bluuebunny/crossref_metadata_2025_split huggingface dataset:
Comparison of pKR values of fluorenyl and anthracenyl cations
Authors: Courtney Michael C. , MacCormack Aoife C. , More O'Ferrall Rory A. 
Published: October 2002

AbstractA value of pKR = −5.1 for the anthracenonium ion in 50:50 (v/v) aqueous trifluorethanol is reported based on a ratio of measured rate constants kH for acid‐catalysed dehydration of the 9,10‐hydrate of anthracene and k for the reverse hydrolysis of the carbocation (KR = k/kH). Comparison with pKR = −15.9 for the fluorenyl cation indicates that the latter ion is less stable by more than 10 log units (15 kcal mol−1). This difference is (a) considerably larger than that between the benzhydryl (pKR = −11.7) and fluorenyl cations (ΔpK = 4.2), which has been considered too small to indicate antiaromatic character for the fluorenyl cation, and (b) comparable to that between pKas for the ionization (in DMSO) of fluorene (22.6) and diphenylmethane (32.2), which has been interpreted as implying aromatic character for the fluorenyl anion. It is shown that a difference in stability of anthracene hydrate and 9‐hydroxyfluorene makes only a minor contribution to the difference in pKR values and that the fluorenyl cation is destabilized by ca 10 kcal mol−1. A smaller difference in pKas for protonation of fluorenimine (5.85) and benzhydrylimine (7.0) is consistent with the expected moderating effect of an electron‐donating substituent on relative carbocation stabilities. Evidence from calculations relating to the antiaromaticity of the fluorenyl cation is reviewed in the light of these measurements. An additional comparison between equilibrium constants for the ionization of aralkylazides (Kaz) and alcohols (KR) reveals the influence of differences in geminal σ‐bond interactions for the hydroxy and azido groups in their respective reactants. Copyright © 2002 John Wiley & Sons, Ltd.
Theoretical study of the gas‐phase decomposition of neutral α‐amino acid ethyl esters. Part 1—The elimination of N,N‐dimethylglycine ethyl ester and ethyl 1‐piperidineacetate
Authors: Quijano Jairo , Notario Rafael , Chamorro Eduardo , León Luis A. , Sánchez Claudia , Alarcón Gustavo , Quijano J. Camilo , Chuchani Gabriel 
Published: October 2002

AbstractTheoretical studies of the thermolysis of two α‐amino acid ethyl esters in the gas phase were carried out using ab initio theoretical methods, at the HF/6–31G(d) and the MP2/6–311 + G(2d,p)//MP2/6–31G(d) levels of theory. The reactions studied have two steps: the first one corresponds to the formation of ethylene and a neutral amino acid intermediate via a six‐membered cyclic transition state, and the second is the rapid decarboxylation of this intermediate via a five‐membered cyclic transition state. The progress of the first step of the reactions was followed by means of the Wiberg bond indices. The results indicate that the transition states have an intermediate character between reactants and products, and the calculated synchronicities show that the reactions are concerted and slightly asynchronous. The bond‐breaking processes are more advanced than the bond‐forming processes, indicating a bond deficiency in the transition states. The kinetic parameters calculated for both reactions agree very well with the available experimental results. Copyright © 2002 John Wiley & Sons, Ltd.
Hydrolysis of N‐aryl thioncarbamate esters. Modified Marcus equation for reactions with asymmetric intrinsic barriers
Authors: Humeres Eduardo , Zucco Cesar , Nunes Mauricea , Debacher Nito A. , Nunes Ricardo J. 
Published: October 2002

