above 300°C.

c) Polyacrylonitrile (PAN):
✓ The addition polymerisation of acrylonitrile in presence of a peroxide catalyst leads to the formation of
polyacrylonitrile. CH
2
CH
CN
[ ]
n
n
peroxide catalyst
CHCH
2
CN
Acrylonitrile Partial structure of PAN
Polymerisation

✓ Polyacrylonitrile is used as a substitute for wool in making commercial fibres such as orlon or acrilan.
✓ Acrylic fibres have good resistance to stains, chemicals, insects and fungi.

II. Condensation Polymerisation or Step Growth polymerisation:
✓ This type of polymerisation generally involves a repetitive condensation reaction between two bi-
functional or tri-functional monomers on a large scale with the elimination of some simple molecules
such as water, alcohol, etc. to form a polymer.
✓ In these reactions, the product of each step is again a bi-functional species and the sequence of
condensation goes on. Since, each step produces a distinct functionalised species and is independent of
each other, this process is also called as step growth polymerisation.

5. Preparation of some important condensation polymers:
a) Polyamides: These polymers possessing amide linkages are important examples of synthetic fibres
and are termed as nylons. The general method of preparation consists of the condensation
polymerisation of diamines with dicarboxylic acids and also of amino acids and their lactams.
(i) Nylon-6,6: It is prepared by the condensation polymerisation of hexamethylenediamine with
adipic acid under high pressure and at 553K. CH
2
N NH
H
H
H
( )
6 +
OH CCH
2
C OH
O O
( )
4
CH
2
N N
H H
CCH
2
C
O O
( )
6
( )
4
[ ]
n
Hexamethylene diamine Adipic acid Partial structure of Nylon-6,6
n n
553K
High pressure

✓ The numbers 6,6 stand for number of carbon atoms in each of the monomers.
✓ Nylon 6, 6 is used in making sheets, bristles for brushes and in textile industry.
(ii) Nylon-6: It is obtained by heating caprolactam with water at 533-543K. Initially, caprolactam
hydrolyses to give amino caproic acid, which then undergoes repetitive condensation reaction to
give Nylon-6. NH
O
H
2
O
533-543K
OH C CH
2
N
O
H
H
( )
5
n n C CH
2
N
O
H
( )
5
[ ]
n
Caprolactam Amino caproic acid
Nylon-6
- n H
2
O

✓ The number 6 indicates the number of carbon atoms in its monomer, caprolactam (lactam = cyclic
amide)
✓ Nylon 6 is used for the manufacture of tyre cords, fabrics and ropes.

b) Polyesters: These are the polycondensation products of dicarboxylic acids and diols. Dacron or
terylene is the best known example of polyesters.
(i) Terylene (Dacron): It is manufactured by heating a mixture of ethylene glycol and terephthalic acid
at 420 to 460 K in the presence of zinc acetate-antimony trioxide catalyst. CH
2
OH CH
2
OH+
n n
Zinc acetate-antimony trioxide
420-460K
[ ]
n
Ethylene glycol
(Ethane-1,2-diol)
Terephthalic acid
(Benzene-1,4-dicarboxylic acid)
Terylene (Dacron)
COH
O
C OH
O
C
O
C
O
OCH
2
CH
2
O

✓ Dacron fibre (terylene) is crease resistant and is used in blending with cotton and wool fibres and also
as glass reinforcing materials in safety helmets, etc.

(ii) Glyptal: It is manufactured by heating a mixture of ethylene glycol and phthalic acid.
✓ Used in the manufacture of paints and lacquers. CH
2
OH CH
2
OH+
n n
[ ]
n
Ethylene glycol
(Ethane-1,2-diol)
Phthalic acid
(Benzene-1,2-dicarboxylic acid)
C COH
OO
OH
D
C
O
OCH
2
CH
2
O C
O
Glyptal

c) Phenol - formaldehyde polymer (Bakelite and related polymers):
✓ Phenol - formaldehyde polymers are the oldest synthetic polymers.
✓ These are obtained by the condensation reaction of phenol with formaldehyde in the presence of
either an acid or a base catalyst.

6. ✓ The reaction starts with aromatic electrophilic substitution to give o-and/or p-hydroxymethylphenol
derivatives which further react to form compounds having rings joined to each other through –CH2–
groups. The initial product could be a linear product – Novolac used in paints.

✓ Novolac on heating with formaldehyde undergoes cross linking to form an infusible solid mass called
bakelite.

