Search

Transfer hydrogenation of azobenzene with ammonia borane mediated by pincer bismuth complex 1 was systematically investigated through density functional theory calculations. An unusual metal-ligand cooperation mechanism was disclosed, in which the saturation/regeneration of the C=N functional group on the pincer ligand plays an essential role. The reaction is initiated by the hydrogenation of the C=N bond (saturation) with ammonia borane to afford 3

The Ni-B complex 1BCF with a facilely accessible monophosphine (Pt Bu3 ) unit was theoretically designed, which was found to be more active than that with an ambiphilic ligand for hydrogenation of styrene. Substituting Pt Bu3 with a stronger electron donating ligand N-heterocyclic carbene largely improves the activity of the Ni-B complex.

Electrochemical organic synthesis has attracted increasing attentions as a sustainable and versatile synthetic platform. Quantitative assessment of the electro-organic reactions, including reaction thermodynamics, electro-kinetics, and coupled chemical processes, can lead to effective analytical tool to guide their future design. Herein, we demonstrate that electrochemical parameters such as onset potential, Tafel slope, and effective voltage can be utilized as electro-descriptors for the evaluation of reaction conditions and prediction of reactivities (yields). An "electro-descriptor-diagram" is generated, where reactive and non-reactive conditions/substances show distinct boundary. Successful predictions of reaction outcomes have been demonstrated using electro-descriptor diagram, or from machine learning algorithms with experimentally-derived electro-descriptors. This method represents a promising tool for data-acquisition, reaction prediction, mechanistic investigation, and high-throughput screening for general organic electro-synthesis.

A new efficient metal-based frustrated Lewis pair constructed by (Pt Bu3 )2 Pt and B(C6 F5 )3 was designed through density functional theory calculations for the catalytic dehydrogenation of ammonia borane (AB). The reaction was composed by the successive dehydrogenation of AB and H2 liberation, which occurs through the cooperative functions of the Pt(0) center and the B(C6 F5 )3 moiety. Two equivalents of H2 were predicted to be liberated from each AB molecule. The generation of the second H2 is the rate-determining step, with a Gibbs energy barrier and reaction energy of 27.4 and 12.8 kcal/mol, respectively.

Tandem Friedel-Crafts (FC) and C-H/C-O coupling reactions catalyzed by tris(pentafluorophenyl) borane (B(C6 F5 )3 ) were achieved without using any other additive in the absence of solvent. This process can be used for the reactions between a series of dialkylanilines and vinyl ethers with good isolated yields of bis(4-dialkylaminophenyl) compounds. Based on combined theoretical and experimental studies, the possible reaction mechanism was proposed. B(C6 F5 )3 can activate the C=C and C-O bond for FC and C-H/C-O coupling reactions respectively. The FC reaction is slow, which is followed by a fast C-H/C-O coupling.

Boron Lewis acid-catalyzed and catalyst-free hydroboration reactions of imines are attractive due to the mild reaction conditions. In this work, the mechanistic details of the hydroboration reactions of two different kinds of imines with pinacolborane (HBpin) are investigated by combining density functional theory calculations and some experimental studies. For the hydroboration reaction of N-(α-methylbenzylidene)aniline catalyzed by tris[3,5-bis(trifluoromethyl)phenyl]borane (BAr

A B(C6 F5 )3 -catalyzed hydroarylation of a series of 1,3-dienes with various phenols has been established through a combination of theoretical and experimental investigations, affording structurally diverse ortho-allyl phenols. DFT calculations show that the reaction proceeds through a borane-promoted protonation/Friedel-Crafts pathway involving a π-complex of a carbocation-anion contact ion pair. This protocol features simple and mild reaction conditions, broad functional-group tolerance, and low catalyst loading. The obtained ortho-allyl phenols could be further converted into flavan derivatives using B(C6 F5 )3 with good cis diastereoselectivity. Furthermore, this transformation was applied in the late-stage modification of pharmaceutical compounds.

A large number of EGFR mutant non-small cell lung cancer patients primordially benefit from first-line treatment with first-generation EGFR-tyrosine kinase inhibitors (TKIs), such as gefitinib and erlotinib. However, multiple acquired resistance mechanisms have been described that limit the clinical efficacy of first-generation EGFR-TKIs. Herein, we report a rare case of lung adenocarcinoma harboring an EGFR exon 19-deletion mutation before the administration of target therapy. This patient acquired resistance to first-generation EGFR-TKIs through small cell lung cancer (SCLC) transformation accompanied by the T790M mutation. Unexpectedly, this SCLC patient maintained a sensitive response to the third-generation EGFR-TKI osimertinib. This special case may indicate that osimertinib represents an effective target drug for SCLC patients who harbor an EGFR T790M mutation.

