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

Microwave-Assisted Polymer Synthesis: Recent Developments in ...
benzoguanamine and pyromellitic anhydride resulting in
the formation of the p-p conjugated poly(amic acid). [12]
Although the polymer was synthesized under microwave
irradiation, no comparison was made to thermal heating.
The side chains of the resulting poly(amic acid) were
further functionalized using both azo as well as isocyanate
coupling procedures at moderate temperatures (no microwave
irradiation) to study the influence on the fluorescence
and non-linear optical properties of the materials.
Direct synthesis of polyimides via the step-growth
polymerization of isocyanates and anhydrides under
microwave irradiation was reported by Yeganeh et al. [13]
The polymerizations were performed in a closed Teflon
mold inside a domestic microwave oven. The feasibility of
this approach was first demonstrated by the model
reaction of phthalic anhydride with p-phenylene diisocyanate
followed by the model polymerization of benzophenone
tetracarboxylic dianhydride and p-phenylene
diisocyanate [Scheme 1(c)]. The effects of microwave
power, solvent amount, reaction time, and catalyst were
optimized for this model polymerization reaction. Subsequently,
the optimal microwave-assisted polymerization
procedure was applied for the synthesis of novel
polyimides.
Most of the recent investigations on microwave-assisted
polycondensations have been performed on the synthesis
of poly(amide-imides). The reported polymerizations were
all performed using domestic microwave ovens. The
monomers were placed and ground in a porcelain dish.
After the addition of a small amount of o-cresol, the mixture
was ground again followed by microwave heating.
The microwave-assisted synthesis of a series of optically
active poly(amide-imides), in which the chirality resulted
from the incorporation of amino acids, was reported by
Faghihi et al. [14–16] N,N 0 -(pyromellitoyl)-bis(amino acid
chloride)s were reacted with eight hydantoin derivatives
as depicted in Scheme 2(a). The polycondensations
proceeded rapidly compared to the conventional polymerization
method under thermal heating and was
completed within 10 min. Nevertheless, the occurrence
of non-thermal microwave effects was not investigated
and no direct comparison between the different heat
sources was made. To provide the optical activity,
L-leucine, L-valine, or L-alanine were incorporated as amino
acids. The resulting poly(amide-imide)s might, e.g., be
suitable as column material for the separation of
enantiomeric mixtures. Similarly, Faghihi and Hajibeygi
reported the polycondensation reaction of N,N 0 -(4,4 0 -
diphenyl ether) bistrimellitimide diacid chloride with
the same hydantoin derivatives under microwave irradiation.
[17] The increased solubility of the resulting
poly(amide-imide)s could be the basis for a novel class
of processable high-performance plastics. Mallakpour and
Kowsari investigated the synthesis of optically active
poly(amide-imide)s by the polycondensation of N,N 0 -(4,
4 0 -oxydiphthaloyl)-bis(amino acid chloride)s (amino acids
used are L-leucine, L-valine, and L-isoleucine) and aromatic
diamines under microwave irradiation [Scheme 2(b)]. [18–20]
It was demonstrated that comparable polymers can be
obtained under both microwave and thermal heating
(different procedures), although shorter reaction times
were required when using microwave irradiation.
The resulting optically active polymers were readily soluble
in organic solvents and exhibited good thermal
stability. Similarly, Mallakpour and Shahmohammadi
replaced the central N,N 0 -(4,4 0 -oxydiphthaloyl) group by
Scheme 2. Poly(amide-imide)s that were synthesized under microwave irradiation: (a) Reaction of N,N 0 -(pyromellitoyl)-bis(amino acid
chloride)s with hydantoin; (b) copolymerization of N,N 0 -(4,4 0 -oxydiphthaloyl)-bis(amino acid chloride)s with aromatic diamines; (c)
synthesis of flame-retardant polymers based on N,N 0 -(3,3 0 -diphenylphenylphosphine oxide) bistrimellitimide and aromatic diamines.
Macromol. Rapid Commun. 2007, 28, 368–386
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