AbstractThe hydrolysis of ethyl N‐p‐substituted arylthioncarbamates was studied at 100 °C in the pH range 6.5–12.5. No general catalysis was found, and the presence of an isothiocyanate intermediate was detected, indicating that the alkaline hydrolysis occurs by an E1cb mechanism. From the pH–rate profiles, the first‐order rate constants kE for the elimination step of the thioncarbamate anion forming the isothiocyanate intermediate were determined. The alkaline hydrolysis of p‐substituted arylisothiocyanates was studied at 25 °C in 0.1–0.3 M solutions of NaOH and in 0.1–0.3 M aqueous ethanol solutions, at different concentrations of NaOH. The second‐order rate constants for the addition reaction with hydroxide (kOH) and ethoxide (kA) ions were obtained. Leffler plots for the elimination of the ethoxide ion from the arylthioncarbamate anion and for the addition of the ethoxide ion to the arylisothiocyanate were linear. From Leffler's equation, with the sole condition that dαL/dΔG should be constant, a modified Marcus equation (MME) was obtained, where a parameter p (or q for the reverse reaction) defined the asymmetry of the intrinsic barrier. (When p = 1/2 the barrier is symmetric and the MME becomes the Marcus equation in the usual form.) For the addition–elimination reaction studied, both Leffler plots were adjusted to MME with the asymmetric parameter p = 0.694 ± 0.002 for the addition and q = 0.307 ± 0.002 for the elimination reaction. The intrinsic barrier was $\Delta G_{0}^{\neq} = 24.75 \pm 0.02\,\hbox{kcal mol}^{-1}$ and |ΔGmax| = 438 ± 4 kcal mol−1 (1 kcal = 4.184 kJ). The addition reaction was exoergic and, as expected from the high intrinsic barrier, αL changed very little in the series (0.679–0.683); the transition state was product‐like, and it moved towards the reagents with increasing exoergicity. Copyright © 2002 John Wiley & Sons, Ltd.
Crosslinking of vinyl‐terminated biphenyl and naphthalene side‐chain liquid‐crystalline poly(epichlorohydrin) derivatives
Authors: Callau Lourdes , Mantecón Ana , Reina José Antonio 
Published: October 2002

AbstractWe performed the crosslinking of vinyl‐terminated biphenyl and naphthalene side‐chain liquid‐crystalline polyethers using peroxide‐type initiators with and without the addition of tertiary amine promoters. The crosslinking temperatures were chosen in the range of mesophase stability to allow the mesophase order to be frozen. The biphenyl derivatives, with a high isotropization temperature, were crosslinked to a large extent. This led to anisotropic thermosets. To crosslink naphthalene derivatives, amine promoters were needed, but degrees of crosslinking were lower, and anisotropic elastomers were obtained. Crosslinking processes were studied by differential scanning calorimetry, polarized optical microscopy, and Fourier transform infrared spectroscopy. The nature of the frozen mesophase was confirmed by X‐ray diffraction studies on mechanically oriented samples. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2237–2244, 2002
Benzyl alcohols as accelerators in the photoinitiated cationic polymerization of epoxide monomers
Authors: Crivello James V. , Acosta Ortiz Ricardo 
Published: October 2002

AbstractBenzyl alcohols display marked accelerating effects on the onium salt photoinitiated cationic ring‐opening polymerizations of epoxide monomers. The participation of radicals in these polymerizations was demonstrated, and a pronounced accelerating effect on the polymerization rate was observed depending on the character of the substituent on the benzene ring of the alcohol. 4‐Methoxybenzyl alcohol was considerably more reactive than benzyl alcohol, whereas the analogous chloro‐ and nitrosubstituted compounds were less reactive. Piperonyl alcohol was even more reactive as an accelerator in these polymerizations. The UV irradiation of an onium salt cationic photoinitiator generates Brönsted acids that initiate the cationic ring‐opening polymerization of the epoxide monomer. In the presence of benzyl alcohol, the usual polymerization mechanism is diverted and proceeds by a more‐rapid activated monomer mechanism. As a result, benzyl ether groups are incorporated at the termini of the polymer chains. Aryl radicals that are also generated by photolysis of the onium salt abstract labile benzyl ether protons, and the benzyl ether radicals that are produced are subsequently oxidized by the onium salt to afford the corresponding benzyl carbocations. This takes place by a chain process. The polymer bound benzyl cations serve as additional sites for the initiation of cationic polymerization. It is proposed that these two mechanisms acting in synergy with one another account for the observed accelerating effect on the polymerization rate. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2298–2309, 2002
Chain‐transfer reaction in the radical polymerization of di‐n‐butyl itaconate at high temperatures
Authors: Hirano Tomohiro , Takeyoshi Ryoko , Seno Makiko , Sato Tsuneyuki 
Published: October 2002

