Divergent growth method

Scheme 23. Polyesters having a pendant double bond prepared by the divergent growth method (adapted from [156])...

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

The synthesis of radially layered poly(amidoamine-organosilicon) (PAMAMOS) copolymeric dendrimers starts from amine terminated PAMAM dendrimers, which, in turn, are obtained by a well-known excess-reagent divergent growth method that involves a reiterative sequence of (a) Michael addition reactions of methyl acrylate (MA) to primary amines, and (b) amidation of the resulting methyl ester intermediates with ethylenediamine (EDA), as shown in Reaction Scheme 1 (39-41). These PAMAM dendrimers are commercially obtained from Dendritech Inc., (Midland, MI) and they can be used for PAMAMOS preparation without any further purification. The synthesis then involves another Michael addition reaction, this time of a silylated acryl ester, such as (3-acryloxypropyl)dimethoxymethylsilane, as shown in Reaction Scheme 2 (4). 

While the divergent-growth approach allowed an entry into the field of dendritic macromolecules and its use continues to produce novel and fascinating structures, the need existed to develop a complementary approach to synthesize dendritic macromolecules. To develop an alternate strategy, we decided to apply the disconnection method in organic... 

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]. 

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. 

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

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... 

Since the blunt-notched specimen undergoes the same viscoelastic deformation as the sharp-notched specimen, but without crack propagation, the crack initiation time can be identified as the time at which the two curves diverge. Determination of the subsequent crack growth is a somewhat more difficult task direct optical observation was not possible, due to the wrapping bag. An indirect method based on compliance analysis was therefore adopted. 

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