CHAIN POLYMERIZATION Free radical polymerization
Download
Report
Transcript CHAIN POLYMERIZATION Free radical polymerization
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