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

R. Hoogenboom, U. S. Schubert
polymerization method yielded higher molecular weight
polymers and slightly lower polydispersity indices,
whereby the direct comparison was performed at different
polymerization temperatures. The Suzuki C–C coupling
method was investigated under microwave irradiation
(monomode microwave reactor) by Scherf et al. [39]
Three differently substituted naphthalene boronic ester
monomers were coupled to 4,4 0 -didecyl-2 0 ,5 0 - dibromoterephthalophenone
using a palladium catalyst. The polymerization
of the less sterically hindered 2,6-diboronic
ester naphthalene under both microwave irradiation and
thermal heating gave similar results. However, the more
sterically hindered 1,5-diboronic ester naphthalenes could
not be polymerized using conventional heating, but the
use of microwave irradiation allowed their polymerization.
All resulting conjugated polymers served as precursors
for the formation of ladder-type polymers, which
have desirable optical and emitting properties. Another
palladium-catalyzed C–C coupling procedure, namely the
Heck reaction, was studied for the formation of conjugated
polymers under both microwave (monomode microwave
reactor) and thermal heating by Ritter and coworkers. [40] The
copolymerization of divinylbenzene and 1,4-diiodo-2,5-
dibutoxybenzene [Scheme 4(c)] was studied in refluxing
dioxane to ensure the same polymerization temperature in
both open vessel reactions. This direct comparison revealed
a slight acceleration of the polymerization under microwave
irradiation. In addition, the molecular weight of the
resulting poly(2,5-dibutoxy-1, 4-phenylenevinylene) was
also a little higher when microwave irradiation was applied
as heat source.
The previous examples were all based on C–C coupling
reactions that require two different functional groups. On
the contrary, Ni(0)-catalyzed coupling reactions can be
used for homocoupling reactions. Yamamoto and coworkers
used a Ni(0) catalyzed polymerization procedure for
the synthesis of poly(pyrazine-2,5-diyl) starting from
2,5-dibromopyrazine. [41] The thermal polymerization
required 2 d polymerization time, while the microwaveassisted
procedure was finished within 10 min. Unfortunately,
no temperature was given for the microwave
procedure and, thus, it is not clear whether the observed
acceleration is due to thermal or microwave effects.
Nevertheless, the polymer obtained under microwave
irradiation had a higher molecular weight. Carter et al.
reported the use of a Ni(0) catalyzed C–C coupling for
the synthesis of poly(biphenylmethylene)s starting from
bistriflate monomers under microwave irradiation (monomode
microwave reactor). [42] It was demonstrated that the
polymerization could be performed using both thermal
and microwave heating, whereby no significant differences
were observed in the resulting polymers. Nevertheless,
the microwave-assisted polymerizations were
performed for only 10 min at 200 8C, whereas the conventional
polymerizations were performed for 16–24 h at
80 8C. Surprisingly, endcapping the polymerization by the
presence of 4-bromostyrene could also be performed at
200 8C under microwave heating without coupling or
degradation of the vinyl groups. The Ni(0)-mediated
polymerization procedure was also applied by Scherf
and coworkers for the synthesis of polyfluorenes
with electrophosphorescent platinum-salen chromophores
[Scheme 4(d)]. [43] The polymerization required
3 d under thermal heating in tetrahydrofuran (THF)
at 80 8C and could be accelerated down to 12 min
under microwave irradiation at 115 or 220 8C in THF of
a mixture of N,N-dimethylformamide (DMF) and toluene,
respectively. The resulting copolymers revealed high
electroluminescence efficiencies due to energy transfer
from the polyfluorene to the salen complex.
Other Step-Growth Polymerizations
Besides poly(amide-imide)s and conjugated polymers,
several other polymers have been prepared using stepgrowth
polymerization mechanisms under microwave
irradiation in the last couple of years.
The synthesis of biodegradable aliphatic polyesters was
investigated under microwave irradiation (monomode
microwave reactor) by Nagahata and coworkers. [44] Direct
polycondensation of succinic acid and butanediol was
investigated in the presence of a stannyl catalyst
[Scheme 5(a)]. The polymerization conditions were investigated
in detail, whereby the (absence of) solvent, catalyst
concentration, polymerization temperature, stoichiometry,
and reaction time were varied. The optimal polymerization
conditions were also tested in a conventionally
heated reaction demonstrating a much lower polymerization
rate compared to the microwave-assisted polymerization
method. The authors speculate that the ten-fold
acceleration under microwave irradiation might be due to
specific microwave absorption by the released water
molecules leading to quicker evaporation of the water and
thus a shift in equilibrium towards the polymer. Chatti
et al. investigated the synthesis of poly(ether-ester)s from
an isosorbide-based aliphatic diol and two different diacid
chlorides under microwave irradiation (monomode microwave
reactor). [45] The used isosorbide-based material is an
interesting building block for polymer structures because
it represents a renewable resource. The bulk polymerizations
revealed a remarkable acceleration under microwave
irradiation compared to thermal heating under the same
conditions that was ascribed to the enhanced polarity of
the transition state during the ester formation. In addition,
it was found that the microwave-assisted polymerization
procedure gave less degradation at longer reaction times
(8 h) compared to conventional heating. The synthesis of
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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