No abstract

A divergent hydrosilation/Grignard-reaction sequence, followed by hydroboration of the allyl end groups was employed to prepare a series of novel dendritic carbosilane polyols. Dendrimers with 4, 12, 36, and 108 hydroxyl end groups have been prepared. The hydroboration reactions were monitored by -NMR and MALDI-TOF and were quantitative in all cases. Using MALDI-TOF, the carbosilane dendrimers as well as the novel dendritic polyols have been characterized directly with respect to polydispersity. Whereas G2-OH consisted of mainly two species, the desired 36-ol (80%) and the 34-ol, G3-OH consisted of polyols with 92-108 hydroxyl groups. The dendritic polyols possess low glass transition temperatures (233-241 K) and thus constitute flexible polyols without polar interactions between the dendrons. The molecules appear suitable as chemically stable, flexible molecular scaffolds for the construction of unusual supermolecular architectures.

A series of carbosilane dendrimer generations, G0 to G3, with 4, 12, 36, and 108 end groups, respectively, has been functionalized with perfluorohexyl (C6F13−) groups on the surface. Quantitative perfluoroalkylation of the dendrimer surface was achieved via free radical addition of 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-n-octyl mercaptan. Quantitative perfluoroalkylation of the dendrimers was demonstrated by NMR and SEC. The thermal properties of the dendrimers with core−shell structure were investigated by DSC, polarizing microscopy, and WAXS. Whereas G0 was obtained as a crystalline material that did not show the formation of mesophases, G1 (12 end groups) exhibited the formation of a highly ordered smectic mesophase between −15 and −30 °C. In contrast, G2 and G3 did not form mesophases; however, a transition to a most probably hexagonally ordered array of columns could be observed. The generation-dependent thermal behavior is ascribed to the increasingly dense packing of perfluorohexyl groups on the dendrimer surface.

The first dendritic liquid crystalline polymer (DLCP), consisting of a flexible, second‐generation dendritic carbosilane scaffold substituted with 36 rid cyanobiphenyl groups (see Figure), has been prepared. The synthesis and characterization of the mesophase are reported as an initial approach to determining how the formation of liquid crystal phases is related to the mesogen density and the spacer length in DLCPs.

Self-assembled ultrathin (5−15 nm) films of carbosilane dendrimers with mesogenic units on the periphery, obtained after deposition on mica surfaces, have been studied with atomic force microscopy. Dendritic polymers with 12, 36, and 108 mesogenic cholesteryl end groups, first, second, and third generation, respectively, were employed. Solvent casting was used to generate the films. At high concentrations of the dendrimers in solution, flat, homogeneous films of 2−4 dendrimer layers were found. For low dendrimer concentrations, a single dendrimer monolayer exhibiting an irregular cellular pattern of holes was observed. The thickness of the monolayer correlated well with the diameter of the dendrimers and with wide-angle X-ray scattering results obtained on crystalline powders. Annealing of the films of G1 and G2 in the liquid crystalline phase caused reorientation of the molecules at the surface. The third-generation dendrimer with 108 end groups (G3), the periphery of which is densely packed with mesogens, behaved differently and did not show dewetting or reorientation upon annealing, which is attributed to lower molecular mobility.