AbstractRadical polymerizations of di‐n‐butyl itaconate were investigated. Unexpected resonances (C resonances) were observed in 13C NMR spectra of CO of poly(di‐n‐butyl itaconate)s [poly(DBI)s] obtained at temperatures higher than 60 °C, although two kinds of carbonyl groups showed splittings due to triad tacticities in the spectra of polymers obtained at lower temperatures. The poly(DBI)s formed by the different kinds of initiators or formed in the presence of chain‐transfer agents showed hardly any changes in the intensities of the C resonances; this indicated that the C resonances were not due to the structures formed through initiating and terminating reactions. The poly(DBI)s obtained at different yields showed only a slight increase in the intensities of the C resonances with the yield, which suggested that the C resonances were not attributable to the intermolecular chain‐transfer reaction to the monomer and/or polymer. However, the intensities of the C resonances significantly increased with a decreasing feed monomer concentration; this suggested that intramolecular chain‐transfer reactions took place at high temperatures. Furthermore, a Cu complex‐catalyzed atom transfer radical polymerization mechanism was revealed to be effective for suppressing the intramolecular chain‐transfer reaction at 60 °C. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2415–2426, 2002
Controlled synthesis and chemical modification of unsaturated aliphatic (Co)polyesters based on 6,7‐dihydro‐2(3H)‐oxepinone
Authors: Lou Xudong , Detrembleur Christophe , Lecomte Philippe , Jérôme Robert 
Published: October 2002

AbstractA pure unsaturated cyclic ester, 6,7‐dihydro‐2(3H)‐oxepinone (DHO2), was prepared by a new synthetic route. The copolymerization of DHO2 with ϵ‐caprolactone (ϵCL) was initiated by aluminum isopropoxide [Al(OiPr)3] at 0 °C as an easy way to produce unsaturated aliphatic polyesters with nonconjugated CC double bonds in a controlled manner. The chain growth was living, as certified by the agreement between the experimental molecular weight at total monomer conversion and the value predicted from the initial monomer/initiator molar ratio. The polydispersity was reasonably low (weight‐average molecular weight/number‐average molecular weight ≤ 1.2). The homopolymerization of DHO2 was, however, not controlled because of fast intramolecular transesterification. Copolymers of DHO2 and ϵCL were quantitatively oxidized with the formation of epoxides containing chains. The extent of the epoxidation allowed the thermal properties and thermal stability of the copolyesters to be modulated. The epoxidized copolyesters were successfully converted into thioaminated chains, which were then quaternized into polycations. No degradation occurred during the chemical modification. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2286–2297, 2002
Synthesis and properties of polyacetylenes with salicylideneaniline groups
Authors: Karim S. M. Abdul , Nomura Ryoji , Masuda Toshio 
Published: October 2002

AbstractAcetylenes containing salicylideneaniline groups—N‐salicylidene‐3‐ethynylaniline (1), N‐(3‐t‐butylsalicylidene)‐3‐ethynylaniline (2), and N‐(3‐t‐butylsalicylidene)‐4‐ethynylaniline (3)—polymerized smoothly and gave yellow to red polymers in excellent yields when a rhodium catalyst was employed. Polymers with alkyl substituents on the aromatic rings [poly(2) and poly(3)] were soluble in CHCl3, tetrahydrofuran, and so forth, whereas the polymer without alkyl substituents [poly(1)] was insoluble in any solvent. N‐(3‐t‐Butylsalicylidene)propargylamine did not provide any polymer. Thermogravimetric analyses of the resultant polymers exhibited good thermal stability (To, onset temperature of weight loss > 300 °C). The ultraviolet–visible spectra of the polymers showed absorption maxima and cutoff wavelengths around 360 and 520 nm, respectively. The polymers exhibited largely Stokes‐shifted fluorescence (emission wavelength ≃ 550 nm) upon photoexcitation at 350 nm, which resulted from the photoinduced intramolecular proton transfer. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2458–2463, 2002
Poly(4‐vinylpyridine) as the host ligand of metal‐containing chromophores for second‐order nonlinear optical active materials
Authors: Caruso Ugo , De Maria Antonella , Panunzi Barbara , Roviello Antonio 
Published: October 2002

