Focal point group

As the name impHes, the core unit, or focal point group, is located at the center of the dendritic macromolecule and, as such, constitutes only a small proportion of the overall dendrimer structure. However the role of functional groups at the central core can have a profound effect on a number of physical properties and be extremely useful in investigating the structure of dendrimers. The central role played by the focal point group in the success of the convergent growth approach has also been well documented in the above discussions. 

The synthesis of polyether quinoxaline by nucleophilic displacement discussed in Section 5.4.2.2 has also been used for the preparation of hyperbranched polyquinoxaline based on AB2 or A2B monomers. In the AB2 monomer the focal point is a single fluoro group (Fig. 5.42)172 in the A2B the focal point is a single... 

Ionic interactions have been used to prepare lanthanide-core dendrimers. This has been achieved using a convergent synthesis, in which polyether den-drons with a carboxylic acid group at the focal point were assembled around a lanathanide cation. This involved a metathetical reaction with compounds such as Er(OAc)3, Tb(OAc)3 or Eu(OAc)3 to introduce the appropriate lanthanide ion. 

Given that the presence of three alkoxy chains in the phenyl group produces such a dramatic change in the properties of the material to the point that columnar mesophases are formed at room temperature, the structure of the aryl isocyanide ligand has been further modified to introduce more paraffinic chains, and examples of metallodendrimers containing monodendrons with an isocyanide group in the focal point, and its gold compound 9, have been reported [26]. 

A potential source of structural imperfection is the rapid increase of reactive groups as growth is pursued. Their incomplete conversion leads to defects inside the molecule [27]. In convergent-iterative syntheses these problems are avoided by directing the dendritic growth from the surface inwards to a focal point. In a final step several dendrons are connected with a multifunctional core to yield the desired dendrimer (Fig. 9). 

Scheme 3. Frechet-type dendrons of generations one to three (2a-c) with a functional group (X) at the focal point... 

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