An efficient method for the metal-free deaminative borylation of alkylamines, using bis(catecholato)diboron as the boron source, to directly synthesize various alkyl potassium trifluoroborate salts is introduced. The key to this high reactivity is the utilization of pyridinium salt activated alkylamines, with a catalytic amount of a bipyridine-type Lewis base as a promoter. This transformation shows good functional-group compatibility (e.g., it is unimpeded by the presence of a ketone, indole, internal alkene, or unactivated alkyl chloride) and can serve as a powerful synthetic tool for borylation of amine groups in complex compounds. Mechanistic experiments and computations suggest a mechanism in which the Lewis base activated B2 cat2 unit intercepts an alkyl radical generated by single-electron transfer (SET) from a boron-based reductant.

A fully optimized implementation of the cluster-in-molecule (CIM) local correlation method for faster and more accurate electron correlation calculations of large systems is reported. The speedup comes from the new procedure of constructing virtual localized molecular orbitals of clusters. In the new procedure, Boughton-Pulay projection method is employed instead of a much more expensive Boys localization procedure. In addition, basis set superposition error correction for binding energy calculations and parallelized electron correlation calculations of clusters are now implemented. Benchmark calculations and illustrative applications at the Moller-Plesset perturbation theory, coupled cluster singles and doubles (CCSD), and CCSD with perturbative triples correction levels show that this newly optimized CIM approach is a reliable theoretical tool for electron correlation calculations of various large chemical systems. © 2018 Wiley Periodicals, Inc.

For the first time have boron-containing weak Lewis acids been demonstrated to be active components of Frustrated Lewis Pair (FLP) catalysts in the hydrogenation of ketones to alcohols. Combining the organosuperbase (pyrr)3 P=NtBu with the Lewis acid 9-(4-CF3 -C6 H4 )-BBN generated an "inverse" FLP catalyst capable of hydrogenating a range of aliphatic and aromatic ketones including N-, O- and S-functionalized substrates and bio-mass derived ethyl levulinate. Initial computational and experimental studies indicate the mechanism of catalytic hydrogenation with "inverse" FLPs to be different from conventional FLP catalysts that contain strong Lewis acids such as B(C6 F5 )3 .

Density functional theory (DFT) investigations revealed that 4-cyanopyridine was capable of homolytically cleaving the B-B σ bond of diborane via the cooperative coordination to the two boron atoms of the diborane to generate pyridine boryl radicals. Our experimental verification provides supportive evidence for this new B-B activation mode. With this novel activation strategy, we have experimentally realized the catalytic reduction of azo-compounds to hydrazine derivatives, deoxygenation of sulfoxides to sulfides, and reduction of quinones with B2 (pin)2 at mild conditions.

BACKGROUND Resveratrol and its glucoside polydatin are the main stilbenes in Polygonum cuspidatum. Resveratrol has become the subject of intensive research over the past two decades owing to its outstanding pharmacological properties. However, its lower concentration in plants compared to polydatin limits its application. In this study, the polydatin-β-d-glucosidase (PBG) that hydrolyzes the β-d-glucosyl residue of polydatin with release of resveratrol was purified to homogeneity and characterized. RESULTS The molecular weight of PBG was estimated to be 125 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and 128 kDa by size-exclusion chromatography- multi-angle laser light scattering/ultraviolet/refractive index. The optimal PBG activity was observed at 70 °C and pH 4.5. The enzyme showed around 50% stability at 60 °C for 12 h and residual activity was over 80% at pH 3.0-5.0. Ca(2+) , Mg(2+) , Mn(2+) , Zn(2+) , Ba(2+) , Ni(2+) , Co(2+) and Cu(2+) ions had no significant effect on the enzyme activity. The PBG presented higher affinity to polydatin (Km = 0.74 mmol L(-1) ) than p-nitrophenyl-β-d-glucopyranoside (Km = 2.9 mmol L(-1) ) and cellobiose (Km = 8.9 mmol L(-1) ). CONCLUSION With this enzyme, nearly all polydatin in P. cuspidatum was converted to resveratrol. Although several β-D-glucosidases (BGLs) have been obtained from other sources, PBG is distinguished from other BGLs by its outstanding thermal stability and high catalytic efficiency. © 2015 Society of Chemical Industry.

