Chemistry Reference
In-Depth Information
peroxide is estimated to be 10
4
to 10
5
s. Past that time, they decompose into phenyl radicals and
carbon dioxide [
49
]. This is sufficient time for the benzoyloxy radicals to be trapped by fast-reacting
monomers. Slow-reacting monomers, however, are more likely to react with the phenyl radicals that
form from the elimination reaction. In effect, there are two competing reactions [
50
]:
1. Decomposition of the free radical:
O
K'
O
CO
2
+
2. Two types of radicals can add to the monomer (where x represents any typical substituent of vinyl
monomers, such as halogens, or esters, or aromatic groups, or nitriles, etc.):
O
O
X
X
K"
O
O
+
X
K'"
X
+
K
0
) depends upon the reactivity of the
monomers. It is shown in Table
3.5
[
51
,
52
]. The benzoyloxy radical is used in this table as an
illustration. A similar comparison is possible for a redox initiating system. An initiating sulfate
radical ion from a persulfate initiator can react with another reducing ion or add to the monomer:
K
00
/
K
0
, (or
K
000
/
The ratio of the rates of the two reactions,
K'
SO
4
SO
4
+
Fe
+
Fe
X
X
K"
SO
4
+
SO
4
In Table
3.6
are shown the relative reaction rates of SO
4
with some monomers at 25
C[
53
-
57
]. As
explained, the rate of addition of a radical to a double bond is affected by steric hindrance from bulky
substituents. Polar effect, such as dipole interactions also influence the rate of addition.
Forbes and Yashiro studied the addition of the initiating radials to methyl methacrylate in liquid
supercritical carbon dioxide [
58
]. They demonstrated that the rate of addition of the initiating radicals
to the monomers,
k
add
values, can be measured in liquid CO
2
.
Phenyl or methyl groups located on the carbon atom that is under a direct attack by a free radical
can be expected to interfere sterically with the approach. For instance, due to steric hindrance,
trans
-
b
trans
isomer is more stable of the two. While 1,1-disubstituted olefins homopolymerize readily, the 1,2-
disubstituted olefins are hard to homopolymerize [
59
]. Some exceptions are vinyl carbonate [
61
] and
maleimide derivatives [
62
]. Also, perfluoroethylene and chlorotrifluoroethylene polymerize readily.
Table
3.6
shows the relative reaction rates of SO
4
-methylstilbene is more reactive toward a radical attack than is its
cis
isomer [
58
]. Yet, the
with some monomers at 25
C[
50
].
