Molecular dendrimer

Fig. 8 Nanosystems that may function as simultaneous drug delivery and imaging agents for targeting T cells (a) liposomal systems, (b) solid biodegradable nanoparticulates, and (c) macro-molecular dendrimer complexes. PEG polyethylene glycol, Gd-DTPA gadolininum-diethylene triamine penta acetic acid. (Adapted from [48])...

Encapsulation of any of these c/s-conformers into libraries of columnar supra-molecular dendrimers eliminates the intramolecular electrocyclization and replaces the hehx-coil transition with an unprecedented helix-helix transition and a reversible transition from cw-transoidal to cis-cisoidal. When the repeat unit of the dendronized polymer also contains a stereocenter, this reversible process can be monitored by circular dichroism (CD) and visualized by different methods [104-111]. This concept was used to elaborate molecular machines that were interfaced for the first time with the real world to lift heavy objects [111]. 

However, dendrimeric and hyperbranched polyesters are more soluble than the linear ones (respectively 1.05, 0.70, and 0.02 g/mL in acetone). The solution behavior has been investigated, and in the case of aromatic hyperbranched polyesters,84 a very low a-value of the Mark-Houvink-Sakurada equation 0/ = KMa) and low intrinsic viscosity were observed. Frechet presented a description of the intrinsic viscosity as a function of the molar mass85 for different architectures The hyperbranched macromolecules show a nonlinear variation for low molecular weight and a bell-shaped curve is observed in the case of dendrimers (Fig. 5.18). 

The synthesis and study of dendrimers is a relatively new branch of macro-molecular chemistry. It began in 1985 with the publication of two landmark papers (D.A. Tomalia, H. Baker, J. Dewald, J.M. Hall, G. Kallos, R. Martin and J. Ryder, Polym. J., 1985,17,117-132 and G.R. Newkome, Z. Yao, G.R. Baker and V.K. Gupta, J. Org. Chem., 1985, 50, 2003-2004), and has grown to become a very vibrant research field. The word dendrimer comes from the Greek word dendra, meaning tree, and was applied to these compounds by Tomalia et al. in their very first paper. Newkome s team, by contrast, called their molecules arborols from the Latin word arbor, which also means a tree. The term cascade molecule has also been used, but the word dendrimer is the one that is used most widely throughout the literature, and is also used in the present chapter. 

Polynuclear dendrimer complexes of this type can undergo redox reactions at the metal centre and have luminescent properties. They have been proposed as molecular photochemical devices, although no practical examples have yet been produced. 

Dendrimer micelles of this type have been formulated as drug delivery vehicles. Dendrimers with a hydrophobic interior have been used to entrap a hydrophobic drug such as indomethacin. This is retained because of the hydrophilic periphery containing ethylene glycol functional groups, and is released slowly because of the collapsed configuration of the interior, through which molecular diffusion is obstructed. 

X-ray diffraction patterns from dendrimers tend to lack sharp features and are similar to those from amorphous linear polymers. This suggests a molecular arrangement in dendrimers that is fairly disordered. There is also the problem that these molecules can exist in a large number of energetically equivalent conformations and that in solution there can be rapid interchange between these conformations. This contributes to the overall amorphous structure of dendrimers. 

SANS is a valuable research technique, since it provides information about the size and shape of the molecular scale structures in the size range 1-10 nm found in polymer or dendrimer systems. Neutrons interact with atomic nuclei, hence are sensitive not only to the details of the way such nuclei are arranged but also to the presence of different isotopes. 

This is a very new field, but given the current interest in both dendrimer chemistry and supramolecular chemistry, it is one that is likely to receive attention in the future. The prospect of preparing well-defined molecular assemblies, rather than ill-defined clusters, is attractive, and is expected to give access to new properties that can be controlled by the molecular architecture of the assembly. New and useful materials seem likely to emerge as this chemistry grows and is exploited in the years to come. 

Sugiura K (2003) An Adventure in Macromolecular Chemistry Based on the Achievements of Dendrimer Science Molecular Design, Synthesis, and Some Basic Properties of Cyclic Porphyrin Oligomers to Create a Functional Nano-Sized Space. 228 65-85 Sun J-Q, Bartlett RJ (1999) Modern Correlation Theories for Extended, Periodic Systems. 203 121-145... 

When tetranuclear platinum complex (35) bridged by tetra(4-ethynylphenyl)-methane was used as the core (Fig. 6), dendrimers having more platinum atoms than those prepared from trinuclear platinum core 30 in the molecule were obtained up to the third generation. GPC analysis of these dendrimers revealed that the dendrimers with a tetraplatinum core have a similar molecular size to those with the triplatinum core 30. 

The ability of a dendritic shell to encapsulate a functional core moiety and to create a specific site-isolated microenvironment capable of affecting the molecular properties has been intensively explored in recent years [19]. A variety of experimental techniques have been employed to evidence the shielding of the core moiety and to ascertain the effect of the dendritic shell [19, 20]. Dendrimers with a fullerene core appear to be appealing candidates to evidence such effects resulting from the presence of the surrounding dendritic branches. Effectively, the lifetime of the first triplet excited state of fullerene derivatives... 

Dendrimers 5-8 were obtained by taking advantage of the versatile regiose-lective reaction developed in the group of Diederich [24], which led to macro-cyclic bis-adducts of Cgg by a cyclization reaction at the C sphere with bis-mal-onate derivatives in a double Bingel cyclopropanation [25]. Reaction of the dendritic malonates with Cgg, I2, and l,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in toluene at room temperature afforded the corresponding cyclization products 5-8 (Fig. 2). The relative position of the two cyclopropane rings in 5-8 on the Cgo core was determined based on the molecular symmetry deduced from the and NMR spectra (Cs) as well as on their UV/Vis spectra. It is well estabhshed... 

Keywords. Rotaxane dendrimers. Host-guest interaction, Recognition, Self-assembly, Supra-molecular chemistry... 

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