C O M M U N I C A T I O N S
Thermal properties of the samples 1-3 have also been studied,
using differential scanning calorimetry (DSC). The solid-state
properties of all samples were similar, despite their different
topology. The melting points of all polymers 1 to 3 were in the
range of 48-54 °C (see Supporting Information), with similar
melting enthalpies ∆H ) 61-79 J/g; glass transitions (Tg) were
observed at -15 to -25 °C. This is not unexpected, since the linear
material is atactic and the alkyl side chains determine the crystal-
lization behavior for the linear and hyperbranched analogues.
Our results underline the crucial role of the hyperbranched
topology and the resulting solution conformation in supramolecular
guest encapsulation and phase transfer. We conclude that the
unusually compact (“collapsed”) structure assumed by hyper-
branched core-shell amphiphiles in apolar media is responsible
for the formation of a hydrophilic compartment, capable of
irreversibly taking up guest molecules.
Figure 1. UV-vis spectra of 1, 2, and 3 (linear) after transfer of Congo
Red dye into chloroform solution ([Congo Red]/[polymer] ) 2).
Acknowledgment. H.K and H.F. are grateful for fellowships
from the Fonds der Chem. Industrie. S.-E.S. acknowledges an
Alexander von Humboldt fellowship.
Supporting Information Available: Synthetic procedures and
additional supporting data (UV-vis spectra, NMR and DSC data)
(PDF). This material is available free of charge via the Internet at http://
pubs.acs.org.
Figure 2. Specific viscosity vs concentration of 1-3 in toluene.
References
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the organic phase by the polyglycerol-amphiphiles was monitored
by UV-vis spectroscopy at different dye concentrations. The results
(Figure 1) demonstrate unambiguously the crucial role of the
hyperbranched topology. Both hyperbranched polymers 1 and 2
exhibit the expected phase transfer,7 with saturation concentrations
of 0.9 and 1.3 dye molecules per amphiphilic polymer molecule,
respectively. In contrast, the analogous linear esterified polyglycerol
showed no phase transfer at all for Congo Red and several other
water-soluble dyes. Thus, molecular encapsulation is clearly a
peculiarity of the hyperbranched topology and is related to the
core-shell-type amphiphilicity of these polymers.
To gain insight into the nature of this strikingly different
encapsulation behavior, specific and intrinsic viscosities of 1 to 3
in various apolar solvents were measured. Determination of the
concentration-dependent specific viscosity is an important method
to investigate the solution conformation of macromolecules. As a
typical example, the behavior in toluene solution is shown in Figure
2. On one hand, the specific viscosity ηsp of the hyperbranched
samples 1 and 2 is considerably lower than ηsp of the linear sample
3 at all concentrations. For instance, ηsp values of 10 wt % toluene
solutions of 1, 2, and 3 were 6.2, 7.0, and 15.8 mL/g, respectively.
In addition, the slope of the ηsp vs concentration plot is considerably
lower for both hyperbranched nanocapsules 1 and 2 in comparison
to that of 3 despite the three times higher molar mass of 2 compared
to 3. Extrapolation of ηsp to c ) 0 yields intrinsic viscosities [η] )
3.4 (1), 4.3 (2), and 7.8 mL/g (3). These viscosity data point to an
extremely compact (dense) structure for the hyperbranched nano-
capsules 1 and 2 in apolar media, in contrast to the linear sample
3 that possesses an open solution conformation.12 This observation
is explained by the preferential interaction of the polar core hydroxyl
groups with themselves and the unfavorable interaction with
nonpolar media.
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(10) Nomenclature: hyperbranched molecular nanocapsule; P(GxCYR), x )
DPn of polyglycerol, Y ) length of respective alkyl chain, i.e., the number
of carbon atoms, R ) degree of alkyl substitution per hydroxy group.
The same nomenclature was used for the linear esterified LP(GxCYR).
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The results show that a conformational collapse in solution
leading to hard-sphere behavior is only possible in the case of the
hyperbranched core-shell architecture.
(12) Elias, H.-G. An Introduction to Polymer Science, VCH: Weinheim, 1997;
Chapter 5.8.
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