Angewandte
Chemie
microenvironment that further enhanced the strength of H-
bonding.[12] In practice, the G3 dendrons were too bulky, and
prevented sufficiently close contacts among the polymer
chains to form a stable H-bond network structure in the
solution state, as confirmed by the smaller red shift values of
À
=
N H and C O stretchings in the FT-IR spectrum of AmDP3
in toluene solution. With regard to the gelation-specificity of
the AmDP2 dendronized polymer, the G2 branching hydro-
carbon residue seemingly possessed the optimized size to
allow close contacts of the dendronized polymer chains, and
hence the formation of a stronger H–bond-mediated network
structure in the solution state, as confirmed by FT-IR study of
AmDP2 in toluene solution. At the same time, the internal
voids created within the network were of the appropriate size
to accommodate solvent molecules and therefore physical
gels were formed. For AmDP1, with the smallest hydro-
carbon side chains that did not hinder chain associations,
interchain H-bonding was very strong and hence network
packing was much denser. Hence AmDP1 is a highly
insoluble compound and the internal voids within it are too
small to accommodate any solvent molecules. In addition, the
role of solubility should also play a role in controlling
gelation. The different hydrophobic side chains could mod-
ulate the solubility of the resulting dendronized polymers.
Hence, AmDP1 does not have enough hydrophobic function-
ality to dissolve and therefore is a solid, whereas AmDP3 has
too much hydrophobicity, interacts too effectively with the
solvent, and simply dissolves.
Figure 4. SEM Images of a) freeze-dried 1% AmDP2 gel in p-xylene at
ꢀ35000 magnification and b) the same sample at ꢀ121000 magnifi-
cation.
were super-bundles formed from the intertwining of many
polymer chains. However, it is not certain whether these were
present in the native gel or formed during the sample drying
stage.
Based on the facts that only AmDP2 formed strong
organogels and that the monomers were all non-aggregating
in the solution state, a gelation model was postulated
(Figure 5). Since the monomers contain only two H-bonding
amide units that are not pre-organized by a rigid spacer, there
is little cooperative binding effect. Hence AmM1–AmM3
showed little self-association in the solution state, as con-
firmed by FT-IR study. After click polymerization, the
dendronized polymers contain a large number of H-bonding
amide units packed at regular intervals along the polymer
chain, producing a zip templating effect, as initial interchain
H-bonding between a few amide units would facilitate
bindings of those located further down the polymer chain,
provided that the steric size of the dendrons did not interfere
with the binding process. In addition, aliphatic hydrocarbon
dendrons also provided a highly nonpolar hydrophobic
In summary, we have reported herein the first efficient
synthesis of G1–G3 dendronized polymers starting from AB-
type heterobifunctional macromonomers using click poly-
merization. A significant functional-group synergistic effect
was noted on the interchain hydrogen-bonding capability of
the many amide functionalities in the resulting dendronized
polymers. Despite their structural similarities, the strength of
the hydrogen-bond networks, and hence the phys-
ical properties of the three dendronized polymers,
were different and were controlled by the size of
the dendritic appendage. It is of interest to note
that our findings are reminiscent to those made for
the hydrogelating properties of poly(N-alkyl)acry-
lamides, wherein their gelating properties are
À
dependent on the amide N H hydrogen bonding
and the nature of the alkyl side chains.[13] Notably,
the G2 dendronized polymer AmDP2 was found
to have very strong organogelating properties, with
MGC values down to 5 mgmLÀ1. We believe the
new findings can provide valuable insights into the
self-assembly of dendronized polymers, and offer
new understandings of the intricate mechanism of
polymer–polymer hydrogen-bonding interactions.
Received: April 22, 2008
Published online: July 24, 2008
Keywords: click chemistry · dendrimers · gels ·
.
Figure 5. Proposed gelation model of AmDP2. For clarity, some dendrons are
omitted in some of the structures.
hydrogen bonding · polymerization
Angew. Chem. Int. Ed. 2008, 47, 6912 –6916
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6915