Chemistry Reference
In-Depth Information
radicals by the excess amount of radicals present [
53
]. Nishioka and Kosai [
41
]
investigated the grafting of two monomers AN and MMA separately onto cellulose
in DMSO/PF system (in homogeneous medium) using two types of initiators: APS
and BPO. It is known that DMSO/PF system is a non-degrading solvent for
cellulose [
41
]. The nature of the initiator has an important effect on the grafting.
AIBN is known to show resonance stabilization, but no such resonance exists in the
peroxide initiators [
41
]. For that reason, they reported that higher grafting yield is
obtained with APS, 87.3 % for AN and 52 % for MMA, in comparison to those with
AIBN, i.e., 10 % for AN and 48 % for MMA. The number of grafts per cellulose
chain by APS and AIBN initiator were found to be 3.9 and 0.5 for AN and 3.4 and
1.3 for MMA monomers. They [
41
] suggested from the results that grafting
occurred in higher parts of cellulose chain in homogeneous medium than heteroge-
neous medium in which the number of grafts per cellulose chain rarely exceed the
unity. They also found that the grafting onto cellulose hardly proceeded with
AN-AIBN system, but appreciably in MMA-AIBN system. The solution of cellu-
lose in DMSO/PF had a high viscosity, and the maximum amount of cellulose
which could be dissolved in DMSO/PF solution was 4 wt% [
41
]. At 2 wt% cellulose
concentrations in DMSO/PF solvent, the optimum molar ratios of monomer/initia-
tor ([M]/[I]) were found to be 22 and 46 for AN and MMA, respectively. Nishioka
et al. [
39
] investigated the grafting of methyl acrylate (MA) onto cellulose in
homogeneous (DMSO/PF) medium and determined that grafting proceeded with
APS (the highest number of grafts per cellulose chain was 1.5), but hardly at all
with BPO and AIBN initiators. In addition, BPO was found to be unsuitable as
initiator for the grafting of MA onto cellulose in homogeneous medium because it
led to degradation of cellulose chains [
39
]. It is known that while redox initiators
and radiation lead to the oxidative degradation of cellulose in the absence of
monomer [
127
-
129
], the degradation of cellulose is minimized in the presence of
monomer [
24
]. Nishioka et al. [
55
] investigated the grafting of 2-hydroxyethyl
methacrylate (HEMA) onto cellulose in DMSO/PF system using various initiators,
APS, KPS, and AIBN. They [
55
] determined that the grafting hardly proceeded
with AIBN initiator similar to their previous works [
39
-
41
], and KPS is not a
suitable initiator for the grafting of HEMA in this system since it led to degradation
of cellulose. The maximum rate of propagation in the grafting of MMA onto EC in
benzene/DMSO solvent system was determined depending on the kind of initiator
[
42
] and found to be 0.2
10
2
min
1
for APS and KPS,
respectively. For that reason, higher grafting yield was obtained with APS than KPS
[
42
].
10
2
min
1
and 0.08
2.4.4 Effect of Solvent
The reaction medium or solvent has also an effect on the grafting yield or percent-
age in the grafting reactions. Jun et al. [
142
] investigated the effect of solvent such
as alcohols and acetone in the grafting of
N
-isopropylacrylamide (NIPAM) onto
