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

R. Hoogenboom, U. S. Schubert
Figure 2. Polymerization kinetics that were obtained for the enzyme-catalyzed ring-opening
polymerization of e-caprolactone under microwave heating and oil bath heating in refluxing
benzene (left) and diethyl ether (right). Reprinted with permission from ref. [50]
the initial stage and thus limits the number of polymer
chains, whereas thermal heating favored the formation of
growing centers in the initial stage. As a result, the
microwave-assisted polymerization procedure resulted in
higher molecular weight polymers. Kerep and Ritter
investigated the enzyme-catalyzed (novozym 435) polymerization
of e-caprolactone under reflux conditions in toluene,
benzene, and diethyl ether using both thermal and
microwave (monomode microwave reactor) heating. [50] A
strong effect of the used solvents was observed: The
polymerization was decelerated under microwave irradiation
in toluene and benzene (Figure 2, left), while it was
accelerated under microwave irradiation in diethyl ether
(Figure 2, right). In this particular case, the boiling point
rather than the nature of the solvents (all apolar), seems to
be a critical factor. The authors proposed that the observed
effects might be due to a microwave- enhanced fit between
the active center of the enzyme and the e-caprolactone
substrate under mild conditions.
Loupy et al. investigated the effect of microwave
irradiation on the polymerization of e-caprolactone with
lanthanide halide catalysts using different heat profiles. [51]
When 200 W microwave power was applied constantly,
broader molecular weight distributions were obtained
compared to the use of an initial power boost (300 W). This
observed effect was attributed to the faster heating with
the higher initial microwave power, which inhibits
secondary transfer reactions. Moreover, direct comparison
of thermal and microwave heating demonstrated the
necessity of longer reaction times with thermal heating as
well as lower molecular weights and broader molecular
weight distributions. In degradation studies of the synthesized
poly(e-caprolactone), it was found that the
presence of lanthanide catalysts accelerates hydrolytic
degradation but it inhibits enzymatic degradation. In
addition, the synthesized acid-functional polymers were
converted into macromonomers by esterification with
2-hydroxyethyl methacrylate. Ritter and coworkers
demonstrated the direct synthesis of similar poly(ecaprolactone)
macromonomers via the stannous octanoate
catalyzed ring-opening polymerization of e-caprolactone
under microwave irradiation. [52] Although no microwaveacceleration
was observed for the polymerization, the use
of microwave irradiation allowed rapid optimization of
the polymerization conditions. Fang et al. also applied a
microwave-assisted polymerization (domestic microwave
oven) procedure for the stannous octanoate catalyzed
ring-opening polymerization of e-caprolactone. [53] The
amino groups of chitosan were protected with phthalic
anhydride allowing the use of the primary hydroxyl-groups
as initiators for the ring-opening polymerization
as depicted in Scheme 6. After the polymerization, the
amino groups were deprotected resulting in poly(ecaprolactone)
grafted chitosan. Compared to conventional
heating, higher grafting densities were obtained with
microwave heating. Moreover, the grafting procedure was
greatly accelerated when more than 450 W of microwave
power was applied. Yu and Liu have used a catalystfree
microwave-assisted polymerization procedure for the
preparation of poly(e-caprolactone)-block-poly(ethylene glycol)-block-poly(e-caprolactone)
copolymers. [54] The monomer
conversion as well as the resulting molecular weight of the
polymers could be adjusted by changes in the irradiation
time, microwave power as well as length and amount of
added poly(ethylene glycol). The resulting triblock copolymers
were studied for the encapsulation and release of
ibuprofen revealing an almost linear release in time.
Besides e-caprolactone, a variety of other cyclic esters
have been used in microwave-assisted polymerization
procedures. Shu et al. described the stannous octanoatecatalyzed
ring-opening polymerization of D,L-lactide under
ambient atmosphere and vacuum using a domestic
microwave oven. [55] It was found that the efficient heating
of microwave irradiation resulted in a successful polymerization
without the need for vacuum or an inert
atmosphere as required with thermal heating. In addition,
higher temperatures were attainable before thermal
decomposition occurred when using microwave irradiation.
This is most likely due to the
more homogeneous heat profile.
Nevertheless, the observed
acceleration under microwave
irradiation could not be completely
designated to the higher
temperatures and, thus, it was
concluded that non-thermal
microwave effects also played a
role. The same polymerization
system was studied by Liu
and coworkers. [56] The focus of
this study was the effect of
microwave power on the polymerization
process under micro-
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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