Dendrimers functional terminal groups

One disadvantage of this synthetic methodology is seen in the exponentially increasing number of functional terminal groups (KxMn see Section 1.2), since they cannot always be made to react quantitatively and thus give rise to structural defects. Such defects cannot always be avoided, even on addition of large excesses of reactants. Moreover, purification and separation of structurally perfect from defective dendrimers are problematic because the compounds have very similar properties. 

Various strategies for the synthesis of peripherally functionalised dendrimers are presented and evaluated in the following sections. The expression peripheral group is used here as a neutral term. It can refer both to a peripheral function-bearing unit and to a peripheral functionality. Terminal groups linked covalently to the periphery can also be - partly - backfolded (see Sections 1.2 and 7.63). 

All the experiments are conducted on aldehyde terminal functions. Depending on the solubility of the resulting phosphorylated dendrimers, anchorage of phosphorus moieties has been carried out on generation 1 (6 phosphate or phosphinite groups) and up to generation 5 [(96 aminophosphate (Fig. 6, Scheme 15), amino phosphite or functionalized phosphonate groups)] [17b]. 

Dendrimers possessing terminal phosphino groups but also other functional chain ends such as allylamine present the same classical behavior towards Fe2(CO)9 and W(CO)5(THF) [21]. 

Dendrimers with terminal functional groups represent mode compact precursors that are spherical and almost monodisperse, with reactive groups placed on their periphery. Their synthesis, structure and properties have been reviewed in monographs and review articles often together with hyperbranched polymers (cf., e.g. [15-20]), as well as in this book. Application of dendrimers as precursors for conventional materials is limited at this time by their relatively high cost. 

In summary, dendrimers are a unique class of monodispersed synthetic molecules reminiscent of proteins or nucleic acids. If they can be functionalized to be soluble in water with appropriately charged terminal groups, they are generally ideal candidates for gel electrophoretic analyses. 

The terminal groups of a dendrimer are large in number and can have functionalities capable of chemical reactions. If the terminal reactive terminal groups were near the periphery, they would be readily accessible for attachment to surfaces or to reagents. Block copolymers or networks with dendrimers as crosslink points would benefit from having them on the outside. 

By manipulating the chemical properties of dendrimer functional groups, hydrophilic guest molecules can be dissolved in nonpolar solvents and hydro-phobic molecules can be dissolved in polar solution. This is possible because, independent of an encapsulated guest, dendrimers terminated in hydropho-... 

A special form of homofimctional hnking utihzes so called dendrimers. Dendrimers are nanospherical structures for which the exact size depends on the number of branching points and which carry reactive functional units in their periphery (for example aldehyde-, thiol-, epoxy groups etc). The structure of dendrimers is similar to a tree, and their ramifications consist of repetitive units. It should be noted that their size is limited due to the fact that the packing density of their terminal groups increases. With increasing size, their macroscopic structure approximates the form of a sphere. 

Thiol-functionalized dendrimers have also been used as surface stabilizers to obtain dendrimer stabilized Au NPs (DSNs) of very small size (1.5-2.1 nm). The study shows that dendrimer molecules are highly flexible and can undergo a conformational change to accumulate thiol terminal groups on one side of the molecule interacting with the nanoparticle surface [124] (Scheme 3.19). 

Kim et al. first reacted dendrimers bearing hydroxy terminal groups with amine-protected a-amino acids after basic cleavage of the protective group they were able to introduce a second functionality by means of carboxylic acids [62]. 

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). 

The molecular scaffolds of carbohydrate dendrimers can be varied in many different ways carbohydrates can act as core unit, serve for branching, or function as terminal groups [47]. Moreover, they are also suitable for supramolecular bonding and for the transport of active ingredients. 

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