Dendrons

Each step in dendrimer synthesis occurs independent of the other steps therefore, a dendrimer can take on the characteristics defined by the chemical properties of the monomers used to construct it. Dendrimers thus can have almost limitless properties depending on the methods and materials used for their synthesis. Characteristics can include hydrophilic or hydrophobic regions, the presence of functional groups or reactive groups, metal chelating properties, core/shell dissimilarity, electrical conductivity, hemispherical divergence, biospecific affinity, photoactivity, or the dendrimers can be selectively cleavable at particular points within their structure. 

The synthesis and structure of a dendrimer can be illustrated by the well-known poly (amidoamine) type (called PAMAM), which describes the monomers making up the complete polymer. The synthesis starts from a core diamine (or ammonia) molecule. The diamine can be of various lengths and spacer arm properties and even contain cleavable components. Typically, the core 

Finally, dendrimers have been synthesized using solid phase peptide synthesis resins, wherein the core is linked to the resin and the half-dendrimer (dendron) is built out from it in sequential steps (Marsh et al., 1996 Swali et al., 1997 Wells et al., 1998). The advantage of this method 

One of the most important advances in synthesizing dendrimers is through the use of the cycloaddition reaction between azides and alkynes, which has become known as click 

Regardless of how they are made, the higher the dendrimer generation the greater the density of its branching becomes. Dendrimers of small size have an internally open configuration that freely permits the flow of small molecules within their inner structure. As dendrimers increase in diameter from G-0 through G-7, their appearance and size becomes more and more similar to 

For example, PPI dendrimers functionalized with azobenzene chromophores appended with aliphatic side chains assemble into large spherical vesicles in water below pH 8 (Fig. 11.14 Tsuda et al. 2000). 

Cryo-transmission electron spectroscopy (TEM), scanning electron spectroscopy, and confocal laser scanning microscopy smdies indicated the presence of large. 

Low generations of hydrophobic ally modified PAMAM dendrhners also exhibit a tendency to deform at air-water interfaces in order to present the hydrophilic dendrimer interior toward water and the aliphatic chains toward the air (Sayed-Sweet et al. 1997). Similarly, the tendency of poly(aryl ether) dendrhners with peripheral carbox-ylate end groups to aggregate in aqueous media decreased with increasing generation (Fig. 11.16 Laufersweiler et al. 2001). 

Similar generation-dependent aggregation behavior was reported for a series of dendrons having a phosphate focal group and functionalized at the periphery with azobenzene chromophores in water (Zhang et al. 2007). In this system, the stability 

Schematic representation of the monolayer organization of amphiphilic dendrimers on the water surface. 

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In addition to alkyl-substituted derivatives, soluble PPPs 6 are also known today containing alkoxy groups as well as ionic side groups (carboxy and sulfonic acid functions) [18]. Schliiter et al. recently described the generation of soluble PPPs decorated with densely packed stcrically demanding dendrons on the formation of cylindrically shaped dendrimers, so-called cylinder dendrimers ] 19]. 

Some of the more remarkable examples of this form of topologically controlled radical polymerization were reported by Percec et cii.231 234 Dendron maeromonomers were observed to self-assemble at a concentration above 0.20 mol/L in benzene to form spherical micellar aggregates where the polymerizable double bonds are concentrated inside. The polymerization of the aggregates initiated by AIBN showed some living characteristics. Diversities were narrow and molecular weights were dictated by the size of the aggregate. The shape of the resultant macroniolecules, as observed by atomic force microscopy (ATM), was found to depend on Xn. With A, <20, the polymer remained spherical. On the other hand, with X>20, the polymer became cylindrical.231,232... 

Dendrimers have structures similar to that of hyperbranched polymer and can be taken as the perfectly branched polymer with monodispersity. However, they need to be prepared by a multistep procedure. Therefore, very little work has been done on dendritic polyfarylcnc ether)s. Morikawa et al. prepared a series of monomers with a various number of phenylene units.164,165 These monomer were used to prepare poly(ether ketone) dendrons with graded structures (Scheme 6.24). 

Scheme 6.24 Synthesis of poly(ether ketone) dendrons with graded structures...

Hyperbranched and dendronized polymers such as 40, 41, and 42 have also been synthesized using the transition metal coupling strategies in recent years.32 These polymers are fundamentally different from those traditional linear polymers. They possess dendritic arms within die polymer or along the polymer backbone. It is believed that they possess interesting properties and have potential applications in many fields such as nanotechnology and catalysis ... 

