bluuebunny/crossref_metadata_2025_split huggingface dataset:
Synthesis of polyisocyanide derived from phenylalanine and its temperature‐dependent helical conformation
Authors: Yamada Yuki , Kawai Tadashi , Abe Jiro , Iyoda Tomokazu 
Published: August 2002

AbstractScrew‐sense‐selective polymerization of the chiral isocyanide monomers derived from phenylalanine with NiCl2 as a catalyst in methanol to yield helical‐conjugated polyisocyanide was investigated with respect to the thermal stability of its helical conformation. Poly(1‐tert‐butoxycarbonyl‐2‐phenylethyl isocyanide) (poly1c) took a stable helical conformer independent of the polymerization temperature. In poly(1‐ethoxycarbonyl‐2‐phenylethyl isocyanide) (poly2c), which had slightly smaller side groups, the helical conformation was thermally destabilized. The specific rotation and circular dichroism of poly2c prepared at temperatures greater than 40 °C were considerably depressed in comparison with the values for poly2c prepared at or below room temperature. Additionally, poly2c prepared at low temperatures exhibited reversible temperature‐dependent specific rotation and circular dichroism, whereas poly1c showed few changes. It is suggested that polyisocyanide derived from phenylalanine takes various helical conformers (i.e., from tightly to loosely coiled helices), the thermal stability of which depends on the size of the side group. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 399–408, 2002
Free‐radical copolymerization of styrene and m‐isopropenyl‐α,α′‐dimethylbenzyl isocyanate studied by 1H NMR kinetic experiments
Authors: Barner Leonie , Barner‐Kowollik Christopher , Davis Thomas P. 
Published: August 2002

AbstractThe free‐radical copolymerization of m‐isopropenyl‐α,α′‐dimethylbenzyl isocyanate (TMI) and styrene was studied with 1H NMR kinetic experiments at 70 °C. Monomer conversion vs time data were used to determine the ratio kp × kt−0.5 for various comonomer mixture compositions (where kp is the propagation rate coefficient and kt is the termination rate coefficient). The ratio kp × kt−0.5 varied from 25.9 × 10−3 L0.5 mol−0.5 s−0.5 for pure styrene to 2.03 × 10−3 L0.5 mol−0.5 s−0.5 for 73 mol % TMI, indicating a significant decrease in the rate of polymerization with increasing TMI content in the reaction mixture. Traces of the individual monomer conversion versus time were used to map out the comonomer mixture composition drift up to overall monomer conversions of 35%. Within this conversion range, a slight but significant depletion of styrene in the monomer feed was observed. This depletion became more pronounced at higher levels of TMI in the initial comonomer mixture. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1064–1074, 2002
Terminology of kinetics, thermodynamics, and mechanisms of polymerization
Authors: Penczek Stanislaw 
Published: August 2002

AbstractThis article, after a short introduction, written by a coordinator of the International Union of Pure and Applied Chemistry (IUPAC) Project devoted to the Terminology of the Kinetics, Thermodynamics, and Mechanisms of Polymerization presents the Provisional Document prepared by the IUPAC Macromolecular Division (Commission Nomenclature). It contains 95 entries, from “activated monomer polymerization” to “zwitterionic polymerization.” © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1665–1676, 2002
Synthesis and properties of segmented main‐chain liquid‐crystalline polyurethanes with a high aspect ratio mesogenic diol as a chain extender
Authors: Padmavathy T. , Srinivasan K. S. V. 
Published: August 2002

