Microwave-Assisted Polymer Synthesis: Recent Developments in a ...
Microwave-Assisted Polymer Synthesis: Recent Developments in a ...
Microwave-Assisted Polymer Synthesis: Recent Developments in a ...
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R. Hoogenboom, U. S. Schubert<br />
the more homogeneous heat<strong>in</strong>g under microwave irradiation<br />
that becomes more apparent at high conversions due<br />
to the <strong>in</strong>creased viscosity of the polymerization mixture. In<br />
contrast, Zhu et al. observed a clear acceleration of the NMP<br />
of styrene <strong>in</strong> bulk under microwave irradiation (monomode<br />
microwave reactor). [116] The rate of polymerization<br />
<strong>in</strong>creased with <strong>in</strong>creas<strong>in</strong>g microwave power while the<br />
control over the polymerization was ma<strong>in</strong>ta<strong>in</strong>ed. The<br />
microwave-assisted RAFT polymerization of both styrene<br />
and methyl methacrylate was <strong>in</strong>vestigated by Perrier and<br />
coworkers. [117] For both monomers an <strong>in</strong>crease <strong>in</strong> polymerization<br />
rate was observed when microwave irradiation<br />
was applied as heat source. Moreover, l<strong>in</strong>ear first-order<br />
k<strong>in</strong>etics were observed and a l<strong>in</strong>ear <strong>in</strong>crease <strong>in</strong> molecular<br />
weight with conversion demonstrat<strong>in</strong>g a good control over<br />
the polymerizations under microwave irradiation was also<br />
observed. Zhu et al. reported similar observations for the<br />
RAFT polymerization of styrene us<strong>in</strong>g a domestic microwave<br />
oven. [118]<br />
microwave irradiation that were observed <strong>in</strong> recent years.<br />
In addition, several open questions such as specific microwave<br />
absorption <strong>in</strong> copolymerizations as well as upscal<strong>in</strong>g<br />
issues have to be addressed <strong>in</strong> the com<strong>in</strong>g years.<br />
Nevertheless, it is expected that the use of microwave<br />
irradiation will evolve from a research topic <strong>in</strong>to a common<br />
research tool <strong>in</strong> polymer science. Whether microwave-assisted<br />
polymerization procedures will be <strong>in</strong>corporated<br />
<strong>in</strong>to future commercial processes is still uncerta<strong>in</strong><br />
and would only be feasible for processes that show a clear<br />
(economical) advantage over thermal heat<strong>in</strong>g.<br />
Acknowledgements: This study was supported by the Dutch<br />
<strong>Polymer</strong> Institute (DPI) and the Fonds der Chemischen Industrie.<br />
Received: October 30, 2006; Accepted: December 8, 2006; DOI:<br />
10.1002/marc.200600749<br />
Keywords: microwave irradiation; polymer; radical polymerization;<br />
r<strong>in</strong>g-open<strong>in</strong>g polymerization; step-growth polymerization<br />
Summary and Outlook<br />
The use of microwave irradiation <strong>in</strong> polymer chemistry is a<br />
rapidly expand<strong>in</strong>g field of research. In the last two years,<br />
the number of publications per year has doubled. The ma<strong>in</strong><br />
reason for this <strong>in</strong>creased <strong>in</strong>terest <strong>in</strong> the use of microwave<br />
irradiation is the often observed acceleration of polymerizations<br />
when performed with microwave irradiation.<br />
Nevertheless, <strong>in</strong> many cases the acceleration is due to<br />
<strong>in</strong>creased polymerization temperatures compared to thermal<br />
heat<strong>in</strong>g, whereby it should be noted that these<br />
<strong>in</strong>creased temperatures are much easier accessible when<br />
us<strong>in</strong>g microwave irradiation. In addition to the <strong>in</strong>creased<br />
temperatures, many of the observed advantages of microwave<br />
heat<strong>in</strong>g can be ascribed to the fast and homogeneous<br />
heat<strong>in</strong>g that can prevent thermal decomposition and/or<br />
other side reactions caused by local overheat<strong>in</strong>g over the<br />
polymerization mixtures with thermal heat<strong>in</strong>g. Besides<br />
these improvements that can be ascribed to thermal<br />
effects under microwave irradiation, several examples<br />
have been reported <strong>in</strong> which non-thermal microwave<br />
effects were observed. These non-thermal microwave<br />
effects can be rationalized by specific microwave absorption<br />
of polar <strong>in</strong>termediates and or reagents mak<strong>in</strong>g them<br />
more reactive. However, the observation of such effects<br />
seems to depend on the choice of microwave reactors<br />
(domestic, monomode, or multimode microwaves) as well<br />
as the choice of polymerization conditions. In addition,<br />
attempts to change monomer reactivity and reactivity<br />
ratios <strong>in</strong> copolymerizations of monomers with different<br />
polarities have not succeeded so far.<br />
The field of microwave-assisted polymerizations is<br />
believed to cont<strong>in</strong>ue its rapid expansion <strong>in</strong> the com<strong>in</strong>g<br />
years <strong>in</strong>spired by the large number of beneficial effects of<br />
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ß 2007 WILEY-VCH Verlag GmbH & Co. KGaA, We<strong>in</strong>heim<br />
DOI: 10.1002/marc.200600749