Organocatalytic living ring-opening polymerization of cyclic carbosiloxanes

[reaction: see text] An organocatalytic route to narrowly dispersed poly(carbosiloxanes) of predictable molecular weight and end group fidelity is described. N-Heterocyclic carbenes (NHC) and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) catalyze the ring opening of cyclic carbosiloxanes. The pK(b) of the catalyst is important in preventing adverse transetherification reactions and obtaining well-defined polymers. Mechanistic studies indicate that hydrogen bonding to TBD or the NHC activates alcohols or silanols for ring-opening reactions.     

New bimetallic complexes of late transition metals involving pyrazole-bridged bis N-heterocyclic carbene ligands

Bimetallic Ni, Rh, and Ir complexes of pyrazolate biscarbene containing bulky substituents have been synthesized and characterized by X-ray crystallography; the Ni complex dimerizes to a highly congested L(2)Ni(2) structure, whereas the corresponding Rh and Ir complexes form bimetallic LM(2) structures.     

The FT-IR study of the brain tissue of Labeo rohita due to arsenic intoxication

The brain is believed to be particularly vulnerable to arsenic due to its high oxygen consumption rate and high level of polyunsaturated fatty acids and relatively high rate of oxygen free radical generate without commensurable level of arsenic. Hence, in the present work an attempt is made to study the changes in the biochemical contents in the brain tissues of edible fish Labeo rohita due to arsenic intoxication using Fourier Transform Infrared (FT-IR) spectroscopy. FT-IR spectra reveal significant differences in absorbance intensities between the control and arsenic intoxicated brain tissues, reflecting an alteration on the major biochemical constituents, such as lipids, proteins and nucleic acids of the brain tissues of L. rohita due to arsenic intoxication. Further, the administration of antidote DMSA improves the protein and lipid contents significantly in the brain tissues when compared to arsenic intoxicated tissues. The decrease in α-helix structure due to arsenic intoxication might be responsible for the increase in β-sheet secondary structures, which is consistent with the mechanism of β-sheet formation.     

Effects of temperature and chemical modification on polymer Langmuir films

We previously reported on a rheological study of Langmuir films of poly(tert-butyl methacrylate) (PtBMA), and this work describes further studies on this system. Here, surface pressure-area (Pi-A) isotherms and interfacial shear rheology experiments are performed to better understand the effects of two modifications: varying the temperature between 5 and 40 degrees C and introducing small amounts of carboxylic acid groups by partially hydrolyzing the polymer. Increased temperature produced isotherms with lower plateau surface pressures, Pip, and Pi values shifted to lower areas above Pip. Film properties transition from being primarily viscous to being dominated by elasticity as Pip is crossed for all temperatures studied, even as the value of Pip changes. Increasing the hydrolysis fraction leads to isotherms shifted to lower areas throughout the curve and higher Pip values. Both temperature variation and chemical modification are believed to affect the relative importance of polymer-polymer and polymer-subphase interactions.     

Ring opening metathesis polymerization on non-covalently functionalized single-walled carbon nanotubes

Norbornene polymerization has been initiated selectively on the surface of single-walled carbon nanotubes (SWNTs) via a specifically adsorbed pyrene-linked ROMP initiator, resulting in a homogeneous non-covalent poly(norbornene) coating.     

Frequency scan for the analysis of high mass ions generated by matrix-assisted laser desorption/ionization in a paul trap

An ion trap has been modified for the analysis of high mass ions generated by matrix-assisted laser desorption/ionization. Samples are deposited on a probe tip and introduced directly onto the hyperbolically shaped surface of one endcap. All three electrodes - both the endcaps and the ring electrode - are insulated so that the radio frequency (Rf) voltage may be applied to the center ring electrode and the inverted Rf voltage to the endcaps. By using low frequencies (below 100 kHz) and low amplitudes (below 200 V), high mass singly charged ions may be trapped and analyzed by a frequency sweep at constant amplitude. In the high mass range (60-160 kDa), this instrument showed good sensitivity, signal-to-noise ratios, and mass resolution. Copyright 1999 John Wiley & Sons, Ltd.