AbstractMain‐chain liquid‐crystalline polyurethanes were synthesized based on a high aspect ratio mesogenic diol (4‐{[4‐(6‐hydroxyhexyloxy)‐phenylimino]‐methyl}‐benzoic acid 4‐{[4‐(6‐hydroxyhexyloxy)‐phenylimino]‐methyl}‐phenyl ester) as a chain extender; polycaprolactone (PCL) diol soft segments of different number‐average molecular weights (530, 1250, or 2000); and different diisocyanates, including 1,4‐hexamethylene diisocyanate (HMDI), 4,4′‐methylene bis(cyclohexyl isocyanate) (H12MDI), and 4,4′‐methylene bis(phenyl isocyanate) (MDI). The structure of the polymers was confirmed with Fourier transform infrared spectroscopy, and differential scanning calorimetry and polarizing microscopy measurements were carried out to examine the liquid‐crystalline and thermal properties of the polyurethanes, respectively. The mesogenic diol was partially replaced with 20–50 mol % PCL. A 20 mol % mesogen content was sufficient to impart a liquid crystalline property to all the polymers. The partial replacement of the mesogenic diol with PCL of various molecular weights, as well as the various diisocyanates, influenced the phase transitions and the occurrence of mesophase textures. Characteristic liquid‐crystalline textures were observed when a sufficient content of the mesogenic diol was present. Depending on the flexible spacer length and the mesogenic content, grained and threadlike textures were obtained for the HMDI and H12MDI series polymers, whereas the polyurethanes prepared from MDI showed only grained textures for all the compositions. The polymers formed brittle films and could not be subjected to tensile tests. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1527–1538, 2002
Synthesis and structure of wholly aromatic liquid–crystalline polyesters containing meta‐ and ortholinkages
Authors: He Chaobin , Lu Zhihua , Zhao Lun , Chung Tai‐Shung 
Published: August 2002

AbstractWholly aromatic main‐chain rigid‐chain polymers containing meta‐ and orthokink linkages were synthesized. The thermal property, liquid crystallinity, and crystalline structure were studied using DSC, polarized light microscopy, and wide‐angle X‐ray scattering. These polymers exhibited liquid crystallinity up to 50 mol % of meta‐ and orthokink linkages. The existence of liquid crystallinity in these polymers may be attributed to the adoption of cis conformation in kink units in these polymers because the energy penalty for doing so can be compensated by the formation of the liquid–crystalline phase. The crystallinity of the polymers was low, and the crystal structure was quite similar to that of the pure polymers (without kink units). This can be explained by that fact that the crystal region mainly consists of the nonkink units, and the kink units disrupt the crystallization of the polymers and form defects. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1242–1248, 2001
Ferrocene‐based organophosphorus liquid–crystalline polymers: Synthesis and characterization
Authors: Senthil S. , Kannan P. 
Published: August 2002

AbstractFerrocene‐containing polyphosphate and phosphonate esters were synthesized by the solution polycondensation method. The structure of the polymers was confirmed using various spectroscopic techniques. The formation of two types of chain blocks was confirmed by 31P NMR spectroscopy. Hot stage optical polarized microscope (HOPM) analysis revealed that all the polymers have a liquid–crystalline property. The char yields of the synthesized similar polymers were much higher than those of nonphosphorus polymers already reported in the literature. DSC analysis confirmed our predictions over the liquid–crystalline property, glass‐transition temperature, isotropization temperature, and thermal stability of the polymers. The effects of substitution on the side chain, structure of the liquid‐–crystalline phase, and thermal stability of the polymers have also been discussed. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2396–2403, 2001
Atom transfer radical polymerization through N‐chlorosulfonamides
Authors: Filiz Şenkal B. , Hizal Gürkan , Biçak Niyazi 
Published: August 2002

AbstractControlled polymerizations of vinyl monomers such as methyl methacrylate and styrene are achieved using N‐chloro,N‐propyl‐p‐toluenesulfonamide (NCPT) together with a cuprous bromide/hexahexyl triethylenetetramine (CuBr/H‐TETA) complex. Although N‐halosulfonamides are known to decompose radically to give free chlorine, NCPT alone (without a cuprous complex) does not initiate any polymerization even in prolonged reaction times. Instead these add to the double bonds to give 2‐chloroethylsulfonamides. In the present polymerization system a good chlorine donator (NCPT) is combined with an organic soluble complex (CuBr/H‐TETA) to perform atom transfer radical polymerizations (ATRPs) in homogenous conditions. The linear proportionality of the molecular weights to the conversions and straight lines observed in ln(M0/M) (where M0 and M are the monomer contents at the beginning and at any time, respectively) versus time plots indicate typical controlled polymerization characteristics. The use of freshly prepared NCPT is advisable due to its slow and spontaneous decomposition when standing at room temperatures. Because of their easy preparation, N‐chlorosulfonamides can be used and are preferred instead of special halogen compounds commonly used in copper mediated ATRP. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2691–2695, 2001
Synthesis and properties of fluorene‐based fluorinated polymers
Authors: Kameshima Hisamitsu , Nemoto Nobukatsu , Endo Takeshi 
Published: August 2002

