144-nitrile dendrimer

A very interesting series of dendrimers containing 24 transition metal sandwich units has been synthesized from the 24-nitrile dendrimer by reduction of the nitrile groups to primary amines followed by reaction of the 24-amine dendrimer with chlorocarbonylferrocene or with [Fe( 5-C5Me5)( 6-C6H5F)][PF6] (Scheme 15) [54]. Both 24-branch metallodendrimers proved very useful and complementary for molecular recognition, as will be discussed later in this chapter. 

Nonetheless, it was a fairly short step from octopus compounds to dendrimers, and the step was taken by Vogtle in the late 1970s when he attempted to use a cascade reaction to prepare a molecule of the dendrimer type that would now be considered a dendron rather than a fully developed dendrimer. It began with the addition of acrylonitrile to an anfine, followed by reduction of the nitrile to amine. This was followed by a further reaction with acrylonitrile, and the process was repeated several times to yield highly branched macromolecules. There were initially problems with the reduction step but these were overcome, and the preparation of these poly(propylene imine) dendrimers was later commercialized. 

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. 

Combinatorial approach to unsymmetrically tiered macromolecules [214] is a brand new area of research which would allow dendrimerization of materials (e. g., glass, classical polymers, fibers) and thus enable fine tuning of macromole-cular properties. For example, treatment of an amine terminated dendrimer with a mixture of complementary, isocyanate-based monomers [215, 216] affords a heterogeneous surfaced dendrimer. Selective transformation of the surface nitrile moieties via metal-catalyzed reduction to obtain a new polyamine dendrimer allows further combinatorial-based elaboration as illustrated in Fig. 42. 

Apart from DAB-dendr-(CN)4, which is a white crystalline solid, all generations are colorless to slightly yellow oils. The amine-terminated dendrimers are transparent, whereas the nitrile-terminated products are somewhat turbid. The solubility of the dendrimers is determined primarily by the nature of the end-group DAB-dendr-(NH2)n is soluble in H20, methanol and toluene, whereas DAB-dendr-(CN)n is soluble in a variety of common organic solvents. 

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...

Titration experiments on the nitrile-terminated dendrimers in water show for DAB-dendr-(CN)A pKa values of 3.2 and 4.8. The corresponding calculated pKa values are 3.1 and 4.1 respectively (using the pKalc program, version 2.0, Com-pudrug chemistry). For DAB-dendr-(CN)s only the two inner nitrogen atoms can be protonated in acetonitrile, due to the low basicity of the four other ones. This is confirmed with calculated pKa values of the four outer tertiary nitrogen atoms in DAB-dendr-(CN)s, ranging from 2.0 to 3.2. The presence of the electropositive nitrile-functions and the protonated inner tertiary amines can account for this phenomenon. 

PPI dendrimers are synthesized via the reaction sequence as shown in Fig. 2. This repetitive reaction sequence involves a Michael addition of two equivalents of acrylonitrile to a primary amine, followed by hydrogenation of the nitrile groups to primary amines. Commercially available PPI dendrimers are usually terminated with amine groups. 

Related dendrimers possessing primary amines were synthesized135 from hexaacrylo-nitrile, a by-product in acrylonitrile polymerization, which was converted to the corresponding hexaester. Aminolysis with ethylenediamine and Michael addition with ethyl acrylate follow Tomalia s process the sequence was repeated to generate the higher generations. 

Scheme 5.18. Statistical preparation of dendrimers possessing 1, 2, or 3 nitrile surface groups.

Figure 4 Plot of [)j] versus logMW of the various nitrile-terminated poly(propylene imine) dendrimers (in THF)...

---