Pulsed-field gradient NMR spectroscopy was used to study the diffusion of three different
poly(propyleneimine) dendrimers with hydrophilic triethylenoxy methyl ether terminal groups (generations
2, 4, and 5) in poly(vinyl alcohol) aqueous solutions and gels. The effects of the diffusant size, polymer
concentration (from 0 to 0.26 g/mL), and temperature on the self-diffusion coefficients have been studied,
and the model of Petit et al. [Macromolecules
1996, 29, 6031] was used to fit the experimental data. The
Stokes−Einstein hard-sphere radii were also calculated in the zero concentration limit and were compared
with those of the linear poly(ethylene glycol)s under the same conditions. The proton NMR relaxation
times (T
1 and T
2) were measured to study the mobility of the dendrimer core part and terminal group as
a function of the dendrimer size.
A molecular necklace of polypseudorotaxanes was prepared by threading β-cyclodextrins (β-CD) onto biodegradable and thermoresponsive polyurethanes derived from bile acids. These polyurethanes were synthesized via a simple step condensation of bile acid-based dicarbonate with poly(ethylene glycol)-diamine. The β-CD rings slide onto the poly(ethylene glycol) segments and selectively recognize the bile acid units of the polyurethane chains, whereas the poly(ethylene glycol) segments remain crystalline with a lower crystallinity. This bio-compound-derived molecular necklace can be visualized by scanning tunneling microscopy. The polypseudorotaxanes show thermosensitivity in water and the phase transition temperature may be fine-tuned by varying the molar ratios of β-CD to the bile acid units. Such an interesting necklace model of polypseudorotaxane constructed from natural compounds may lead to the further exploration of their applications, such as as an enzyme model, due to their biological nature.
Pd(II)-directed self-assembly of a 3-pyridyl grafted Lindqvist hexavanadate led to the formation of a unique trimeric species, as confirmed by a variety of techniques, including pulsed-field gradient NMR spectroscopy and high-resolution ESI mass spectrometry.
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