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

R. Hoogenboom, U. S. Schubert
Cationic Ring-Opening Polymerizations
Cationic ring-opening polymerizations seem to be very
well suited to address non-thermal microwave effects due
to the cationic propagating species. In recent literature,
several studies were reported to address this topic for the
cationic ring-opening polymerization of 2-oxazolines,
which will be discussed in this section.
The cationic ring-opening polymerization of 2-ethyl-2-
oxazoline under microwave irradiation (monomode microwave
reactor) was first reported by Schubert and coworkers.
[60] It was demonstrated that the polymerization in
acetonitrile could be tremendously accelerated under
superheated conditions up to 180 8C without losing the
living character of the polymerization. Reference experiments
in a pressure NMR tube and larger pressure
reactors [61] revealed that the polymerization was only
accelerated due to thermal effects. This study was further
expanded to other 2-oxazoline monomers (methyl, nonyl,
and phenyl substituted) and it was demonstrated that the
polymerizations of these monomers could also be accelerated
when going to superheated conditions. [62] The
Arrhenius parameters were found to be in the same range
as for thermal polymerizations and, thus, it was concluded
that the observed accelerations are only due to the
increased polymerization temperatures. Nevertheless, it
was found that the control over the polymerizations was
slightly better under microwave irradiation due to the
more homogeneous heat profiles. Additional studies were
performed on different solvents for the polymerization of
2-nonyl-2-oxazoline revealing that the polymerization rate
and the Arrhenius parameters were comparable in both
acetonitrile and dichloromethane. [63] Sinnwell and Ritter
also found that the cationic ring-opening polymerization
of 2-phenyl-2-oxazoline [64] and 2-phenyl-2-oxazine [65]
could be accelerated in both open and closed reactors
under microwave irradiation (monomode microwave
reactor). Surprisingly, reference experiments with thermal
heating revealed that the acceleration was due to nonthermal
effects, which was attributed to specific microwave
absorption of the cationic propagating species. In the
case of 2-phenyl-2-oxazine, the microwave-acceleration
was demonstrated using methyl tosylate and butyl iodide
as initiators. Although these initiators result in different
polymerization rates due to the different counterions, the
observed acceleration was in both cases a factor 1.8. In
addition, it was found that the initiating group can be used
to tune the glass transition temperature of the resulting
polymers. A careful evaluation of these opposite results on
the (non)-existence of non-thermal microwave effects
revealed that the polymerization rates for 2-phenyl-2-
oxazoline under microwave irradiation were almost the
same in both studies, but different polymerization rates
were found in the thermal reference. [66] Another claimed
Figure 4. Size exclusion chromatograms as well as pictures of
upscaling the cationic ring-opening polymerization of 2-ethyl-2-
oxazoline from 4 mmol (a) via 200 mmol (b) to 1 000 mmol
(c) under superheated microwave conditions. Reprinted with
permission from ref. [67]
advantage of microwave heating, namely direct scalability,
was addressed by Schubert et al. [67] The superheated
cationic ring-opening polymerization of 2-ethyl-2-oxazoline
was performed under microwave irradiation at scales from
4 mmol up to 1 mol using both monomode and multimode
microwave reactors yielding highly comparable poly(2-
ethyl-2-oxazoline)s with low polydispersity indices regardless
of the scale (Figure 4). Further upscaling using continuous
flow microwave reactors resulted in broadening of the
molecular weight distribution due to the residence time
distribution of the reactors. [68] Nonetheless, reasonably
well-defined poly(2-ethyl-2-oxazoline)s could be prepared
using continuous flow microwave set-ups. Very recently,
Schubert and coworkers investigated the cationic ringopening
polymerization of 2-ethyl-2-oxazoline and 2-phenyl-
2-oxazoline under microwave irradiation using ionic liquids
as solvent to exclude the use of organic solvents. [69,70] The use
of ionic liquids as strongly polar solvents resulted in an
acceleration of the polymerization due to a better stabilization
of the ionic propagating species. However, the same
acceleration could be reproduced with thermal heating.
Despite the current debate on non-thermal microwave
effects for the polymerization of 2-oxazolines, the
improved microwave-assisted polymerization procedure
was applied for the fast synthesis of libraries of poly(2-
alkyl-2-oxazoline)s, [71] quasi-diblock [72] and diblock copoly(2-oxazoline)s
[73] as well as triblock terpoly(2-oxazoline)s
[74] to elucidate structure-property relationships (this
work was recently featured in ref. [75] ). The first successful
synthesis of triblock terpoly(2-oxazoline)s with polydispersity
indices below 1.30 was claimed to be facilitated
by the improved control over the polymerization due to
more homogeneous heating. In addition, a 2-oxazoline
378
Macromol. Rapid Commun. 2007, 28, 368–386
ß 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/marc.200600749