Microwave-Assisted Polymer Synthesis: Recent Developments in a ...

Microwave-Assisted Polymer Synthesis: Recent Developments in ...
Scheme 9. Grafting of methyl methacrylate on k-carrageenan using potassium persulfate.
the removal of these ions from waste water. Prasad et al.
used microwave irradiation as heat source for the grafting
of methyl methacrylate on k-carrageenan using a
potassium persulfate initiating system as depicted in
Scheme 9. [94] The most important advantages of the
microwave-assisted grafting procedure were the very
short polymerization times and the simplicity, whereby
the good control over the polymerization conditions
prevented thermal degradation of the k-carrageenan.
Superabsorbing materials have been prepared under
microwave irradiation by the copolymerization of cornstarch,
sodium acrylate, and poly(ethylene glycol) diacrylate
by Zheng et al., where it was found that both the
inhibitor and oxygen did not have to be removed from the
polymerization mixture when using microwave irradiation.
[95] Optimization of the microwave-assisted polymerization
method resulted in the preparation of a superabsorber
with a swelling ratio of 520 to 620 g water per
gram polymer. When the optimal conditions were applied
for the preparation of the hydrogel using conventional
heating, the material exhibited lower swelling ratios
demonstrating the advantage of microwave heating. Xu
and coworkers prepared superabsorbers by the copolymerization
of starch, sodium acrylate, and 2-acrylamido-2-
methylpropanosulfonic acid. [96] Besides the faster polymerization
under microwave irradiation, it was found
that the swelling rate of the material prepared under
microwave irradiation was
much higher than for the material
prepared with conventional
heating. Scanning electron
microscopy revealed that the
material produced under microwave
irradiation consisted of
many evenly distributed pores,
whereas the material prepared
with thermal heating contained
irregular pores. This
observation was correlated to
the different temperatures that
were used: A higher temperature
with microwave heating
causes in situ drying and thus the pores resulted from
water evaporation during the preparation while thermal
heating gave a hydrogel that was dried afterwards.
Another current topic in microwave-assisted free radical
polymerizations is the preparation of composite materials
in which the solid materials act as microwave absorbers
and initiate the polymerization. Zhu and Zhu reported the
simultaneous one-step preparation of polyacrylamidemetal
nanocomposites under microwave irradiation as
depicted in Figure 5. [97] Using this method, monodisperse
nanocomposites could be obtained with silver, platinum,
and copper. During these studies, ethylene glycol was used
as solvent, reducing agent, and microwave absorber. This
novel method excludes the use of additional initiators,
surfactants, and stabilizers.
Polystyrene-silica composites were prepared by Liu and
Su. [98] Silica particles were dispersed in bulk styrene with
azoisobutyronitrile as initiator followed by microwave
irradiation resulting in the polymerization of styrene. The
effect of microwave power and initiator on the styrene
conversion and grafting efficiency were investigated.
Moreover, it was mentioned that the microwave-assisted
polymerization procedure is the shortest and easiest
method known in the literature: it results in almost the
best grafting parameters. Ritter and coworkers used the
selective heating of iron fibers under microwave irradiation
for the synthesis of channel containing polymeric
Figure 5. Schematic representation of the one-pot synthesis of polyacrylamide-metal nanocomposites.
Macromol. Rapid Commun. 2007, 28, 368–386
ß 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.mrc-journal.de 381