Orthogonal synthesis of poly(aryl ether amide) dendrons

Article abstract of DOI:10.1021/ma051538y

A simple orthogonal convergent methodology for the synthesis poly(aryl ether amide) dendrons was introduced. It was found that nucleophilic aromatic substitution and amidation were chosen as orthogonal coupling reactions because each coupling reaction do

Full text of DOI:10.1021/ma051538y

Volume 38, Number 23
November 15, 2005
© Copyright 2005 by the American Chemical Society
Communications to the Editor
Orthogonal Synthesis of Poly(aryl ether
amide) Dendrons
Insik In and Sang Youl Kim*
Department of Chemistry and School of Molecular
Science (BK21), Korea Advanced Institute of Science and
Technology, 373-1, Guseong-dong, Yuseong-Gu, Daejeon
305-701, Korea
Received July 14, 2005
Revised Manuscript Received September 18, 2005
Dendrimers are monodisperse and perfectly branched
macromolecules that are the objects of continuously
increasing interests because of their predetermined
structures at the nanoscale region and the resulting
unique properties.1 Generally, dendrimers are synthe-
sized by the divergent and the convergent synthetic
methods. These synthetic methods inevitably require a
repeated deprotection step in addition to the coupling
step for the synthesis of the next generation dendron.2
Several efficient approaches have been proposed to
accelerate the dendrimer synthesis. These include the
double-stage convergent and double-exponential growth
approaches as well as the hypermonomer and orthogo-
nal monomer approaches.2a Among these approaches,
the orthogonal monomer approach, which excludes the
need for activation of peripheral or focal point moieties
to continue dendrimer growth, is the most powerful
since it gives a new generation at each step.3 For
utilizing the orthogonal monomer approach for a certain
dendrimer synthesis, two types of orthogonal AB2 units,
which contain two pairs of complementary coupling
functionalities, must be prepared.
Figure 1. Orthogonal strategy for the synthesis of poly(aryl
ether amide) dendrons.
knowledge, such accelerating synthetic strategies have
never been attempted for the synthesis of poly(aryl
ether) dendrimers.
In this study, we introduce a simple orthogonal
convergent methodology for the synthesis of poly(aryl
ether amide) dendrons. Nucleophilic aromatic substitu-
tion (SNAr) and amidation were chosen as the orthogo-
nal coupling reactions because each coupling reaction
does not interfere with the other coupling reaction
(Figure 1). Here, both hydroxy nucleophiles and acti-
vated halide moieties (A and B) for the SNAr reaction
are latent during the amidation reaction. Also, both
amine and carboxylic acid moieties (C and D) for the
amidation reaction are inert under the conditions of the
SNAr reaction. Such orthogonal characteristics of both
reactions have been successfully utilized for the syn-
thesis of hydroxy-functionalized linear polyamides and
amine-functionalized linear poly(aryl ether ketones).6
Alternative stepwise reactions using these two or-
thogonal monomers can produce poly(aryl ether amide)
dendrons. A2C (two hydroxy groups (A) and one amine
(C)) and BD′2 (one activated fluorine (B) and two
carboxylic acid chlorides (D′)) monomers were selected
as the orthogonal monomer units for the synthesis of
poly(aryl ether amide) dendrons (Figure 2). Instead of
BD2 monomers having two carboxylic acid groups, BD′2
Recently, dendrimers based on the high-performance
polymers such as aromatic polyamides and poly(aryl
ethers) have been developed by several research groups.4
For the synthesis of dendritic polyamides, the double-
stage convergent approach, the orthogonal monomer
approach, and the unprotected AB2 monomer approach
have been successfully exploited.5 But to the best of our
* Corresponding author. E-mail: kimsy@kaist.ac.kr.
10.1021/ma051538y CCC: $30.25 © 2005 American Chemical Society
Published on Web 10/20/2005

