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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 polymerizationsare chain polymerizationsin 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.
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Example
The formation of polyvinyl monomer.
CH2= CHX
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INITIATION
This stage is a two steps stage
1. The formation of free radicals from aninitiator.
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 ofa single bond.
2. Single electron transferred to or from an ionor molecule (redox reactions)
Homolytic can be achieved by heat (thermolysis)
or by light such as U.V. (photolysis).
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Example
Sometimes the radicals undergo further breakdown (-
scissions) such as
O O O
-C-0-0-C- 2 -C-O
Benzoly peroxide Benzolyooxy radicals
(CH3)2C-N = N- C (CH3)2 2(CH3)2C + N2
CN CN CN
Azobisisobutyronitril 2-Cyanopropyl radicals
O O
-C-0 + C = O
(CH3)2 - C-O CH3 + (CH3)2-C = 0
CH3 Methyl acetone
radical
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REDOX REACTION
Redox reaction defined as the generation of freeradicals by electron transfer and it is use when
polymerization performed at low temperature. Example
CH3 CH3
-C-0-0H + Fe2+` -C-O + OH + Fe3+
CH3 CH3
Cumyl Ferrous Cumyloyloxy
hydroperoxide ion radical
O O O O O O
O-S-O-O-S-O + HO-S-O 0-S-O + O-S-O + OH-S-O
O O O O
Presulphate Bisulphate Sulphate Sulphate Bisulphate
ion ion ion radical radical
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An active center is crated when a free radical (Ro)which is generated from an initiator attacks the -bondof the monomer molecules.
R + CH2= CH R-CH2-CH or R-CH - CH2
X X X
This is more This is more
likely stable
Sometime free radical react with each other such as:
O O O
2 -C-0 -C-O- + C = O
OR
2
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PROPAGATION The addition of monomer molecules to the active center to grow
the polymer chain.
There are two modes of chain propagation
Again mode (1) are more dominant.
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.
Therefore polymer structure are like-----CH2-CH-CH2-CH-CH2-CH-CH2-CH------
X X X X
1. Head to Tail
R-CH2-CH + CH2=CH R-CH2-CH-CH2-CH
X X X X
2. Head to Head
R-CH2-CH + CH2=CH R-CH2-CH-CH-CH2
X X X X
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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
2. Disproportionation
when a hydrogen atom move from one growing chain toanother
--CH2-CH + CH - CH2--- ---- CH2-CH- CH-CH2---
X X X X
H H
--CH2-CH + C - C--- ----CH2-CH2+ CH=CH2---
X X H X X
Saturated end Unsaturated end
group polymer group polymer
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Thank You
See You Next Lecture