AbstractFluorene‐based polymers containing various fluorinated benzene (fluorobenzene, p‐difluorobenzene, and tetrafluorobenzene) moieties were synthesized. In addition, perfluorooctylation of poly‐[(9,9‐dioctylfluorene‐2,7‐diyl)‐co‐(fluorene‐2,7‐diyl)] was carried out to afford fluorene‐based polymers with perfluorooctyl moiety at the 9‐position on the fluorene ring. To evaluate the effect of fluorine moiety, polymers containing nonfluorinated benzene moieties and nonfluorinated octyl groups were synthesized. The photoluminescence measurements indicated that all these polymers exhibited blue emission in solution, but a polymer containing a perfluorooctyl group did not emit in the film state. Polymers containing various fluorinated benzene moieties showed higher fluorescence quantum yields and thermal stability than those containing nonfluorinated benzene. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3143–3150, 2001
Self‐initiating performance of maleic anhydride on surface photografting polymerization
Authors: Deng Jian‐Ping , Yang Wan‐Tai 
Published: August 2002

AbstractMaleic anhydride (MAH) was photografted onto low‐density polyethylene (LDPE) films with a grafting efficiency of about 70% in the absence of a photoinitiator. The self‐initiating performance was attributed to a mechanism of abstracting hydrogen atoms from LDPE chains by excited MAH dimers. The supporting experimental results were as follows: (1) the far‐UV radiation (200–300 nm) was indispensable for the graft polymerization and 2) the crosslinking reaction of LDPE inevitably accompanied the grafting of MAH. In addition, the initiation performance of MAH was further confirmed by surface photografting of acrylic acid in the presence of MAH, where MAH was used as the photoinitiator. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3246–3249, 2001
Synthesis and luminescence of polyphosphazenes with carbazolyl side chains
Authors: Li Zhen , Qin Jingui , Deng Xianyu , Cao Yong 
Published: August 2002

AbstractTwo new blue‐light‐emitting polyphosphazenes (1 and 2) containing carbazolyl groups as side chains were synthesized from a highly reactive macromolecular intermediate by a nucleophilic substitution reaction. Molecular structural characterization for the polymers was presented by 1H NMR, IR, and ultraviolet–visible spectra, gel permeation chromatography, and differential scanning calorimetry. The polymers exhibited excellent solubility in common organic solvents and were thermally stable. A fluorescence analysis of the two materials in tetrahydrofuran showed a strong blue light emitting. The quantum yields of the polyphosphazenes were 0.55 for 1 and 0.64 for 2, relative to quinoline (in 0.1 N H2SO4). An electroluminescent diode was fabricated, and a bright blue light was observed; the maximum external quantum efficiency was about 0.026% at an applied forward voltage of 23 V. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3428–3433, 2001
Theoretical consideration of the molecular mass distribution of starlike polymers formed by attachment of linear polymers to dendritic cores
Authors: Liu Hewen , Wilén Carl‐Eric 
Published: August 2002

AbstractStar‐shaped polymers can be synthesized by condensation of linear polymers to dendritic cores. The distribution of molecular masses of such hybrid star polymers and the factors influencing it have been investigated by calculations based on population balance equations and Laplace transformations. Moreover, the expressions for numerical calculations of molecular mass distributions have been derived for both Poisson and Schultz–Zimm types of initial molecular mass distributions. According to the calculation results, the molecular mass distribution of the star polymers is strongly dependent on the distribution of the length of linear polymers. Beyond this, the molecular mass development is affected by several parameters that describe the reaction degree of dendritic cores, thus, by the fraction of all functional groups of dendritic cores involved in grafting, denoted by p; the average branching degree of one starlike molecule, represented by ρ; or the fraction of dendritic molecules becoming the cores of star polymers, expressed by p/ρ, provided the star molecule contains only one core. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2751–2758, 2001
Inelastic neutron scattering from isotactic polypropylene
Authors: Annis B. K. , Kim Man‐Ho , Alamo R. , Pyda M. 
Published: August 2002

AbstractWe used inelastic neutron scattering to probe the low‐energy excitations in semicrystalline isotactic polypropylenes with different degrees of crystallinity. The contributions from the amorphous and crystalline regions to the total scattering intensity were extracted under the assumption of a weighted linear contribution of the two regions in a simplified two‐phase system. The resulting intensity from the amorphous region showed a peak at 1.2 meV that was in good agreement with the previously determined boson peak characteristic of atactic polypropylene. The possibility of a contribution to the boson peak region by longitudinal acoustic mode modes that are characteristic of semicrystalline polymers and appear in the same low‐frequency region is discussed. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2852–2859, 2001
Interface development in polycarbonate/poly(methyl methacrylate) bilayer films
Authors: Hutchings L. R. , Richards R. W. , Thompson R. L. , Clough A. S. , Langridge S. 
Published: August 2002

