Convergent growth method

FIGURE 30.6 (a) The divergent growth method of dendrimers. (b) The convergent growth method of... 

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. 

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 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... 

Scheme 12-3 Method for the preparation of phenylacetylene dendrimers that combines a linear increase in monomer core size per generation with the convergent growth process (SYNDROME strategy). Adapted from [23].

A second method, which was pioneered by Frdchet et al. in 1989, is referred to as the convergent growth process [78] and proceeds in the opposite direction inward from what will become the... 

The monodispersed nature of dendrimers has been verified extensively by mass spectroscopy, size exclusion chromatography, gel electrophoresis, and transmission electron microscopy (TEM) [55, 115]. As is always the case, the level of monodispersity is determined by the skill of the synthetic chemist, as well as the isolation or purification methods utilized. 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 eliminates cumulative effects due to failed couplings [85, 116]. Appropriately purified, convergent dendrimers are probably the most precise synthetic macromolecules that exist today. 

Recent measurements of accommodation coefficients of H20(g) to H20(l) have been made mainly using a liquid droplet train flow reactor or liquid droplet growth method. Although the obtained values converge within 0.1 values obtained by both methods. For example, the values of a obtained by the liquid droplet train method of Li et al. (2001) are 0.17 0.03 and 0.32 0.04 at 280 and 258 K, while the value by... 

Dendritic macromolecules are hyperbranched fractal-like structures that emanate from a central core and contain a large number of terminal groups. Two synthetic approaches have been reported for the preparation of these macromolecules the divergent [76-78] and convergent growth approaches [79-82]. In both methods many synthetic steps are necessary to produce high molecular weight materials. To avoid synthetic problems, the macromonomer with hyperbranched dendritic moiety may be one of the most useftil materials for the dendritic macromolecules. 

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. 

Rapid growth can be achieved by improved iterative methods, especially following a convergent-divergent pathway. In future this will be the method of choice as it facilitates the separation of unreacted material and minimizes the total number of reaction steps needed for the synthsesis of large molecules [50,61]. 

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