The effect of crystallinity and crosslinking on the depression of the glass transition temperature in nylon 6 by water

The influence of crystallinity and crosslinking on the depression of the glass transition temperature in nylon 6 by water has been investigated by dynamic mechanical methods. Radiation crosslinking by high-energy electrons was effective in preventing morphological changes during the measurement of the incremental change in heat capacity (ΔC_p) at T_g, which was performed by differential scanning calorimetry. The experimentally determined value of ΔC_p, when normalized to account for the crystalline phase, was found to deviate from a linear two-phase relation and was reduced further than would be expected based on this model. It is proposed that nylon 6 is best described by a three-phase model which consists of a crystalline domain, a wholly amorphous domain, and an “intercrystalline” region. The importance of this in explaining the relatively large depression of T_g by small quantities of water is illustrated by applying equations derived to account for the compositional dependence of T_g in polymerdiluent mixtures, based on a classical thermodynamic interpretation of the glass transition phenomenon.     

Mechanics of peeling of rubbery materials. II. Initiation and propagation of peeling

The peel strength of a joint with flexible materials bonded by an elastic adhesive was evaluated in relation to the fracture mechanism. It was found that initiation and propagation of peeling are governed by different mechanisms. Initiation (the formation of an initial crack) occurs when the maximum stress in the adhesive layer reaches a definite value. In this case, the strength f_i in a trousers-type peeling is given by 2f_i = y₀σ_b?_b, where y₀ is the half-thickness of the adhesive layer, σ_b is the tensile strength, and ?_b is the tensile elongation of the adhesive. On the other hand, propagation is governed by the surface energy of the adhesive. In this case, the peeling strength f_s is determined by a balance of energies. For trousers-type peeling it is given by 2f_s = Γ, where Γ is twice the surface energy. These results were verified experimentally.     

On the representations of the rotation group

The matrices of the irreducible representations of the 3-dimensional rotation group are shown to be related to Krawtchouk's orthogonal polynomials of a discrete variable x = j – m', whose degrees are given by n = j + m. The relation follows directly from the recurrence formulas satisfied by the matrix elements and permits a concise development of the formal properties of the rotation matrices. In particular, an asymptotic relation for large j is developed that generalizes a formula first discussed for a special case by Wigner.     

Nitrosation of Phenolic Compounds: Effects of Alkyl Substituents and Solvent

Summary.  Nitrosation reactions of phenol, o-cresol, 2,6-dimethylphenol, o-tert-butylphenol, 2-hydroxyacetophenone, and 2-allylphenol in water and water/acetonitrile were studied. Kinetic monitoring of the reactions was accomplished by spectrophotometric analysis of the nitrosated products at 345 nm. The dominant reaction was C-nitrosation via a mechanism consisting of an attack on the nitrosatable substrate by NO⁺/NO₂H₂ ⁺ followed by a slow proton transfer. The values of the rate constants of phenolic C-nitrosation were increased by electron donating substituents, and a good Hammett correlation was observed with ρ = −6.1. The results also revealed the strong effect of pH and the permitivity of the reaction medium on the rate constant, whose maximum values were observed for pH ≈ 3, decreasing strongly for higher pH values. The study in water/acetonitrile with up to 25% acetonitrile showed that it is possible to inhibit the reaction strongly by increasing the percentage of the organic component. The conclusions drawn show that (i) it is possible to predict the rate of nitrosation of phenolics as a function of the meta-substituents on the phenol ring and (ii) the nitrosation of phenolics can be strongly inhibited by increasing the pH of the reaction medium as well as by lowering its dielectric constant. Received July 13, 2001. Accepted (revised) September 18, 2001     

Synthesis of silicon nitride based ceramic nanoparticles by the pyrolysis of silazane block copolymer micelles

