Dendrimer Features

Other examples for the use of this template activated method include the formation of hexakis-adducts [15,16] with, for example, dendrimers featuring a fully buried fullerene core [8,17,18],... 

For a general review on supramolecular chemistry with dendrimers, the reader is referred to an excellent paper of Zimmerman et al. [18]. Because of our acquaintance with the polylpropylene imine) dendrimers, we will restrict ourselves in this Chapter to some examples of supramolecular behaviour of these systems as investigated in our laboratory. Three systems will be discussed (Figure 1) the dendritic box, which can encapsulate guest molecules, the polystyrene-poly(propy-lene imine) block copolymer superamphiphiles, and alkyl-decorated dendrimers, which function as unimolecular micelles, and show surprising aggregation behaviour. However, first the synthesis and properties of the poly(propylene imine) dendrimers will be discussed to demonstrate some typical dendrimer features. 

Since the pioneering studies reported by van Koten and coworkers in 1994 [20], dendrimers as catalyst supports have been attracting increasing attention. The metaUodendrimers and their catalytic applications have been frequently reported and reviewed [7-15]. As a novel type of soluble macromolecular support, dendrimers feature homogeneous reaction conditions (faster kinetics, accessibility of the metal site, and so on) and enable the application of common analytical techniques such as thin-layer chromatography (TLC) and nuclear magnetic resonance... 

The dendrimer featuring a rhenium core was solubilised by the complexation of the terminal adamantane groups by / -cyclodextrins generating a water-soluble complex with potential applications in radiotherapy (Figure 8.9). 

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. 

The interest in hyperbranched polymers arises from the fact that they combine some features of dendrimers, for example, an increasing number of end groups and a compact structure in solution, with the ease of preparation of hn-ear polymers by means of a one-pot reaction. However, the polydispersities are usually high and their structures are less regular than those of dendrimers. Another important advantage is the extension of the concept of hyperbranched polymers towards vinyl monomers and chain growth processes, which opens unexpected possibilities. 

The alkynyl-metal (metal-acetyhde) complex is one of the best building blocks for organometallic dendrimers, since it has some advantages compared to other organometallic complexes [18]. Most of the metal-acetylide complexes are thermally robust and stable, even when exposed to air and moisture. Metal-acetylide complexes are fairly accessible in high yields by well-established synthetic methodology [19]. These features are essential to the construction of dendrimers. 

Scheme 2. Classification of rotaxane dendrimers Type I, II and III rotaxane dendrimers incorporating rotaxane-like features at the core, termini, and branches, respectively...

Template effects have been used in rotaxane synthesis to direct threading of the axle through the wheel. Since macrocycHc compounds such as cyclodextrins, crown ethers, cyclophanes, and cucurbiturils form stable complexes with specific guest molecules, they have been widely used in the templated synthesis of rotax-anes as ring (wheel) components. Here, we briefly discuss macrocycles used in the synthesis of rotaxane dendrimers and their important features. 

As described earlier, Type II pseudorotaxane dendrimers have pseudorotaxane-like features at the periphery of dendrimers. Depending on whether the terminal units of the dendrimers serve as rod components or ring components, they can be further classified as Type II-A and Type II-B pseudorotaxane dendrimers, respectively. 

In this review, we tried to cover all the supramolecular species that maybe classified as rotaxane dendrimers. We classified them by their structures - where in dendrimer rotaxane-hke features are introduced. Several different types of macrocycles have been employed as a ring component in the templated synthesis of rotaxane dendrimers. While the synthesis of Type I and II rotaxanes dendrimers is relatively straightforward, that of well-defined Type III rotaxane dendrimers, particularly those of second and higher generations, is still challenging. 

The PBE dendron has a glass transition at about 40 °C and is soluble in various organic solvents (e.g., THF, acetone, toluene). It is therefore a moldable, thermoplastic, film-forming material. This practical feature is maintained for the lanthanide-cored dendrimer complexes. The complexes are partially miscible with poly(methyl methacrylate), affording transparent luminescence compositions by mixing in solvent. 

Dendrimers are taking their space in the tool box of the modern synthetic chemist. Dendritization might offer solutions to problems yet unsolved. Dendritic wedges, i. e. dendryl substituents of well-chosen size and generation allow us to tune molecular properties like solubility, steric accessability of reactive sites, redox behaviour, and other features. Easy-to-make dendrimers and dendrons will thus become extremely helpful for any chemist in the covalent as well as in the supramolecular world . 

A dendrimer of generation 1 is also characterized by X-ray diffraction studies (Fig. 2). Three main features can be emphasized. First each OC6H4CH=N-N(Me)P... 

Practically all dendrimers [ 1 ] known today have cores with a few, typically three to six functional groups to which the corresponding number of dendrons (dendritic wedges) are attached. The fact that these dendrons are connected to one another by a small, almost dot-like molecule results in considerable steric congestion in the space around the core. This congestion is a unique structural feature and has one led to view dendrimers, specifically those of high genera-... 

Fig. 4. Phase contrast SFM images of an ultrathin film of dendrimer 22a on HOPG. The dendrimers are grouped in domains in which they are packed parallel to each other, separated by a periodicity of 4.8 ( 0.5) nm. The nature of the dark features is not yet understood... 

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