Covalent dendrimer micelle

Fig. 3.3 TEM and model of an assembly of fluid kanamycin micelles. The picture shows no signs of fusion or collapse. It looks similar to pictures of much smaller covalent dendrimer micelles.

In contrast to dendrimers built up from aliphatic chains, polyphenylene dendrimer micelles possess shape and size persistent cavities due to their rigid scaffold which strongly depends on the type of dendrimer. In this case a selective incorporation of guest molecules, e.g., fluorescent dyes, should be possible, dependent on the size of the guest molecule and the cavity of the host. The non-covalent uptake of dyes with an appropriate size thus allows the investigation of their interactions within the dendritic micelle. In our case we made the second-generation polyphenylene dendrimer 48, which bears 16 carboxy-functions at the periphery, by starting from a tetrahedral core and an appropriately... 

The dendrimers form small micelles, which transport small molecules within their matrices or act as hubs for covalent bonding to drug molecules, extending like dendrites. In this way, they can shepherd high concentrations of drugs to targets. 

Recently, several articles have been published dealing with water soluble dendrimers, due to their outstanding structure and properties which are similar to natural micellar systems, e. g., liposomes. Scheme 17 shows such an example of a dendritic micelle containing a hydrophobic inner core surrounded by a hydrophilic layer of carboxylate groups. The most important difference to natural micelles is that dendritic micelles are covalently bound structures and not dyn-... 

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... 

The structure and properties of water soluble dendrimers, such as 46, is, in itself, a very promising area of research due to their similarity with natural micellar systems. As can be seen from the two-dimensional representation of 46 the structure contains a hydrophobic inner core surrounded by a hydrophilic layer of carboxylate groups (Fig. 12). However these dendritic micelles differ from traditional micelles in that they are static, covalently bound structures instead of dynamic associations of individual molecules. A number of studies have exploited this unique feature of dendritic micelles in the design of novel recyclable solubilization and extraction systems that may find great application in the recovery of organic materials from aqueous solutions [84,86-88]. These studies have also shown that dendritic micelles can solubilize hydrophobic molecules in aqueous solution to the same, if not greater, extent than traditional SDS micelles. The advantages of these dendritic micelles are that they do not suffer from a critical micelle concentration and therefore display solvation ability at nanomolar... 

One of the possible alternative to micelles are spherical dendrimers of diameter generally ranging between 5 and 10 nm. These are highly structured three-dimensional globular macromolecules composed of branched polymers covalently bonded to a central core [214]. Therefore, dendrimers are topologically similar to micelles, with the difference that the strnctnre of micelles is dynamic whereas that of dendrimers is static. Thus, unlike micelles, dendrimers are stable nnder a variety of experimental conditions. In addition, dendrimers have a defined nnmber of fnnctional end gronps that can be functionalized to prodnce psendostationary phases with different properties. Other psendostationary phases employed to address the limitations associated with the micellar phases mentioned above and to modnlate selectivity include water-soluble linear polymers, polymeric surfactants, and gemini snrfactant polymers. 

Tomalia DA, Berry V, HaU M, Hedstiand DM. Starburst dendrimers. 4. Covalently fixed unimolecular assemblages reiminiscent of spheroidal micelles. Macromolecules 1987 20 1164-1167. 

Non-covalent bonding of guest molecules in the interior of a dendrimer molecule can also proceed dynamically. In micelle-like amphiphilic dendrimers, guests can diffuse into and out of the interior of the dendrimer scaffold via hydrophilic and hydrophobic interactions (Fig. 6.18). 

Another approach is to assemble multiple chelates either covalently (11) (oligomer, polymer, and dendrimer) or non-covalently (12, 13) (micelle, liposome, and emulsion). These approaches all yield higher molecular relaxivities because of the assembly, but the per-ion relaxivity also is increased because motion is slowed. Fast internal motions can limit these relaxivity gains, but this limitation can be overcome by rigidifying the structure in some way (14). 

In contrast to the above discussion, theoretical treatments favour the model of the molecular micelle [195, 207]. At least an analogue to the latter has been realized recently in some dendrimers, arborols (Fig. 35) and hypercrosslinked polymers, replacing the self-organization of hydrophobic moieties by covalent bonding [41-45],... 

PAMAM dendrimers, synthesized subsequently, are likely the most widely investigated and used dendritic polymers to date. The first dendrimers commercialized in that family were the Starburst systems. These species were developed in the mid-1980s by Tomalia [5], at about the same time when Newkome developed similar dendritic architectures named Arborols [6]. A major incentive for the development of these molecules was the creation of covalently bonded (unimolecular) micelles comparable to the well-known multi- or intermolecular micellar systems. 

Figure 2 Schematic representation of various modes of attachment of catalyticaUy active complexes to soluble polymers covalent binding to a PPI dendrimer (upper left) electrostatic binding to linear poly(diallyldimethyl-ammonium chloride) (upper right) physisorption in a unimolecular micelle of amphiphillically modified hyperbranched polyglycerol (lower left) specific hydrogen-bonding to a urea functionalized PPI dendrimer.

Small assemblies that topologically mimic the micelle architecture but that present covalently bound amphiphilic chains have been synthesized as an alternative approach to provide intrinsic stability These nanocarriers are referred to as unimolecular polymeric micelles (UPM) and consist of single macromolecules such that their formation and dissociation are intrinsically independent on polymer concentration (no apparent CMC). UPM can be obtained from both dendrimers and star-block copol)miers, with the latter form involving fewer s)mthetic steps and being simpler to prepare. 

The self-assembly of natural and synthetic multifunctional unimers occurs, in fact, by a combination of classical molecular recognition and growth mechanisms. Self-assembled, reversible structures include linear, helical, columnar, and tubular polymers micelles monolayers and three-dimensional phases and networks. Self-assembled systems based on an interplay between covalent chains and supramolecular interactions are also important. Typical examples are side-chain SPs (Chapter 5), host-guest polymeric composites and dendrimers (Chapters 2 and 7), polymers with mechanical bonds (Chapter 8), and block copolymers (Chapters 9-11). 

Tomalia, D. A. Berry, V. Hall, M. Hedstrand, D. M. (1987). Starburst Dendrimers. 4. Covalently Fixed Unimolecular Assemblages Reminiscent of Spheroidal Micelles. Macromolecules, 20,1164-1167... 

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