Do you think that daily life would have been easier and

colourful without the discovery and varied applications

of polymers? The use of polymers in the manufacture

of plastic buckets, cups and saucers, children’s toys,

packaging bags, synthetic clothing materials, automobile

tyres, gears and seals, electrical insulating materials and

machine parts has completely revolutionised the daily

life as well as the industrial scenario. Indeed, the

polymers are the backbone of four major industries viz.

plastics, elastomers, fibres and paints and varnishes.

The word ‘polymer’ is coined from two Greek words:

poly means many and mer means unit or part. The

term polymer is defined as very large molecules having

high molecular mass (10

-10

u). These are also referred

to as macromolecules, which are formed by joining of

repeating structural units on a large scale. The repeating

structural units 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

After studying this Unit, you will be

able to

• explain the terms - monomer,

polymer and polymerisation and

appreciate their importance;

• distinguish between various

classes of polymers and different

types of polymerisation processes;

• appreciate the formation of

polymers from mono- and bi-

functional monomer molecules;

• describe the preparation of some

important synthetic polymers and

their properties;

• appreciate the importance of

polymers in daily life.

Objectives

“Copolymerisation has been used by nature in polypeptides which

may contain as many as 20 different amino acids. Chemists are still

far behind”.

UnitUnit

Unit

PolymersPolymers

Polymers

PolymersPolymers

Polymers

15.115.1

15.1

ClassificationClassification

Classification

of Polymersof Polymers

of Polymers

There are several ways of classification of polymers based

on some special considerations. One of the common

classifications of polymers is based on source from which

polymer is derived.

Under this type of classification, there are three sub

categories.

1. Natural polymers

These polymers are found in plants and animals.

Examples are proteins, cellulose, starch, some resins

and rubber.

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2. Semi-synthetic polymers

Cellulose derivatives as cellulose acetate (rayon) and cellulose

nitrate, etc. are the usual examples of this sub category.

3. Synthetic polymers

A variety of synthetic polymers as plastic (polythene), synthetic

fibres (nylon 6,6) and synthetic rubbers (Buna - S) are examples

of man-made polymers extensively used in daily life as well as

in industry.

Polymers can also be classified on the basis of their structure, molecular

forces or modes of polymerisation.

There are two broad types of polymerisation reactions, i.e., the addition

or chain growth polymerisation and condensation or step growth

polymerisation.

In this type of polymerisation, the molecules of the same monomer or

diferent monomers add together on a large scale to form a polymer. The

monomers used are unsaturated compounds, e.g.

, alkenes, alkadienes

and their derivatives. This mode of polymerisation leads to an increase in

chain length and 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.

1. Free radical mechanism

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. 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. This step is called chain

initiating step. As this radical reacts with another molecule of ethene,

another bigger sized radical is formed. The repetition of this sequence

with new and bigger radicals carries the reaction forward and the step

is termed as chain propagating step. Ultimately, at some stage the

product radical thus formed reacts with another radical to form the

polymerised product. This step is called the chain terminating step.

The sequence of steps involved in the formation of polythene are

depicted as follows:

Chain initiation steps

15.2.1.1

Mechanism of

Addition

Polymerisation

15.215.2

15.2

Types ofTypes of

Types of

PolymerisationPolymerisation

Polymerisation

ReactionsReactions

Reactions

15.2.1

Addition

Polymerisation

or Chain Growth

Polymerisation

15.1 What are polymers ?

Intext QuestionsIntext Questions

Intext Questions

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Polymers

Chain propagating step

Chain terminating step

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:

The addition polymers formed by the polymerisation of a single

monomeric species are known as homopolymers, for example

polythene discussed above is a homopolymer.

The polymers made by addition polymerisation from two different

monomers are termed as copolymers. Buna-S, which is formed by

polymerisation of buta–1, 3–diene and styrene is an example of

copolymer formed by addition polymerisation.

15.2.1.2

Some Important

Addition Polymers

(a)Polythene

Polythenes are linear or slightly branched long chain molecules.

