Solubility dendritic macromolecules

Initial efforts gave rise to well-characterized dendritic macromolecules, but applications remained limited because of the lack of specific functionalities. An exponential increase of publication volume observed for about 15 years testified the growing interest for dendrimers and has led to versatile and powerful iterative methodologies for systematically and expeditiously accessing complex dendritic structures. The perfect control of tridimensional parameters (size, shape, geometry) and the covalent introduction of functionalities in the core, the branches, or the high number extremities, or by physical encapsulation in the microenvironment created by cavities confer such desired properties as solubility, and hydrophilic/hydrophobic balance. Thus, creativity has allowed these structures to become integrated with nearly all contemporary scientific disciplines. 

Undoubtedly, the most notable feature of these new dendrimeric organometallic molecules is their ability to act successfully as effective homogeneous catalysts for the Kharasch addition reaction of polyhalogenoalkanes to olefinic C=C double bonds. Indeed, they show catalytic activity and clean regiospecific formation of 1 1 addition products in a similar way to that observed in the mononuclear compounds. Likewise, the nanoscopic size of these first examples of soluble dendritic catalysts allows the separation of such macromolecules from the solution of the products by ultrafiltration methods. 

Reichert and Mathias prepared related branched aramids, to those of Kim,t5-34] from 3,5-dibromoaniline (23) under Pd-catalyzed carbonylation conditions (Scheme 6.7). These brominated hyperbranched materials (24) were insoluble in solvents such as DMF, DMAc, and NMP, in contrast to the polyamine and polycarboxylic acid terminated polymers that Kim synthesized, which were soluble. This supports the observation that surface functionality plays a major role in determining the physical properties of hyperbranched and dendritic macromolecules J4,36 A high degree of cross-linking could also significantly effect solubility. When a four-directional core was incorporated into the polymerization via tetrakis(4-iodophenyl)adamantanc,1371 the resultant hyperbranched polybromide (e.g., 25) possessed enhanced solubility in the above solvents, possibly as a result of the disruption of crystallinity and increased porosity. 

M. Kramer, N. Perignon, R. Haag, J. D. Marty, R. Thomann, N. Lauth-de Viguerie, and C. Mingotaud, Water-soluble dendritic architectures with carbohydrate shells for the temptation and stabilization of catalytically active metal nanoparticles. Macromolecules, 38 (2005) 8308-8315. 

As a result of their ready solubility, dendritic materials can be analyzed 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 that can analyze such macromolecules, the structural perfection assumed in many depictions of dendrimers has been shown to be highly idealized. Indeed, spectrometric analysis of many samples has revealed that imperfections and defects are, in fact, very common. 

These distinguishing features of dendritic macromolecules render these novel materials a reliable alternative to traditional polymers in a wide range of potential applications. The primary focus of this chapter is to cover the range of these potential uses of water-soluble dendrimers - it does not provide comprehensive coverage of these applications but has targeted a number of notable examples of the utilisation of water-soluble hyperbranched macromolecules. 

A comparison of the physical properties of hyperbranched and dendritic macromolecules with linear polymers and the linear analogs of these 3-dimensional polymers is presented. It is found that thermal properties, such as glass transition temperature and degradation, are the same regardless of the macromolecular architecture but are very sensitive to the number and nature of chain end functional groups. However, other properties, such as solubility, melt viscosity, chemicd reactivity, intrinsic viscosity were found to be very dependent on the macromolecular architecture. 

A number of conclusion can be drawn from the above studies. Firstly some physical properties such as glass transition temperature and thermal degradation are independent of macromolecular architecture but are dependent on the nature of the chain ends. Other physical properties such as solubility, chemical reactivity, viscosity, etc. are dependent on macromolecular architecture and definite differences are observed between linear, hyperbranched and dendritic macromolecules based on the same building block. 

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