We have extended the generalized energy-based fragmentation (GEBF) method to condensed phase systems with periodic boundary condition (PBC). The so-called PBC-GEBF method provides an alternative way of calculating electronic structures of condensed phase systems, whose accuracy is comparable to standard periodic electronic structure methods for some types of condensed phase systems such as molecular crystals and ionic liquid crystals. Within the PBC-GEBF approach, the unit cell energy (or properties) of a condensed phase system can be evaluated as a linear combination of ground-state energies (or corresponding properties) of a series of electrostatically embedded subsystems, which can be routinely calculated with existing molecular quantum chemistry packages. With the PBC-GEBF approach, one can routinely perform ab initio calculations at post-Hartree–Fock levels, including Moller–Plesset perturbation theory (MP2) or coupled cluster singles and doubles, on certain types of condensed phase systems, in which periodic post-Hartree–Fock methods are not available or not feasible computationally. This review will offer an overview of the methodology and implementation of the PBC-GEBF method and its applications in predicting the structures, lattice energies, and vibrational spectra of a wide range of molecular and ionic liquid crystals. Our results show that the PBC-GEBF approach at post-Hartree–Fock theory level can generally provide highly accurate descriptions on the structure and properties of crystals under study. For example, the vibrational spectra of the crystalline BH3NH3 predicted by the PBC-GEBF approach at the MP2 level are in better agreement with the experimentally observed spectra, than those based on density functional theory calculations. WIREs Comput Mol Sci 2017, 7:e1297. doi: 10.1002/wcms.1297 For further resources related to this article, please visit the WIREs website.

A low scaling quantum chemistry program, called LSQC, is described in this article. This version includes two linear scaling methods, generalized energy-based fragmentation (GEBF) approach and cluster-in-molecule (CIM) approach. In the GEBF approach, a variety of electron structure methods (including ab initio and density functional theory) are available for the calculations of ground-state energies, geometry optimizations, vibrational spectra, and other molecular properties for large systems. In the CIM approach, the electron correlation energies of large systems can be approximately obtained at the second-order Moller−Plesset perturbation theory and couple cluster levels. In this article, the details and the capabilities of the LSQC program are presented. © 2014 Wiley Periodicals, Inc.

Objectives/Hypothesis To explore the difference in the course of the lingual arteries between the tongue in the fully extended position and in the resting position in obstructive sleep apnea hypopnea syndrome (OSAHS) patients. Study Design Prospective nonrandomized study. Methods Seventeen patients with OSAHS were included in the study. Each patient received two lingual artery computed tomography angiography (CTA) examinations. For the first CTA examination, each subject was requested to keep the mouth closed with the tongue in a resting position. For the second CTA examination, each subject was asked to keep the mouth open with the tongue in a fully extended position. The difference in the distances between the bilateral lingual arteries and the depths of the lingual artery beneath the lingual surface, based on three reference marks, was analyzed with the tongue in a resting and extended position using the CTA images. Results Both the depths of the lingual artery beneath the lingual surface and the distances between the bilateral lingual arteries based on three reference marks in the fully extended position were all significantly smaller than those in the resting position (P

Objectives/Hypothesis To compare polysomnography (PSG) data after nasopharyngeal tube (NPT) insertion and uvulopalatopharyngoplasty (UPPP), and explore the possibility of predicting UPPP outcomes with NPT-PSG. Study Design Prospective nonrandomized study. Methods PSG was performed after NPT insertion in 73 inpatients with obstructive sleep apnea-hypopnea syndrome (OSAHS). All patients accepted UPPP surgery. PSG was repeated after 12 months of UPPP to assess curative effects. Data were then compared to correlate NPT-PSG with the efficacy of UPPP. Results NPT insertion decreased the overall apnea-hypopnea index (AHI) and increased the lowest oxygen saturation (LaSO2). NPT-PSG results correlated well with surgical outcomes 12 months post-UPPP. The correlation coefficient for AHI and LaSO2 were 0.571 (P

