Divergent dendritic construction

The dendrimer is a new type of cross-linked nanoparticle prepared by repetitive reactions to compose dendritic structure. There are two kinds of preparative methods, divergent (30) and convergent (31) methods, as shown in Figure 12.2.7. The dendrimer by the former method is constructed from the center of sphere to the outer layers, and therefore the surface chemistry is decided by the final reaction. When a dendrimer is formed from a central ammonia molecule, the forth-generation dendrimer of about 3 nm diameter has 48 functional groups at the surface. If a crowd of functional groups is not desirable on the outermost layer, they can be thinned down properly by replacing a two-functionality compound with a one-functionality one. 

Balzani et al. prepared dendrimers with metal complexes serving both as core [36] and as branching unit The metallodendrimer in Fig. 2.10 is constructed solely from polypyridine ligands and transition metal ions. Such dendritic transition metal complexes can be synthesised both convergently and divergently and different transition metal ions (ruthenium/osmium) can be incorporated. This provides a means of influencing the luminescence properties of the den-drimer. Thus the energy transfer process proceeds from the inside outwards in... 

The simplest synthetic approach to the preparation of monofunctional dendritic systems is to functionalise the periphery of existing molecular scaffolds, generally constructed by divergent or convergent growth (Fig. 3.2, Route a) or b), respectively). 

However, the divergent synthetic strategy also permits covalent construction of a dendritic shell around a function-bearing core unit (Fig. 6.8 further examples are given in Section 4.1). 

Meijer et al.123 constructed a series of novel dendritic boxes by surface modification of divergently prepared, amine-terminated poly(propyleneimine) dendrimers 24 (e.g., 30 Scheme 7.7) by attachment of A-protected chiral amino acid caps (see Section 4.3.1). 

This phenoltriallyl dendron has been functionalized at both the phenolic and allylic positions. For instance, the dendron can be bound, after suitable molecular engineering, to the branches of a phenolic-protected dendron (convergent construction), onto stars and dendritic cores (divergent construction), nonaparticles, surfaces, and polymers (Scheme 11.9). An example is... 

The conventional synthesis of dendrimers involves the divergent method, in which branch cells are constructed in situ arotmd the initiator core or preformed branch cells are attached to the core. Successive generations are then built. On the other hand, in the convergent method [131-133] the dendritic fragments are prepared by starting firom firag-... 

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