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

Microwave-Assisted Polymer Synthesis: Recent Developments in ...
monomer based on unsaturated soy bean fatty acids
was polymerized successfully under microwave irradiation,
whereby the unsaturated sites were not affected
by the polymerization of the 2-oxazoline ring. [76]
The resulting polymers with unsaturated side-chains
could be cross-linked by UV-irradiation. This soy-based
2-oxazoline monomer was also used for the microwaveassisted
two-step synthesis of amphiphilic poly(2-ethyl-
2-oxazoline)-block-poly(2-soy-alkyl-2-oxazoline) block
copolymers that were successfully applied for the preparation
of cross-linked micelles. [77,78]
Radical Polymerizations
Scheme 7. Schematic representation of the different selectivity in
the synthesis of (meth)acrylamides that was found by Ritter and
coworkers using microwave and thermal heating. [79,80]
Radical polymerization techniques are widely used in both
industry and academia because of the broad range of
possible monomers and the relatively simple polymerization
procedures. Moreover, radical polymerizations are
compatible with water and therefore emulsion and dispersion
polymerization procedures that exclude the use of
organic solvents can be applied.
This section will discuss the recent developments in
microwave-assisted free radical polymerizations, free
radical polymerizations in emulsion as well as controlled
radical polymerizations. The section on free radical polymerization
will specifically address the issue of copolymerizing
different monomers under microwave irradiation
since specific microwave absorption may lead to
changes in monomer reactivity or in transition states and
thus in the reactivity ratios.
Free-Radical Polymerizations
The free-radical polymerization of vinylic monomers is one
of the major processes for the industrial production of bulk
polymers like polystyrene and poly(methyl methacrylate).
The previous review on microwave-assisted polymerizations
covered a wide range of investigations on the effect
of microwave irradiation on free radical homopolymerizations.
[7] However, in recent years only a few of such
investigations were reported and the major focus has
shifted to copolymerizations.
Nevertheless, Ritter and coworkers have investigated
the microwave-assisted direct synthesis and polymerization
of a series of chiral (meth)acrylamides. [79,80] The direct
synthesis of chiral meth(acrylamide) from (meth)acrylic
acid and 1-phenylethylamine under microwave irradiation
yielded the desired vinyl monomers, whereas the
same reaction under thermal heating resulted mainly in
the formation of the Michael addition product (Scheme 7).
The preferred formation of the desired (meth)acrylamides
under microwave irradiation could be rationalized by the
(zwitter) ionic intermediates that lead to the formation of
the meth(acrylamide)s. The direct synthesis of (meth)acrylamides
under microwave irradiation represents a
major improvement compared to the conventional methods
that make use of acid chloride reagents and/or coupling
agents. In addition, it was demonstrated that the
synthesis and polymerization of 1-phenylethyl (meth)acrylamide
can be performed simultaneously in a one-pot
reaction under microwave irradiation.
Similarly, Bezdushna and Ritter reported a microwave
acceleration for the direct synthesis of N-phenylmaleimide
from maleic anhydride and aniline based on specific
microwave absorption of the ionic intermediates. [81] Moreover,
the synthesis of N-(2-ethoxyethyl)maleimide from its
corresponding maleic acid in acetic anhydride as reactive
solvent was also investigated under microwave irradiation.
[82] Although this reaction also proceeds via ionic
intermediates, no acceleration was observed due to
specific microwave absorption in this case. Fischer et al.
investigated the free radical polymerization of N-alkylacrylamides
with 3-mercaptopropionic acid as chain
transfer agent in methanol with thermal heating at
ambient pressure and under superheated conditions as
well as under microwave irradiation. [83] The chain transfer
polymerization could be accelerated from 5 to 1 h when
going to superheated conditions with thermal heating.
When changing the heat source to microwave irradiation,
the polymerization was further accelerated down to
several seconds. However, the microwave-assisted polymerizations
were performed without solvent in a domestic
microwave oven under power control. Therefore, it is not
clear whether the acceleration is due to thermal effects or
not. Nevertheless, the accelerated microwave polymerization
procedure was incorporated into university education
allowing synthesis, isolation and characterization of the
polymers within a one-day laboratory session. [84]
The copolymerization of monomers with significantly
different microwave absorption characteristics is believed
Macromol. Rapid Commun. 2007, 28, 368–386
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