Chemistry Reference
In-Depth Information
may accelerate the rate of decomposition of persulfates. Their useful temperature
range is generally between 40 and 90
C.
Redox systems are used for polymerizations at lower temperatures. Many of
these redox initiator couples were developed for the emulsion copolymerization
of butadiene and styrene, since the 5
10
C “cold recipe” yields a better rubber
than the “hot” 50
C emulsion polymerization.
Common redox systems include the persulfate
bisulfite couple:
S
2
O
2
8
1
HSO
3
-
SO
2
4
1
SO
4
1
HSO
3
(10-4)
and ferrous ion reducing agents combined with persulfate or hydroperoxide oxi-
dizing agents:
S
2
O
2
8
1
Fe
2
1
-
Fe
3
1
1
SO
2
4
1
SO
4
(10-5)
Fe
2
1
-
Fe
3
1
1
OH
2
1
ROOH
1
RO
The rate of radical generation by an initiator is greatly accelerated when it is
coupled with a reducing agent. Thus, an equimolar mixture of FeSO
4
and K
2
S
2
O
8
at 10
C produces radicals about 100 times as fast as an equal concentration of
the persulfate alone at 50
C. Redox systems are generally used only at lower
temperatures. The activation energy for the decomposition reaction is usually
12
15 kcal/mol so that it is very difficult to obtain a controlled and sustained
generation of radicals at an acceptable rate at high temperatures.
It is possible to prevent a too rapid depletion of the initiator system by making
repeated additions of the oxidizer and reducer or by having one component soluble
in the monomer and the other in the aqueous phase. Thus, organic peroxides like
cumyl hydroperoxide (10-1) are soluble in the monomer droplets but the redox
reaction itself occurs in the aqueous phase where the hydroperoxide encounters
the water-soluble reducing agent. The rate of production of radicals is then con-
trolled by the diffusion of the oxidizing component from the organic phase.
CH
3
C
OOH
CH
3
10-1
During polymerization there is often an induction period of variable length
caused by traces of oxygen (see Section 8.9), followed by an accelerating rate.
10.2.1
Harkins
Ewart Mechanism
Emulsion polymerization first gained industrial importance during World War II
when a crash research program in the United States resulted in the production of
styrene-
Smith
co
-butadiene [SBR] synthetic rubber. The Harkins
Smith
Ewart model