Diblock copolymer poly(1,1,3,N,N′‐pentamethyl‐3‐vinylcyclodisilazane)‐block‐polystyrene (polyVSA‐b‐polySt) and triblock copolymer poly(1,1,3,N,N′‐pentamethyl‐3‐vinylcyclodisilazane)‐block‐polystyrene‐block‐poly(1,1,3,N,N′‐pentamethyl‐3‐vinylcyclodisilazane) (polyVSA‐b‐polySt‐b‐polyVSA), consisting of silazane and nonsilazane segments, were prepared by the living anionic polymerization of 1,1,3,N,N′‐pentamethyl‐3‐vinylcyclodisilazane and styrene. PolyVSA‐b‐polySt formed micelles having a poly(1,1,3,N,N′‐pentamethyl‐3‐vinylcyclodisilazane) (polyVSA) core in N,N‐dimethylformamide, whereas polyVSA‐b‐polySt and polyVSA‐b‐polySt‐b‐polyVSA formed micelles having a polyVSA shell in n‐heptane. The micelles with a polyVSA core were core‐crosslinked by UV irradiation in the presence of diethoxyacetophenone as a photosensitizer, and the micelles with a polyVSA shell were shell‐crosslinked by UV irradiation in the presence of diethoxyacetophenone and 1,6‐hexanedithiol. These crosslinked micelles were pyrolyzed at 600 °C in N₂ to give spherical ceramic particles. The pyrolysis process was examined by thermogravimetry and thermogravimetry/mass spectrometry. The morphologies of the particles were analyzed by atomic force microscopy and transmission electron microscopy. The chemical composition of the pyrolysis products was analyzed by X‐ray fluorescence spectroscopy and Raman scattering spectroscopy. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4696–4707, 2006     

Nitrosation kinetics of phenolic components of foods and beverages

The kinetics of the reactions between sodium nitrite and phenol or m-, o-, or p-cresol in potassium hydrogen phthalate buffers of pH 2.5–5.7 were determined by integration of the monitored absorbance of the C-nitroso reaction products. At pH > 3, the dominant reaction was C-nitrosation through a mechanism that appears to consist of a diffusion-controlled attack on the nitrosatable substrate by NO⁺/NO₂H₂⁺ ions followed by a slow proton transfer step; the latter step is supported by the observation of basic catalysis by the buffer which does not form alternative nitrosating agents as nitrosyl compounds. The catalytic coefficients of both anionic forms of the buffer have been determined. The observed order of substrate reactivities (o-cresol ≈ m-cresol > phenol ≫ p-cresol) is explained by the hyperconjugative effect of the methyl group in o- and m-cresol, and by its blocking the para position in p-cresol. Analysis of a plot of ΔH^# against ΔS^# shows that the reaction with p-cresol differs from those with o- and m-cresol as regards the formation and decomposition of the transition state. The genotoxicity of nitrosatable phenols is compared with their reactivity with NO⁺/NO₂H₂⁺. © 1997 John Wiley & Sons, Inc.     

Molecular parameters of hyperbranched polymers made by self‐condensing vinyl polymerization of macroinimers

The molecular weight averages and the degree of branching, DB, of a hyperbranched polymer obtained by self‐condensing vinyl polymerization (SCVP) of a macroinimer A‐(m)_γ‐B* is calculated by modifying the existing equations for SCVP. The polydispersity is lowered by a factor (γ + 1), where γ is the degree of polymerization of the macroinimer. DB decreases with γ, however, at full conversion the DB of the polymacroinimer is approximately 60% higher than expected from the “dilution” of an AB* inimer with linear m units. This is the result of the existence of a new kind of branched units. The structure of polymacroinimers is similar to the pattern of a highly branched copolymer obtained by self‐condensing vinyl copolymerization. However, the polydispersity index and the DB of these two processes are different, for a given molecular weight the polydispersity and the DB of the macroinimer is lower than the corresponding parameters for SCVCP at the same value of γ.     

Total Synthesis of (+)‐Gracilamine Based on an Oxidative Phenolic Coupling Reaction and Determination of Its Absolute Configuration

The enantioselective total synthesis of (+)‐gracilamine ( 1 ) is described. The strategy features a diastereoselective phenolic coupling reaction followed by a regioselective intramolecular aza‐Michael reaction to construct the ABCE ring system. The configuration at C3a in 1 was controlled by the stereocenter at C9a, which was selectively generated (91 % ee) by an organocatalytic enantioselective aza‐Friedel–Crafts reaction developed by our research group. This synthesis revealed that the absolute configuration of (+)‐gracilamine is 3aR, 4S, 5S, 6R, 7aS, 8R, 9aS.     