✓ It is used for making combs, phonograph records, electrical switches and handles of various utensils.

d) Melamine – formaldehyde polymer:
✓ Melamine formaldehyde polymer is formed by the condensation polymerisation of melamine and
formaldehyde. N
N
N
NH
2
NH
2
NH
2
+
CH H
O
Nucleophilic addition
[

]
n
Melamine Formaldehyde
Resin intermediate
Melamine-formaldehyde polymer
n n n polymerisation
N
N
N
NHN
N
H
H
CH
2
N
N
N
NHN
N
H
H
CH
2
OHH
H

✓ It is used in the manufacture of unbreakable crockery.
Copolymerisation:
✓ Copolymerisation is a polymerisation reaction in which a mixture of more than one monomeric
species is allowed to polymerise and form a copolymer.
✓ The copolymer can be made not only by chain growth polymerisation but by step growth
polymerisation also. It contains multiple units of each monomer used in the same polymeric chain. For
example, a mixture of 1, 3 – butadiene and styrene can form a copolymer. CH
2
CHCHCH
2+
CH
2
CH
C
6
H
5
CH
2
CHCH CH
2
CH
2
CH
C
6
H
5
[ ]
n
n n
Polymerisation
1,3-Butadiene Styrene Partial structure of Buna-S (Copolymer)

✓ Copolymers have properties quite different from homopolymers. For example, butadiene - styrene
copolymer is quite tough and is a good substitute for natural rubber.

7. Rubber: There are 2 types of rubber.
(i) Natural rubber :
✓ Rubber is a natural polymer and possesses elastic properties. It is also termed as elastomer and has a
variety of uses.
✓ It is manufactured from rubber latex which is a colloidal dispersion of rubber in water. This latex is
obtained from the bark of rubber tree and is found in India, Srilanka, Indonesia, Malaysia and South
America.
✓ Natural rubber may be considered as a linear polymer of isoprene (IUPAC name: 2-methyl-1, 3-
butadiene) and is also called as cis - 1, 4 - polyisoprene.

✓ The cis-polyisoprene molecule consists of various chains held together by weak van der Waals
interactions and has a coiled structure. Thus, it can be stretched like a spring and exhibits elastic
properties.
Note: Gutta-percha is trans-1,4 - polyisoprene. It differs from natural rubber in being almost incapable of
vulcanization, and in that it becomes thermoplastic when heated to about 45-60
0
C. Gutta-percha has been
the predominant root canal filling material for over a century.
Vulcanisation of rubber:
Following properties of raw rubber limits its applications. They are-
▪ Natural rubber becomes soft at high temperature (>335 K) and brittle at low temperatures (<283 K).
▪ Shows high water absorption capacity.
▪ It is soluble in non-polar solvents and is non-resistant to attack by oxidising agents.
To improve upon these physical properties, a process of vulcanisation is carried out.
✓ The process of heating a mixture of raw rubber with sulphur and an appropriate additive at a
temperature range between 373 K to 415 K in order to improve its physical properties, is called as
Vulcanisation of rubber.
✓ On vulcanisation, sulphur forms cross links at the reactive sites of double bonds and thus the rubber
gets stiffened.
✓ In the manufacture of tyre rubber, 5% of sulphur is used as a crosslinking agent. The probable
structures of vulcanised rubber molecules are depicted below:


(ii) Synthetic rubber: Synthetic rubber is any vulcanisable rubber like polymer, which is capable of
getting stretched to twice its length. However, it returns to its original shape and size as soon as
the external stretching force is released. Buna-S, Buna-N, Neoprene are some of the examples for
synthetic rubbers.

8. (a) Buna-S: It is obtained by the copolymerisation of 1,3-butadiene and styrene in the presence of
peroxide catalyst. CH
2
CHCHCH
2+
CH
2
CH
C
6
H
5
CH
2
CHCH CH
2
CH
2
CH
C
6
H
5
[ ]
n
n n
Peroxide catalyst
1,3-Butadiene Styrene Buna-S

✓ It is used for the manufacture of autotyres, floortiles, footwear components, cable insulation, etc.

(b) Buna-N: It is obtained by the copolymerisation of 1, 3 – butadiene and acrylonitrile in the presence of
a peroxide catalyst. CH
2
CHCHCH
2+
CH
2
CH
CN
CH
2
CHCH CH
2
CH
2
CH
CN
[ ]
n
n n
Peroxide catalyst
1,3-Butadiene Acrylonitrile Buna-N

✓ It is resistant to the action of petrol, lubricating oil and organic solvents. It is used in making oil
seals, tank lining, etc.

