Dendritic Skeleton Research Articles

Dendrimeric phosphines in asymmetric catalysis

Dendrimers are hyperbranched nanosized and precisely defined molecules, attracting increasing attention each year due to their numerous properties in catalysis, materials science, and biology. This tutorial review concerns the use of dendrimers as catalysts and focuses more precisely on their properties as enantioselective catalysts. Emphasis is put on chiral phosphine complexes constituting the core or the end groups of various types of dendrimers. The effect of the location of the catalytic entities, the effect of the size (generation) and the nature of the dendritic skeleton on the enantiomeric excesses are discussed.

PGSE NMR Studies on DAB-Organo-Rhodium Dendrimers: Evaluation of the Molecular Size, Self-Aggregation Tendency, and Surface Metal Density

PGSE NMR measurements have been carried out for DAB-dendr-(NH2)n [n = 4 (Dab4), 8 (Dab8), 16 (Dab16), 32 (Dab32), and 64 (Dab64)] and DAB-dendr-[NH(O)COCH2CH2OC(O)C5H4Rh(NBD)]n [n = 4 (Rh-Dab4), 8 (Rh-Dab8), 16 (Rh-Dab16), 32 (Rh-Dab32), and 64 (Rh-Dab64)] in CD2Cl2 and CD3OD as a function of the concentration. The hydrodynamic radius (rH) and, consequently, the hydrodynamic volume (VH) of all the species are determined from the measured translational self-diffusion coefficients (Dt). In CD2Cl2, both Dab and Rh-Dab dendrimers show a tendency toward self-aggregation that increases with the generations. In addition, while the radii rH for Dab dendrimers is ca. 20−30% higher in CD3OD than in CD2Cl2, the rH values for Rh-Dab dendrimers are only slightly influenced by solvent variation. To estimate the Rh−Rh spatial proximity (dRh/Rh) on the surface, the internal radius (rint) of the Dab skeleton in Rh-Dab dendrimers was (i) considered equal to that of the Dab dendrimers (model A) or (ii) evaluated assuming that the additional solvent molecules derived from the attachment of Rh to Dab dendrimers were incorporated into the elongated dendritic skeleton (model B). It was found that dRh/Rh decreases from 17.2−19.8 Å (Rh-Dab4) to about 14.0 Å (Rh-Dab64) with the increase in dendrimer generation.

Ethylene polymerization behavior of monometallic complexes and metallodendrimers based on cyclopentadienyl-aryloxy titanium units

Titanium-containing carbosilane dendrimers of type nG-Si[(CH 2) 3[{C 6H 4 − y (OMe) y }O]Ti(C 5H 5)Cl 2] x , in which phenoxy group is anchored to the dendritic skeleton in para position through eugenol or 2-allyl-2,6-dimethoxyphenol precursors, were used in ethylene polymerization. Fresh toluene solutions of these dendrimers in conjunction with MAO behaved as moderate active systems. However, their aged toluene solutions, when they were activated with MAO became highly active catalysts as result of dendrimer aggregation processes. However, when the peripheral unit are replaced by [Ti(C 5Me 5)Cl 2] or [M(C 5H 5) 2Cl] groups or 2-allyl-6-methylphenol are incorporated to the peryphery of a carbosilane dendrimer no aggregation was observed upon aging.

Diels–Alder reaction of anthracene on carbosilane dendrimer

The dendritic macromolecule with 4, 8, 16 and 32 bicyclo end groups on the periphery has been created by the Diels–Alder reaction of anthracene and maleimide. The dendritic skeleton with triple bonds has been prepared by the hydrosilation and the alkynylation of bis(phenylethynyl)dimethylsilane as a core. The peripheral anthracene on dendrimers has been substituted by the reaction of chlorosilyl groups containing dendritic generations and 9-anthracenecarbinol. NMR and MALDI-TOF mass spectrum has characterized these Diels–Alder products.

Synthesis of conjugated dendrons with nonlinear optical activity.

New dipolar conjugated dendrons with electron-withdrawing groups on the periphery and electron-donating groups distributed at the core and throughout the dendritic skeleton have been conveniently synthesized. The pi-conjugated dendritic skeleton provides extended conjugation between each electron donor-acceptor pair, resulting in numerous traditional nonlinear optically active chromophores in one single dendron.