Preparation of L-Lyxo-Hexos-5-Ulose Through C-3 Epimerization of Bis-Glycopyranosides of L-Arabino-Hexos-5-Ulose1

Abstract

The unreported title compound and its 2,6-di-O-benzyl derivative have been prepared from methyl β-D-galactopyranoside through a sequence involving the bisglycoside methyl 2,6-di-O-benzyl-5-O-methoxv-β-D-galactopyranoside 8, the precursor of L-orabino-hexos-5-ulose, that was converted to the L-lyxo series by inversion at C-3. The inversion was achieved in acceptable yields by selective triflation, followed by displacement with benzoate, and by an oxidation/reduction sequence. Whereas 2,5-di-O-benzyl-L-lyxo-hexos-5-ulose exists entirely as a mixture of the two anomeric 1,4-furanosic forms, the unprotected hexos-5-ulose involves at equilibrium in CD₃CN/D₂O at least eight tautomers, one of which is predominant.     

Studies on the self-assembly behavior of the amphiphilic block copolymer of PSt-b-PAA in apolar solvents with polar fluorescent probe

The block copolymer of polystyrene-b-poly(butyl acrylate) (PSt-b-PBA) with a well-defined structure was synthesized by atom transfer radical polymerization (ATRP); its structure was characterized, and the living polymerization was also validated by gel permeation chromatography, Fourier transform infrared, and ¹H NMR measurements. Then, the amphiphilic block copolymer of polystyrene-b-poly(acrylic acid) (PSt-b-PAA) has been prepared by hydrolysis of PSt-b-PBA, and copolymers of PSt-b-PAA with longer PSt blocks and shorter PAA blocks were obtained by controlling the conditions of ATRP polymerization. The reversed micelle solution of PSt-b-PAA in toluene was prepared by using the single-solvent dissolving method, and the reverse micellization behavior of PSt-b-PAA in toluene was mainly investigated in this paper. The fluorescent probe technique was used by using polar fluorescence compound N-(1-Naphthyl)ethylenediamine dihydrochloride (NEAH) as a polar fluorescent probe to study the reverse micellization behavior of PSt-b-PAA. It was found that the reverse micellization behaviors of PSt-b-PAA in toluene can be clearly revealed by using NEAH as a polar fluorescence probe, and the critical micelle concentrations (cmcs) can be well displayed. The experimental results showed that the self-assembling behavior of PSt-b-PAA in toluene depends apparently on the microstructure of the macromolecules and is also influenced by the temperature. For the copolymers of PSt-b-PAA with the same length of hydrophobic PSt blocks, the copolymer with a longer hydrophilic block PAA has lower cmc, and at higher temperature, the copolymer has lower cmc.     

Quenching of chlorophyll a singlets and triplets by carotenoids in light-harvesting complex of photosystem II: comparison of aggregates with trimers

Laser-induced changes in the absorption spectra of isolated light-harvesting chlorophyll a/b complex (LHC II) associated with photosystem II of higher plants have been recorded under anaerobic conditions and at ambient temperature by using multichannel detection with sub-microsecond time resolution. Difference spectra (ΔA) of LHC II aggregates have been found to differ from the corresponding spectra of trimers on two counts: (i) in the aggregates, the carotenoid (Car) triplet–triplet absorption band (ΔA>0) is red-shifted and broader; and (ii) the features attributable to the perturbation of the Q_y band of a chlorophyll a (Chla) by a nearby Car triplet are more pronounced, than in trimers. Aggregation, which is known to be accompanied by a reduction in the fluorescence yield of Chla, is shown to cause a parallel decline in the triplet formation yield of Chla; on the other hand, the efficiency (100%) of Chla-to-Car transfer of triplet energy and the lifetime (9.3 μs) of Car triplets are not affected by aggregation. These findings are rationalized by postulating that the antenna Cars transact, besides light-harvesting and photoprotection, a third process: energy dissipation within the antenna. The suggestion is advanced that luteins, which are buried inside the LHC II monomers, as well as the other, peripheral, xanthophylls (neoxanthin and violaxanthin) quench the excited singlet state of Chla by catalyzing internal conversion, a decay channel that competes with fluorescence and intersystem crossing; support for this explanation is presented by recalling reports of similar behaviour in bichromophoric model compounds in which one moiety is a Car and the other a porphyrin or a pyropheophorbide.     

