Ordered Dendritic Networks

Potentially, a limited number of juxtaposed bridging amide bonds could be possible at the initial junction locus, depending on the contact surface area defined by the interaction of the initial reagents. Other methods of direct surface-to-surface connectivity that have been reported 79,80 include the reaction of olefinic terminated dendrimers with initiator-terminated dendrimers. 

Connection of dendrimers via treatment with multifunctional cross-linking-type reagents such as the addition of polyhalides to amine-terminated dendrimers 78 results in randomly positioned dendritic assemblies. Dendrimers have also been bridged by the incorporation of copolymerizable units into reactions with dendrimers possessing polymerizable, terminal olefins. 79,80  

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Figure 9.5. An idealized representation of an ordered dendritic network constructed by the procedure of Watanabe and Regen. 81 ...

As new synthetic methods are pursued in future to synthesize dendrimers more efficiently and cheaply on an industrial scale, one of the most promising areas for these macromolecules is seen in the construction of higher order assemblies or dendritic networks [213]. 

As defined, dendritic networks are considered to result from the one-, two-, and three-dimensional orientation of dendrimers thus ordering can be geometrically likened to rods surfaces or sheets and cubes, tetrahedrons, or spheres, respectively. Due to the broad scope and breadth of potential macromolecular architectures that can be obtained by application of different modes of connectivity, we will herein concentrate on networks constructed from the simplest dendritic structures, namely those that are pseudospheri-cal or globular. The principles that are presented here pertaining to network formation should be easily adaptable to non-spheroidal dendritic structures as well as macromolecular assemblies possessing only limited dendritic character. 

All ordered and random dendritic networks that are constructed via covalent or non-covalent means result from positioning of one dendrimer relative to another. Thus, macroassembly positioning can be effected via at least one of three different methods of connectivity. These methods are geometrically rooted in dendritic chemistry. 

Fig. 4. Distributions of equivalent orders of primary (Va), secondary (Vb), and tertiary (Vc) vertices in control and 4% lead-exposed Purkinje cell dendritic networks. The frequency of secondary vertices relative to primary and tertiary vertices is increased in the lead-exposed animals, particularly over the lower order vertices (i.e., in the proximal portion of the tree), indicating an increase in the degree of collateral branching relative to symmetrical branching.

Attention will focus in two primary modes of network assembly (1) random, uncontrolled connectivity analogous to classical polymer preparation, whereby monomer units or building blocks are essentially positioned relatively unsystematically via single-pot-type reactions and (2) ordered, controlled connectivity analogous to tessellated dendritic polymer construction whereby elements, or building blocks, are precisely juxtaposed into a coherent motif. 

The ordering procedure does not rely on site-specific reactions but rather the packing efficiency of the dendritic species, or building blocks. This example possesses characteristics of both random and ordered networks and exemplifies the broad spectrum of the random-ordered network continuum. 

Incorporation of //-bonding moieties capable of self-assembly into or onto a dendritic superstructure can lead to ordered networks. One of the first dendrimers to be developed for the purpose of exploring potential network formation is depicted in Scheme 9.3. Aminopyridine triester 16 was reacted with a tetraacyl halide core to afford the first generation tetrapyridine 17, which was subsequently deprotected with HC02H and treated with amino triester 18 to give the second tier, 36-ester 19. As envisioned, treatment... 

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