Dendrimers convergent growth approach

The convergent growth approach to dendrimers [1] first introduced [2] in 1989 at the IUPAC meeting on macromolecules in Seoul, Korea, has provided a useful alternative to the divergent methods exemplified by the work of Tomalia et al. on PAMAM dendrimers [3] and Meijer et al. on polypropylene imine) dendrimers [4], Today several hundred papers have exploited the convergent approach to dendrimers to prepare a variety of synthetic functional macromolecules of unparalleled structural precision. 

The near monodispersity typically associated with dendrimers prepared by both the divergent or convergent growth approaches presents a number of interesting characterization traits when compared to traditional narrow polydisper-sity linear polymers. These features are further enhanced by the high degree of symmetry normally associated with dendritic macromolecules and permit an unprecedented amount of structural information to be obtained using standard characterization techniques. 

As the name impHes, the core unit, or focal point group, is located at the center of the dendritic macromolecule and, as such, constitutes only a small proportion of the overall dendrimer structure. However the role of functional groups at the central core can have a profound effect on a number of physical properties and be extremely useful in investigating the structure of dendrimers. The central role played by the focal point group in the success of the convergent growth approach has also been well documented in the above discussions. 

The first part of this review deals with the synthesis and characterization of dendritic aliphatic polyesters based on 2,2-bis(dimethylol)propionic acid (bis-MPA) and 1,1,1 -tris(hydroxyphenyl)ethane (THPE), as core molecule. The convergent growth approach described earlier was applied. We also present H NMR self-diffusion studies of dendrimers of first, second, and third generation. 

The chosen ABj monomer for the synthesis of die dendritic aliphatic polyesters Dl, D2, and D3 was bis-MPA. In the convergent growth approach, dendrons of certain generations were initially synthesized. In a final step these dendrons were coupled to the polyfunctional core molecule (see Scheme 2). To get an acceptable overall yield it is important that all reactions such as coupling, protection, and deprotection are selective and proceed in high yields since a laige number of steps are involved in the synthesis of the final dendrimers. 

The convergent growth approach showed to be a useful method in the synthesis of monodisperse dendrimers. The use of bis-MPA, as a building block, resulted in dendrimers with simple H NMR and NMR spectra where different generations... 

While the preparation of large dendrimers (>G6) is complicated, an attractive feature of the convergent growth approach is the ability to construct well-defined, bifunctional dendrimer motifs, where two dendrons displaying different chain-end functionalities are coupled together (Figure 7). 

In 1989, Fr chet and co-workers first reported the convergent growth approach [8,9]. In contrast to the divergent growth approach, dendrimer construction is initiated at what will eventually become the outer surface shell of the ideally branched macromolecule and proceeds inward, by a stepwise addition of branching monomers, followed by the final attachment of each branched dendritic sub-unit (or dendron ) to a poly-functional core. This synthesis generated a poly(aromatic ether) dendrimer and a repetitive sequence of Williamson ether coupling and bromination reactions were employed as shown in Scheme 8.1. 

Irrespective of these differences, both approaches have proved to be extremely successful for the preparation of dendritic macromolecules. By the use of either the divergent or convergent growth approaches a number of different groups have prepared dendrimers based on a wide variety of functional groups i.e. dendritic poly(amides) (5), poly(etherketones) (6), poly(amines) (7), poly(phenylenes) (8), poly(silanes) (9), poly(phosphonium salts) (10), poly(esters) (11), poly(phenylacetylenes) (12), poly(alkanes) (13), etc. Interesting variations on this theme have been the preparation of optically active dendrimers (14), dendritic poly(radicals) (15), and dendrimers based on co-ordination chemistry with... 

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. 

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