Three remarkable new genera of Tethyidae (Porifera, Demospongiae) from Australia

Three new genera and species of Tethyidae (Porifera, Demospongiae) are described from shallow water Australian stations of the Pacific and Indian Oceans. Oxytethya mirabilis gen. n. sp. n., from the southeastern Australian coast, is a large sponge, hemispherical or spherical, characterized by radiate, branching and sometimes anastomosing, tracts of giant oxeas, a megasclere type new for Tethyidae, a pinnacled and crested surface texture, lack of megasters, varied micrasters and polyrhabds. Anthotethya fromontae gen. n. sp. n. from the Dampier Archipelago along the Western Australian coast shows an irregular somewhat conical body, irregular tubercles and sinuous ridges on the surface, a thin cortex, megascleres, generally styles, in ascending tracts branching and sometimes anastomozing, but also abundant and scattered in the interstices, anthasters as megasters, large oxyasters and small strongylasters as micrasters. Laxotethya dampierensis gen. n. sp. n., also from the Dampier Archipelago, is characterized by a compressed cushion shape, irregular tubercles and sinuous ridges on the surface, indistinct cortex, megascleres, generally short subtylostyles, sometimes tylostyles, forming a loose dendritic reticulate skeleton with winding and more or less compact ascending, branching and anastomosing tracts but also many spicules scattered in the interstices, lack of megasters, tylasters and oxyasters as micrasters.

Skeletal dendritic structure of dust microparticles and of their agglomerates in tokamak T-10

The dendritic skeletons are found in submicron dust particles contained in various types of dust deposit in tokamak T-10. These skeletons are composed of few-several nanometers thick fibers and are partly/fully embedded in an amorphous component. The agglomerates of visually separate particles (VSPs) are shown to possess a common dendritic skeleton with the distribution of amorphous component being peaked around basic blocks of the dendrite to form the VSPs. The dendricity of above skeletons is compatible with a hypothetical streamer-like mechanism of assembling a skeleton from nanotubular blocks.

Effect of undercooling on the microstructure of Ni-35 at. pct Mo (eutectic) and Ni-38 at. pct Mo (hypereutectic) alloys

Ni-35 at. pct Mo (eutectic) and Ni-38 at. pct Mo (hypereutectic) alloy specimens have been solidified from various levels of undercooling in the differential thermal analysis (DTA) and the electromagnetic levitation (EML) units in a pyrex/vycor bed. The evolution of the microstructure in the solidified specimens has been examined in terms of the degree of undercooling, the nature of the first phase to nucleate from the melt, and the specimen cooling rate. The melt has been observed to undercool more in the presence of intermetallic NiMo (β) phase as compared to that in the presence of nickel-rich solid solution (γ). The “anomalous eutectic” type of microstructure has been shown to result from the initial formation of the dendritic skeleton of either of the two phases, its segmentation due to convection and ripening, and the subsequent nucleation of the other phase in the interdendritic liquid regions. The recalescence behavior has been examined as a function of undercooling and the nature of the phase nucleating first in the melt.

On Growth and Structure of Vapor‐Grown Cadmium Iodide Crystals

Cadmium iodide crystals have been grown by the vapor phase technique. Under different thermal and supersaturation conditions, cadmium iodide crystals of the same morphological form and shape, but of different size and thickness, have been obtained. No spirals are observed on the crystals which grow with their basal planes normal to the wall of the growth tube, whereas, spirals have been observed on crystals grown on a glass substrate at high super‐saturation. The observations indicate that a screw dislocation mechanism is not operative in the growth of crystals in the present case. The growth starts with a dendritic skeleton and then changes to flat platelets by the filling in process and by two dimensional nucleation. Formation of spirals on crystals grown on glass slides has been attributed to adsorbed impurities or debris and irregularities on the substrate surface. An x‐ray study of the crystals has revealed that nearly all vapor‐grown crystals show the most stable basic polytype (4H) of . Observations indicate that conditions of growth play an important role in deciding the structure.