Dendrimers stepwise synthesis

Opposite to the divergent method, the convergent method [3] consists in the stepwise synthesis of dendrimer branches first. They are then attached to the core yielding the dendrimer. Today the name dendrimer has been extended to... 

Masamune and coworkers reported a divergent stepwise synthesis of siloxane dendrimers as shown in Scheme 15155. The third generation dendrimer was the largest obtained. Characterization techniques included 1H, 13C and 29Si NMR spectroscopy, mass spectroscopy and size-exclusion chromatography. 

As a general statement it can be concluded that macromolecular conformations different fi om the predominant coil structure are still the exception. Defined spherical secondary-structures have not been obtained by means of noncovalent interactions, since there is no synthetic concept available, distinguishing between inter- and intra-molecular interactions. Formation of globular structures by linear macromolecules is still a privilege of biomolecules where intermolecular interactions are counteracted by well-coordinated intermolecular interactions [50,51]. Synthetic nanospheres can be obtained by the stepwise synthesis of dendrimers [15] or by polymerization of microgels [52] (see below). 

A graphical comparison of the structure of dendrons, dendrimers, hyperbranched polymers, and dendrigraft polymers is shown in Figure 30.1. The stepwise synthesis of dendrimers involves multiple cycles of protection, condensation, and deprotection reactions to produce strictly... 

Fig. 1 Structures of dendritic nanomaterials. (a) Dendrimers have a well-defined hierarchical structure consisting of a core, branch, and periphery, (b) Each generation is added to the structure by repeating the reaction sequence of stepwise synthesis, which generates two new branching points, (c) Dendrimers can be encapsulated into relatively large nanoparticles such as liposomes and polymeric nanoparticles (50-200 nm in diameter), which can aim both passive and active targeting strategies.

Standard nuclear magnetic resonance (NMR, and spectroscopy is the most commonly used technique for the routine analysis of dendrimers and for monitoring end groups in the stepwise synthesis and functionalization of the structures. It is also noteworthy that NMR spectroscopy is a versatile method to monitor the generational growth of the most commonly used dendrimers. Figure 25 depicts H- and C-NMR spectra of a PBisMPA dendron, which serves as an illustrative example of how well the layered dendritic structure can be probed by NMR spectroscopy. In... 

Another novel feature of dendrimers is that, because of their rational, stepwise synthesis, the surface can be modified in very specific ways. For example, you could build up a very hydrophobic dendrimer, but then at the last step decorate the surface with a large number of hydrophilic species such as carboxylates or sulfonates. Such a system would then be water soluble, and would very much resemble a unimolecular micelle. 

Two different methods have been developed for the stepwise synthesis of starburst dendrimers ... 

Unlike dendrimers prepared by stepwise synthesis, hyperbranched polymers have diverse molecular weights, isomerism, and geometrical shapes. 

General Procedure for the Stepwise Hydroformylation/Reductive Amination on Allylated Hyperbranched Polyglycerols (PG). Synthesis of Hydroaminomethylated Hyperbranched PG-dendrimers. PG-Allyl, Rh(acac)(CO)2 and XANTPHOS were dissolved in dry toluene and placed in an autoclave. The autoclave was pressurized with CO/H2 (1 1, 30 bar), heated at 70 °C for 5d. After cooling, the amine was added to the crude PG-aldehyde (1H NMR was used to confirm full conversion) and stirred for 1-2 h. After stirring, Rh(acac)(CO)2 was added and the autoclave was pressurized with CO/H2 (1 6, 70 bar) and heated at 85 °C for 2-5 days. After cooling, the solvent was removed in vacuo and the crude mixture was purified by dialysis (benzoylated cellulose tubing) to give the re-... 

One astute way to obtain macrocyclic systems with TTF is the stepwise method of deprotection/alkylation of cyanoethyl-protected TTF-thiolates. With this method molecular units can be built but with the precaution of preserving one cyanoethyl group in order to be able to iteratively proceed with the oligomerization. Combining such units, larger units can be produced. An example of a TTF dendrimer containing 21 TTFs is shown in Fig. 2.15 (Christensen et al, 1998). Here only the main philosophy of the synthesis is discussed. 

Synthesis of a dendrimer according to the divergent method (Fig. 2.1) proceeds stepwise starting from a multi-functionalised core building block, to whose reac-... 

After these initial reports appeared, Marschner and Hengge152 reported the synthesis of a series of first generation dendrimers via another stepwise approach (Scheme 12). 13C and 29Si NMR characterization data were included in this report. 

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