AbstractWe report the effect of annealing over a range of temperatures and times on the mixing, stability, and interfacial width in thin bilayer films of bisphenol A–polycarbonate (PC) on deuterated poly(methyl methacrylate) (dPMMA). These bilayer films were highly stable when annealed at temperatures of up to 438 K, the temperature at which the degradation of the blends of these materials was first detectable by thermogravimetric analysis. At higher temperatures, dewetting of the PC upper layer of the film occurred at an increasing rate. Nuclear reaction analysis showed that the PC and dPMMA layers remained segregated. Neutron reflectometry data showed that the interfacial width between the two polymer layers grew rapidly from 0.5 nm for an unannealed sample to approximately 4.0 nm, the latter value being in good agreement with the predicted value for the interfacial width in the absence of any reaction. Extended annealing at 438 K and lower temperatures had no effect on the interfacial width, whereas at higher temperatures, the interfacial width increased to approximately 5.5 nm before the films became unstable. The broadening of the interface found at higher annealing temperatures was attributed to an increase in the miscibility of the polymers induced by the monomer from the unzipping of the dPMMA chains. There was no evidence of a thermally induced chemical reaction between the two polymers. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2351–2362, 2001
Investigation at the chain segmental level of the miscibility of poly(vinyl chloride)/atactic poly(methyl methacrylate) blends
Authors: Lau Cheryl , Mi Yongli 
Published: August 2002

AbstractWe employed high‐resolution 13C cross‐polarization/magic‐angle‐spinning/dipolar‐decoupling NMR spectroscopy to investigate the miscibility and phase behavior of poly(vinyl chloride) (PVC)/poly(methyl methacrylate) (PMMA) blends. The spin–lattice relaxation times of protons in both the laboratory and rotating frames [T1(H) and T1ρ(H), respectively] were indirectly measured through 13C resonances. The T1(H) results indicate that the blends are homogeneous, at least on a scale of 200–300 Å, confirming the miscibility of the system from a differential scanning calorimetry study in terms of the replacement of the glass‐transition‐temperature feature. The single decay and composition‐dependent T1ρ(H) values for each blend further demonstrate that the spin diffusion among all protons in the blends averages out the whole relaxation process; therefore, the blends are homogeneous on a scale of 18–20 Å. The microcrystallinity of PVC disappears upon blending with PMMA, indicating intimate mixing of the two polymers. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2390–2396, 2001
Structural and functional properties of hemocyanin from Cyanagraea praedator, a deep‐sea hydrothermal vent crab
Authors: Chausson Fabienne , Bridges Christopher R. , Sarradin Pierre‐Marie , Green Brian N. , Riso Ricardo , Caprais Jean‐Claude , Lallier François H. 
Published: August 2002

AbstractCyanagraea praedator (Crustacea: Decapoda: Brachyura) is an endemic species of the East Pacific Rise hydrothermal vents, living in the upper part of black smoker chimneys. Because we were seeking species that have made respiratory adaptations to the hydrothermal environment, we looked at Cyanograea hemocyanin (Hc) and determined its quaternary structure and the oxygen‐binding properties in relation to temperature, pH, and lactate. C. praedator Hc is composed of dodecamers and hexamers, with dodecamers formed by the perpendicular association of two hexamers. The composition of these polymers was determined by electrophoresis and, for the first time, by electrospray mass spectrometry. Dodecamers and hexamers are composed of six subunits common to the two forms, with molecular mass ranging from 75,008 Da to 75,534 Da. In addition, we found two dodecamer‐specific subunits, at 75,419 Da and 75,629 Da. The native hemocyanin possesses a high oxygen affinity (P50 varies between 4 and 10 Torr at pH 7.5, 15°C) and a large Bohr coefficient (Δ log P50/ΔpH ∼ −1.8). Oxygen affinity is not affected by lactate or, surprisingly, temperature between 5°C and 35°C (ΔH = 1.16 kJ/mol1 5–35°C). Dialysis of native hemolymph elicited a significant increase in Hc‐O2 affinity (ΔP50 = 2.5 Torr at pH 7.5), an effect opposite the usual trend observed for crustacean hemocyanins. In this article these functional properties are interpreted in relation to characteristics of the environment. Proteins 2001;45:351–359. © 2001 Wiley‐Liss, Inc.
Design and initial characterization of a circular permuted variant of the potent HIV‐inactivating protein cyanovirin‐N
Authors: Barrientos Laura G. , Louis John M. , Hung Jason , Smith Trent H. , O'Keefe Barry R. , Gardella Roberta S. , Mori Toshiyuki , Boyd Michael R. , Gronenborn Angela M. 
Published: August 2002