These are capable of repeatedly softening on heating and

hardening on cooling and are thus thermoplastic polymers.

There are two types of polythene as given below:

(i) Low density polythene: 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) is obtained

through the free radical addition and H-atom abstraction.

It has highly branched structure. These polymers have

straight chain structure with some branches as

shown below.

Low density polythene is chemically inert and tough but

flexible and a poor conductor of electricity. Hence, it is used

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436Chemistry

in the insulation of electricity carrying wires and manufacture

of squeeze bottles, toys and flexible pipes.

(ii) High density polythene: 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 as shown below and has a high density due to

close packing. Such polymers are also called linear polymers.

High density polymers are also chemically inert and more

tough and hard. It is used for manufacturing buckets,

dustbins, bottles, pipes, etc.

(b)Polytetrafluoroethene (Teflon)

Teflon is manufactured by heating tetrafluoroethene with a free

radical or persulphate catalyst at high pressures. 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.

The addition polymerisation of acrylonitrile in presence of a

peroxide catalyst leads to the formation of polyacrylonitrile.

Polyacrylonitrile is used as a substitute for wool in making

commercial fibres as orlon or acrilan.

Is a homopolymer or a copolymer?

It is a homopolymer and the monomer from which it is obtained

is styrene C

CH = CH

Example 15.1Example 15.1

Example 15.1

SolutionSolution

Solution

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Polymers

This type of polymerisation generally involves a repetitive

condensation reaction between two bi-functional or trifunctional

mono-meric units. These polycondensation reactions may result in

the loss of some simple molecules as water, alcohol, hydrogen

chloride, etc., and lead to the formation of high molecular mass

condensation polymers.

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.

The formation of terylene or dacron by the interaction of ethylene

glycol and terephthalic acid is an example of this type

of polymerisation.

(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 or

condensation of amino acids or their lactams.

Nylons

(i) Nylon 6,6: It is prepared by the condensation

polymerisation of hexamethylenediamine with adipic acid

under high pressure and at high temperature.

Nylon 6, 6 is fibre forming solid. It possess high tensile

strength. This characteristic can be attributed to the strong

intermolecular forces like hydrogen bonding. These strong

forces also lead to close packing of chains and thus impart

crystalline nature.

Nylon 6, 6 is used in making sheets, bristles for brushes

and in textile industry.

(ii) Nylon 6: It is obtained by heating caprolactum with water

at a high temperature.

15.2.2

Condensation

Polymerisation

or Step Growth

Polymerisation

n CH + n COOH

–HOH C OH HOOC

Ethylene glycol

(Ethane-1, 2 - diol)

Terephthalic acid

(Benzene-1,4 - di

carboxylic acid)

Terylene or dacron

OCH CH

2 2

– –O–

15.2.2.1

Some Important

Condensation

Polymers

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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. 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 as per the reaction given

earlier. 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.

Novolac on heating with formaldehyde undergoes cross linking

to form an infusible solid mass called bakelite. It is

thermosetting polymer which cannot be reused or remoulded.

Thus, bakelite is formed by cross linking of linear chains of the

polymer novolac. Bakelite is used for making combs, phonograph

records, electrical switches and handles of various utensils.

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. The reaction starts with the initial formation of

o-and/or p-hydroxymethylphenol derivatives, which further

react with phenol to form compounds having rings joined to

each other through–CH

groups. The initial product could be a

linear product – Novolac used in paints.

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Polymers

(d) Melamine — formaldehyde polymer

Melamine formaldehyde polymer is formed by the condensation

polymerisation of melamine and formaldehyde.

It is used in the manufacture of unbreakable crockery.

Bakelite

H C

15.2 Write the names of monomers of the following polymers:

15.3 Classify the following as addition and condensation polymers: Terylene, Bakelite,

Polythene, Teflon.

Intext QuestionsIntext Questions

Intext Questions

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.

15.2.3

Copolymerisation

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For example, a mixture of buta–1, 3–diene and styrene can form

a copolymer.

