Dendrimer structurally uniform

Figure 5.21 Self-assembly of two building blocks A and six building blocks B to a structurally uniform dendrimer. 

Because of their high molecular weight and their defined structure, dendrimers offer themselves for studying the expression of chirality on a macromolecular level. The construction of configurationally uniform macromolecules is otherwise a complex task but can be achieved more easily with dendrimers because of repetitive synthesis from identical (chiral) building blocks. Comparison of optical rotation values and circular dichroism (CD) spectra should demonstrate what influence there is of the chiral building blocks on the structure of the whole dendrimer. 

Dendrimers are structurally defined branched polymeric molecules and can form good X-ray quality crystals. This is particularly useful in the correlation of structure and properties. Two main synthetic approaches have been used divergent, which is appropriate for smaller dendrimers, and convergent, which is better for larger dendrimers. Branched polysilanes with a structurally defined branched core were described in 1994, but the growth from the core was not controlled and non-uniform structures were obtained.357... 

The development of mass spectroscopic techniques such as matrix assisted laser desorption (MALDI) and electrospray mass spectrometry has allowed the absolute determination of dendrimer perfection [7,8], For divergent dendrimers such as PAMAM and PPI, single flaws in the chemical structure can be measured as a function of generation to genealogically define an unreacted site of or a side reaction producing a loop at a particular generation level. Mass spectromet-ric results on dendrimers, not only demonstrate the extreme sensitivity of the technique, but also demonstrate the uniformity of the molecular mass. The polydispersity index of Mw/Mn for a G6 PAMAM dendrimer can be 1.0006 which is substantially narrower than that of living polymers of the same molecular mass [7],... 

Therefore, within the PAMAM family, the shapes of dendrimers span the range from stars to spheres. Small G3 dendrimers have a diffuse, open structure while large G10 dendrimers are spheres with a uniform interior, sharp outside transition, and low polydispersity. 

In contrast to the uniform dendrimer, a polydisperse hyperbranched polymer with OH-terminal groups showed only one transition corresponding to the collapse from the monolayer of flatly spread molecules to a disordered liquid. Dendrimer 1 is similar to dendrimer 2 in Fig. 8, except the difference in branch ends (CH3 vs OH), did not spread and exhibited no structural transitions upon compression. 

In contrast to polymers, the structural precision and hence more uniform properties of dendrimers predestine them primarily for applications in the biomedical sector. A further reason for interest in these compounds is that effects such as luminescence can be enhanced by straightforward substitution and high local concentrations of certain structural elements and functionalities can be obtained. 

For self-assembled dendrimers which are uniform in size but not necessarily in structure see Franz, A., Bauer, W. and Hirsch, A. (2005) Angew. Chem., 117, 1588-1592 Angew. Chem. Int. Ed., 44, 1564-1567. 

The size and shape of dendrimers have been estimated using transmission electron microscopy [14]. Producing better images of dendrimers may supply answers to fundamental questions such as whether dendrimers have truly uniform sizes, spherical shapes, and dense cores. The visualization of dendrimer assemblies would also be helpful to confirm the structure and morphology for many applications of dendrimers. The positively stained PAMAM dendrimers are shown in Fig. 2 for G10 to G5, respectively. The dendrimer molecules appear as dark objects on a light background of the amorphous carbon substrate and they are well separated from each other. The shapes of the stained molecules are spherical to a first approximation for G10 to G7, with some molecules showing... 

Dendrimers are a class of macromolecules with highly branched and well-defined structures, and have recently attracted much attention as soluble supports for (chiral) catalyst immobilization [55-65]. As stated above, the catalysts anchored onto or into insoluble supports often possess an uneven catalytic site distribution and partly unknown structures, and generally suffer from diminished activity due to the mass transfer hmitations. Dendrimers, on the other hand, allow for the precise construction of catalyst structures with uniformly distributed catalytic... 

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