Hyperbranched Polymers and Dendrimers

HigMy branched polymeric materials with large number of end groups can offer unique physical properties. Dendrimers (described in Chap. 1) differ from linear polymers in viscosity and thermal behavior. A variety of applications have been forecast for these highly branched globular macromolecules. One early approach to dedrimer synthesis relied upon the Michael reaction using repeated sequential additions of an amine to a,(3-unsaturated ester [248]  

In the first step, ammonia is added to an excess of methyl acrylate. The product is reacted with an excess of ethylenediamine to form a star molecule with three arms. The product of the second step is then reacted with an excess of methyl acrylate. This is followed by a reaction with an excess of ethylenediamine to form a molecule with six arms. The sequences are repeated to yield a star polymer. Such star polymers are sometimes referred to as starburst polymers. 

A recent publication described preparation of dendrimers from functional aliphatic polyesters that are based on 2,2-bis(hydroxymethyl)propionic acid [249]. A, Al -dicyclohexylcarbodiimide was used as the coupling agent in a double-stage convergent approach that reduced the number of synthetic and Uquid chromatographic steps required in the preparations and purifications. The hydroxyl functional dendrimers were then subjected to a variety of surface modifications by reactions with different acid chlorides [249]. 

Another publication describes formation of polyphenylene dendrimers that formed nanoparticles with 102 benzene rings [250]. In this preparation, an A—B type monomer, 2,3,4,5-tetrakis(4-triisopropylsilylethynylphenyl)-cyclopenta-2,3-dienone, is condensed in a [4 -i- 2]cycloaddition with 3,3 5,5 -tetraethynyl-biphenyl  

The term dendrimer derives from the Greek word dendro for tree, and this indicates their three-dimensional branched structure. The molecules have a regularly branched structure, as shown in Fig. 2.32. In this idealized structure, each monomer group is cormected to three others, except at the periphery, giving a structure described as a Cayley tree with functionality f = 3. It is possible to synthesize dendrimers with different core, branch and surface imits. Due to steric effects there is a limit on the number of generations that can be grown . The structure shown in Fig. 2.32 is a fourth generation dendrimer. 

Flyperbranched polymers have an irregular branched structure and are prepared by the one-pot self-condensation of multifunctional ABj (X 2) type monomers. 

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Monomers of die type Aa B. are used in step-growth polymerization to produce a variety of polymer architectures, including stars, dendrimers, and hyperbranched polymers.26 28 The unique architecture imparts properties distinctly different from linear polymers of similar compositions. These materials are finding applications in areas such as resin modification, micelles and encapsulation, liquid crystals, pharmaceuticals, catalysis, electroluminescent devices, and analytical chemistry. 

Son, D. Y. Silicon-Based Dendrimers and Hyperbranched Polymers. In Chemistry of Organic Silicon Compounds-, Rappoport, Z., Apeloig, Y., Eds. Wiley Chichester, 2001 Vol. 3, pp 745-803. 

Frechet, J. M. J., Hawker, C. J., Synthesis and properties of dendrimers and hyperbranched polymers, in Aggarwal, S. L. and Russo, S. (eds), Comprehensive Polymer Science, Second Supplement, Elsevier Science, Oxford 1996, pp. 71-133. 

In the search to develop new materials for immobilization of homogeneous transition metal catalyst to facilitate catalyst-product separation and catalyst recychng, the study of dendrimers and hyperbranched polymers for application in catalysis has become a subject of intense research in the last five years [68], because they have excellent solubility and a high number of easily accessible active sites. Moreover, the pseudo-spherical structure with nanometer dimensions opens the possibility of separation and recycling by nanofiltration methods. Although dendrimers allow for controlled incorporation of transition metal catalysts in the core [69] as well as at the surface [70], a serious drawback of this approach is the tedious preparation of functionalized dendrimers by multi-step synthesis. 

Although some make a distinction between dendrimers and hyperbranched polymers, we will not do so here. In essence, hyperbranched polymers are formed under conditions that give... 

How do dendrimers and hyperbranched polymers compare from an industrial viewpoint Dendrimers offer the potential for producing polymers whose molecular size and structure are more regular and less polydisperse. 1 lyperbranched polymers are easier and cheaper to synthsize—a one-pot synthesis compared to the multipot synthesis for dendrimers. However, not too many AB/ monomers are readily available, and this may modify the overall economics. Hyperbranched polymers will probably find use in larger-scale or commodity applications where lower cost is a necessity and dendrimers in specialty applications where higher cost is justified. 

Dendrimers and hyperbranched polymers are two groups of materials resembling each other. The architectural difference is that dendrimers are perfectly branched structures, while hyperbranched polymers contain defects. Dendrimers are mono-dispersed while hyperbranched polymers are more dispersed which can be an advantage in some applications. 

Fig. 4 Perfect PAMAM dendrimer and hyperbranched polymer (HYPAM). Reproduced with permission from [117]. Copyright 2002 Elsevier...

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]. 

Fig-1 Schematic comparison of perfect dendrimers and hyperbranched polymers... 

Tonic Dendrimers and Related Materials [R. Engel, Chapt. 3, pp. 73-99]. Silicon-based Stars, Dendrimers, and Hyperbranched Polymers [L. J. Mathias, T. W. Carothers, Chapt. 4, pp. 101-121]. 

The syntheses and applications of dendrimers and hyperbranched polymers have attracted considerable scientific interest in recent years. These highly branched polymers possess... 

In this review, complete coverage of the syntheses and applications of silicon-based dendrimers and hyperbranched polymers will be provided. For organizational purposes, dendrimers and hyperbranched polymers will be reviewed in separate sections, and each section will be further divided according to polymer class. For this review, the various classes will be defined according to the branch backbone substituents as follows ... 

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