We propose an improved fragmentation scheme for the generalized energy-based fragmentation (GEBF) approach, which improves the accuracy of the GEBF approach in total energy calculations and intermolecular interactions. The main modification is to introduce some two-fragment-centered primitive subsystems, which are neglected in the previous GEBF implementation. Numerical calculations demonstrate that the present GEBF approach can provide more accurate ground-state energies and intermolecular interactions. The present GEBF approach with the M06-2X functional and the cc-pVTZ basis set are employed to investigate the structures and binding energies in two dimeric species, which are related to pseudopolymorphism of a phenyleneethynylene-based π-conjugated molecule. A comparison of the binding free energies in a dimeric species and its corresponding model without C-H···F contacts reveal that the substitution of fluorine atoms weakens the binding of monomers in the dimeric species formed by intermolecular O-H···O hydrogen bonds, but strengthens the binding in the dimer formed by the π-π stacking interaction. Therefore, the C-H···F contacts in these two dimeric species are demonstrated to play a less significant role.

The title coumarins are obtained under sunlight via a cascade of radical iodination, spirocyclization, and ring expansion through ester migration affording the kinetically favoured product.

The scalable, robust, and catalyst-free method has a broad substrate scope, thus offering access to a broad range of styrene derivatives, α,β-unsaturated carbonyl compounds as well as 1,2 and 1,2-disubstituted and even trisubstituted alkenes, including cyclic ones.

A highly (E)-stereoselective photoredox process for the direct C(sp2)—H bond difluoroalkylation of aldehyde-derived hydrazones is reported.

This thesis is devoted to first-principles simulations of bio- and nano-materials,focusing on various soft x-ray spectra, ground-state energies and structures of isolated largemolecules, bulk materials, and small molecules in ambient solutions. K-edge near-edge x-ray absorption fine structure (NEXAFS) spectra, x-ray emission spectra, andresonant inelastic x-ray scattering spectra of DNA duplexes have been studied by means oftheoretical calculations at the density functional theory level. By comparing a sequence of DNAduplexes with increasing length, we have found that the stacking effect of base pairs has verysmall influence on all kinds of spectra, and suggested that the spectra of a general DNA can bewell reproduced by linear combinations of composed base pairs weighted by their ratio. The NEXAFS spectra study has been extended to other realistic systems. We have used cluster modelswith increasing sizes to represent the infinite crystals of nucleobases and nucleosides, infinitegraphene sheet, as well as a short peptide in water solution. And the equivalent core holeapproximation has been extensively adopted, which provides an efficient access to these largesystems. We have investigated the influence of external perturbations on the nitrogen NEXAFSspectra of guanine, cytosine, and guanosine crystals, and clarified early discrepancies betweenexperimental and calculated spectra. The effects of size, stacking, edge, and defects to theabsorption spectra of graphene have been systematically analyzed, and the debate on theinterpretation of the new feature has been resolved. We have illustrated the influence of watersolvent to a blocked alanine molecule by using the snapshots generated from molecular dynamics. Multi-scale computational study on four short peptides in a self-assembled cage is presented. It isshown that the conformation of a peptide within the cage does not corresponds to its lowest-energyconformation in vacuum, due to the Zn-O bond formed between the peptide and the cage, and theconfinement effect of the cage. Special emphasis has been paid on a linear-scaling method, the generalized energy basedfragmentation energy (GEBF) approach. We have derived the GEBF energy equation at the Hartree-Focklevel with the Born approximation of the electrostatic potential. Numerical calculations for amodel system have explained the accuracy of the GEBF equation and provides a starting point forfurther refinements. We have also presented an automatic and efficient implementation of the GEBFapproach which is applicable for general large molecules.

The Kekule-based valence bond (VB) method, in which the VB model is solved using covalent Kekule structures as basis functions, is justified in the present work. Ibis method is demonstrated to provide satisfactory descriptions for resonance energies and bond lengths of benzenoid hydrocarbons, being in good agreement with SCF-MO and experimental results. In addition, an alternative way of discussing characters of localized substructures within a polycyclic benzenoid system is suggested based upon such simplified VB calculations. Finally, the symmetries of VB ground states for nonalternant conjugated systems are also illustrated to be obtainable through these calculations, presenting very useful information for understanding the chemical behaviors of some nonalternant conjugated molecules.

The mechanism of a typical Petasis-type boronic mannich reaction (the styrylboronic acid, dibenzylamine, and α-hydroxylpropionaldehyde) has been investigated using density functional theory calculations. According to our calculations, the reaction is most likely to proceed through the following steps: 1) the nucleophilic addition of the amine to the aldehyde to form the carbinolamine; 2) the dehydration of the carbinolamine; 3) the formation of the tetra-coordinated borate intermediate; 4) the CC bond formation by the intramolecular transfer of the styryl group; 5) the hydrolysis of the resulting intermediate to give the final products. The highest point on the energy profile is the transition state for the CC bond formation (118.8 kJ·mol−1 above the reactants in ethanol). Our results can give reasonable explanations on some experimental facts observed for many Petasis-type boronic Mannich reactions.

