Dendrimers controlled surface chemistry

The use of ordered supramolecular assemblies, such as micelles, monolayers, vesicles, inverted micelles, and lyotropic liquid crystalline systems, allows for the controlled nucleation of inorganic materials on molecular templates with well-defined structure and surface chemistry. Poly(propyleneimine) dendrimers modified with long aliphatic chains are a new class of amphiphiles which display a variety of aggregation states due to their conformational flexibility [38]. In the presence of octadecylamine, poly(propyleneimine) dendrimers modified with long alkyl chains self-assemble to form remarkably rigid and well-defined aggregates. When the aggregate dispersion was injected into a supersaturated... 

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. 

This lecture exposed these rather revolutionary concepts to an elite scientific community in Europe. Secondly, an invitation by Dr. P. Golitz (Editor, Angew. Chem.) to publish an important review [20] entitled Starburst dendrimers molecular-level control of size, shape, surface chemistry, topology and flexibility from atoms to macroscopic matter provided broad exposure to the basic concepts underlying dendrimer chemistry. Finally, important contributions by key researchers significantly expanded the realm of dendrimer chemistry with the convergent synthesis approach of Frechet and Elawker [37] (Figure 4), as well as the systematic and critical photophysical characterization of Turro et al [38],... 

Tomalia DA, Naylor AM, Goddard HI WA. Starburst dendrimers control of size, shape, surface chemistry, topology and flexibility in the conversion of atoms to macroscopic materials. Angew Chem 1990 102 119-157. 

These seven italicized criteria are integrated into a variety of (GDS) schemes thus allowing construction of hyperbranched macromolecular structures referred to as dendrons or dendrimers . A direct consequence of this strategy is a systematic molecular morphogenesis [1] with an opportunity to control "critical molecular design parameters (CMDP s) (i.e., size, shape, surface chemistry, topology and flexibility) as one advances with covalent connectivity from molecular reference points (seeds) of picoscopic/sub-nanoscopic size (i.e.. 0.01-1.0 nm) to precise macromolecular structures of nanoscopic dimensions (i.e., 1.0-100 nm) [2]. Genealogically directed synthesis offers a broad and versatile approach to the construction of precise, abiotic nanostructures with predictable sizes, shapes and surface chemistries. 

In this fashion, it is possible to control the critical molecular design parameters (CMDPs, i.e., size, shape, topology, flexibility, and surface chemistry) and grow predictable, stoichiometric structures up to a self-limited dimension (generation) which is determined by Nc and Nb as well as by the dimensions of the structural components. Such space-filling, terminally functionalized molecular organizations have been coined Starburst dendrimers [2]. Two dimensional projections of such molecular morphogenesis [1] are as illustrated in Fig. 2. 

Triazine dendrimers—which are usually cationic in nature—have been largely explored as targets of synthesis and platforms for manipulation. Their application in fields including regenerative medicine will likely derive from the ability of investigators to tune molecular parameters including size, surface chemistry, and ligand density in a controlled and reproducible manner. 

Tomalia, D.A., Naylor, A.M., Goddard, W.A. Starburst Dendrimers - Molecular-Level Control of Size, Shape, Surface-Chemistry, Topology, and Flexibility from Atoms to Macroscopic Matter. Angew. Chem. Int. Edit. 29(2), 138-175 (1990). doi 10.1002/Anie. 199001381... 

The stmcture-controlled features manifested by dendrons/dendrimers, such as size, shape, surface chemistry, flexibility/rigidity, elemental composition, and architecture, have provided a unique window to a new systematic concept for unifying nanoscience and will be described later in Section 6. These nanolevel structure-controlled features are referred to as critical nanoscale design parameters (CNDPs). 

Strategy based on rational biomimiciy as a means for creating a repertoire of structure-controlled, size- and shape-variable dendrimer assemblies. Successful demonstrations of such a biomimetic approach has proved it to be a versatile and powerful synthetic strategy for systematically accessing virtually any desired combinatimi of size, shape, and surface chemistry in the nanoscale region. Future extensions will involve combinational variation of dendrimer module parameters such as families (interior compositiOTis), surfaces, generational levels, or architectural shapes (i.e., spheroids, rods, etc.). 

The starburst point occurs at a well-defined limit for each dendrimer system, and its occurrence is dependent mainly on (a) the functionality of the core, (b) the multiplicity of the branches and (c) the branch length. However, the volume of the core itself, and the length of the monomer branches also have an influence. The end groups may occupy the outer surface of the dendrimer, or the branches may fold inwards, thus distributing the end-groups within the dendrimer. The factors that control this behaviour of the branches are not fully understood, but include the nature of the solvent and the detailed chemistry of the dendrimer branches. 

Since 1985, Newkome and co-workers [92-94] have reported the synthesis of related symmetrically branched macromolecules which they refer to as arborols . These macromolecules are highly branched dendrimers which are not amenable to size control as a function of generation in that reiterative chemistry for advancing concentric growth has not been reported. These prototypes have been used successfully for controlling molecular shape and, in some cases, surface moieties. To date, examples of uni-, di- and tri-dendron arborols have been reported [92-94] (see Sect. 5). 

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