Conjugated polymers, including optically active polymers and dendronized polymers that are very useful in electrical and optical fields and asymmetric catalysis, will continue to attract interest from chemists and materials scientists. It is well anticipated that more and more polymers with interesting structures and properties will be synthesized from the transition metal coupling strategy. 

The main chain of dendronized polymers, due to die large size of the mon-odendrons, is usually forced to take a stretched shape thus the whole molecule exists as a rigid rod architecture both in solution and in the solid state.32d Depending on the backbone stiffness, the degree of monodendron coverage, and the size of die monodendron, the architecture of these macromolecules is no longer a sphere but a cylinder this dictates die properties of the dendronized polymers. 

Poly(ether ketone), synthesis of, 343 Poly(ether ketone) dendrons, synthesis of, 352-353... 

Poly f p-oxybenzoyl-co-p-phenylene isophthalate]), 113-114 Poly(2,2 -oxydiethylene adipate), 29 Polyoxymethylene glycol, aqueous, 377 Poly(oxytetramethylene) (PTMO), 53 Poly (p-pheny lene). See also Poly(para-phenylene)s dendronized, 520-521 synthesis of, 491-494 synthesis of water-soluble, 493 Poly(phenylene ether sulfone) chains,... 

Poly(phenylenethylene), dendronized, 522 Poly(phenylenevinylene) optically active, 510-511 synthesis of, 495-496 Poly(/ ara-phenylenevinylene)s, 472 Polypheny lquinoxaline (PPQ) hyperbranched, 312-314 synthesis of, 309-313 Polyphosphoric acid, 333 Polypropylene oxide) polyol, 223 Polypropylene polyols, 220 Poly (pyridine), synthesis of, 503-505 Polyquinoxaline (PQ), synthesis of, 309-313... 

Dendrimers have distinctive properties, such as the ability to entrap small molecules in their core region and very low intrinsic viscosities in solution. Such properties require molecules to have achieved a particular size, and not all molecules with branches radiating from a core are large enough to develop the characteristic properties of true dendrimers. Branched molecules below this critical size are called dendrons and are the equivalent in dendrimer chemistry of oligomers in polymer chemistry. 

Nonetheless, it was a fairly short step from octopus compounds to dendrimers, and the step was taken by Vogtle in the late 1970s when he attempted to use a cascade reaction to prepare a molecule of the dendrimer type that would now be considered a dendron rather than a fully developed dendrimer. It began with the addition of acrylonitrile to an anfine, followed by reduction of the nitrile to amine. This was followed by a further reaction with acrylonitrile, and the process was repeated several times to yield highly branched macromolecules. There were initially problems with the reduction step but these were overcome, and the preparation of these poly(propylene imine) dendrimers was later commercialized. 

HPLC is limited to relatively low molar mass compounds, Le. below 3000. In practice this tends to restrict the technique to dendrons rather than fully formed dendrimers, but it is nonetheless a useful technique, given the importance of establishing high analytical purity of products at each step in a dendrimer synthesis. 

G. Newkome, C.N. Moorefield, F. Vogtle and N.C. Moorefield, Dendrimers and Dendrons Concepts, Synthesis, Applications, Wiley-VCH, Weinheim, Berlin, 2001. 

Kawa M (2003) Antenna Effects of Aromatic Dendrons and Their Luminescene Applications. 228 193-204... 

A dendrimer-based approach for the design of globular protein mimics using glutamic (Glu) and aspartic (Asp) acids as building blocks has been developed [151]. The preassembled Glu/Asp dendrones were attached to a 1,3-bifunctional adamantyl based on a convergent dendrimer synthesis strategy (see Fig. 28). 

The reaction of the first-generation dendron 29 with the core, which is a trin-uclear platinum complex (30) bridged by 1,3,5-triethynylmesitylene, in a molar ratio of 3 1 resulted in the formation of the first-generation dendrimer (31). Since the reaction proceeded quantitatively and no side products were observed, the first-generation dendrimer was easily isolated by alumina column chromatography. 

The first-generation dendron 29 was grown up to the second- and third-generation dendrons 33 and 34 by the successive reaction with dinuclear platinum complex 32 followed by treatment with TBAF for desilylation (Scheme 11). Reactions of the second- and third-generation dendrons 33 and 34 with the core 30 gave the second- and third-generation dendrimers, respectively. Although... 

Newkome GR, Moorefield CN, Vdgtle F (2001) Dendrimers and dendrons, concept, synthesis, applications. Wiley-VCH, Weinheim... 

Fullerenes possess electronic and photophysical properties which make them natural candidates for the preparation of functional dendrimers. The attachment of a controlled number of dendrons on a core provides a compact insulating layer around the carbon sphere, and the... 

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