Dendrimers convergent method

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. 

C. Ozawa, H. Hojo, Y. Nakahara, H. Katayama, K. Nabeshima, T. Akahane, and Y. Nakahara, Synthesis of glycopeptide dendrimer by a convergent method, Tetrahedron, 63 (2007) 9685-9690. 

Conventional sulfur vulcanization cure systems, 21 801 Convention du Mtre, 24 434 Convergent dendrimer synthesis method, 26 787-788... 

In general, convergent methods produce the most nearly isomolecular dendrimers. This is because the convergent growth process allows purification at each step of the synthesis and therefore no cumulative effects of failed couplings are found [85, 89]. Appropriately purified convergent dendrimers are probably the most precise synthetic macromolecules that exist today. 

Figure 5 Schematics of dendrimer growth by the divergent and the convergent methods...

Like the divergent approach, the convergent method also involves repetition of several basic chemical reactions. However, the reaction cycles are used to synthesize individual dendrons (dendrimer branches) instead of complete dendrimers. The dendrons have a protected focal point which can be activated in the last synthetic step and linked to two or more attachment points of a core molecule. Dendrimers synthesized by either method contain defects, but the problem is less pronounced for materials prepared by the convergent method. 

The convergent method also allows the synthesis of dendrimers of regular shapes other than spheres. Ellipsoid- and rod-shaped dendrimers can be synthesized hy using appropriate core molecules [Tomalia, 2001]. 

Opposite to the divergent method, the convergent method [3] consists in the stepwise synthesis of dendrimer branches first. They are then attached to the core yielding the dendrimer. Today the name dendrimer has been extended to... 

Path B in Fig. 2 is the convergent method. It is the outside-inward method, proposed independently by Miller and Neenan [9] and by Hawker and Frechet [10]. This method is well suited when the branch point is an aromatic ring. As an example of the convergent process we show in Scheme 3 the preparation of poly(benzyl ether) dendrimers. The phenol functionality of 2,5-dihydroxyben-zyl alcohol is first protected by Williamson reaction with benzyl bromide to provide the first generation dendron [G-l]-OH. The benzyl alcohol in [G-l]-OH is then converted to the benzyl bromide form [G-1]-Br. This in turn reacts with... 

In the convergent method, the synthesis of dendrimers starts from the periphery and ends at the core. In this method, a constant and low number of reaction sites is warranted in every reaction step throughout the synthesis. As a consequence, only a small number of side products can be formed in each reaction. It is necessary to employ a purification process in order to remove unreacted dendrons or defected dendrons after reaction. Accordingly it seems difficult to produce dendrimers on a large scale by this method. However, dendrimers synthesized convergently can provide defect-free ones. 

More recently an interesting dendron construction method was reported independently by Frechet et al. [102] as well as by Neenan and Miller [103] which has been referred to as the 1(B) Convergent Method. This method differs from the divergent dendron approach in that exponential proliferation of surface groups is accomplished by use of a limited number of growth reactions per sequence. Its advantages and limitation will be discussed later. With this method a variety of dendron families have been successfully synthesized and subsequently coupled to various point-like as well as hyper-branched cores, to produce dendrimers. This... 

Measurements of molecular density and intrinsic viscosity of PAMAM dendrimers indicate an unusual variation with dendrimer generation. Minimum density and maximum intrinsic viscosity were observed at around G4, which suggests that the fully developed dendrimers have a high accessible internal surface area in a solvent-filled intramolecular free volume that may consist of internal cavities and channels. Similar findings were reported by Mourey et al. for PBE monodendrons (based on dihydroxybenzyl alcohol) and tridendrons produced by coupling these dendrons to a trifunctional core, l,l,l-tris(4 -hydro-xyphenyl)ethane prepared by the convergent method. A maximum intrinsic viscosity occurred at G3 for tridendrons and G5 for monodendrons, consistent with the model developed by Lescanec and Muthukumar... 

Dendrimers containing cyclopentadienylruthenium complexes have been reported by Moss and co-workers via a convergent method.322 This reaction has led to a fourth-generation den-drimer, 275, containing 48 organometallic moieties. Low generation dendrimers containing six terminal ruthenium complexes were also reported. 

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