Synthesis of Dendrimers and Hyperbranched Polymers

For the synthesis of dendrimers, mainly the divergent and the convergent methods are used (Fig. 5.13). In the divergent method, a dendritic macromolecule is synthesized that starts from the core and is expanded in a stepwise fashion. Further generations (G) are built with well-defined core-shell structures in iterative stages. A basic example of the divergent 

FIGURE 5.13 Divergent and convergent synthetic routes toward dendrimers. G generation. 

Hyperbranched polymers are synthesized in a one-step method, often from AB monomers but also by combining A +B (x 3) monomers or variations of those. Polymerization methods have been applied that involve polycondensation, polyaddition, and ring-opening or self-condensing vinyl polymerization. Even though the one-pot synthetic approach leads to imperfectly branched structures because of uncontrolled growth, it is more suitable for the preparation on a larger scale and thus for commercial use. Nowadays, different 

FIGURE 5.14 Synthetic steps toward PAMAM dendtimers by the divergent method. 

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Until now, thio-bromo click chemistry has only been used for the synthesis of dendrimers and hyperbranched polymers. Without any doubt, further research will lead to other complex architectures, such as dendronized polymers, dendrigrafts and unsymmetrical dendrimers. 

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. 

The comparable architecture and chemical functionality of dendrimers and hyperbranched polymers lead to similar applications for these two families of dendritic polymers. The main benetit in using hyperbranched polymers to replace dendrimers lies in their simpler synthesis, provided that the perfect structure of dendrimers can be sacrificed for their broadly distributed hyperbranched analogs. The one-pot syntheses require less time and resources, resulting in less expensive processes that make hyperbranched polymers excellent candidates for commercial applications. Pertinent to the hyperbranched architecture are applications as electronic, magnetic, and catalytic materials, as well as numerous uses in the biomedical field some of these are considered herein. 

Synthesis of the hyperbranched poiyethoxysiloxane. Viewed from a standpoint of the chemistry of dendrimers and hyperbranched polymers, triethoxysilanol (regarded in (20) as a primary product of hydrolysis) is no more than a reactant AB3 according to the Flory condition. This signifies that, by generating this product under the conditions of a heterofunctional condensation, one can direct the reaction such that a hyperbranched poiyethoxysiloxane is formed, that is, to make the process structurally selective. It is known that, in hyperbranched polymers, cyclization is a minor contributor to the molecular structuring because of the paucity of A-type functionalities. In other words, with allowance made for structural imperfection of the hyperbranched polymer and for the fact that proportions of the dentritic, linear and end chains depend on a number of factors, it is possible in principle to obtain an end product with desired properties by monitoring structure, rather than process parameters, of the polymer formed. 

The fundamental requirements for the synthesis of dendrimers and hyperbranched macromolecules is examined. Examples of the divergent and convergent approaches are presented and a comparison of both methods with respect to each other and also the one-step procedure for hyperbranched macromolecules is made. The structural similarities and differences between dendrimers and hyperbranched macromolecules are described and the effect of this on the physical properties of these novel three-dimensional materials is discussed. Finally, a comparison of these materials with linear polymers is examined. 

Recently the development of dendritic and hyperbranched polymers (HBPs) has attracted much attention (Tomalia, 1985, Newkome et al, 1985, Webster, 1991, Chu and Hawker, 1993, Wooley et al, 1994, Feast and Stanton, 1995, Malmstrom et al, 1995, Kim, 1998). The key features of the macromolecular architecture of dendrimers and HBPs are given in Section 1.2, and their synthesis by stepwise polymerization is discussed in Section 1.2.1. Dendrimers and HBPs are globular macromolecules that have a highly branched structure with multiple reactive chain ends (shell), which converge to a central focal point (core) see Figure 5.1, where I is the core, 11 is the structure and 111 is the shell. 

The general area of dendritic and hyperbranched polymers has received remarkable attention over the past decade. New properties not available with linear polymers have been demonstrated. For example, evidence has been provided that supports the existence of considerable space for the encapsulation of small molecules, and this has led to the idea of a dendritic box" (69j. A severe problem with dendrimers is their timesynthetic methods that form hyperbranched materials that may exhibit many of the advantageous properties of dendritic macromolecules have been receiving significant attention (70j. 

Perfectly branched dendrimers have potentially better properties for applications in the field of biomedicine than hyperbranched polymers due to their well-defined and predictable structure and narrow mass distribution, which is important for in vitro and in vivo applications. However, hyperbranched polymers have one very significant advantage, which is their easier preparation by a one-step synthesis. Therefore, hyperbranched polymers are also used in technical applications, for example, as additives, blends, or coating components and as multifunctional cross-linkers. But both, dendrimers and hyperbranched polymers, have been extensively studied in the fields of encapsulation and delivery of drugs, dyes, and genes because of their original branched architecture (Fig. 5.15). Small molecules of interest can be incorporated in the interior cavities of dendritic molecules or bound to their outer functional groups. 

In this chapter we consider the properties of synthetic polymers. First, the main techniques of polymer synthesis are outlined (Section 2.2). Then the conformation of polymer molecules is discussed in Section 2.3. We move on to a summary of the main methods for characterization of polymeric materials in Section 2.4. Then the distinct features of the main classes of polymer are considered, i.e. solutions (Section 2.5), melts (and glasses) (Section 2.6) and crystals (Section 2.7). Then the important properties of plastics (Section 2.8), rubber (Section 2.9) and polymer fibres (Section 2.10) are related to microscopic structure and to rheology. Polymer blends and block copolymers form varied structures due to phase separation, and this is compared and contrasted for the two types of system in Section 2.11. Section 2.12 is concerned with dendrimers and hyperbranched polymers. Section 2.13 and 2.14 deal with polyelectrolytes and (opto)electronic polymers respectively. 

The s)mthesis, characterization, and applications of dendritic and hyperbranched polymers is an exciting active area of investigation. The one-pot synthesis of hyperbranched polymers suggests that their commercial exploitation will outpace that of dendrimers for many applications. 

Dendrimers produced by divergent or convergent methods are nearly perfectly branched with great structural precision. However, the multistep synthesis of dendrimers can be expensive and time consuming. The treelike structure of dendrimers can be approached through a one-step synthetic methodology.31 The step-growth polymerization of ABx-type monomers, particularly AB2, results in a randomly branched macromolecule referred to as hyperbranch polymers. 

The DB obtainable in SCVP is DB=0.465 for r=kjk =l and reaches its maximum, DB=0.5, for r=2.6 [70,78]. This value is identical to that obtained in AB2 polycondensation when both B functions have the same reactivity [70,78]. Thus, hyperbranched polymers prepared by bulk polycondensation or polymerization contain at least 50% linear units, making this approach less efficient than the synthesis of dendrimers. 

Very recently, highly regular, highly controlled, dense branching has been developed. The resulting dendrimers often have a spherical shape with special interior and surface properties. The synthesis and properties of dendrimers has been reviewed (see e.g. G.R. Newkome et al. Dendritic Molecules , VCH, 1996). In this series, a chapter deals with the molecular dimensions of dendrimers and with dendrimer-polymer hybrids. One possible development of such materials may be in the fields of biochemistry and biomaterials. The less perfect hyper-branched polymers synthesized from A2B-type monomers offer a real hope for large scale commercialization. A review of the present status of research on hyperbranched polymers is included. 

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