Copolymers have properties quite different from homopolymers.

For example, butadiene - styrene copolymer is quite tough and is

a good substitute for natural rubber. It is used for the manufacture

of autotyres, floortiles, footwear components, cable insulation,

etc.

1. Natural rubber

Rubber is a natural polymer and possesses elastic

properties. It is also termed as elastomeric polymer. In

elastomeric polymers, the polymer chains are held together by

the weak 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.

Rubber 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 rubber tree which is found in India,

Srilanka, Indonesia, Malaysia and South America.

Natural rubber may be considered as a linear polymer of

isoprene (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.

Vulcanisation of rubber: Natural rubber becomes soft at

high temperature (>335 K) and brittle at low temperatures (<283

K) and 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. This process consists of heating a

mixture of raw rubber with sulphur and an appropriate additive

15.2.4 Rubber

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Polymers

at a temperature range between 373 K to 415 K. 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:

2. Synthetic rubbers

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. Thus, synthetic rubbers are either

homopolymers of 1, 3 - butadiene derivatives or copolymers

of 1, 3 - butadiene or its derivatives with another unsaturated

monomer.

Preparation of Synthetic Rubbers

1. Neoprene

Neoprene or polychloroprene is formed by the free radical

polymerisation of chloroprene.

It has superior resistance to vegetable and mineral oils. It is

used for manufacturing conveyor belts, gaskets and hoses.

2. Buna – N

You have already studied about Buna-S, in Section 15.1.3. Buna

–N is obtained by the copolymerisation of 1, 3 – buta–1, 3–diene

and acrylonitrile in the presence of a peroxide catalyst.

It is resistant to the action of petrol, lubricating oil and organic solvents.

It is used in making oil seals, tank lining, etc.

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15.315.3

15.3

MolecularMolecular

Molecular

Mass ofMass of

Mass of

PolymersPolymers

Polymers

15.415.4

15.4

BiodegradableBiodegradable

Biodegradable

PolymersPolymers

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 molecular mass of polymers can be determined

by chemical and physical methods.

A large number of polymers are quite resistant to the environmental

degradation processes and are thus responsible for the

accumulation of polymeric solid waste materials. 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. These 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:

1. Poly

ββ

-hydroxybutyrate – co-

ββ

-hydroxy valerate (PHBV)

It is obtained by the copolymerisation of 3-hydroxybutanoic

acid and 3 - hydroxypentanoic acid. PHBV is used in speciality

packaging, orthopaedic devices and in controlled release

of drugs. PHBV undergoes bacterial degradation in the

environment.

2. Nylon 2–nylon 6

It is an alternating polyamide copolymer of glycine (H

N–CH

–

COOH) and amino caproic acid [H

N (CH

)

COOH] and is

biodegradable. Can you write the structure of this copolymer?

15.4 Explain the difference between Buna-N and Buna-S.

15.5 Arrange the following polymers in increasing order of their intermolecular forces.

Nylon 6,6, Buna-S, Polythene.

Intext QuestionsIntext Questions

Intext Questions

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Polymers

Name of Polymer Monomer Structure Uses

Polypropene Propene Manufacture of

ropes, toys, pipes,

fibres, etc.

Polystyrene Styrene As insulator, wrapping

material, manufacture

of toys, radio and

television cabinets.

Polyvinyl chloride Vinyl chloride Manufacture of rain

(PVC) coats, hand bags, vinyl

flooring, water pipes.

Urea-formaldehyle (a) Urea For making unbreak-

Resin (b) Formaldehyde able cups and

laminated sheets.

Glyptal (a) Ethylene glycol Manufacture of

(b) Phthalic acid paints and lacquers.

Bakelite (a) Phenol For making combs,

(b) Formaldehyde electrical switches,

handles of utensils and

computer discs.

Table 15.1: Some Other Commercially Important Polymers

Besides, the polymers already discussed, some other

commercially important polymers along with their structures and

uses are given below in Table 15.1.