Density functional theory calculations have been carried out to investigate the addition mechanism for the reactions of “frustrated Lewis pairs” with olefins. Several reactions are studied in this work, which include a three-component reaction between a sterically demanding phosphane [P(tBu)3], borane [B(C6F5)3], and ethylene, and a two-component reaction between an olefin derivative of phosphane [CH2=CH–(CH2)3P(tBu)2] and B(C6F5)3. For the two-component reaction, we find a concerted addition mechanism, in which the formation of the B–C and P–C bonds takes place simultaneously. For the three-component reaction, our calculations show that the reaction may be initiated by the weak association of B(C6F5)3 with ethylene (to form a transient species) and then proceeds in a concerted transition state similar to that in the two-component reaction under study. The natural population analyses for the corresponding transition states indicate that the CH2=CH group (in the two-component reaction) and C2H4 (in the three-component reaction) seem to act as a bridge for electron transfer from the Lewis base center P to the Lewis acid center B. We also investigate the reaction between P(tBu)3, propylene, and B(C6F5)3. The results account well for the experimentally observed regioselectivity. In addition, our calculations also indicate that the presence of fluorine atoms in the borane is essential for stabilizing the addition product.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Oncogenic K-ras mutations are frequently observed in colon cancers and contribute to transformed growth. Oncogenic K-ras is detected in aberrant crypt foci (ACF), precancerous colonic lesions, demonstrating that acquisition of a K-ras mutation is an early event in colon carcinogenesis. Here, we investigate the role of oncogenic K-ras in neoplastic initiation and progression. Transgenic mice in which an oncogenic K-ras(G12D) allele is activated in the colonic epithelium by sporadic recombination (K-rasLA2 mice) develop spontaneous ACF that are morphologically indistinguishable from those induced by the colon carcinogen azoxymethane (AOM). Similar neoplastic changes involving the entire colon are induced in transgenic mice constitutively expressing K-ras(G12D) throughout the colon (LSL-K-ras(G12D)/Villin-Cre mice). However, the biochemistry and fate of K-ras-induced lesions differ depending upon their location within the colon in these mice. In the proximal colon, K-ras(G12D) induces increased expression of procarcinogenic protein kinase C beta II (PKC beta II), activation of the MEK/ERK signaling axis and increased epithelial cell proliferation. In contrast, in the distal colon, K-ras(G12D) inhibits expression of procarcinogenic PKC beta II and induces apoptosis. Treatment of K-rasLA2 mice with AOM leads to neoplastic progression of small ACF to large, dysplastic microadenomas in the proximal, but not the distal colon. Thus, oncogenic K-ras functions differently in the proximal and distal colon of mice, inducing ACF capable of neoplastic progression in the proximal colon, and ACF with little or no potential for progression in the distal colon. Our data indicate that acquisition of a K-ras mutation is an initiating neoplastic event in proximal colon cancer development in mice.

In C3 plants, carbon isotope discrimination (Δ) has been proposed as an indirect selection criterion for grain yield. Reported correlations between Δ and grain yield however, differ highly according to the analyzed organ or tissue, the stage of sampling, and the environment and water regime. In a first experiment carried out in spring wheat during two consecutive seasons in the dry conditions of northwest Mexico (Ciudad Obregon, Sonora), different water treatments were applied, corresponding to the main water regimes available to spring wheat worldwide, and the relationships between Δ values of different organs and grain yield were examined. Under terminal (post-anthesis) water stress, grain yield was positively associated with Δ in grain at maturity and in leaf at anthesis, confirming results previously obtained under Mediterranean environments. Under early (pre-anthesis) water stress and residual moisture stress, the association between grain Δ and yield was weaker and highly depended on the quantity of water stored in the soil at sowing. No correlation was found between Δ and grain yield under optimal irrigation. The relationship between Δ and grain yield was also studied during two consecutive seasons in 20 bread wheat cultivars in the Ningxia region (Northern China), characterized by winter drought (pre-anthesis water stress). Wheat was grown under rainfed conditions in two locations (Guyuan and Pengyang) and under irrigated conditions in another two (Yinchuan and Huinong). In Huinong, the crop was also exposed to salt stress. Highly significant positive associations were found between leaf and grain Δ and grain yields across the environments. The relationship between Δ and yield within environments highly depended on the quantity of water stored in the soil at sowing, the quantity and distribution of rainfall during the growth cycle, the presence of salt in the soil, and the occurrence of irrigation before anthesis. These two experiments confirmed the value of Δ as an indirect selection criterion for yield and a phenotyping tool under post-anthesis water stress (including limited irrigation).

