Macro-azo-initiators composed of various polyesters: Their syntheses,thermal properties,and application to block copolymerization

Various aliphatic polyesters such as poly (ethyleneadipate), poly (tetramethylene adipate), poly (caprolactone), and poly (carbonate) were condensed with 4,4'-azobis-4-cyanopentanoyl chloride to prepare macro-azo-initiators. Their thermal properties, observed by differential scanning calorimetry, showed similar decomposition behavior to each other. Block copolymers containing each of these polyesters as a block segment combined with polystyrene or poly (methylmethacrylate) have been derived by the polymerization of monomers initiated with these macro-azo-initiators. © 1994 John Wiley & Sons, Inc.     

Miscibility and gas permeability of poly(ethylene-co-5,4 mol% 3,5,5-trimethylhexyl methacrylate)-polydimethyl-siloxane blends

 Blends of poly(ethylene- co-5.4 mol% 3,5,5-trimethylhexyl methacrylate) (PE-TMHM) with poly(dimethylsiloxane) (PDMS) were prepared in the PDMS content range from 0 to 20%. The miscibility was studied for PE-TMHM–PDMS blends by DSC, dynamic mechanical and microscopic spectroscopy, and the gas permeability was measured for O₂, N₂ and CO₂ as function of PDMS content. PE-TMHM and PDMS were partially miscible with each other below 20 wt% of PDMS content. The permeability coefficients (P) for O₂, N₂ and CO₂ were increased by the blending of PDMS to PE-TMHM. The change of P for O₂ with PDMS content well reflected the partially miscible phase separation behavior. Received: 10 June 1996 Accepted: 14 August 1996     

Effect of nonpolar field formed by polymer ligand on the oxidation of 2,6-xylenol catalyzed by Cu complexes

The oxidative polymerization catalyzed by Cu(II)-poly(vinylpyridine) derivatives and the effect of the nonpolar field formed by the polymer ligand on the catalysis was studied in dimethyl sulfoxide (DMSO) solvent. The catalytic activity of the Cu complex increases in the following order; Cu-(styrene-vinylpyridine copolymer) (St·VP) complex > Cu-partially quarternized (by benzyl chloride) poly(vinylpyridine)(BCQP) complex > Cu-poly(vinylpyridine) (PVP) complex > Cu-pyridine (Py) complex > Cu-partially quarternized (by ethyl bromide) poly(vinylpyridine) (EBQP) complex. This order is the same as that of the reoxidation rate constant of the catalyst. Acceleration of k_o process in Cu-St>VP and Cu-BCQP systems, where the active site is in the nonpolar field formed by the polymer ligand, is explained by an increase in strength of the coordination bonds.     

Searching ultimate nanometrology for AlOx thickness in magnetic tunnel junction by analytical electron microscopy and X-ray reflectometry.

Practical analyses of the structures of ultrathin multilayers in tunneling magneto resistance (TMR) and Magnetic Random Access Memory (MRAM) devices have been a challenging task because layers are very thin, just 1-2 nm thick. Particularly, the thinness (approximately 1 nm) and chemical properties of the AlOx barrier layer are critical to its magnetic tunneling property. We focused on evaluating the current TEM analytical methods by measuring the thickness and composition of an AlOx layer using several TEM instruments, that is, a round robin test, and cross-checked the thickness results with an X-ray reflectometry (XRR) method. The thickness measured by using HRTEM, HAADF-STEM, and zero-loss images was 1.1 nm, which agreed with the results from the XRR method. On the other hand, TEM-EELS measurements showed 1.8 nm for an oxygen 2D-EELS image and 3.0 nm for an oxygen spatially resolved EELS image, whereas the STEM-EDS line profile showed 2.5 nm in thickness. However, after improving the TEM-EELS measurements by acquiring time-resolved images, the measured thickness of the AlOx layer was improved from 1.8 nm to 1.4 nm for the oxygen 2D-EELS image and from 3.0 nm to 2.0 nm for the spatially resolved EELS image, respectively. Also the observed thickness from the EDS line profile was improved to 1.4 nm after more careful optimization of the experimental parameters. We found that EELS and EDS of one-dimensional line scans or two-dimensional elemental mapping gave a larger AlOx thickness even though much care was taken. The reasons for larger measured values can be found from several factors such as sample drift, beam damage, probe size, beam delocalization, and multiple scattering for the EDS images, and chromatic aberration, diffraction limit due to the aperture, delocalization, alignment between layered direction in samples, and energy dispersion direction in the EELS instrument for EELS images. In the case of STEM-EDS mapping with focused nanoprobes, it is always necessary to reduce beam damage and sample drift while trying to maintain the signal-to-noise (S/N) ratio as high as possible. Also we confirmed that the time-resolved TEM-EELS acquisition technique improves S/N ratios of elemental maps without blurring the images.     

