Dendrimers surface congestion

Dendrimer surface congestion can be appraised mathematically as a function of generation, from the following simple relationship ... 

As a consequence of the excluded volume associated with the core, interior and surface branch cells, steric congestion is expected to occur due to tethered connectivity to the core. Furthermore, the number of dendrimer surface groups, Z, amplifies with each subsequent generation (G). This occurs according to geometric branching laws, which are related to core multiplicity (iVc) and branch cell multiplicity (iVb). These values are defined by the following equation ... 

While retentions of the moleailarly enlarged systems of up to 99.75% were observed, the formation of insoluble purple species occurred under continuous-flow conditions. Addition of [Bu4N]Br prevented catalyst precipitation but a fast decrease in the conversion was detected. After 45 cycles, the activity of the catalyst dropped to almost zero, while the retention of the catalyst under the applied conditions was 98.6% (Figure 3). The authors state that the main decrease in activity was due to formation of inactive Ni(III) species. Furthermore, the carbosdane support plays a pivotal role in the accessibility of the active sites surface congestion can lead to the formation of mixed-valence Ni(II)/Ni(III) complexes on the dendrimer periphery that compete for reactions with substrate radicals. 

Unimolecular container/scaffolding behavior appears to be a periodic property that is specific to each dendrimer family or series. These properties will be determined by the size, shape, and multiplicity of the construction components that are used for the core, interior and surface of the dendrimer. Higher multiplicity components and those that contribute to tethered congestion will hasten the... 

The dendrimer framework also plays an important role. The catalytic performance measured by activity, selectivity, stability, and recyclability depends on the dendritic architecture, and it is important to distinguish periphery-functionalized, core-functionalized, and focal point-functionalized dendrimers (Fig. 1). Periphery-functionalized dendrimers have catalytic groups located at the surface where they are directly available to the substrate. In contrast, when a dendrimer is functionalized at its core, the substrate has to penetrate the dendrimer support before it reaches the active center, and this transport process can limit the rate of a catalytic reaction if large and congested dendrimers are involved. 

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