The molecular structure and chemical reactivities of the condensation products of o-substituted benzylidenacetylacetone with hydrazine dihydrochloride

Reaction of o-nitrobenzylideneacetylacetone ( 1a ) with hydrazine dihydrochloride in methanol gave 4-(α-methoxy-o-nitrobenzyl)-3,5-dimethylpyrazole hydrochloride ( 4a ), whose structure was unambigously confirmed by an X-ray crystallographic analysis, via 4-(o-nitrobenzylidene)-3,5-dimethylisopyrazole ( 2a ). Compound 2a was synthesized by condensation of 1a with hydrazine dihydrochloride in acetonitrile. Analogously the corresponding o-chloro derivatives ( 2b, 4b ) were obtained. These were converted to N-methyl ( 6b ) and N-acetyl ( 7a,b ) derivatives and the behaviors on bromination and pyrolysis were investigated.     

The Rates of Diazomethane Formation from Methylnitrosoamides. The stability of diazomethane solutions towards aqueous alkalis

The rate of hydrolysis of N-methyl-N-nitrosoamides by aqueous alkalis varies greatly. Methylnitrosourea (1) is hydrolyzed rapidly by aqueous KOH-solutions at low temperatures to give a high yield of diazomethane. Under similar conditions, N,N′-dimethyl-N,N′-dinitroso-oxamide (3) is hydrolyzed more slowly, but also gives a good yield of diazomethane. N,N′-Dimethyl-N,N′-dinitrosoterephthal-amide (4) , and (N-methyl-N-nitroso)-4-amino-4-methyl-2-pentanone (5) are less easily hydrolyzed by aqueous KOH-solutions. N-Methyl-N-nitroso-p-toluenesulfonamide (2) was the least reactive out of those tested. The hydrolysis of diazomethane in toluene with aqueous bases follows first order kinetics. The hydrolysis rate is greatly influenced by the concentration and strength of the base and temperature.     

Some aspects of the polymerization of N-vinylcarbazole by grignard reagents

The polymerization of N-vinylcarbazole (NVC) initiated by PhMgBr in benzene was studied at 32°C. R_p is second order with respect to PhMgBr concentration but increases with NVC concentration (up to 0.06 M) and falls thereafter. R_p and P _n are depressed by the addition of thiophene and water. Modifiers such as benzaldehyde, butanone, and ethylene glycol practically inhibit the polymerization. Carbon tetrachloride and carbon dioxide, when passed through the NVC solution first, enhance the R_p and P _n increases with increasing PhMgBr and NVC concentrations, respectively. R_p increases with temperature, but P _n shows a maxium at a certain temperature. A cationic mechanism has been proposed where the polymerization is initiated by RMg⁺ cations produced from the ionization of PhMgBr by the Ashby and Smith mechanism.     

Stereochemical effects in mass spectrometry. 7. Determination of absolute configuration of some organic molecules by reaction mass spectrometry

It was found that in the chemical ionization (isobutane) mass spectra of some asymmetric secondary alcohols and α-amino acids, when a pair of enantiomers (such as R- and S-2-phenyIbutyric anhydride, R- and S-mandelic acid, R- and S-2-methylbutanoic acid or R- and S-α-phenyl ethyl amine) were used as reaction reagents, the relative abundances of characteristic ions formed by the stereoselective reaction between sample and reagent of the same configuration were much higher than those ions formed by the sample and a reagent of a different configuration. The absolute configuration of the sample molecule may be predicted by examination of mass spectra of the sample measured with R- and S-reagent respectively. This approach proved to be a convenient way for determination of the absolute configurations of organic molecules on a micromole level by mass Spectrometry.     