15.515.5

15.5

Polymers ofPolymers of

Polymers of

CommercialCommercial

Commercial

ImportanceImportance

Importance

Polymers are defined as high molecular mass macromolecules, which consist of

repeating structural units derived from the corresponding monomers. These polymers

may be of natural or synthetic origin and are classified in a number of ways.

In the presence of an organic peroxide initiator, the alkenes and their derivatives

undergo addition polymerisation or chain growth polymerisation through a free

radical mechanism. Polythene, teflon, orlon, etc. are formed by addition polymerisation

of an appropriate alkene or its derivative. Condensation polymerisation reactions

are shown by the interaction of bi – or poly functional monomers containing – NH

, –

OH and – COOH groups. This type of polymerisation proceeds through the elimination

of certain simple molecules as H

O, CH

OH, etc. Formaldehyde reacts with phenol and

melamine to form the corresponding condensation polymer products. The condensation

SummarySummary

Summary

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15.1 Explain the terms polymer and monomer.

15.2 What are natural and synthetic polymers? Give two examples of each type.

15.3 Distinguish between the terms homopolymer and copolymer and give an

example of each.

15.4 How do you explain the functionality of a monomer?

15.5 Define the term polymerisation.

15.6 Is ( NH-CHR-CO )

, a homopolymer or copolymer?

15.7 Why do elastomers possess elastic properties?

15.8 How can you differentiate between addition and condensation polymerisation?

15.9 Explain the term copolymerisation and give two examples.

15.10 Write the free radical mechanism for the polymerisation of ethene.

15.11 Define thermoplastics and thermosetting polymers with two examples of

each.

15.12 Write the monomers used for getting the following polymers.

(i) Polyvinyl chloride (ii) Teflon (iii) Bakelite

15.13 Write the name and structure of one of the common initiators used in free

radical addition polymerisation.

15.14 How does the presence of double bonds in rubber molecules influence their

structure and reactivity?

15.15 Discuss the main purpose of vulcanisation of rubber.

15.16 What are the monomeric repeating units of Nylon-6 and Nylon-6,6?

15.17 Write the names and structures of the monomers of the following polymers:

(i) Buna-S (ii) Buna-N (iii) Dacron (iv) Neoprene

15.18 Identify the monomer in the following polymeric structures.

(i)

Exercises

polymerisation progresses through step by step and is also called as step growth

polymerisation. Nylon, bakelite and dacron are some of the important examples of

condensation polymers. However, a mixture of two unsaturated monomers exhibits

copolymerisation and forms a co-polymer containing multiple units of each monomer.

Natural rubber is a cis 1, 4-polyisoprene and can be made more tough by the processof

vulcanisation with sulphur. Synthetic rubbers are usually obtained by copolymerisation

of alkene and 1, 3 butadiene derivatives.

In view of the potential environmental hazards of synthetic polymeric wastes, certain

biodegradable polymers such as PHBV and Nylon-2- Nylon-6 are developed as

alternatives.

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Polymers

(ii)

15.19 How is dacron obtained from ethylene glycol and terephthalic acid ?

15.20 What is a biodegradable polymer ? Give an example of a biodegradable

aliphatic polyester.

Answers of Some Intext Questions

15.1 Polymers are high molecular mass substances consisting of large numbers

of repeating structural units. They are also called as macromolecules. Some

examples of polymers are polythene, bakelite, rubber, nylon 6, 6, etc.

15.2 (i) Hexamethylene diamine and adipic acid.

(ii) Caprolactam.

(iii) Tetrafluoroethene.

15.3 Addition polymers: Polyvinyl chloride, Polythene.

Condensation polymers: Terylene, Bakelite.

15.4 Buna-N is a copolymer of 1,3-butadiene and acrylonitrile and Buna-S is

a copolymer of 1,3-butadiene and styrene.

15.5 In order of increasing intermolecular forces.

Buna-S, Polythene, Nylon 6,6.

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Note

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