Dendrimer polypropylene imine

The history of dendrimer chemistry can be traced to the foundations laid down by Flory [34] over fifty years ago, particularly his studies concerning macro-molecular networks and branched polymers. More than two decades after Flory s initial groundwork (1978) Vogtle et al. [28] reported the synthesis and characterization of the first example of a cascade molecule. Michael-type addition of a primary amine to acrylonitrile (the linear monomer) afforded a tertiary amine with two arms. Subsequent reduction of the nitriles afforded a new diamine, which, upon repetition of this simple synthetic sequence, provided the desired tetraamine (1, Fig. 2) thus the advent of the iterative synthetic process and the construction of branched macromolecular architectures was at hand. Further growth of Vogtle s original dendrimer was impeded due to difficulties associated with nitrile reduction, which was later circumvented [35, 36]. This procedure eventually led to DSM s commercially available polypropylene imine) dendrimers. 

The latter effect has been demonstrated by Meijer et al., who attached chiral aminoalcohols to the peripheral NH2-groups of polypropylene imine) dendrimers of different generations [100]. In the enantioselective addition of diethyl-zinc to benzaldehyde (mediated by these aminoalcohol appendages) both the yields and the enantioselectivities decreased with increasing size of the dendrimer (Fig. 28). The catalyst obtained from the 5th-generation dendrimer carrying 64 aminoalcohol groups at its periphery showed almost no preference for one enantiomer over the other. This behavior coincides with the absence of measurable optical rotation as mentioned in Sect. 3 above. The loss of activity and selectivity was ascribed to multiple interactions on the surface which were... 

Polypropylene imine) dendrimer of 3rd generation with 16 end groups... 

Kaneda et al. reported substrate-specific hydrogenation of olefins using the tri-ethoxybenzamide-terminated polypropylene imine) dendrimers (PPI) as nanoreactors encapsulating Pd nanoparticles (mean diameter 2-3 nm) [59]. The catalytic tests were performed in toluene at 30 °C under dihydrogen at atmospheric pressure (Table 9.3). The hydrogenation rates were seen to decrease with increasing generation of dendrimers, from G3 to G5. 

Meijer et al [6] reported on inverted unimolecular micelle type dendrimers (Figure 13.4) which have a hydrophilic interior and a hydrophobic shell, synthesized by modifying the end groups of hydrophilic polypropylene imine) dendrimer with alkyl chains. It was shown that these dendrimers could host... 

These dendritic boxes (Figure 13.7) were synthesized by the conjugation of a chiral shell of protected amino acids onto a flexible polypropylene imine) dendrimer with 64 amino end groups. In solution, the shell was highly hydrogen-bonded and dense-packed, displaying a solid-phase behavior, which was indicated by the low NMR relaxation time of the surface groups [11]. 

Vogtle and co-workers first reported a photoswitchable dendrimer [33] with six peripheral azobenzene groups, which took advantage of the efficient and fully reversible photoisomerization reaction of azobenzene-type compounds (Scheme 7). In a follow-up study [34], polypropylene imine) dendrimers bearing azobenzene moieties (p-Im-Gn, n = 1-4) on the periphery were synthesized. These dendrimers displayed similar photoisomerization properties as the azobenzene monomers. Irradiation of the all-E azobenzene dendrimers at 313 nm led to the Z-form dendrimers, while irradiation at 254 nm or heating could convert the Z-form dendrimers back to the E-form dendrimers. The observation that the... 

Weener et al. prepared photo-responsive monolayers from azobenzene modified polypropylene imine) dendrimers which also hold promise in the area of optical data storage [74], A fifth generation polypropylene imine) dendrimer was functionalized with equal amounts of palmitoyl and azobenzene containing alkyl chains, resulting in the formation of an amphiphilic copolymer with a random shell structure (Figure 16.5). 

Baars et al. functionalized polypropylene imine) dendrimers (generation 1, 3, 5) with pentaoxycyanobiphenyl and decyloxycyanobiphenyl mesogens [132], All dendrimers were found to exhibit smectic A mesophases. The SA-layer... 

Figure 16.10 Conformational changes of polypropylene imine) dendrimers in LC materials from spherical to (a) cylindrical, in hexagonal columnar mesophase [130], and (b) ellipsoid, in smectic A mesophase [132]...

Figure 16.11 The pH-dependent conformational behaviour of polypropylene imine) dendrimers. At low pH (left) the occurrence of a soft-core, dense-shell dendrimer, whereas at high pH (right) severe back-folding occurs leading to a dense-core structure [57]...

Several studies have been devoted to determine the localization of end-groups in modified polypropylene imine) dendrimers. Goddard et al. [ 157] and Cavallo and Fraternali [158] investigated the properties of the dendritic box, a fifth-generation polypropylene imine) dendrimer functionalized with (t-BOC)-pro-tected L-phenylalanine residues (Figure 16.12a) [159]. 