An Efficient Synthesis of Enantiomeric Ribonucleic Acids from D-Glucose

Enantiomeric oligoribonucleotides (= ent-RNA) up to a sequence length of thirty-five and consisting of the (L -configurated) nucleosides ent-adenosine, ent-guanosine, ent-cytidine, ent-uridine, and 1-(β-L -ribofuranosyl)thymine were prepared by automated synthesis from appropriate building blocks, carrying a known photo-labile 2′-O-protecting group. A simple large-scale synthesis of the new, prefunctionalized L -ribose derivative 5 from D -glucose (Scheme 1) and its straightforward conversion into the five phosphoramidites 28 – 32 and five solid supports 38 – 42 , respectively, were elaborated (Scheme 4). Within this project, a novel, superior strategy for the synthesis of the 2′-O-{[(2-nitrobenzyl)oxy]methyl}-substituted key intermediates 18 – 22 by regioselective alkylation of their 5′-O-dimethoxytritylated precursors 13 – 17 was developed. Furthermore, an improved set-up for the final light-induced cleavage of the 2′-O-protecting groups from the oligonucleotide sequences was designed (Scheme 5 and Fig. 1). The correct composition of all ent-oligoribonucleotides prepared was established by their MALDI-TOF mass spectra. The ¹H-NMR-spectroscopic data of a dodecameric ent-RNA sequence was in excellent agreement with the published data of its natural counterpart, synthesized by conventional methods. The known specific cleavage of a tetradecamer sequence by a 35mer ribozyme structure could be reproduced by ent-oligoribonucleotides, synthesized by the presented methods (Fig. 4).     

Collective Total Synthesis of Mavacuran Alkaloids through Intermolecular 1,4-Addition of an Organolithium Reagent**

We report a synthetic endeavor towards the highly strained pentacyclic caged framework of the mavacuran alkaloids which culminated with the concise total synthesis of C-fluorocurine, C-profluorocurine, C-mavacurine, normavacurine, 16-epi-pleiocarpamine and taberdivarine H. We designed a strategy involving late-stage construction of the D ring by Michael addition of a vinylic nucleophile to a 2-indolyl acrylate moiety. While the intramolecular Michael addition did not succeed, we were able to perform a diastereoselective unusual intermolecular 1,4-addition of a functionalized vinyl lithium reagent to a readily accessible Michael acceptor with the assistance of the piperidine nitrogen atom through the formation of a complex as suggested by DFT computations. Final cyclization was achieved by nucleophilic substitution to form an ammonium intermediate. The first total syntheses of C-profluorocurine and C-fluorocurine were finalized by the dihydroxylation of C-mavacurine and a pinacol rearrangement, respectively.     

Polymerization of N-(p-aminobenzoyl)caprolactam: Block and alternating copolymers of aromatic and aliphatic polyamides

The polymerization behavior of N-(p-aminobenzoyl)caprolactam was studied. It was found that polymerization could proceed by either elimination of caprolactam or by ring opening. Polymers prepared at temperatures above 200°C showed a greater tendency for ring opening to produce alternating aromatic/aliphatic copolymers than did polymers prepared at lower temperatures. Block copolymers of poly(p-benzamide) and nylon 6 were prepared by a two-stage hydrolytic polymerization process or by anionic polymerization at temperatures > 200°C. Polymer microstructures were determined using ¹³C-NMR spectroscopy by comparison with homopolymers and model alternating copolymers. The alternating copolymer prepared by condensation of N-(p-aminobenzoyl)-6-caproic acid showed a melting transition at 300–305°C in the DSC and a T_g in subsequent heating cycles of 116–119°C. Copolymers made with the two-stage process were rich in p-benzamide sequences and showed no T_g or T_m below 400°C. Copolymer made with NaH was rich in nylon 6 units, showed a T_m of 175–180°C and a T_g of 80–81°C, and was homogeneous in both the melt and solid.     

Preparation of Regioselectively Protected Hydroquinones by Phosphorylation of p-Benzoquinones with Trialkyl Phosphites

The title reaction has been applied to 10 monosubstituted p-benzoquinones (Scheme 2, Table). The regioselectively of the O-phosphorylation is influenced by bulky substituents (t-butyl and trimethylsilyl) and, electronically, by the methoxy group. The regioselectivity, which is high in nonpolar media (benzene), is lower in polar solvents (CH₂Cl₂) and (CH₃CN). The synthetic potential of this transformation, exemplified by the preparation of compounds 29 (Scheme 3) and 32 (Scheme 4), is considerably extended by applying milder methods for the phosphate hydrolysis and by using the reagent couple P(OCH₃)₃/trimethylsilyl chloride, which gives clean access to p-hydroxyphenyl phosphates. p-Benzoquinones 4th and 4i with strong π-acceptor substituents react in a different way, giving phosphonates. The electronically induced regioselectivity of the O-phosphorylation is in according with the preferences expected for the attack by a nucleophilic phosphorylation agent.