Dendritic materials

Host-guest systems made from dendritic materials have potential in the areas of membrane transport and drug delivery [68, 84, 85]. In a recent report [136] Tomalia and coworkers investigated structural aspects of a series of PAM AM bolaamphiphiles (e.g., 50) with a hydrophobic diamino do decane core unit. Fluorescence emission of added dye (nile red) was significantly enhanced in an aqueous medium in the presence of 50 unlike the cases when 51 and 52 were added (Fig. 23). Addition of anion surfactants to this mixture generated supramolecular assemblies which enhanced their ability (ca.by 10-fold) to accommodate nile red (53). Further increase in emission was noted by decreasing the pH from the normal value of 11 for PAMAM dendrimers to 7. At lower pH values the... 

The overwhelming number of dendrimer-related reports flooding the chemical arena, particularly, in the last five years, has made it a difficult task to summarize all important developments in one treatise. The restricted scope of this chapter - supramolecular chemistry within dendritic materials - denotes the utilitarian character to the unique infrastructure of these materials. Surface coatings and attachments to molecular spheres should possess a common theme respective of their frameworks, and thus there should be less differentiation between the mode of construction but rather what is the surface functionality. 

These materials have possible utility in a number of specialty applications and are being explored by Guan et al. [37], They have used these catalysts, and their unique chain-walking characteristics to synthesize a variety of dendritic materials (Fig. 4), which could find potential application as processing aids, rheological modifiers, and amphiphilic core-shell nanoparticles for drug delivery and dye formulation. 

Fig. 4 Dendritic materials made via chain-walking mechanism. Left processing aid, rheology modifier. Right ampiphilic core-shell...

In October 2002, CDT acquired the OLED technology from Oxford-based Opsys, which is known for its dendritic materials development. As of July 2003, no patents have been granted to Opsys. 

In the Mukaiyama aldol additions of trimethyl-(l-phenyl-propenyloxy)-silane to give benzaldehyde and cinnamaldehyde catalyzed by 7 mol% supported scandium catalyst, a 1 1 mixture of diastereomers was obtained. Again, the dendritic catalyst could be recycled easily without any loss in performance. The scandium cross-linked dendritic material appeared to be an efficient catalyst for the Diels-Alder reaction between methyl vinyl ketone and cyclopentadiene. The Diels-Alder adduct was formed in dichloromethane at 0°C in 79% yield with an endo/exo ratio of 85 15. The material was also used as a Friedel-Crafts acylation catalyst (contain-ing7mol% scandium) for the formation of / -methoxyacetophenone (in a 73% yield) from anisole, acetic acid anhydride, and lithium perchlorate at 50°C in nitromethane. 

The continued interest in dendritic materials as well as the related hyperbranched polymers has sparked the imagination of researchers in many different areas. The incredible increase in annual publications in this topic is best shown in the Figure and thus as the number on new building blocks and core molecules proliferate, the structural composition of precise and controlled design will grow to meet the imagination of molecular architects. This review series was initially conceived to cover the synthesis and supramolecular chemistry of dendritic or cascade supermolecules as well as their less perfect hyperbranched cousins. 

The third International Dendrimer Symposium took place at Berlin Technical University in 2003. Interdisciplinary lectures demonstrated the extent to which dendritic molecules branch ouf into other areas of science, such as physics, biology, medicine, and engineering. The possibilities of functionalisation and resulting applications in industry were at the focus of this symposium. For example, nano-dimensioned dendrimer-based contrast agents were presented as multilabels for visualisation of blood vessels (see Chapter 8). Potential applications of dendritic materials as luminescence markers in diagnostics attracted lively interest (see Chapter 8). Consideration of the differences between dendrimers and hyperbranched polymers from the viewpoint of their cost-favourable application was also a topic of discussion [18]. 

After the initial disclosure of a viable iterative synthetic method for the construction of polyfunctional macromolecules, a small number of articles explored the use of repetitive chemistry for the preparation of dendritic materials. Figure 2.4 illustrates the early branched architectures that were constructed, in most cases, by employing protection-deprotection schemes. Detailed descriptions of the synthetic procedure can be found in Chapters 4 (structures 7-9) and 5 (structures 10 and 11). 

These initial theoretical investigations, as well as the early alternative architectures that were synthesized, serve as an exordium for the expanding and synergistic discourse of dendritic material science. In-depth discussion pertaining to more recent synthetic and theoretical reports can be found beginning at Chapter 4. 

Since these organometallic dendrimers have non-interacting ferrocenyl redox centers, Moran and coworkers1401 successfully modified electrode surfaces by electrodeposition of these dendritic materials in their oxidized forms. Electrodeposition was accomplished by controlled potential electrolysis or by repetitive anodic and cathodic cycling the amount of electroactive dendrimer deposited could thus be regulated. 

Cheaper, high loading hyperbranched polymers were introduced by Kantchev and Parquette and used to prepare oligosaccharides (301) their potential as solid supports is considerable. Newkome et al. have reported the combinatorial synthesis of dendritic materials with different properties (302, see also Section 11.3.2). 

This award is considered to have generated a new field known as nanotechnology, as worldwide exposure was instantly aware of these nanoscale molecules, and other developments in this size regime were found shortly thereafter. It should be noted that the discovery of dendrimers by Denkwalter and coworkers from AlUed Corporation was disclosed in 1981, 4 years before bucky balls were discovered. It may be expected that these nanopolymeric materials will be extremely influential toward the nanotechnology revolution (see Chapter 5 for more information on dendritic materials). 

These procedures generally maintain the versatility and product monodispersity offered by the traditional convergent method, but reduce the number of linear synthetic steps required to access larger dendritic materials. 

Poly(ethylene oxide) (PEO) has been employed frequently as a water-soluble catalyst support [9]. Further water-soluble polymers investigated include other linear polymers such as poly(acrylic acid) [10], poly(N-alkylacrylamide)s [11], and copolymers of maleic anhydride and methylvinylether [12], as well as dendritic materials such as poly(ethyleneimin) [10a, c] or PEO derivatives of polyaryl ethers [13]. The term dendritic refers to a highly branched, tree-like structure and includes perfectly branched dendrimers as well as statistically branched, hyperbranched macromolecules. 

Chromophore-containing dendritic structures have emerged as an alternative solution to achieve spherical shape modification of chromophores [108], In spite of any conventional EO polymer, the void-containing structure of dendrimers provides the site isolation needed for chromophores to independently reorient under the external poling field [109]. Moreover, these dendritic materials possess a mono-disperse and well-defined globular geometry. Their structure is synthetically controllable in size and shape, allowing wide control over solubility, processability, viscosity, and stability. 

As a result of their ease of solubility, characterization of dendritic materials can be readily achieved by means of standard polymer characterization techniques, such as NMR and GPC. However, despite the excellent characterization that pervades the materials discussed in this chapter, it should be noted that with the widespread introduction of mass spectrometric techniques which can characterize such macromolecules, the structural perfection assumed in many depictions of dendrimers have been shown to be highly idealized. Indeed, spectrometric analysis of many samples has revealed that imperfections and defects are, in fact, very common. In this section, organometallic dendrimers, a subset of metallodendrimers, are discussed. 

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