Figure 16.15 Ethyleneglycol-functionalized polypropylene imine) dendrimers as water-soluble hosts of TCF (top) and RB (bottom) [191]...

Baars et al. recently investigated the host-guest properties of polypropylene imine) dendrimers functionalized with tris-3,4,5-tri(tetraethyleneoxy)benzoyl units (Figure 16.15) [191]. These hosts are highly soluble in a broad range of solvents, from apolar solvents such as toluene to polar aqueous media. 

In contrast to Bosman et al., who only found metal complexation in the periphery of polypropylene imine) dendrimers, Tomalia and co-workers reported on the incorporation of copper ions into the interior of PAMAM dendrimers judging from EPR and UV/Vis studies [220, 221]. Metal binding in the dendrimer interior has also been observed for dendrimers carrying multiple ligands for metal complexation within their framework such as crown-ethers [222, 223] (Cs(I)-complexes), piperazine [224] (Pd(II)- and Cu(II)-complexes) or triazocyclononane [225] (Cu(II)- and Ni(II)-complexes). In most cases addition of the metal-salt to the dendrimer led to the formation of 1 1 complexes. 

Figure 16.17 Amine-terminated polypropylene imine) dendrimers act as tridentate ligands for the complexation of transition metals [217] (a), and can function as templates for the assembly of Troger s base dizinc(ll) bis-porphyrin molecules, (b) [218]...

A number of groups have reported the preparation and in situ application of several types of dendrimers with chiral auxiliaries at their periphery in asymmetric catalysis. These chiral dendrimer ligands can be subdivided into three different classes based on the specific position of the chiral auxiliary in the dendrimer structure. The chiral positions may be located at, (1) the periphery, (2) the dendritic core (in the case of a dendron), or (3) throughout the structure. An example of the first class was reported by Meijer et al. [22] who prepared different generations of polypropylene imine) dendrimers which were substituted at the periphery of the dendrimer with chiral aminoalcohols. These surface functionalities act as chiral ligand sites from which chiral alkylzinc aminoalcoholate catalysts can be generated in situ at the dendrimer periphery. These dendrimer systems were tested as catalyst precursors in the catalytic 1,2-addition of diethylzinc to benzaldehyde (see e.g. 13, Scheme 14). 

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. 

In 1993 we have published a method which met most of these requirements and for the first time allowed for the large-scale synthesis of dendrimers [2], Since that time, each of the steps have been optimized in the reaction scheme. In this chapter, we present state-of-the-art procedures for the large-scale production of the polypropylene imine) dendrimers. 

Figure 26.1 Synthetic scheme for polypropylene imine) dendrimers using 1,4-diaminobutane as core...

Dendrimers are regarded as macromolecules with a structural precision comparable to proteins or organic compounds. Accurate analysis and quantitative identification of side products are required to optimize and adjust the reaction conditions for the synthesis of DAB-dendr-(NH2)n and DAB-dendr-(CN)n. Therefore, it is a prerequisite to characterize the products obtained unambiguously. To achieve complete molecular characterization of the polypropylene imine) dendrimers and the possible side-products, NMR- and IR-spectroscopy, HPLC, GPC and electrospray mass spectrometry are used. 

The viscosity of the polypropylene imine) dendrimers is investigated both in THF (intrinsic viscosity, Figure 26.3) and in the neat form. For the intrinsic... 

Figure 26.3 Intrinsic viscosity of the various nitrile-terminated polypropylene imine) dendrimers in THF as a function of molar mass...

Figure 26.4 Plot of the glass transition temperatures of the polypropylene imine) dendrimers with nitrile ( ) and amine ( ) end-groups...

All dendrimers consist of inner tertiary amines, located at the branching points of the various dendritic shells (layers). The amine-terminated dendrimers, furthermore, have basic primary amine end-groups. Basicity is therefore one of the most dramatic properties of the polypropylene imine) dendrimers, and has been studied via titration experiments and calculations. Titration experiments of the dendrimers have been performed in water using 1 M hydrochloric acid. Only two equivalence points are observed for DAB-J nJr-(NH2)4 in a ratio of 2 1. From these titrations, pKa values of 10.0 (primary amine groups) and 6.7 (tertiary... 

Molecular modeling techniques are a powerful tool to obtain a very detailed insight in the three-dimensional structure of dendrimer molecules at the atomic level. They have been applied to calculate sizes of the polypropylene imine) dendrimers and radial density profiles in order to estimate the free volume inside the dendrimers, as well as to make predictions about de Gennes dense-packed generations. The molecular modeling work by Coussens and co-workers [20] was focused on the generations 1-5 of the DAB-dendr-(CN)n and DAB-dendr-(NH2)n (n = 4, 8, 16,32, 64). 

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