Free radical chain polymerization

1. 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.

2. Example
 The formation of polyvinyl monomer.
 CH2 = CHX

3. 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).

4. 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)2 C-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

5. 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.

6. 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 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

7.  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 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

8. 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

9. 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 to
another
 --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

10. Thank You
See You Next Lecture