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

R. Hoogenboom, U. S. Schubert
the more homogeneous heating under microwave irradiation
that becomes more apparent at high conversions due
to the increased viscosity of the polymerization mixture. In
contrast, Zhu et al. observed a clear acceleration of the NMP
of styrene in bulk under microwave irradiation (monomode
microwave reactor). [116] The rate of polymerization
increased with increasing microwave power while the
control over the polymerization was maintained. The
microwave-assisted RAFT polymerization of both styrene
and methyl methacrylate was investigated by Perrier and
coworkers. [117] For both monomers an increase in polymerization
rate was observed when microwave irradiation
was applied as heat source. Moreover, linear first-order
kinetics were observed and a linear increase in molecular
weight with conversion demonstrating a good control over
the polymerizations under microwave irradiation was also
observed. Zhu et al. reported similar observations for the
RAFT polymerization of styrene using a domestic microwave
oven. [118]
microwave irradiation that were observed in recent years.
In addition, several open questions such as specific microwave
absorption in copolymerizations as well as upscaling
issues have to be addressed in the coming years.
Nevertheless, it is expected that the use of microwave
irradiation will evolve from a research topic into a common
research tool in polymer science. Whether microwave-assisted
polymerization procedures will be incorporated
into future commercial processes is still uncertain
and would only be feasible for processes that show a clear
(economical) advantage over thermal heating.
Acknowledgements: This study was supported by the Dutch
Polymer Institute (DPI) and the Fonds der Chemischen Industrie.
Received: October 30, 2006; Accepted: December 8, 2006; DOI:
10.1002/marc.200600749
Keywords: microwave irradiation; polymer; radical polymerization;
ring-opening polymerization; step-growth polymerization
Summary and Outlook
The use of microwave irradiation in polymer chemistry is a
rapidly expanding field of research. In the last two years,
the number of publications per year has doubled. The main
reason for this increased interest in the use of microwave
irradiation is the often observed acceleration of polymerizations
when performed with microwave irradiation.
Nevertheless, in many cases the acceleration is due to
increased polymerization temperatures compared to thermal
heating, whereby it should be noted that these
increased temperatures are much easier accessible when
using microwave irradiation. In addition to the increased
temperatures, many of the observed advantages of microwave
heating can be ascribed to the fast and homogeneous
heating that can prevent thermal decomposition and/or
other side reactions caused by local overheating over the
polymerization mixtures with thermal heating. Besides
these improvements that can be ascribed to thermal
effects under microwave irradiation, several examples
have been reported in which non-thermal microwave
effects were observed. These non-thermal microwave
effects can be rationalized by specific microwave absorption
of polar intermediates and or reagents making them
more reactive. However, the observation of such effects
seems to depend on the choice of microwave reactors
(domestic, monomode, or multimode microwaves) as well
as the choice of polymerization conditions. In addition,
attempts to change monomer reactivity and reactivity
ratios in copolymerizations of monomers with different
polarities have not succeeded so far.
The field of microwave-assisted polymerizations is
believed to continue its rapid expansion in the coming
years inspired by the large number of beneficial effects of
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DOI: 10.1002/marc.200600749