Dendrimers initial development

Dendrimers were initially developed in the field of organic chemistry. More recently, a number of dendrimers based on or containing metal ions have been prepared and investigated. The study of these species has considerably expanded the scope of metal coordination chemistry. 

Since the production of perfect monodisperse dendrimers is too complicated and expensive, several researchers at DuPont Experimental Station working on dendritic polymers as rheology control agents and as spherical multifunctional initiators developed a route for a one-step synthesis of dendritic polymers. These... 

Nonetheless, it was a fairly short step from octopus compounds to dendrimers, and the step was taken by Vogtle in the late 1970s when he attempted to use a cascade reaction to prepare a molecule of the dendrimer type that would now be considered a dendron rather than a fully developed dendrimer. It began with the addition of acrylonitrile to an anfine, followed by reduction of the nitrile to amine. This was followed by a further reaction with acrylonitrile, and the process was repeated several times to yield highly branched macromolecules. There were initially problems with the reduction step but these were overcome, and the preparation of these poly(propylene imine) dendrimers was later commercialized. 

The architecture of macromolecules is another important synthetic variable. New materials with controlled branching sequences or stereoregularity provide tremendous opportunity for development. New polymerization catalysts and initiators for controlled free-radical polymerization are driving many new materials design, synthesis, and production capabilities. Combined with state-of-the-art characterization by probe microscopy, radiation scattering, and spectroscopy, the field of polymer science is poised for explosive development of novel and important materials. New classes of nonlinear structured polymeric materials have been invented, such as dendrimers. These structures have regularly spaced branch points beginning from a central point—like branches from a tree trunk. New struc-... 

The fact that the dendritic shell can produce localized microenvironments has been used by Diederich et al. who developed water-soluble dendritic cyclophanes (dendrophanes) as models for globular proteins (Figure 16.13) [5, 170, 171], These dendrimers contain well-defined cyclophane recognition sites as initiator cores for the complexation of small aromatic guests [172-174] and steroids [174-176], Enlargement of the cyclophane core could be used as a tool to complex larger steroid molecules. 

Recently another family of dendrimers has become commercially available. These polyamines were developed by Meijer and de Brabander-van den Berg of DSM Research and are based on Vogtle s initial synthesis [7]. In this case the troublesome reduction step was performed using a Raney cobalt hydrogenation catalyst and other process improvements have permitted this synthesis to be continued up to the fifth generation with multikilogram quantities available. 

This review summarizes pioneering relevant works of expert groups on oiganomet-allic dendritic mactomolecules, together with our initial effmts in this developing field. Throughout the chapter, the synthetic aspects are especially emphasized. Likewise, we describe in detail the redox properties exhibited by a new family of silicon-based ferrocenyl-containing dendrimers and we also mention some of their relevant applications. 

Convergent syntheses developed by Moore et al. provided access to rigid higher-generation phenylacetylene dendrimers (Fig. 4.14). Dendrons with peripheral p-tert-butylphenyl units used in initial work showed only limited solubility and were therefore unsuitable for assembly of higher generations. However, minimal modification of the periphery with 3,5-di-tert-butylphenyl groups improves the solubility so dramatically that it even proved possible to prepare a dendri-mer with 94 monomeric units [24]. 

A decade after the initial pioneering studies, dendrimer catalysis has developed into a highly varied and complex field of research. While much of the fascination is derived from the aesthetic appeal of dendrimers (as with other applications of this class of macromolecule), dendrimer catalysts have vindicated many of the utilitarian aspects of this research. This volume provides a comprehensive overview of the current state of the art. 

Recently, Mullen et al. have reported a convergent approach134 to poly(phenylene) dendrimers (Scheme 5) similar to the divergent [4 + 2] cycloaddition route they had previously developed.135 The process was initiated by a Diels—Alder reaction between tetra-substituted cyclopentadienone 61 and dialkynyl mono-... 

Using a similar type of branch, developed by the Frechet group, they have published extensively on chromophore labeled dendrimers [12], The dendrimers possessing coumarin-2 dyes at the periphery, and either coumarin-343 (Fig. 1.4) or a heptathiophene dye at the core, were studied by time-resolved fluorescence and transient absorption spectroscopy. It was revealed that upon excitation of the rim chromophores almost no direct fluorescence occurred from these initially excited chromophores. Instead, only the center... 

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