Chemistry Reference
In-Depth Information
Comparatively, the most commonly used initiation system is redox system
because activation energy for the redox initiation is quite low and it can initiate
the reaction under ambient condition, and the reaction rate is faster and the energy
consumption is low [
108
]. The type and activity of initiator usually decide the
grafting reaction efficiency and rate, and the development of new initiator has long
been the subject of great interests. Because of the different types of oxidants and
reductants, the formation mechanism of free radicals in redox system is distinct, and
the sorts of redox initiators are important factor to decide the graft efficiency.
Figure
5.12
depicts the most frequently used redox initiation system and depicts
the reaction and formation mechanism of radicals [
34
,
58
,
59
,
62
,
67
,
109
-
123
]. For
instance, in the KHSO
5
/Fe
2+
redox system, the divalent Fe
2+
may lose one electron
under the action of oxidant KHSO
5
to form Fe
3+
, and simultaneously the S-O or
O-H bonds of KHSO
5
were broken to form -OH
•
and -SO
4
•
radicals. In the
potassium chromate/malonic acid initiation system, the CrO
4
2
ions may transform
with H
2
CrO
4
each other at acidic condition. The oxidant H
2
CrO
4
may react with
reductant CH
2
(COOH)
2
to form Cr
4+
midbody with higher activity. The Cr
4+
may
capture the active H atom of CH
2
(COOH)
2
(the strong electron withdrawing
capability of -COOH render the conjoint -CH
2
- higher reactivity) to form
•
CH
(COOH)
2
radicals, and the Cr
4+
ion was reduced as Cr
3+
ion. In a word, the
formation process of radicals is an electron transport process induced by a redox
reaction.
As discussed above, although the formation mechanism of radicals for various
initiation systems is different, the radicals have the same effect when they initiate
the gum to perform a graft reaction and form a graft copolymer. Figure
5.13
gives
the typical grafting mechanism of vinyl monomers onto gum backbone. Firstly, the
primary radicals were generated by the decomposition of thermal initiator or the
reaction of redox initiators (Fig.
5.12
). These radicals striped down the hydrogen
atoms of the -OH groups on gum chains to form macro-radicals. After added vinyl
monomers, the active radical sites on gum chains may initiate vinyl groups of the
monomers to process chain propagation. This is a typical “graft from” reaction. In
the grafting process, the type and activity of initiators, the concentration, viscosity
and activity of gum solution, the concentration of monomers, the reaction tempera-
ture, and time may greatly affect the graft ratio and efficiency. Table
5.1
shows the
different gum-g-copolymers prepared by using various redox initiator systems as
well as the grafting efficiency.
Besides the direct grafting reaction of -OH groups, the saccharide ring of gum may
be opened. Typically, when the initiator is multivalent metal ions such as Ce
4+
ion, the
metal ions may interact with the C2-C3 glycol and the C6 hydroxyl of the anhydro-
D
-
glucose unit of saccharide ring to form a gum-Ce
4+
complex [
134
]. The Ce
4+
ion in
the complex can then be reduced as Ce
3+
ion with the release of a proton and a
subsequent formation of a free radical on the backbone of gum. These free radicals
could then react with the end vinyl groups of monomer to initiate graft copolymeriza-
tion (Fig.
5.14
)[
135
]. Termination of the graft copolymer was carried out through the