The macroscopic viscoelastic properties of a physical hydrogel are reversibly modulated by tuning the microscopic hydrogen-bonding interactions with pH. The hydrogel forms at a rather low concentration of the multi-pyridyl-based gelator, N, N', N"-tris(3-pyridyl)trimesic amide. The yield stress of the hydrogel is greatly enhanced from 10 to 769 Pa by changing the pH from 7.0 to 5.0. At pH7.0, the amide molecules are assembled into an ordered structure as a result of the hydrogen bonds between the amide N-H bond and the nitrogen on the pyridyl group (N-H···Py). Fourier transform (FT) IR spectroscopy indicates that hydrogen bonds of N-H···Py are partially broken because the pyridyl groups are partly protonated at pH5.0. This condition leads to a highly branched and homogeneous fibrillar network, which is confirmed by X-ray diffraction (XRD) measurements and field-emission scanning electron microscopy (FESEM) images. Highly branched fibrillar networks create more compartments and greatly increase the interfacial tension that is required to hold the solvent in the gel, thereby increasing the yield stress to 769 Pa. By further increasing the acidity of the hydrogel to pH < 3.0, the gel becomes a sol. Both the change in the viscoelastic properties and the sol-gel transition are reversible and controllable in the material.

A multireference configurational interaction method based on the separated electron pair (SEP) wave functions, SEP-CI approach, has been developed as an approximation to the traditional CASSCF method. It differs from the CASSCF method in that active orbitals are obtained from the SEP wave function without further optimization in the subsequent CI calculations, and the active space is automatically constructed according to the occupation coefficients of SEP natural orbitals. These features make the present SEP-CI method computationally much less demanding than the CASSCF method. The applicability of the SEP-CI method is illustrated with sample calculations on the insertion reaction of BeH2 and dissociation energies of LiH, BH, FH, H2O, and N2. © 2005 Wiley Periodicals, Inc. J Comput Chem 27: 39–47, 2006

In the past decade, we devoted time to the valence bond (VB) study of huge conjugated hydrocarbons by employing the Heisenberg model. Due to the dimension of the covalent space exponentially increasing with the size of molecule, the exact solution of the VB model for large molecules encounters a fundamental difficulty. Before 1990, Alexander and Schmalz published ground-state energy results on as et of benzenoids of up to 24 -electrons using the graphical unitary group approach. Around 1995, we employed Lanczos technique to diagonalize the VB Hamiltonian and presented the low-lying state energies and wave functions of benzenoids with up to 22 -centers. In terms of wave functions, the physical and chemical behaviors of these conjugated species have been discussed in detail. In succession, we generalized the VB calculation to larger systemsbyproposinganalgorithmincodingallSlaterdeterminantsofthecovalentspacewithconservedspin quantum number. The exact solutions of the VB model have been obtained for conjugated species with up to 30 -electrons. Recently, we broke through this limitation by extending the density-matrix renormalization group (DMRG) method of White to the quasi-one-dimensional system. The VB energies of the ground states for two series of polycyclic aromatic hydrocarbons, polyacene and polyphenanthrene, of arbitrary lengths defined by the number of constituent hexagons have been obtained. The effective conjugated length for these infinite long-chain polymers could be estimated.

Poly(ethylene glycol)-block-poly(butyl acrylate) synthesized by radical polymerization in a one-step procedure were characterized by gel permeation chromatography, infrared, IH-NMR spectroscopy, and differential scanning calorimetry (DSC). The crystalline property, emulsifying property, and phase transfer catalytic effect in the Williamson reaction were studied. It was found that the crystallinity of the copolymer increased with an increase in both the content and molecular weight of poly( ethylene oxide) (PEO) sequences. DSC curves showed two distinct crystallization temperature due to the heterogeneous nucleation and homogeneous nucleation crystallization. The casting solvent significantly affected the morphology and crystallinity of the solvent cast films. Both the emulsifying volume and the phase transfer catalytic efficiency in the Williamson reaction increased with the amount and PEO content of the block copolymers used, but decreased with an increase in the molecular weight of PEO sequences. (C) 1998 John Wiley & Sons, Inc.

