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排序方式: 共有815条查询结果,搜索用时 15 毫秒
1.
Vishal Sipani Ann Kirsch Alec B. Scranton 《Journal of polymer science. Part A, Polymer chemistry》2004,42(17):4409-4416
The effective propagation rate constant (kp; averaged over all the propagating active centers) was characterized for solvent‐free cationic photopolymerizations of phenyl glycidyl ether over the entire range of conversions, including the high conversion regime in which mass transfer limitations become important. The profile for the kp as a function of conversion was found to exhibit a constant plateau value at low to intermediate conversions, followed by a monotonic increase above a threshold value of conversion. To explain this trend, it is proposed that at high conversion the diffusional mobility of the photoinitiator counterion is reduced whereas the mobility of the cationic active center remains high because of reactive diffusion. Therefore, with increasing conversion, the average distance between the active centers and counterions may increase, resulting in an increase in the propagation rate constant. The profiles for the kp values were investigated as a function of the temperature, photoinitiator anion, and photoinitiator concentration. As the photoinitiator concentration was increased, the plateau value of the effective propagation rate constant decreased whereas the threshold conversion increased. All of the experimental trends are consistent with the proposed increase in ion separation at high conversions. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4409–4416, 2004 相似文献
2.
Vishal A. Mahajan 《Tetrahedron》2006,62(6):1258-1272
A short synthetic strategy for synthesis of lignan analogues involving 5-methylene-4-substituted-2(5H)-furanones as the key intermediates has been developed. Various lignans including the natural product solafuranone and analogues of dihydrotaiwanin C and deoxydehydropodophyllotoxin were synthesized in good yields. 相似文献
3.
Xiao Huang Bishwa Ranjan Nayak Tao Lu Lowe 《Journal of polymer science. Part A, Polymer chemistry》2004,42(20):5054-5066
A series of novel multifunctional hydrogels that combined the merits of both thermoresponsive and biodegradable polymeric materials were designed, synthesized, and characterized. The hydrogels were copolymeric networks composed of N‐isopropylacrylamide (NIPAAM) as a thermoresponsive component, poly(L‐lactic acid) (PLLA) as a hydrolytically degradable and hydrophobic component, and dextran as an enzymatically degradable and hydrophilic component. The chemical structures of the hydrogels were characterized by an attenuated total reflection–Fourier transform infrared spectroscopy (ATR–FTIR) technique. The hydrogels were thermoresponsive, showing a lower critical solution temperature (LCST) at approximately 32 °C, and their swelling properties strongly depended on temperature changes, the balance of the hydrophilic/hydrophobic components, and the degradation of the PLLA component. The degradation of the hydrogels caused by hydrolytic cleavage of ester bonds in the PLLA component was faster at 25 °C below the LCST than at 37 °C above the LCST, determined by the ATR–FTIR technique. Due to their multifunctional properties, the designed hydrogels show great potential for biomedical applications, including drug delivery and tissue engineering. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5054–5066, 2004 相似文献
4.
Some bounds on the entries and on the norm of the inverse of triangular matrices with nonnegative and monotone entries are found. All the results are obtained by exploiting the properties of the fundamental matrix of the recurrence relation which generates the sequence of the entries of the inverse matrix. One of the results generalizes a theorem contained in a recent article of one of the authors about Toeplitz matrices. 相似文献
5.
Paital AR Mitra T Ray D Wong WT Ribas-Ariño J Novoa JJ Ribas J Aromí G 《Chemical communications (Cambridge, England)》2005,(41):5172-5174
Based on a combined theoretical-experimental study, we propose that substituted m-phenylene ligands (m-N-Phi-N) can act as tuneable strong ferromagnetic couplers connecting Cu(II) ions; a new complex presenting that bridge with J close to +15 cm(-1) has been suggested and synthesized. 相似文献
6.
