Dendrimers covalently assembled

Alternatively, dendrimers may be synthesized directly by our original II Divergent Core Proliferation method. This method may involve the exponential covalent assembly of monomer units around a multi-valent core to produce branch cells in situ or it may involve the direct use of pre-formed branch cell reagents. In either case the resulting covalent structure consists of precise numbers of dendrons organized around the initiator core. 

Some attention has arisen to consider dendrimers as building blocks for the construction of nanoarchitectures possessing higher complexity and dimensions beyond the dendrimer. In an early paper, Tomalia et al referred to their starburst polymers as a class of macromolecules obtained from the chemical bridging of starburst dendrimers. These macromolecules have been termed megamers, which have been defined as architectures derived from the combination of two or more dendritic macromolecules or poly(dendrimers). Many examples of randomly assembled as well as structure-controlled megamers have been reported, such as oligomeric covalent assemblies of dendrimers (i.e., dimers, trimers) referred... 

Several of these topological types (i.e., linear, cyclic, clusters) are remarkably reminiscent of those that are obtained by noncovalent self-assembly of proteins [8]. This offers a glimpse of the possible covalent topologies however, virtually no examples of dendrimer self-assembly by noncovalent, electrostatic type interactions have been reported until recently. Such abiotic, noncovalent PAMAM (G = 9) dendrimer self-assemblies have now been observed by atomic force microscopy (AFM) on freshly cleaved mica surfaces [268] (Figure 42). 

Many opportunities conversely are supported by reversible reactions of QM despite the noted complications. One example includes the synthesis and chiral resolution of binaphthol derivatives by two cycles of QM formation and alkylation.77 The reversibility of QM reaction may also be integrated in future design of self-assembling systems to provide covalent strength to the ultimate thermodynamic product. To date, QMs have already demonstrated great success in supporting the opposite process, spontaneous disassembly of dendrimers (Chapter 5). 

More recently, mathematically defined, structure controlled, covalent megamers have been reported. They are a major subclass of megamers also referred to as core-shell tecto dendrimers) [126-128], Synthetic methodologies to these new architectures have been reported to produce precise megameric structures that adhere to mathematically defined bonding rules [91, 129], It appears that structure controlled complexity beyond dendrimers is now possible. The demonstrated structure control within the dendrimer modules, and now the ability to mathematically predict and synthesize precise assemblies of these modules, provide a broad concept for the systematic construction of nanostructures with dimensions that could span the entire nanoscale region (Figure 1.24). 

While the convergent strategy, as described above, has been exceptionally successful, a considerable amount of effort has been devoted to improving its speed and synthetic efficiency. The remainder of this chapter reviews the major methodological developments and improvements in the accelerated, covalent, convergent synthesis of dendrimers. Dendrimers constructed via self-assembly and noncovalent interactions, as well as inorganic dendrimers [25], are beyond the scope of this brief chapter. 

Laboratory procedures are presented for two divergent approaches to covalent structure controlled dendrimer clusters or more specifically - core-shell tecto(dendrimers). The first method, namely (1) the self assembly/covalent bond formation method produces structure controlled saturated shell products (see Scheme 1). The second route, referred to as (2) direct covalent bond formation method , yields partial filled shell structures, as illustrated in Scheme 2. In each case, relatively monodispersed products are obtained. The first method yields precise shell saturated structures [31, 32] whereas the second method gives semi-controlled partially shell filled products [30, 33],... 

The general chemistry used in this approach involves the combination of a limited amount of an amine-terminated dendrimer core reagent with an excess of carboxylic acid terminated dendrimer shell reagent [31]. These two charge differentiated species are allowed to self-assemble into the electrostatically driven supramolecular core-shell tecto(dendrimer) architecture. After equilibration, covalent bond formation at these charge neutralized dendrimer contact sites is induced with carbodiimide reagents (Scheme 1). 

By covalent linkage of different types of molecules it is possible to obtain materials with novel properties that are different from those of the parent compounds. Examples of such materials are block-copolymers, soaps, or lipids which can self-assemble into periodic geometries with long-range order. Due to their amphiphilic character, these molecules tend to micellize and to phase-separate on the nanometer scale. By this self-assembly process the fabrication of new na-noscopic devices is possible, such as the micellization of diblock-co-polymers for the organization of nanometer-sized particles of metals or semiconductors [72 - 74]. The micelle formation is a dynamic process, which depends on a number of factors like solvent, temperature, and concentration. Synthesis of micelles which are independent of all of these factors via appropriately functionalized dendrimers which form unimolecular micelles is a straightforward strategy. In... 

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