(c) Neoprene: Neoprene or polychloroprene is obtained by the free radical polymerisation of
chloroprene in the presence of peroxide catalyst. CCH
2
CHCH
2
Cl
n
Peroxide catalyst
C CHCH
2
Cl
CH
2
[ ]
n
chloroprene
(2-chloro-1,3-butadiene)
Neoprene or Polychloroprene

✓ It has superior resistance to vegetable and mineral oils. It is used for manufacturing conveyor
belts, gaskets and hoses.
Molecular Mass of Polymers:
✓ Polymer properties are closely related to their molecular mass, size and structure.
✓ The growth of the polymer chain during their synthesis is dependent upon the availability of the
monomers in the reaction mixture. Thus, the polymer sample contains chains of varying lengths and
hence its molecular mass is always expressed as an average. The two ways are-
(i) Number average molecular mass (??????
??????
̅̅̅̅):
If n1, n2, n3.…..are the number of molecules in a polymeric sample with molecular masses M1, M2,
M3…..respectively. Then
M
n
̅̅̅̅ =
??????
1??????
1+??????
2??????
2+??????
3??????
3+⋯
??????
1+??????
2+??????
3+⋯
=
∑??????
????????????
??????
∑??????
??????

(ii) Weight (Mass) average molecular mass:
M
w
̅̅̅̅̅ =
??????
1 ??????
1
2
+??????
2 ??????
2
2
+??????
3 ??????
3
2
+⋯
??????
1??????
1+??????
2??????
2+??????
3??????
3+⋯
=
∑??????
?????? ??????
??????
2
∑??????
????????????
??????

✓ The degree of heterogeneity of a polymer sample is expressed as polydispersity index (PDI). It is given
by,
PDI =
M
w
̅̅̅̅̅
M
n
̅̅̅̅̅

▪ If M
n
̅̅̅̅̅ = M
w
̅̅̅̅̅ , the sample is monodisperse.
▪ If M
n
̅̅̅̅̅ < M
w
̅̅̅̅̅ , the sample is polydisperse.
✓ In general, the natural polymers are monodisperse, as all the polymeric chains are identical in size.
Hence, PDI = 1
✓ Synthetic polymers are polydisperse, as they have varying chain lengths. Hence, PDI > 1.
Biodegradable polymers:
✓ Polymers which undergo environmental degradation by microorganisms are called biodegradable
polymers. e.g. Natural polymers (biopolymers) such as starch, cellulose etc are biodegradable.

9. ✓ A large number of synthetic polymers are quite resistant to the environmental degradation processes
and are thus responsible for the accumulation of polymeric solid waste materials. Such polymers are
called as non-biodegradable polymers. e.g: Polythene, polypropene etc.
These solid wastes cause acute environmental problems and remain undegraded for quite a long time.
✓ In view of the general awareness and concern for the problems created by the polymeric solid wastes,
certain new biodegradable synthetic polymers have been designed and developed.
✓ Biodegradable synthetic polymers contain functional groups similar to the functional groups present in
biopolymers. Aliphatic polyesters are one of the important classes of biodegradable polymers. Some
important examples are given below:

a) Poly- β –hydroxy butyrate – co- β -hydroxy valerate (PHBV): (Aliphatic polyester)
✓ It is obtained by the copolymerisation of 3-hydroxybutanoic acid (β –hydroxy butyric acid) and 3 -
hydroxypentanoic acid (β -hydroxy valeric acid).
CCH
2
CHOH OH
OCH
3
+
CCH
2
CHOH OH
OC
2
H
5
CCH
2
CHO O
OCH
3
CCH
2
CH O
OC
2
H
5
[ ]
n
3-Hydroxybutanoic acid 3-Hydroxypentanoic acid
PHBV
n n

✓ PHBV is used in speciality packaging, orthopaedic devices and in capsules for controlled release of
drugs. PHBV undergoes bacterial degradation in the environment.

b) Nylon 2–nylon 6: (Aliphatic polyamide)
✓ It is an alternating polyamide copolymer obtained by the copolymerisation of glycine (H2N–CH2–COOH)
and amino caproic acid [H2N– (CH2)5– COOH] and is biodegradable. CH
2
N CH
H
OH
O
+
HNCH
2
C OH
H O
( )
5
CH
2
N C
H O
NHCH
2
C
O
( )
5
[ ]
n
n n
Glycine Aminocaproic acid Nylon-2-Nylon-6


c) Polyglycolic acid (PGA): (Aliphatic polyester)
CH
2
O CH OH
O
glycolic acid
polymerisation
n
CH
2
O C
O
[ ]
n
PGA


d) Polylactic acid (PLA) : (Aliphatic polyester)
CHO CH OH
OCH
3
lactic acid
polymerisation
n
CHO C
OCH
3
[ ]
n
PLA

✓ A copolymer of PGA and PLA (90:10) is used to make absorbable sutures.

e) Polycaprolactone (PCL): (lactone = cyclic ester) (Aliphatic polyester)
O
O
OH C CH
2
O
O
H( )
5
n n C CH
2
O
O
( )
5
[ ]
n
Caprolactone
PCL
- n H
2
O
hydrolysis polymerisation

10. Polymers of Commercial Importance:
Some commercially important polymers along with their structures and uses are given below.



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