9400 Communications to the Editor
Macromolecules, Vol. 38, No. 23, 2005
were attempted. Amidation of Gn-NH2 with p-fluoroben-
zoyl chloride quantitatively produced Gn-F.7 All the
dendrons were purified by just simple reprecipitation
of their tetrahydrofuran solution into methanol.
1H NMR spectra well matched with the assigned
structures of these dendrons. All dendrons showed
characteristic amide proton peaks at 10.2 ppm. The
amine protons at the focal point of G1-NH2 or G2-NH2
were well observed near 5.0 ppm, but the presence of
focal groups in G3-NH2 or G4-NH2 were difficult to be
identified because these focal groups contribute too
much small portion compared with these large dendron
structures. FTIR spectra of all dendrons showed the
characteristic absorption peaks of amide groups around
3300 and 1660 cm-1, corresponding to amide N-H
stretching and carbonyl CdO stretching, respectively.
Also, the absorption peaks of aromatic ether linkages
(Ar-O-Ar) were observed at 1240 cm-1 in all cases. All
dendrons showed unimodal molecular weight distribu-
tion with polydispersity values between 1.1 and 1.2,
which shows the effectiveness of this orthogonal syn-
thetic approach of poly(arylene ether amide) dendrons.
But the molecular weights of these dendrons might be
overestimated in GPC measurement with dimethylfor-
maide with lithium bromide (10 mmol/L) as an eluent,5b
and therefore, the exact masses of these dendrons were
monitored by MALDI-TOF spectroscopy. Up to the G3
dendrons, single or quadruple Na+ adducts were ob-
served in the MALDI spectrum using the dithranol
matrix with sodium iodide as ionizing agents. The multi-
cation adducts are not often observed in the MALDI-
TOF spectrum of dendritic polymers but may be origi-
nated from the high amide contents of these dendrons.8
In the case of higher generation dendrons, ionization
might be so difficult and only fragmented polymer-like
species were observed, and the variation of matrix or
ionizing agents was not effective in these cases.
Figure 2. Structure of two orthogonal units A2C and BD2.
monomers having two carboxylic acid chloride groups
were used for the synthesis of dendrons because direct
amidation reaction is possible between A and D′ moi-
eties without additional condensation reagents such as
DBOP. At first, first-generation dendrons with a focal
amine G1-NH2 were effectively synthesized from an A2C
orthogonal monomer unit via the SNAr reaction. Cor-
responding G1-F dendrons were synthesized by the
amidation reaction between G1-NH2 dendrons and p-
fluorobenzoyl chloride.7 From these two G1 dendrons
and two orthogonal monomers, A2C and BD′2, poly(aryl
ether amide) dendrons up to fourth generation were
successively synthesized as shown in Scheme 1.
Scheme 1. Orthogonal Synthesis of Poly(aryl ether
amide) Dendrons
All poly(aryl ether amide) dendrons show enhanced
solubility resulting from dendritic macromolecular struc-
tures compared with the linear or hyperbranched poly-
(aryl ether amides).9 These dendrons were easily soluble
in common organic solvents such as dichloromethane,
chloroform, tetrahydrofuran, and dimethylforamide while
the corresponding linear poly(aryl ether amides) often
meet solubility problem in chlorinated solvents unlike
other linear poly(aryl ethers). Thermal properties of
these dendrons were measured by DSC and TGA. All
eight dendrons were amorphous, and any melting
transition was not observed in DSC. G1 and G2 dendrons
showed glass transition temperatures between 109 and
181 °C, but higher generation dendrons did not show
any thermal transition up to 300 °C. It is not quite
certain but seems that higher generation dendrons show
higher glass temperatures and that the glass transition
temperature difference between Gn-NH2 and Gn-F de-
creases as the generation number increases. All den-
drons were highly thermally stable and showed no
weight loss up to 300 °C. In addition, G3-NH2 and G4-
NH2 showed visible mild fluorescence around 400 nm
by excitation with 280 nm light, which the phenyl
groups strongly absorbs, but lower generation dendrons
showed only weak emission in this wavelength range.
This generation-dependent fluorescence may reflect the
highly globular geometry of higher generation poly(aryl
ether amide) dendrons and the resulting enhanced
energy transfer processes within them.10
After each orthogonal reaction, Gn-NH2 changes to
G(n+1)-F while Gn-F transforms to G(n+1)-NH2. Because
this approach leads to the formation of dendrimers that
are not homogeneous in chemical constituents due to
the presence of different moieties in the focal point,
interconversion reactions between Gn-NH2 and Gn-F

Macromolecules, Vol. 38, No. 23, 2005
Communications to the Editor 9401
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In conclusion, poly(aryl ether amide) dendrons up to
fourth generation were efficiently synthesized through
an orthogonal convergent approach using both SNAr and
amidation reactions. Each single orthogonal reaction
produced next generation dendrons with high purity
after simple precipitation.
Supporting Information Available: A table summariz-
ing the properties of all dendrons, all synthetic procedures,
1H NMR, FTIR, DSC, MALDI-TOF mass, ultraviolet, and
photoluminescence spectrum of dendrons. This material is
available free of charge via the Internet at http://pubs.acs.org.
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