Chemistry Reference
In-Depth Information
2.3 kcal mol
1
[
44
]. Gupta and Sahoo [
45
] investigated the grafting of
N
-vinyl
pyrrolidone (NVP) onto cellulose with the initiator of Co(III) acetylacetonate
(Co(acac)
3
)-HClO
4
complex between 40 and 60
C and found that the activation
energy of (
E
a
) for grafting of NVP onto cellulose was 22.7 kJ mol
1
. The synergis-
tic effect of comonomer was observed by G¨rda
˘
et al. [
99
] in the grafting
of AASO
3
H and AA onto cellulose using CAN-HNO
3
10
3
M
(4
10
3
M) as initiator system at 30
C. The presence of 10 mol% AASO
3
H
in the binary monomer mixture led to twice increase in grafting percentage in
comparison to the grafting of AA alone. In the grafting reactions, normally, the
increase in the monomer concentration increases the grafting percentage and
grafting efficiency up a certain value independent from the kind of initiator,
monomer, and the medium of grafting (homogeneous/heterogeneous) [
41
,
46
,
55
,
89
,
95
,
96
,
98
,
101
,
103
,
124
]. Further increases in monomer concentration beyond
the optimum value decrease the grafting percentage and grafting values since the
homopolymer formation dominates over the grafting, although the optimum mono-
mer concentration differs for each grafting reaction depending on the kind of
monomer and substrate, and the reaction conditions.
7.5
2.4.3 Effect of the Kind and Concentration of Initiator
Except the radiation technique, all chemical grafting reactions require an initiator,
and its nature, concentration, solubility, as well as radical creation mechanism
affect the grafting [
14
]. Various kinds of initiators have been used for grafting
reactions: (Fe
2+
-H
2
O
2
), azobisisobutyronitrile (AIBN), K
2
S
2
O
8
,Ce
4+
, etc. Grafting
percentage can be increased either by increasing the number of grafts (grafting
frequency) per substrate chain or by increasing the molecular weight of grafted
chains at constant number of graft. It is apparent that the initiator concentration
affects both the number of grafts per cellulose chain and the molecular weights of
graft chains. Radicalic sites may also be created on cellulose by some transition
metals such as Ce
4+
,Co
3+
,V
5+
, and Cr
6+
[
14
]. The redox potential of the metal ion
determines its grafting ability. In general, metal ions with low oxidation potential
provides higher grafting efficiency [
14
]. The number of active sites created on the
cellulose backbone depends on the initiator concentration, namely, the ratio of
initiator/cellulose. Gupta and Sahoo [
45
] observed in the grafting of NVP
onto cellulose with Co(acac)
3
-HClO
4
initiator at the concentrations of (2
10
5
10
3
M, respectively, that the amount of grafted NVP and
the conversion of cellulose to graft copolymer increased up to 15
10
5
)
20
2.5
10
5
MCo
(acac)
3
, but beyond this concentration they decreased. While the number of grafts
per cellulose chains increased from 1.23
10
6
to 5.5
10
6
with the increase of
10
5
M, number average molecular
weights (Mn) of graft chains decreased from 105
Co(acac)
3
concentration from 2 to 20
10
3
. The similar
finding, first the increase in grafting with the initiator and then the decrease with
further increase of initiator, observed for Co(acac)
3
-HClO
4
initiator was also
10
3
to 56