AbstractTwo formulas of grafted polymers with metal‐containing chromophores, potentially suitable for second‐order nonlinear optics applications, are described. Two chromophores were obtained from a tridentate ligand coordinated to Cu(II) or Pd(II) ions. The organometallic chromophore fragments were grafted to poly(4‐vinylpyridine) by the pyridinic nitrogen of the host polymer. Some qualities displayed by the new metallated polymers are remarkable: (1) a high value of the first hyperpolarizability coefficient of the chromophores, (2) a high content of the grafted chromophore in the polymers (up to 60 wt %), (3) a considerable increase in the glass‐transition temperatures (up to 240 °C), (4) good thermal stability in air (ca. 280 °C), and (5) good optical transparency of the films. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2987–2993, 2002
Synthesis of biodegradable copolymers with low‐toxicity zirconium compounds. III. Synthesis and chain‐microstructure analysis of terpolymer obtained from L‐lactide, glycolide, and ϵ‐caprolactone initiated by zirconium(IV) acetylacetonate
Authors: Dobrzynski Piotr 
Published: October 2002

AbstractZirconium(IV) acetylacetonate [Zr(acac)4] is a very good initiator for the terpolymerization of glycolide with L‐lactide and ϵ‐caprolactone. The microstructure of the obtained terpolymer was determined by NMR spectroscopy and then compared with terpolymers obtained in the presence of stanous(II) octoate [Sn(oct)2]. Samples obtained with Zr(acac)4 were characterized by a segmental‐chain microstructure. Apart from relatively long lactidyl microblocks, there were also segments made of random copolymer of glycolide with lactide. Such a structure is formed as a result of strong transesterification caused by active caproyl chain endings attacking the glycolidyl groups. Domination of this type of transestrification is shown. The growth of terpolymer chains and the influence of transesterification on gradual changes of the microstructure of the forming terpolymer chain were examined. Significant differences among glycolide, lactide, and the least reactive caprolactone were observed. The results of differential scanning calorimetric examinations of the obtained terpolymers are presented. Differences between the structures of random terpolymers obtained during terpolymerization initiated by Sn(oct)2 and those obtained by Zr(acac)4 influence their thermal properties. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3129–3143, 2002
Polyetherether ketone/polyarylethersulfone blends: Thermal and compatibility aspects
Authors: Nandan Bhanu , Kandpal L. D. , Mathur G. N. 
Published: October 2002

AbstractThe compatibility behavior of polyetherether ketone (PEEK) with poly(ether sulfone) (PES) has been reexamined using differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and extrudate swell measurements. The blends were prepared by both melt‐blending and solution‐blending techniques. The phase behavior of blends is strongly affected by the blending technique used. Blends prepared by solution‐blending are compatible in the entire composition range on the basis of the single composition‐dependent glass transitions and exhibit lower critical solution temperature (LCST) behavior. LCST was near 340 °C around which the crystalline melting point of PEEK exists. Near LCST melting‐induced movement of molecular chains disturbs the initial homogeneous state of the solution blends and leads to a phase‐separated state that is thermodynamically more stable in the absence of strong specific interactions between the homopolymers. Contrary to the solution‐blended samples, melt‐blended samples were in the phase‐separated state even at a lower processing temperature of 300 °C. Two glass transitions corresponding to a PEEK‐rich and a PES‐rich phase were found for all compositions. From the measured glass transition of phase‐separated blends, weight fractions of PES and PEEK dissolved in each phase were determined using the Fox equation. Compatibility is greater in the PEEK‐rich compositions than in the PES‐rich compositions. PEEK dissolves more in PES‐rich phases than does PES in the PEEK‐rich phase. Variation of the specific heat increment (ΔCp) at the glass transition with composition also supports these inferences. Solution‐blended samples, quenched from 380 °C, also indicated similar behavior but were slightly more compatible. The aforementioned results are consistent with those inferred from SEM studies and extrudate swell measurements that show a greater compatibility in PEEK‐rich compositions than in PES‐rich compositions. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1407–1424, 2002
Simulation of orientation of uniaxially stretched poly(vinyl phenol) by molecular dynamics
Authors: Gestoso Patricia , Brisson Josée 
Published: October 2002