AbstractA circular permuted variant of the potent human immunodeficiency virus (HIV)‐inactivating protein cyanovirin‐N (CV‐N) was constructed. New N‐ and C‐termini were introduced into an exposed helical loop, and the original termini were linked using residues of the original loop. Since the three‐dimensional structure of wild‐type cyanovirin‐N is a pseudodimer, the mutant essentially exhibits a swap between the two pseudo‐symmetrically related halves. The expressed protein, which accumulates in the insoluble fraction, was purified, and conditions for in vitro refolding were established. During refolding, a transient dimeric species is also formed that converts to a monomer. Similar to the wild‐type CV‐N, the monomeric circular permuted protein exhibits reversible thermal unfolding and urea denaturation. The mutant is moderately less stable than the wild‐type protein, but it displays significantly reduced anti‐HIV activity. Using nuclear magnetic resonance spectroscopy, we demonstrate that this circular permuted monomeric molecule adopts the same fold as the wild‐type protein. Characterization of these two architecturally very similar molecules allows us to embark, for the first time, on a structure guided focused mutational study, aimed at delineating crucial features for the extraordinary difference in the activity of these molecules. Proteins 2002;46:153–160. © 2001 Wiley‐Liss, Inc.
Interhelical hydrogen bonds and spatial motifs in membrane proteins: Polar clamps and serine zippers
Authors: Adamian Larisa , Liang Jie 
Published: August 2002

AbstractPolar and ionizable amino acid residues are frequently found in the transmembrane (TM) regions of membrane proteins. In this study, we show that they help to form extensive hydrogen bond connections between TM helices. We find that almost all TM helices have interhelical hydrogen bonding. In addition, we find that a pair of contacting TM helices is packed tighter when there are interhelical hydrogen bonds between them. We further describe several spatial motifs in the TM regions, including “Polar Clamp” and “Serine Zipper,” where conserved Ser residues coincide with tightly packed locations in the TM region. With the examples of halorhodopsin, calcium‐transporting ATPase, and bovine cytochrome c oxidase, we discuss the roles of hydrogen bonds in stabilizing helical bundles in polytopic membrane proteins and in protein functions. Proteins 2002;47:209–218. © 2002 Wiley‐Liss, Inc.
Elucidation of the mode of substrate binding to hydroxynitrile lyase from Hevea brasiliensis
Authors: Gruber Karl 
Published: August 2002

AbstractThe hydroxynitrile lyase from Hevea brasiliensis (Hb‐HNL) is used as a catalyst in enantiospecific syntheses of α‐hydroxynitriles from aldehydes and methyl‐ketones. The catalyzed reaction represents one of the few industrially relevant examples of enzyme mediated CC coupling reactions. In this work, we modeled Hb‐HNL substrate complexes that have as yet proven inaccessible to experimental structure determination and were able to identify two binding modes for the natural substrate acetone cyanohydrin in docking simulations. Discrimination of the two alternatives was achieved by modeling complexes with two different chiral cyanohydrins followed by an analysis of the respective relative binding energies from molecular mechanics and thermodynamic integration. Only for one of the alternative binding modes the experimentally established S‐selectivity of the enzyme was correctly predicted. Our results yielded further support for an enzymatic mechanism involving the catalytic triad Ser80, His235, and Asp207 as a general acid/base. A pivotal role was ascribed to Lys236, which seems to be crucial for enzymatic activity at low pH values. In addition, the modeling calculations provided possible explanations for the observed substrate and enantioselectivity of the enzyme that rationalize available mutational data and will be the basis for future protein engineering efforts. Proteins 2001;44:26–31. © 2001 Wiley‐Liss, Inc.