Some examples of dendrimer synthesis

Editor s Note. Professor Tomalia is one of the pioneers of dendrimer chemistry, the importance of the work described here is reflected in the huge interest currently surrounding this topic. - 

In general, the placement of reactive functionalities on the exterior surface of the dendrimers allows introduction of a wide variety of terminal moieties. In alternate synthetic approaches, spacer groups have been deliberately introduced to relieve the steric hindrance in order to faciUtate construction of the next generation. This may provide the possibility of enhancing interior cargo spaces for guest-host type chemistry.  

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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. 

Over the years, molecular design has allowed the synthesis of functional dendrimers possessing metal and semiconductor nanoparticles, metal complexes and macrocycles, dyes and biologically important carrier molecules in various parts of the dentritic structures [31-39], Such dendrimers incorporating functional units are considered as supramolecular species [16, 21], Functional dendrimers have been proved to be very useful for specific applications [40, 41], Some examples of functional dendrimers are shown in Fig.5. 

Figure 10. Some examples of the orthogonal coupling strategy in the synthesis of polymers and dendrimers.

Since 1985, Newkome and co-workers [92-94] have reported the synthesis of related symmetrically branched macromolecules which they refer to as arborols . These macromolecules are highly branched dendrimers which are not amenable to size control as a function of generation in that reiterative chemistry for advancing concentric growth has not been reported. These prototypes have been used successfully for controlling molecular shape and, in some cases, surface moieties. To date, examples of uni-, di- and tri-dendron arborols have been reported [92-94] (see Sect. 5). 

Despite some uncertainty in the expected olefin geometry, the utility of the RBR is anply demonstrated by its application to the synthesis of many diverse targets. These include dendrimers, cyclophanes, modified carbohydrates such as C-glycosides and exoglycals, alkaloids, terpene derivatives, enediynes, amino acids, stilbenes, and macrocycles.The remainder of this chapter presents examples of the RBR that are pertinent to the strategic considerations oudined previously and illustrate its scope as a versatile and powerful synthetic method. 

Ihe literature " describes a number of dendrimers and the closely related star-like 120-123 polysiloxanes. Ihe hyperbranched polysiloxanes are the primary example of more random structures. Although the emphasis has been on synthesis and characterization,i i modeling on hyperbranched polymers has also been carried out. Some of the most interesting species involve polysiloxane chains. Star polymers, some with nanosized silica cores, have also been synthesized.i °-i i Hyperbranched polysiloxanes have been prepared with controllable molecular weights and polydispersities,i -1 - with epoxy terminal groups some are UV-curable ° and some serve as a source of molecular silica. Hyperbranched polysiloxanes have also been used in the sol-gel preparation of polypropylene/silica nanocomposites. ... 

Figure 13.4 shows some specific dendrimer examples. Very early examples of this "cascade synthesis" approach were reported by Vogtle. Here we show two especially well characterized systems. The first has been termed PAMAM [poly(amido amine)]. The second structure, the poly(propylene imine), has been especially well optimized and characterized. Both systems are now available commercially and are produced on the tens of kilogram scale. Many variations of the structures in Figure 13.4 can be obtained by starting with a different core or choosing different reagents. 

As discussed in the first section of this chapter, interest in dendrimers has increased rapidly since the successful synthesis of the first cascade molecules two decades ago. Much of this interest has been driven by the expectation that dendrimers will exhibit unique properties [2-5, 60]. Because dendrimers in many cases interact strongly with metal ions, it seems reasonable to expect that such composite materials might provide additional heretofore unknown or biomimetic functions. This is particularly true in hght of the high number of metal ions that can be complexed to a single dendrimer and (in some cases) their well-defined position in the dendrimer. For example, there has been much recent speculation that these materials will be useful for catalysis [3, 4, 53,... 

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