AbstractMolecular dynamics were performed for the simulation of the uniaxial deformation of poly(vinyl phenol) under periodic boundary conditions with the Parrinello–Rahman scheme followed by relaxation under NVT conditions (constant number of atoms, volume, and temperature). Changes in the orientation of the main chain, benzene segments, and hydrogen bonds were analyzed with the second‐order Legendre polynomial, 〈P2(cos θ)〉. Conformational changes were also followed. During deformation, backbone and phenyl rings both initially orient parallel to the draw direction. After relaxation, the chain is oriented parallel to the deformation direction, and side groups orient approximately perpendicular to this direction, in agreement with experimental data reported in the literature. Orientation values are higher than experimental values, as expected from the limited relaxation time range attainable in the simulations. Deformation proceeds by changes from gauche conformers to nontrans, nongauche, and trans conformers, whereas relaxation mainly allows high‐energy nontrans, nongauche conformers to convert into lower energy trans conformers. Values of the α angle for ring and bonded OH segments agree with those in the literature. Differences observed for free hydroxyl moieties are discussed. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1601–1625, 2002
Melt blending of ethylene‐vinyl alcohol copolymer/clay nanocomposites: Effect of the clay type and processing conditions
Authors: Artzi N. , Nir Y. , Narkis M. , Siegmann A. 
Published: October 2002

AbstractEthylene‐vinyl alcohol copolymer (EVOH)/clay nanocomposites were prepared via dynamic melt blending. The effect of the processing parameters on blends containing two clay types in different amounts was examined. The blends were characterized with a Brabender plastograph and capillary rheometer, differential scanning calorimetry, dynamic mechanical thermal analysis (DMTA), X‐ray diffraction (XRD), transmission electron microscopy (TEM), and thermogravimetric analysis (TGA). XRD showed advanced EVOH intercalation within the galleries, whereas TEM images indicated exfoliation, thereby complementing the XRD data. A dilution process with EVOH and clay treatment in an ultrasonic bath before melt blending did not add to the intercalation level. Different trends were observed for the EVOHs containing two different clay treatments, one claimed to be treated for EVOH and the other for amine‐cured epoxy. They reflected the differences in the amounts of the strongly interacting polymer for the two nanocomposites. Thermal analysis showed that the melting temperature, crystallization temperature, and heat of fusion of the EVOH matrix sharply decreased with both increasing clay content and processing times. Significantly higher viscosity levels were obtained for the blends in comparison with those of the neat polymer. The DMTA spectra showed higher glass‐transition temperatures for the nanocomposites in comparison with those of the neat EVOH. However, at high clay loadings, the glass‐transition temperature remained constant, presumably because of an adverse plasticizing effect of the low moleculared mass onium ions treating the clays. The storage modulus improved when clay treated for EVOH was used, and it deteriorated when amine‐cured epoxy clay was incorporated, except for the sonicated clay. TGA results showed significant improvements in the blends' thermal stability in comparison with that of the neat EVOH, which, according to TEM, was greater for the intercalated structures rather than for exfoliated ones. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1741–1753, 2002
Seasonal variation in respiratory syncytial virus chest infection in the tropics
Authors: Chan P.W.K. , Chew F.T. , Tan T.N. , Chua K.B. , Hooi P.S. 
Published: October 2002

AbstractRespiratory syncytial virus (RSV) is the most important cause of lower respiratory tract infection (LRTI) in young children. We determined if there was a seasonal variation in Malaysia in the incidence of RSV infection in young children admitted with LRTI, and possible associations of RSV infection with local meteorological parameters. A total of 5,691 children, aged less than 24 months and hospitalized with LRTI (i.e., bronchiolitis and pneumonia) between 1982–1997, were included in this study. Nasopharyngeal samples were collected and examined for RSV by immunofluorescence, viral culture, or both. Seasonal variations were determined by analyzing the monthly RSV‐positive isolation rate via time series analysis. Possible correlations with local meteorological parameters were also evaluated.RSV was isolated in 1,047 (18.4%) children. Seasonal variations in RSV infection rate were evident and peaked during the months of November, December, and January (test statistics [T] = 53.7, P < 0.001). This seasonal variation was evident for both bronchiolitis and pneumonia categories (T = 42.8 and 56.9, respectively, P < 0.001). The rate of RSV infection appeared to correlate with the monthly number of rain days (r = 0.26, P < 0.01), and inversely with the monthly mean temperature (r = −0.38, P < 0.001).In the tropics, seasonal variations in the incidence of RSV infection are evident, with an annual peak in November, December, and January. This information provides a guide for healthcare provisions and implementation of RSV prevention. Pediatr Pulmonol. 2002; 34:47–51. © 2002 Wiley‐Liss, Inc.