Dendrimer topology

At about the same time that these fundamental studies of molecular encapsulation were beginning to appear, other groups were exploring dendrimer topology broadly. Historically, at this point, there were few examples of metallodendrimers, so many creative example architectures were possible and were shown. Many different architectures and behaviors were being explored, so thematically these examples are diverse. 

Dendrimer Topology, Branch-Cell Differentiation and Surfaces. . 279... 

The structural subtleties represented in these dendrimer topologies require a variety... 

Tomalia, D. A., et al. (1991), Comb-burst dendrimer topology. New macromolecular architecture derived from dendritic grafting, Macromolecules, 24,1435-1438. 

The amidoferrocenyl or silylferrocenyl monomers do not show any effect, however. Therefore, the dendrimer topology is important for recognition of oxo-anions. The appropriate encapsulation of the anionic host between the dendritic tethers is a key factor that very much increases the interaction between the functional ferrocenyl termini and the guest (Scheme 8.6). 

Dendrimers are complex but well-defined chemical compounds, with a treelike structure, a high degree of order, and the possibility of containing selected chemical units in predetermined sites of their structure [4]. Dendrimer chemistry is a rapidly expanding field for both basic and applicative reasons [5]. From a topological viewpoint, dendrimers contain three different regions core, branches, and surface. Luminescent units can be incorporated in different regions of a dendritic structure and can also be noncovalently hosted in the cavities of a dendrimer or associated at the dendrimer surface as schematically shown in Fig. 1 [6]. 

Dendrimers, a relatively new class of macromolecules, differ from traditional Hnear, cross-Hnked, and branched polymers. The conventional way of introducing an active moiety into polymers is to Hnk it chemically into the polymeric backbone or a polymer branch. This synthetic approach results in a topologically complex material. Therefore, a significant effort has to be devoted to improve the structural complexities and functions of the polymers. 

In dendrimers based on metals as branching centers (Fig. 1 d), the electrochemical behavior is even more complex since (i) each unit of the dendrimer is electro active, (ii) the chemical nature of the metal-based units constituting the dendrimer may be different, (iii) chemically equivalent units can be different from the topological viewpoint, and (iv) the degree of interaction among the moieties depends on their chemical nature and distance. 

In conclusion, the electrochemical data offer a fingerprint of the chemical and topological structure of these dendrimers. Furthermore, the knowledge of the electrochemical properties of the mononuclear components and the synthetic control of the supramolecular structure allow the design of dendrimers with predetermined redox patterns. 

D. A. Tomalia, A. M. Naylor, and W. A. Goddard, Starburst dendrimers-molecular level control of size, shape, surface chemistry, topology, and flexibility from atoms to macroscopic matter, Angew. Chem. Int. Ed. Engl., 29 (1990) 138-175. 

Flory was the first to hypothesize concepts [28,52], which are now recognized to apply to statistical, or random hyperbranched polymers. However, the first purposeful experimental confirmation of dendritic topologies did not produce random hyperbranched polymers but rather the more precise, structure controlled, dendrimer architecture. This work was initiated nearly a decade before the first examples of random hyperbranched4 polymers were confirmed independently in publications by Odian/Tomalia [53] and Webster/Kim [54, 55] in 1988. At that time, Webster/Kim coined the popular term hyperbranched polymers that has been widely used to describe this type of dendritic macromolecules. 

Figure 1.11 Comparison of degree of polymerization as a function of topology and growth process (a) dendrigraft, (b) dendrimer, (c) non-linear straight chain and (d) linear...

Parallel studies on PAMAM dendrimers, the Frechet type polyether den-drons, and other dendrimer families have generated an extensive list of unique properties driven by the dendritic state/ Figure 1.18 compares several significant physical property differences between the linear and dendritic topologies related to conformations, crystallinity, solubilities, intrinsic viscosities, entanglement, diffusion/mobility and electronic conductivity. 

Frechet [49, 89] was the first to compare viscosity parameters for (A) linear topologies, as well as (B) random hyperbranched polymers and (C) dendrimers. More recently, we reported such parameters for (D) dendrigraft polymers [111] as shown in Figure 1.19. It is clear that all three dendritic topologies behave differently than the linear. There is, however, a continuum of behavior wherein random hyperbranched polymers behave most nearly like the linear systems. Dendrigrafts exhibit intermediary behavior, whereas dendrimers show a completely different relationship as a function of molecular weight. 

Unique features offered by the dendritic state , that have no equivalency in the linear topologies, are found almost exclusively in the dendron/dendrimer subset or to a slightly lesser degree in the dendrigrafts. They include ... 

Figure 1.19 Comparison of intrinsic viscosities (log (f/)) versus molecular weight (log M) for (A) linear, (B) random hyperbranched, (C) dendrimers and (D) dendrigraft topologies. Data for A, B, C adapted from Frechet etal.. Ref. 49.

The unique topology and architectural components of dendrimers suggest many important applications that may be possible. We have identified at least five important factors that pose questions concerning these technological applications. Table 11.1 lists these factors along with the applicability of SANS, SAXS and TEM to address the questions. 

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