ADDITION
POLYMERIZATION
Pandya Khyati
Roll no:11
What is polymers!

Polymers:A long chain which is build up of a number of
repeating chemical units (monomers) held to gather by
the chemical bonds (covalent) is known as a polymer.

Polymerization:-
A process in which a simple low molecular weight
compounds units in large number and forms a high
molecular
weight
polymerization.
compound
is
known
as
What is addition polymerization!

Addition polymerization or chain growth reaction
polymerization , requires the monomers to have at least
one double bond.

In addition polymerization, there is no molecule is
eliminated and no by-product is generated.

The molecular weight of the formed polymer is exactly
the same as the sum of all monomers included in the
polymerization.

In, this type of polymerization, the molecular of the same
monomer or different monomer add together on a large
scale to form a polymer.

The monomer normally employed in this polymerization
contain a carbon-carbon double bond or unsaturated
compounds like alkenes and there derivatives, that can
participate in a chain reactions.

These chain reaction consist of mainly three stages:
I.
Initiation
II.
Propagation
III. Termination.
INITIATION

In the initiation step an initiator molecule is thermally
decomposed or allowed to undergo a chemical reaction
to generate an ‘active species’

This ‘active species’ which can be a free radical or a
cation or an anion.

Then initiates the polymerization by adding to the
monomer’s carbon carbon double bond.

The reaction occurs in such a manner that a new free
radical or anion or cation is generated.

The initial monomer becomes the first repeat unit in
the incipient polymer chain.
Propagation step

In the propagation step , the newly
generated active species adds to another
monomer in the same manner as in the
initiation step.

The procedure is repeated over and over
again until the final step of the process,
termination occurs.
Termination:

In the termination step, the growing chain
terminates through reaction with another
growing chain, by reaction which
another
species in the polymerization mixture, or by the
spontaneous decomposition of the active site.

And then the growth of polymer chain is
arrested and hence, it becomes dead polymer
chain.
General characteristics of addition
polymers:

Once initiation occurs, the polymer chain
forms very quickly 10-1 to 10-6 sec.

The
concentration
required
is
polymerization
very
of
low.
mixture
active
species
Hence,
the
consists
of
primarily of newly – formed polymer and
unreacted monomer.

Since the carbon carbon double bonds in the
monomers are, in effect, converted to two
single carbon carbon bonds in the polymer. So
energy is released marking the polymerization
exothermic with cooling often required.

High molecular weight polymers with weight of
about 10,000 to 1 million can be obtained.

Molecular weight is time independent.
Types of chain growth
polymerization

The mechanism of addition polymerization can be
divided broadly upon the nature of initiation and
the initiator required into two main classes,
i.
Free radical polymerization
ii.
Ionic polymerization

Ionic polymerization is divided into two types
cationic and anionic polymerization.
Free radical polymerization:

The polymerization which involves free radical as active
species is known as free radical polymerization.

A verity of alkenes or dienes and their
polymerized in
derivatives are
the presence of a free radical generating
initiator (catalyst) like benzoyl peroxide, acetyl peroxide, tert
–butyl peroxide, etc.

Any compound that undergoes decomposition and form free
radical is known as initiator.

Generally unstable compounds are used as an
initiator.

A free radical may be defined as an intermediate
compound containing an odd number of
electrons, but which do not carry an electric
charge and are not free ions.

For example ,the polymerization of ethene to
polyethene consists of heating or exposing to
light a mixture of ethene with a small amount of
benzoyl peroxide initiator.

Decomposition of initiator is endothermic
process.

Rate of decomposition is depends upon
I.
Temperature
II.
Nature of initiator
III.
Nature of solvent.

Decomposition of initiator can be done by
thermal decomposition, photo decomposition
and catalytic decomposition.
Free radical polymerization:

In, chain growth polymerization reactions require
the presence of initiator, a compound that reacts
with the monomer to form another reactive
compound, which begins the linking process.

The most widely used initiators are compounds
such as peroxides that break down to an unstable
species called a radical ( or free radical )

A radical is a reactive compound that contains an
unpaired electron.

In chemical formulas it is commonly given the
generic designation R.

As shown the reaction diagram below, the most
commonly
used
peroxide
initiator, benzoyl
peroxide, can produce benzoyloxi radical by
cleaving at an oxygen-oxygen bond.

The pair of benzoyloxy radicals thus produced
may initiate a polymer chain, or they may break
down further to yield carbon dioxide and yet a
new initiator, a phenyl radical.

The first step in polymerization involves
addition of the initiator radical (R. ) to the
monomer to form the new radical having the
unpaired electron on a carbon atom, as can be
seen in the polymerization of ethene.

The new radical then adds to a sec.
ethene molecule:
ethene molecules are added successively
to the chain until very little ethene is left.
 The termination occurs by many ways,
like disproportionation, combination or
coupling, by chain transfer agents, and by
inhibitors.


At this point the chain is terminated,
either by a combination of two chains .

Or by a disproportionation reaction
involving the transfer of a hydrogen atom
from one of the growing chains to the
others.

The structure enclosed in brackets CH2=CH2 is
the repeating unit of polymer chain.

The number of repeating units ,n, varies according
to length of the polymer chain. Or in other words
,the molecular weight of polymer chain.

In the termination process collision of chain
depend upon mobility of chains and mobility of
chain depends on temperature and nature of
solvents.
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