Esterase activity of cyclodextrin dithiocarbamates

The catalytic activity of dithiocarbamate appended α- and β-cyclodextrins in the hydrolysis of p-nitrophenyl acetate (PNPA) was studied at alkaline pH. The values of k_cat/k_uncat for the hydrolysis of PNPA ranged from 8 to 165 in the presence of the catalyst and are pH independent in the range 7.5-9.0. Addition of Cu(II) increases the activity by factors of 75-515 and k_cat values increase with increasing pH in the same range. The differences in the obtained kinetic parameters are interpreted on the basis of two different mechanisms.     

Photoinduced Formation of an Azobenzene‐Based CD‐Active Supramolecular Cyclic Dimer

A series of new photo‐responsive amino acid‐derived azobenzenedicarboxylic acid derivatives (S)‐ 1 a – e were synthesized. Compound (S)‐ 1 a in the trans form exhibited no circular dichroism (CD) signal in DMF under ambient conditions, whereas intense Cotton effects were observed upon UV irradiation, indicating the formation of a chiral supramolecular structure in the cis form. The CD signals disappeared when trifluoroacetic acid (TFA) was added to the solution. The ester counterpart [(S)‐ 1 a′ ] showed no CD signal. Hydrogen bonding between the carboxy groups seemed necessary for constructing the supramolecular structure. The kinetic studies of cis to trans isomerization of (S)‐ 1 a demonstrated that the formation of a chiral supramolecule enhances the stability of the cis‐azobenzene structure. The ESI mass spectrum of stilbenedicarboxylic acid (S)‐ 4 , an analogue of (S)‐ 1 b , confirmed the formation of a dimer. A theoretical CD study revealed that (S)‐ 1 a in the cis form should be present as a cyclic chiral dimer.     

Polymorphic and pseudomorphic transformation behavior of acyclovir based on thermodynamics and crystallography

Acyclovir (ACV) has two polymorphs, anhydrate 1 and anhydrate 2, and two hydrates, 2/3 hydrate and dihydrate. The effect of polymorphic transformation of ACV on the temperature and humidity was evaluated by simultaneous XRD-DSC and vapor sorption analysis. Each crystal structure of ACV was analyzed by single crystal analysis and powder X-ray diffraction analysis. On the polymorphic and pseudomorphic transformation, anhydrate 1 did not directly transform to anhydrate 2, but transform through 2/3 hydrate and dihydrate due to relative humidity and temperature. According to the molecular packing for four crystals, there are two packing manners for purine moiety. Anhydrate 1, anhydrate 2, 2/3 hydrate and dihydrate were packed in parallel, anti parallel, mixture of parallel–anti parallel and parallel manners, respectively. Base on the packing manner of ACV, it can be seen why the phase transformation occurs with readily or with difficulty. The thermodynamic relation of anhydrate form 1 and form 2 was evaluated by DSC and microcalorimetry. It was found that these two forms are monotropic forms, with anhydrate form I is stable form and it transform to a new form 3 at 443.2 K.     

Synthetic Studies on Sialoglycoconjugates 104: Synthesis of Kdn-Lewis X Ganglioside Analogs Containing Modified Reducing Terminal and L-Rhamnose in Place of L-Fucose 1 2

Abstract

KDN-Le^x ganglioside analogs (10, 13, 16 and 19) containing the modified reducing terminal and L-rhamnose in place of L-fucose have been synthesized. Glycosidation of methyl 2,3,4-tri-O-benzyl-1-thio-α-L-rhamnopyranoside (1) with 2-(trimethylsilyl)ethyl O-(2-acetamido-4,6-O-benzylidene-2-deoxy-β-D-glucopyranosyl)-(1→3)-O-(2,4,6-tri-O-benzyl-α-D-galacopyranoside (2), followed by reductive ring opening of the benzylidene acetal, gave 2-(trimethylsilyl)ethyl O-(2,3,4-tri-O-benzyl-α-L-rhamnopyranosyl)-(1→3)-O-(2-acet-amido-6-O-benzyl-2-deoxy-β-D-glucopyranosyl)-(1→3)-O-(2,4,6-tri-O-benzyl-β-D-galactopyranosyl)-(1→4)-2,3,6-tri-O-benzyl-β-D-glucopyranoside (4). The tetrasaccharide 4 was coupled with methyl O-(methyl 4,5,7,8,9-penta-O-acetyl-3-deoxy-D-glycero-α-D-galacto-2-nonulopyranosylonate)-(2→3)-2,4,6-tri-O-benzoyl-1-thio-β-D-galactopyranoside(5), using dimethyl(methylthio)sulfonium triflate (DMTST), to give the hexasaccharide 6, which was converted into compound 11 in the usual manner. Compounds 8 and 11 were transformed, via bromination of the reducing terminal, radical reduction, O-deacylation and saponification of the methyl ester, into the desired KDN-Le^x hexasaccharides (10, 13). On the other hand, glycosylation of 2-(tetradecyl)hexadecanol with α-trichloroacetimidates 14 and 17, afforded the target ganglioside analogs 16 and 19.