CHAIN POLYMERIZATION
Free Radical Polymerization

Free radical are independently-existing species that have
unpaired electron. Normally they are highly reactive
with short life time.
 Free radical polymerization’s are chain polymerization’s
in which each polymer molecules grows by addition of
monomer to a terminal free-radical reactive site known
as active center.
 After each addition the free radical is transferred to the
chain end.
 Chain polymerization is characterized by three distinct
stages, Initiation, propagation and termination.
Example

The formation of polyvinyl monomer.

CH2 = CHX
INITIATION

This stage is a two steps stage
 1. The formation of free radicals from an
initiator.
 2. The addition of one of these free radicals to a
monomer molecules.
 Free radical can be formed by two principal
 1. Homolytic scission (homolysis) or breakage of
a single bond.
 2. Single electron transferred to or from an ion
or molecule (redox reactions)
 Homolytic can be achieved by heat (thermolysis)
or by light such as U.V. (photolysis).
Example
O

O
O
-C-0-0-C-
2
Benzoly peroxide

Benzolyooxy radicals
(CH3)2 C-N = N- C (CH3)2
2(CH3)2C + N2

CN
CN
Azobisisobutyronitril

-C-O
CN
2-Cyanopropyl radicals
Sometimes the radicals undergo further breakdown (scissions) such as


O
-C-0
O
+C=O

(CH3)2 - C-O

CH3

CH3 + (CH3)2-C = 0
Methyl acetone
radical
PHOTOLYSIS

Photolysis is the second principle of free
radical formation.
 The advantage of this method is that the
formation of free radicals begins at the
instant of exposure and cases as soon as
the light source is removed.
REDOX REACTION


Redox reaction defined as the generation of free
radicals by electron transfer and it is use when
polymerization performed at low temperature.
Example

CH3

-C-0-0H + Fe2+`

CH3
 Cumyl
Ferrous
 hydroperoxide
ion
CH3
-C-O + OH + Fe3+
CH3
Cumyloyloxy
radical

O
O
O
O-S-O-O-S-O + HO-S-O
O
O
O
0-S-O + O-S-O + OH-S-O

O
O
Sulphate Sulphate Bisulphate
ion
radical
radical
O
O
Presulphate
 ion
Bisulphate
ion

An active center is crated when a free radical (Ro)
which is generated from an initiator attacks the -bond
of the monomer molecules.
R + CH2 = CH
X
R-CH2-CH
or R-CH - CH2
X
X
This is more
likely
This is more
stable
Sometime free radical react with each other such as:
O
O
O
2
OR
2
-C-0
-C-O-
+C=O
PROPAGATION


The addition of monomer molecules to the active center to grow
the polymer chain.
There are two modes of chain propagation
1. Head to Tail
R-CH2-CH + CH2=CH
X
X
2. Head to Head
R-CH2-CH + CH2=CH
X
X
Again
R-CH2-CH-CH2-CH
X
X
R-CH2-CH-CH-CH2
X X
mode (1) are more dominant.
Therefore
polymer structure are like
-----CH2-CH-CH2-CH-CH2-CH-CH2-CH-----X
X
X
X
Time
of addition for each monomer is of the order of a millisecond.
Thus several thousands of additions can take place in a few seconds.
TERMINATION

The last stage of chain reaction in which the growth of
the polymer chain terminated (or stopped).
 There are two mechanisms of termination
 1. Combination
 Coupling together of two growing chains to form a
single polymer molecules. polystyrene


--CH2-CH + CH - CH2-----CH2-CH- CH-CH2--X
X
X X
 2.
Disproportionation
 when a hydrogen atom move from one growing chain to
another

H H
--CH2-CH + C - C------CH2-CH2 + CH=CH2--
X
X H
X
X

Saturated end Unsaturated end

group polymer group polymer
Thank You
See You Next Lecture