Multifunctional dendrimers

The following sections are devoted primarily to the introduction of two different types of functional units. A distinction is made between dendrimers with bi-functionalised molecular periphery (Fig. 3.7 Types A, B, C, D) and those in which one function is located in the core and the other in the branching units or in the periphery (Types E, F). Multifunctional dendrimers of type G with different functional units in the core, scaffold, and periphery have so far played only a minor role and will therefore only be treated briefly here, particularly since compounds of this type will be considered in greater depth in Chapter 6. 

Whereas the bifunctionalisation strategies presented so far are relatively straightforward, the synthesis of multifunctional dendrimers with more than two kinds of functional units requires considerable synthetic effort. Preparation of dendrimers with a functional core and additional functional units in the dendrimer scaffold and in the periphery requires de novo synthesis of the entire dendrimer scaffold, with the synthesis conditions having to be tolerable for all groups (Fig. 3.15). 

The second option is the convergent approach, as introduced in 1990 by Hawker and Frechet [67], In this approach, dendritic wedges, so-called dendrons, are produced first and assembled on the core molecule in the last step, e.g. [68, 69]. A benefit of this route is that it is possible to mix wedges with different terminal groups with the opportunity to synthesize designed dendrimer surfaces and multifunctional dendrimers. 

S. Wen, et al.. Multifunctional dendrimer-entrapped gold nanoparticles for dual mode CT/MR imaging appUcations, Biomaterials 34 (5) (2013) 1570-1580. 

The PPDs thus create a perfect nanoenvironment with increasingly complex function. Admittedly, they define enormous tasks for precision polymer synthesis, but if really sophisticated function is targeted it is worthwhile to take on this challenge in view of the wealth of otherwise inaccessible chemical, physical, and biological properties [10, 46]. Further, there is much to learn when comparing multifunctional dendrimers based upon covalent build-up, with assemblies depending upon weak intermolecular forces. This is indeed another benefit of the structural perfection of the PPDs, that they also allow the fabrication of increasingly complex supramolecular structures. 

Scheme 15 Dendritic hydrogel 48 formed from CuAAC cross-linking of a multifunctional dendrimer 46 and a PEO-derived diazide 47...

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