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261.
262.
Associating polymers which consist of water-soluble long-chain molecules containing a small fraction of hydrophobic groups
(hydrophobes) behave as flocculants in aqueous suspensions. The effects of associating polymers on the rheological behavior
are studied for single suspensions of particles with hydrophilic and hydrophobic surfaces, and their mixtures. For particles
with hydrophilic surfaces, the suspensions are highly flocculated by a bridging mechanism, because the water-soluble chains
adsorb onto hydrophilic surfaces. On the other hand, the particles with hydrophobic surfaces cannot be dispersed in water
without polymer and the additions of a small amount of polymer are required for preparation of homogeneous suspensions. The
associating polymer acts as a dispersant at low concentrations. However, further additions of polymer lead to a drastic increase
in viscosity. Since the hydrophobes on one end of molecules adsorb onto hydrophobic surfaces and other hydrophobes tending
from the particles can form micelles, the particles are connected by linkage of interchain associations. By mixing two suspensions
of particles with hydrophilic and hydrophobic surfaces, the viscosity is substantially reduced and the flow becomes nearly
Newtonian. The associating polymer in complex suspensions acts as a binder between the hydrophilic and hydrophobic surfaces.
The hetero-flocculation which leads to the formation of composite particles may be responsible for the viscosity reduction
of complex suspensions. 相似文献
263.
G Fukuhara T Nakamura Y Kawanami C Yang T Mori H Hiramatsu Y Dan-Oh K Tsujimoto Y Inoue 《Chemical communications (Cambridge, England)》2012,48(73):9156-9158
Photocyclodimerization of 2-anthracenecarboxylate tethered to a cyclic nigerosylnigerose scaffold gave a single chiral cyclodimer (out of two achiral and two chiral stereoisomers) in 99% optical and 96% chemical yields, achieving the ultimate stereocontrol of the supramolecular photochirogenesis in aqueous solution at 25 °C. 相似文献
264.
The synthesis of various types of optically active α-(allenylsilane-containing)glycines via a chirality-transferring ester-enolate Claisen rearrangement of α-acyloxy-α-alkynylsilanes is described. The conversion of the rearranged products into the optically active silicon-free α-(allenyl)- and α-substituted-α-(allenyl)glycines was achieved by the removal of the Me2PhSi- or TMS group from the allene terminus. 相似文献
265.
Umezawa T Hayashi T Sakai H Teramoto H Yoshikawa T Izumida M Tamatani Y Hirose T Ohfune Y Shinada T 《Organic letters》2006,8(21):4971-4974
[reaction: see text] The first total synthesis of 5,6,11-trideoxytetrodotoxin (1) and its 4-epimer were achieved. The synthesis is characterized by the stereoselective construction of the quaternary amino carbon center at C8a by an asymmetric transferring Strecker synthesis and the highly efficient conversion of cyanohydrin 4 to 1 via intramolecular cyclization reactions. 相似文献
266.
Ellis TK Ueki H Yamada T Ohfune Y Soloshonok VA 《The Journal of organic chemistry》2006,71(22):8572-8578
A new generation of modular achiral glycine equivalents have been evaluated with respect to their synthetic utility for the production of tailor-made, sterically constrained alpha-amino acids, which proved to be the most efficient approach developed to date for the synthesis of symmetrical alpha,alpha-disubstituted-alpha-amino acids. Among the new series of achiral glycine equivalents, one was found to be a superior glycine derivative for the Michael additions with various (R)- or (S)-N-(E-enoyl)-4-phenyl-1,3-oxazolidin-2-ones representing a general and practical synthesis of sterically constrained beta-substituted pyroglutamic acids. In particular, the application of these complexes allowed for the preparation of several beta-substituted pyroglutamic acids which include electron-releasing and sterically demanding substituents in the structure thus increasing the synthetic efficiency and expanding the generality of these Michael addition reactions. 相似文献
267.
Ohne Zusammenfassung 相似文献
268.
Kazuhiko Mizuno Nobuyuki Ichinose Toshiyuki Tamai Yoshio Otsuji 《Tetrahedron letters》1985,26(47):5823-5826
The 9,10-dicyanoanthracene-sensitized photooxygenation of biphenyl and its derivatives in the presence of Mg(ClO4)2 in acetonitrile brought about the oxidative cleavage of benzene nucleus to give benzoic acid and its derivatives. 相似文献
269.
The tensile strength of poly(methyl methacrylate) (PMMA), polycarbonate (PC), polychlorotrifluoroethylene, and polysulfone was measured in liquid nitrogen over the strain rate range of 2 × 10?4 to 660 min?1. These polymers deformed by crazing which was induced by the liquid nitrogen. The stress versus log strain rate curve was sigmoidal in that its slope increased and then decreased with strain rate. Above a critical strain rate of about 200 min?1, which varied somewhat with the polymer, crazing was not observed with the optical microscope; the behavior became brittle, and the tensile strength became constant. The nonlinear behavior of stress versus log strain rate at low strain rates was associated with a decrease in activation volume with increasing strain rate whereas the nonlinear behavior at high strain rates was associated with an increase in density and decrease in length of the crazes with strain rate. The strain rate effect was the basis for calculating the diffusion coefficient of nitrogen into the polymers at 77°K. The shear deformation mode of PC was measured under compression and under tension. The compressive strength versus log strain rate was linear throughout the entire range giving a compression shear activation volume of 360 Å3. The shear tensile strength of PC varied only slightly with strain rate when compared to the compressive strength. The brittle fracture stress of PMMA, in the absence of crazing, in compression and in tension, did not vary with strain rate. 相似文献
270.
Toshiyuki Tamai Kazuhiko Mizuno Isao Hashida Yoshio Otsuji 《Photochemistry and photobiology》1991,54(1):23-29
The 9,10-dicyanoanthracene (DCA)-sensitized photooxygenation of biphenyl derivatives in the presence of Mg(CIO4)2 in acetonitrile produces benzoic acid and its derivatives in high yields. In the absence of Mg(CIO4)2, the rates for the consumption of biphenyl derivatives decrease by a factor of 0.5-0.8, compared with those in the presence of Mg(CIO4)2. In these cases, however, both biphenyls and DCA are oxygenated to give benzoic acids and anthraquinone, respectively, indicating that the addition of Mg(CIO4)2 retards the photooxygenation of DCA. With 4-methylbiphenyl, the photooxygenation proceeds efficiently without added Mg(CIO4)2, and benzene rings and methyl groups are competitively oxygenated to give benzoic acid, 4-methylbenzoic acid, 4-phenylbenzoic acid, and 4-phenylbenzaldehyde. The addition of Mg(CIO4)2 facilitates the oxidation of benzene rings, giving benzoic acid and 4-methylbenzoic acid as major products. These photooxygenations are initiated by a one-electron transfer from biphenyls to the excited singlet DCA and proceed via the radical cations of biphenyls and the radical anion of DCA. 相似文献