Surface congestion

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

I) Shape, size of gen. 3 1.0 (]) Surface congestion resulting from branch junctures where N = 2... 

This contrasts with the small number of coupling reactions involved in each cycle of a convergent synthesis. A lower degree of surface congestion also exists for the synthesis of individual dendrons, which favors complete reactions. 

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. 

These effects are a consequence of the collapse of what, in suspension, are highly nonplanar lamellae (27-29). The nonplanarity can take several forms but arises because molecular inclination to lamellar normals increases the surface area per end group or fold and so reduces surface congestion, as is common for the alkane subcell. The simplest polyethylene geometries are hollow p5T amids that form, at higher temperatures, with 312 fold surfaces in 110 sectors and a common c-axis (30). The adoption of 31f planes is thought to reflect the assrmmetric shape of sharp folds, both within and normal to the fold plane. 

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