POPAM

Dendrimer Chemistry. Fritz Vogtle, Gabriele Richardt and Nicole Werner Copyright 2009 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-32066-0 

Generation End groups [number] Diameter [A] Molar mass [g  

Divergent synthesis of POPAM dendrimers is based on the concept developed by Vogtle et al. in 1978 (see Section 1.1) [4]. Acrylonitrile is added to a primary mono- or oligo-diamine in a Michael reaction (Fig. 4.2). Suitable reducing agents are Co(II)-borohydride complexes or diisobutylaluminium hydride [5]. Subsequent iterative reaction cycles permit repeated addition of acrylonitrile followed by reduction until the limiting generation is reached. 

In 1993 Meijer et al. and Miilhaupt et al. almost simultaneously accomplished the preparative synthesis of higher generation monodisperse POPAM dendrimers on the basis of this principle [6]. Thanks to their terminal amino groups, POPAM dendrimers can readily be per-functionalised. Thus Balzani, De Cola, 

For example, Kaifer et al. decorated the periphery of a POPAM dendrimer with ferrocene and cobaltocenium units. Such a fourth-generation dendrimer (Fig. 4.4) proved to be a suitable guest system - with numerous docking sites -for /9-cyclodextrin as host compound (see Section 8.3.6) [8]. 

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Figure 4. First synthesis of poly(propyleneamine) dendrimers (abbreviated POPAM) by Vogtle et al.

Depending upon their molecular structure, dendrimers can be classified as tendentially more rigid (e.g. polyphenylene dendrimers Section 4.1.5) and ten-dentially more flexible (e.g. POPAM, PAMAM dendrimers Sections 4.1.1 and 4.1.2). 

Most studies performed partly on molecular models [33] but also on real POPAM and PAMAM dendrimers support the latter model concept [34]. Careful studies on the three-dimensional structure of flexible dendrimers in solution were performed by Ballauff et al. by means of SANS (Small Angle Neutron Scattering) [35] (see Section 7.6). 

Fig. 1.21 POPAM dendrimer according to the Newkome nomenclature 16-Cascade l, 4-diaminobu tane[4-N, N,N, N ] [ -azabutylide-ne)2 aminopropane [45]...

In the case of complex dendrimers with differing branches or differing dendryl substituents on a non-dendritic scaffold it is necessary to include further details in the name. For this reason, a more detailed cascadane nomenclature [45] has been developed, according to which the above POPAM would be called ... 

Review C.J. Hawker, J.M.J. Frechet, Step-Growth-Polymers for High Performance Materials (Eds. J.L. Hedrick, J.W. Labadie), Oxford Press, Oxford 1996, Chapter 7 M. Seiler, Chem. Eng. Technol. 2002, 3, 237—253 comparison of thermosensitivity properties of POPAM den-drimers and linear analogues Y. Haba,... 

Commercially available POPAM and PAMAM dendrimers (see Chapter 4) bearing terminal primary amino groups are currently the most commonly used divergently grown dendrimer structures, followed by poly(benzyl ether) den-drons or poly(benzyl ether) dendrimers (Frechet type) as representatives of con-vergently synthesised dendritic molecules. 

The principle of dick chemistrf presented by Hawker et al. (see Section 2.3.5) offers an efficient and versatile method of functionalisation. For example, it utilises [3+2]-cycloaddition of azide-functionalised reagents with ethyne end groups of a dendrimer precursor to prepare dendrimers with triazole-functionalised end groups. The mild reaction conditions, almost quantitative reaction, and not least the tolerance towards numerous functional groups permit the use of widely differing molecular frameworks (e.g. poly(benzyl ether), POPAM dendrimer structures or hyperbranched polyesters) and different functionalised azides... 

Newkome et al. were the first to present a combinatorial approach to the synthesis of peripherally heterogeneously functionalised POPAM dendrimers of type B (see Fig. 3.7), which is based on functionalisation of end groups with a... 

Fig. 3.11 Amphiphilic switchable POPAM dendrimer (according to Majoral et al.), whose periphery contains the two functional units in equal proportions but randomly distributed...

However, these bifunctionalisation methods are comparatively laborious and applicable only in special cases, since the monofunctionalisation step is limited to substrates possessing an additional coupling site in protected form for the second functional unit. A more versatile method of local bifunctionalisation, which has no need of a deprotection step and also utilises commercially available dendrimer scaffolds, consists in the functionalisation of POPAM dendrimers bearing amine terminal groups with sulphonyl chlorides and subsequent substitution of the sulphonamide proton with other sulphonyl chlorides [63] or with alkyl- or (dendritic) benzyl bromides [64] (see Fig. 3.13). 

Fig. 4.1 Third-generation (G3) POPAM dendrimer — with tetrafunctional core unit and AB2 branching (the generations are shown in shades of grey)...

Table 4.1 I ncrease of POPAM parameters with the number of generations [3]...

Fig. 4.2 Synthesis of POPAM dendrimers (the key step is Michael addition according to Vogtle et a. )...

It also proved possible to attach aza-crown ethers to the periphery of a POPAM dendrimer via amidoferrocenyl units (Fig. 4.5) [9a] and also to decorate the outer shell of a second-generation POPAM dendrimer alternately with eight F-stilbene and eight 4-tert-bulylbenzenesulphonic acid units [9b]. Various other... 

Fig. 4.3 Fi rst- to fourth-generation per-mono-substituted dan-syl-POPAM dendrimers (the fifth generation omitted here was also isolated)...

Fig. 4.4 Fourth-generation POPAM dendrimer with terminal cobaltocenium units...

The acronym PAMAM for poly ami doamine dendrimers (Fig. 4.6) was intended to emphasise the amide bonds present in the molecule to distinguish them from POPAM dendrimers (from poly(propylenamine)). They are commercially... 

Fig. 4.5 POPAM dendrimer with terminal aza-crown ether and ferrocene units...

Dendritic hybrid architectures of the two dendrimer types - POPAM and PAMAM - designated by Majoros et al. as POMAM dendrimerf [17] are structural rarities. One such dendrimer of this type was assembled with PAMAM branching units starting from a POPAM core unit [18]. Vogtle et al. developed POPAM/PAMAM hybrid dendrimers up to the third generation (Fig. 4.8) [19]. 

The stilbene carbon unit has also been peripherally bound to POPAM cores. Although it does not strictly belong to the hydrocarbon dendrimers, the formula of a G2 dendrimer of this type (Fig. 4.21) is depicted here as an example. It was obtained by alkylation of the corresponding eightfold mono-sulphonamide with 4-(bromomethyl)stilbene. Its fluorescence, E/Z isomerisation, photoisomerisation (see Section 5.2.2), and excimer formation were compared with those of non-dendritic stilbenes. The quantum yields of photoisomerisation (0.30) and fluorescence of the E isomer (0.014) of the dendrimer proved to be substantially lower [38]. 

Fig. 4.21 Second-generation stilbene-decorated POPAM Dendrimers (according to Balzani, Vogtle, et ai)...

J. R. Baker Jr., Synthesis and Characterisation of Novel POPAM-PAMAM (POMAM) Hybrid Dendrimers as a Building Block in Nanotechnology. http //www.forsight.org/conferences/... 

In contrast, the rotation per chiral end group of first- to fifth-generation POPAM dendrimers with sterically demanding Boc-protected amino acids as end groups (Fig. 4.75) decreased significantly with increasing generation number and approached the value zero [30]. 

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