Chemistry Reference
In-Depth Information
Fig. 6.4
Grafting polymerization (%) versus reaction time. Backbone polymer dextran:
open
circle
s D1,
M
w
9,000;
half
-
filled circles
D2,
M
w
61,000;
filled circle
s D3,
M
w
196,000
Figure
6.3
shows that grafting (%) increased rapidly at the beginning of the
reaction when D1 sample (
M
w
9,000) was one of the reactants, but only gradually
with D2 or D3. Graft polymerization (%) plotted against time for the purpose of
obtaining the rate of graft polymerization produces a similar profile (Fig.
6.4
). The
results can be explained in terms of the diffusion control by the increase in viscosity
of the reaction solution due to augmentation of dextran molecular weight [
9
,
27
]. It
is suggested that the lower the molecular weight of dextran, the greater the number
of reactive functions per unit weight. This accords with the interpretation of other
graft polymerizations proposed by Wallace et al. [
5
] that polymerization is initiated
mainly by the breakdown of coordination complexes of ceric ions with 1,2-glycol
groups on the ends of the dextran chains. There are three such functional glycol
neighbours in a terminal pyranose ring, so that three different modes of ring
cleavage are possible. Homolytic bond fission between C1 and C2 may be most
preferable, considering both the
cis
-orientation of the two hydroxyl groups [
28
] and
the powerful electron-withdrawal ability for either oxygen.
The polymerization is initiated by a ceric ion-alcohol redox system by way of an
intermediate complex [
9
].
6.2.2 Rate of Consumption of Ce
4+
Reaction rates were determined by following the absorbance of the reaction
solution (Hitachi model 101 spectrophotometer). The composition of the solution
([Ce
4+
]
10
3
M, [dextran]
¼
2
¼
2%, [HNO
3
]
¼
0.017 N) was similar to that
