POLYMERS | MCAT Notes | UCAT Notes | COMPLETE CLASS NOTES | All Concepts, Tricks & PYQs | Class 12
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Course
MCAT CLASS NOTES
Institution
MCAT CLASS NOTES
After studying this , 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- an...
Polymers
Polymers
Polymers
Polymers
Polymers
complete class notes MCAT | UCAT
with question . Polymers
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 (103-107u). 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.
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.
Intext Questions
1 What are polymers ?
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
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