Poly convergent synthesis

Figure 6. Convergent synthesis of poly(arylether) dendrimers by Frechet et al.

At the center of the dendrimer is the core that represents the first synthetic step in divergent synthesis, or the last step in convergent synthesis. Each concentric shell of branched units is a generation (X), designated (GX), of identical repeat units. Hundreds of chemical repeat units have been reported [3, 4], but our characterization has been focused primarily on poly(amidoamine) (PAMAM), -(CH2CH2CONHCH2CH2N) < [1], or poly(propyleneimine)... 

A 50% functionalization evokes the interesting question, bearing in mind facile transesterification, of how the fluoroalkyl chains will be distributed over the molecules and how they will be distributed on one particular molecule This question has been examined in detail for dendrimers of the poly(propyleneimine) type functionalized with stearic acid [33]. It was proven that the compositional heterogeneity (distribution of degree of substitution) is random, but the positional heterogeneity (spatial distribution of the substituents over the dendrimer molecule) is not random. However, due to flexibility, no particular effect of the spatial distribution can be observed. Unlike the dendrimers, we expect the hyperbranched polyesteramides to be stiffer, so that spatial distribution could lead to interesting effects if the molecule were composed of a functionalized side and a non-func-tionalized side (Fig. 28), as shown possible for dendrimers via a convergent synthesis [34]. 

Tomalia s exhaustive review paper with coloured illustrations [10] did much to popularise the highly branched compounds and to ensure broad general acceptance of the family name dendrimers . In the same year, Frechet and Hawker described the first convergent synthesis of dendrimers. They constructed poly(aryl ether) architectures from the outside inwards (Fig. 1.6 see Section 2.2) [15]. 

Scheme 30.16 The convergent synthesis of a seven-arm star polymer containing a j6-cyclodextrin core and poly( --caprolactone) arms using CuAAC click chemistry. Reproduced with permission from Ref. [Ill] 2006, The Royal Society of Chemistry.

Nearly simultaneously, Frechet et al. pioneered [54] the synthesis and development of a new type of amphiphilic dendrimers wherein, the unimolecular architecture was differentiated. In these instances, Frechet demonstrated that his poly(ether) dendrimers derived via convergent synthesis could be either homogeneously terminated or differentiated into hydrophobic and hydrophilic hemispheres, as illustrated in Figure 22(a) and (b). These dendrimers could be oriented at an interface, as illustrated, or under the influence of an external stimulus. 

Chan, J.W., Yu, B., Hoyle, C.E., and Lowe, A.B. (2008) Convergent synthesis of 3-arm star polymers from RAFT-prepared poly(iV,iV-diethylacrylamide) via a thiol-ene cUck reaction. Chemical Communications, 4959. 

The first example of an iterative synthetic approach toward dendrimers has been reported in 1978 by Vbgtle [3], who called it cascade synthesis . In the mid-1980s, Tomalia [4] and Newkome [5] independently reported the divergent synthesis of two new families of dendrimers poly(amidoamine) and the so-called arborols , respectively. A growing interest on these macromolecules lead in 1990 to the convergent synthesis of aromatic polyether dendrimers by Frechet [6]. The two different synthetic approaches can be explained as follows ... 

Webster et al. l4a expanded on the preparation of the polyphenylenes to develop the one-pot synthesis of hyper-crosslinked poly(triphenylcarbinol). Thus, reaction of 4,4 -dilithiobiphenyl with (CH3)2C03 (— 80 °C — 25 °C, THF) afforded trityl alcohol-based polymer. The absence of carbonyl or methoxycarbonyl NMR resonances led to the speculation that the polymer grows via a branched convergent process. 

Neenan and Miller [118] reported the preparation of poly (arylester) dendrons using a convergent iterative sequence consisting of esterification and hydrolysis reactions as illustrated in Scheme 14. The key intermediate in the synthesis of these dendrons is 5-(te/V-butyldimethylsiloxy)isophthaloyl dichloride, 48, which is first converted to the diester with phenol, followed by hydrolysis to the diester phenol. This dendron is designated [3-OH] indicating it contains three phenyl... 

Recently Moore and Xu [120] reported the successful synthesis of rigid poly-(arylacetylenic) dendrons through generation = 2, using a convergent strategy and the masked l-ethynyl-3,5-dibromobenzene branch cell reagent, 50, as shown in... 

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