Chemistry Reference
In-Depth Information
decomposition always initiates by scission of the bond between the oxygen
atoms:
O
RC
OO
C
R
R
C
(8-40)
O
O
O
The acyloxy radicals produced in reaction (8-40) of diacyl peroxides can initi-
ate polymerization or undergo side reactions as described in
Section 8.5.5
. Other
peroxides behave similarly.
The temperature
half-life relations given in
Table 8.1
may vary with reaction
conditions, because some peroxides are subject to accelerated decompositions by
specific promoters (
Section 8.5.2
) and are also affected by solvents or monomers
in the system.
The choice of reaction conditions has much less effect on the behavior of azo
initiators. The activation energies for decomposition of azo compounds are similar
to those of peroxides although the azo initiators do not contain a weak bond like
the O
O linkage. Decomposition of azodiisobutyronitrile (AIBN), for example,
proceeds because the nitrogen that is formed is a very stable gas and has a very
high enthalpy of formation:
a
CH
3
C
CH
3
CH3
Δ
C
N = N
C
N
2
(8-41)
CH
3
CH
3
2 CH
3
+
CN
CN
CN
This initiator has a 10-h
t
1
/
2
at 64
C.
Azo compounds are preferred for scientific investigations because their
k
d
values do not vary with the particular polymerization system as much as those of
peroxides. Cost considerations generally favor the industrial use of peroxides.
8.5.2
Redox Sources
Oxidation
reduction reactions can also be used to initiate free-radical polymeri-
zations. Such initiators include a metallic ion that can undergo a one-electron
transfer:
(8-42)
-
ROOH + Co
2+
Ro
+ OH + Co
3+
(8-43)
S
2
O
2-
Fe
2+
SO
2-
Fe
3+
+
+
SO
4
+
Redox reactions occur with hydroperoxides, peroxides, peresters, persulfates,
hydrogen peroxide, and other peroxides. A wide variety of metal ions may be
used as reducing agents. Ions which are commonly used include Co
2
1
,Fe
2
1
,
Cr
2
1
, and Cu
1
.