Prepolymers of poly(ethylene oxide) (Pre-PEG) were synthesized by reacting azoisobutyronitrile (AIBN) with poly(ethylene glycol) (PEG), and their structures were characterized by IR and UV. The molecular weight of pre-PEG was related to the feed ratio and reaction time. These prepolymers can be used to prepare block copolymers - poly(ethylene oxide)-block-poly(butyl acrylate) (PEO-b-PBA) by radical polymerization in the presence of butyl acrylate (BA). Solution polymerization was a suitable technique for this step. The yield and the molecular weight of the product were related to the ratio of the prepolymer to BA, the reaction time, and temperature. GPC showed that the molecular weight increased with a higher ratio of BA to pre-PEO. The intrinsic viscosity of the copolymers was only slightly dependent on reaction time, but decreased at higher reaction temperatures, as did the amount of PEA homopolymer. (C) 1997 John Wiley & Sons, Inc.

The thermal properties and crystalline structure of the amphiphilic graft copolymers CR-g-PEG600, CR-g-PEG2000, and CR-g-PEG6000 using chloroprene rubber (CR) as the hydrophobic backbone and poly(ethylene glycol) (PEG) with different molecular weights as the hydrophilic side chains were studied by DSC and WAXD. The results showed that a distinct phase-separated structure existed in CR-g-PEGs because of the incompatibility between the backbone segments and the side-chain segments. For all the polymers studied, T-m2, which is the melting point of PEG crystalline domains in CR-g-PEG, decreased compared to that of the corresponding pure PEG and varied little with PEG content. For CR-g-PEG600 and CR-g-PEG2000, T-m1, which is the melting point of the CR crystalline domains, increased with increasing PEG content when the PEG content was not high enough, and at constant PEG content, the longer were the PEG side chains the higher was the T-m1. The crystallite size L-011 of CR in CR-g-PEGs increased compared to that of the pure CR and decreased with increasing PEG content. (C) 1997 John Wiley & Sons, Inc.

A new amphiphilic polymer i.e., polyethylene glycol (PEG) grafted crystalline neoprene, which was used as compatibilizer to improve the compatibility of elastomer and water-absorbent resin, has been investigated. The synthesis was based on the reaction between chlorine in neoprene and sodium salts of PEG. PEGs with molecular weights of 600 and 2000 were used. The grafting percent and the PEG content were calculated through elemental analysis of chlorine in the resulted copolymers. The maximum grafting percent of copolymers was ca. 24.80%. The molecular parameters such as number-average molecular weight and the average number of grafting chains on one CR backbone were also calculated and discussed. (C) 1996 John Wiley & Sons, Inc.

Low-density polyethylene/ethylene-propylene-diene terpolymer/carbon black blends were irradiated in air with a 60 Co γ ray or an electron beam accelerator. The absorbed dose ranged from 1-400 Mrad. The influences of irradiation on positive temperature coefficient behavior of the blends were analyzed.

In this paper, the results of a systematic study of carbon black (CB)-filled conducting polymer positive and negative temperature coefficient (PTC/NTC) effects are report. The conductivity of the composites jumps by several orders of magnitude at the critical value of carbon black. This critical value, oc, decreases with the increase of melt index and degree of crystallinity of the polymer. The crystalline lamellae just “modify” the distribution of carbon black and make the dispersion heterogeneous. Radiation cross-linking enhances the PTC intensity and decreases the NTC effect of the materials. The electrical reproducibility of compounds is improved by the cross-linking structure that reduces the freedom of carbon black movement at high temperature. The NTC phenomenon is related to the carbon black coagulation that facilitates electrical conduction at high temperature. The larger the melt index, the more easily carbon black coagulates. A new model was set up to explain the results successfully. © 1993 John Wiley & Sons, Inc.

By employing the Lanczos method, the effective valence bond model (EVB) is exactly solved for a series of conjugated hydrocarbons with four-membered rings. Several indices calculated from the EVB ground-state energies and wave functions are applied to interrelate with bond lengths, stabilities, and reactivities of these systems. Our results agree well with related experimental observations and deductions of molecular orbital theory. The failure of the classical valence bond theory in these systems is also exposed.