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

Microwave-Assisted Polymer Synthesis: Recent Developments in ...
Scheme 5. Examples of other polycondensations: (a) Polyester synthesis from succinic acid and butanediol; (b) polycondensation
of dimethylhydrogen phosphonate and poly(ethylene glycol) resulting in poly(alkylene hydrogen phosphonate); (c) poly(urea) or
poly(thiourea) synthesis.
poly(ether)s from trichlorophenol was investigated under
microwave irradiation by Kisakuerek and coworkers
(domestic microwave oven). [46] Surprisingly, it was found
that, depending on the microwave irradiation and polymerization
time, a conjugated polymer was formed next to
the expected poly(dichlorophenylene oxide). Moreover, the
induction period of the polymerization was shorter under
microwave irradiation compared to conventional heating.
Troev and coworkers reported the step-growth polymerization
of dimethyl hydrogen phosphonate with poly(ethylene
glycol) resulting in biodegradable poly(alkylene
hydrogen phosphonate) [Scheme 5(b)]. [47] Thermal and
microwave heating both resulted in comparable polymers,
whereby the microwave polymerization was performed
for 55 min at 140–180 8C and the thermal polymerization
for 9.5 h at 130–140 8C. However, the higher temperatures
used for the microwave polymerization could not be
applied in the thermal polymerization process due to
thermal degradation of the dimethyl hydrogen phosphonate.
The syntheses of poly(urea)s and poly(thiourea)s
from the polycondensation of urea or thiourea with a
series of diamines and a catalytic amount of p-toluenesulfonic
acid were studied under microwave irradiation
(domestic microwave oven) by Banihashemi et al.
[Scheme 5(c)]. [48] The solvent, microwave power as well
as the polymerization time were optimized for the microwave-assisted
polymerizations. Even though no comparison
was made with thermal heating, the authors concluded
that microwave irradiation is a fast and efficient
method for such polymerizations.
Ring-Opening Polymerizations
Polymerization of cyclic monomers is a popular approach
for the synthesis of polymers, because these ring-opening
polymerizations do not suffer from equilibria between the
polymer with elimination products and the monomers
that often limit the attainable molecular weights in
polycondensations. As a result, ring-opening polymerization
methods allow easier preparation of high-molecular
weight polymers. In addition, a variety of ring-opening
polymerizations can be performed via controlled/living
polymerization mechanisms that allow the formation of
well-defined (co)polymers. In the field of microwave-assisted
polymerizations, the main focus has been on
the ring-opening polymerization of cyclic esters as well as
the living cationic ring-opening polymerization of
2-oxazolines that will be discussed in the following
sections.
Aliphatic Polyesters
Ring-opening polymerization of cyclic esters like lactones
and lactides results in the formation of biodegradable
aliphatic polyesters. These materials are promising candidates
for bio-related applications such as drug-delivery or
as scaffolds in tissue engineering. The majority of the
investigations on microwave-assisted ring-opening polymerizations
of cyclic esters were performed using e-caprolactone
or lactides.
The effect of microwave irradiation on the chain propagation
of the benzoic acid initiated polymerization of
e-caprolactone was investigated in detail by Yu and Liu
using closed ampoules in a domestic microwave oven. [49]
The chain propagation was studied as a function of
microwave power, monomer to initiator ratio, and polymerization
temperature. Nevertheless, the most interesting
results were obtained by a direct comparison of the use
of microwave heating and thermal heating demonstrating
that the use of microwave heating favors chain growth in
Macromol. Rapid Commun. 2007, 28, 368–386
ß 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.mrc-journal.de 375