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

R. Hoogenboom, U. S. Schubert
materials. [99] A tube with steel wool, methyl methacrylate,
ethylene glycol dimethacrylate, and azoisobutyronitrile
was heated under microwave irradiation resulting in the
formation of a cross-linked network around the steel wool.
These iron fibers were subsequently removed using
concentrated hydrochloric acid leaving a channel-containing
cross-linked poly(methyl methacrylate) network.
The selective heating of the iron fibers was demonstrated
by applying 25 W power to a monomer-initiator solution
with and without iron powder. Without iron almost no
temperature increase was observed while in the presence
of iron the temperature increased quickly. The use of
thermal heating resulted in polymerization from the wall
of the flask and did not give the desired channel containing
materials.
Emulsion Polymerizations
Free radical polymerizations in emulsion are environmentally
benign alternatives due to the absence of organic solvents.
Many studies were already performed on microwaveassisted
emulsion polymerizations often resulting in faster
polymerizations. Palacios et al. have proposed a modeling
approach of such microwave-assisted polymerizations in
which a conventional model was expanded with a second
free radical chemical initiator to account for the microwave
acceleration. [100] The concentration of this artificial
radical initiator was related to the microwave power and
its half life time was related to the ratio of monomer
concentration to the rate of microwave absorption. With
these parameters it was possible to model and reproduce a
variety of polymerization kinetics for microwave-assisted
emulsion polymerizations that were published in recent
literature. Wu and coworkers have replaced thermal
heating by microwave heating in their recent studies on
the emulsion polymerization of styrene in the presence [101]
and absence [102] of surfactants. However, their main
objectives were related to the principles of emulsion
polymerization and they did not study the effects of
microwave irradiation on the emulsion polymerization
process. Holtze et al. exploited the fast heating and cooling
that is provided by modern microwave reactors for
the synthesis of ultrahigh molecular weight polystyrene
in miniemulsion. [103] Application of alternating short
microwave-irradiation pulses (10 s) and long cooling
intervals (15 min) resulted in radical formation in the
irradiation phase and propagation during the cold phase. It
was demonstrated that after the hot phase, zero or one
radical survives per droplet generating high molecular
weight polystyrene until chain-transfer to monomer
occurs and a new polymer chain is formed. The occurrence
of non-thermal microwave effects is excluded by the
authors and all the observed effects could be explained
with common radical kinetics for heterophase systems.
O’Mealey et al. studied the effect of microwave irradiation
(multimode microwave reactor) on the emulsion polymerization
of styrene and methyl methacrylate. [104] The
polymerization kinetics for styrene were similar under
thermal and microwave heating, whereby only higher
monomer conversions were observed at longer reaction
times. Surprisingly, the emulsion polymerization of
methyl methacrylate was faster with microwave irradiation
at all investigated reaction times. Additionally,
the obtained molecular weights for polystyrene and
poly(methyl methacrylate) were significantly higher when
the polymerizations were performed under microwave
irradiation, which was ascribed to a possible higher degree
of branching when microwave irradiation was applied.
Surfactant-free emulsion polymerization of methyl methacrylate
was studied by Bao and Zhang using both thermal
and microwave heating resulting in very uniform
poly(methyl methacrylate) particles. [105] It was found that
the use of microwave irradiation clearly accelerated the
emulsion polymerizations, which was ascribed to an
increased thermal decomposition rate of the potassium
persulfate due to microwave irradiation. This was confirmed
by an Arrhenius plot of the potassium persulfate
decomposition that showed a decrease in the activation
energy from 128 to 106 kJ mol 1 when going from
thermal heating to microwave heating (Figure 6).
The microwave-assisted emulsion polymerization of
methyl methacrylate was also studied in comparison to
thermal heating by Palacios and coworkers. [106] The
application of only 50 W microwave power accelerated
the polymerization compared to thermal heating. In
addition, higher molecular weight polymers with lower
polydispersity indices, sometimes even in the range of
Figure 6. Arrhenius plot for the decomposition of potassium
persulfate under microwave (40 and 300 W) and thermal heating
(K p ¼ decomposition rate; reprinted with permission from
ref. [105] ).
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Macromol. Rapid Commun. 2007, 28, 368–386
ß 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/marc.200600749