Zhou S Xu C Wang J Gao W Akhverdiyeva R Shah V Gross R 《Langmuir : the ACS journal of surfaces and colloids》2004,20(19):7926-7932
Acidic sophorolipid (SL) molecules derived from yeasts represent a novel type of asymmetrical bolaamphiphiles due to their unique structural features that include an asymmetrical polar head size (disaccharide vs COOH), a kinked hydrophobic core (cis-9-octadecenoic chain), and a non-amide polar-nonpolar linkage. Light microscopy, small- and wide-angle X-ray scattering, FT-IR spectroscopy, and dynamic laser light scattering were used to investigate the supramolecular structures of the self-assembled aggregates of SL molecules at different pH values. In acidic conditions (pH < 5.5), giant twisted and helical ribbons of 5-11 microm width and several hundreds of micrometers length were observed for the first time. Increase in solution pH values slowed ribbon formation, decreased ribbon yield, and increased the helicity and entanglements of the giant ribbons. An interdigitated lamellar packing model of acidic SL-COOH molecules with a long period of 2.78 nm, stabilized by both the strong hydrophobic association between the cis-9-octadecenoic chains and strong disaccharide-disaccharide hydrogen bonding, is proposed. The neutralization of SL-COOH in water to SL-COONa produced clear solutions with the formation of short-range ordered aggregates. At concentrations below 1.0 mg/mL, the size of self-assembled aggregates increased as the concentration increased. At concentrations above 1.0 mg/mL, narrowly distributed micellar aggregates with a constant hydrodynamic radius (R(h)) of about 100 nm are formed. The large micelles show strong angular dependence with the fast mode appearing at scattering angle theta >/= 60 degrees. 相似文献
7.
A visible light promoted, photoredox catalysed, green one-pot approach for the alkylation of amine substrates with sp2 carbon has been developed. This eosin Y based organic transformations, can behave as an effective direct hydrogen-atom transfer catalyst for coupling reaction. The proposed strategy includes simple procedure which can make adduct product with sp2 carbon. This eosin Y based photocatalytic hydrogen-atom transfer strategy may hold great potential for diverse functionalization of a wide range of native CH bonds in an economical and sustainable manner. 相似文献
8.
Proline-catalyzed enantioselective direct intermolecular aldol reactions of tetrahydro-4H-thiopyran-4-one with various aldehydes give anti adducts with high diastereo- and enantioselectivities in moderate to excellent yields. With the aromatic aldehydes best results were obtained in wet DMF whereas dry DMSO generally was superior with the aliphatic aldehydes. Desulfurization of the adducts with Raney Ni provides products equivalent to aldols from 3-pentanone with potential applications in polypropionate synthesis. 相似文献
9.
Paul N Nashine VC Hoops G Zhang P Zhou J Bergstrom DE Davisson VJ 《Chemistry & biology》2003,10(9):815-825
The development of novel artificial nucleobases and detailed X-ray crystal structures for primer/template/DNA polymerase complexes provide opportunities to assess DNA-protein interactions that dictate specificity. Recent results have shown that base pair shape recognition in the context of DNA polymerase must be considered a significant component. The isosteric azole carboxamide nucleobases (compounds 1-5; ) differ only in the number and placement of nitrogen atoms within a common shape and therefore present unique electronic distributions that are shown to dictate the selectivity of template-directed nucleotide incorporation by DNA polymerases. The results demonstrate how nucleoside triphosphate substrate selection by DNA polymerase is a complex phenomenon involving electrostatic interactions in addition to hydrogen bonding and shape recognition. These azole nucleobase analogs offer unique molecular tools for probing nonbonded interactions dictating substrate selection and fidelity of DNA polymerases. 相似文献
10.
It was predicted that the folding space for various protein sequences is restricted and a maximum of 1000 protein folds could be expected. Although, there were about 648 folds identified, general functional features of individual folds is not thoroughly studied. We selected OB-fold, which is supposed to be an oligonucleotide and oligosaccharide binding fold to study the general functional features. OB-fold is a small beta-barrel fold formed from 5 strands connected by modulating loops. We observed consistently 2 or 3 loops on the same face of barrel acting as clamps to bind to their ligands. Depending on the ligand, which could be a single or double stranded DNA/RNA or an oligosaccharide, and their conformational properties the loops change in length and sequence to accommodate various ligands. Different classes of OB-folded proteins were analyzed and found that the functional features are retained in spite of negligible sequence homology among various proteins studied. 相似文献