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排序方式: 共有138条查询结果,搜索用时 15 毫秒
1.
Saito K Masuyama T Oyaizu K Nishide H 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(17):4240-4246
Depolymerization of an engineering plastic, poly(2,6-dimethyl-1,4-phenylene oxide) (PPO), was accomplished by using 2,6-dimethylphenol (DMP) under oxidative conditions. The addition of an excess amount of DMP to a solution of PPO in the presence of a CuCl/pyridine catalyst yielded oligomeric products. When PPO (M(n)=1.0x10(4), M(w)/M(n)=1.2) was allowed to react with a sufficient amount of DMP, the molecular weight of the product decreased to M(n)=4.9x10(2) (M(w)/M(n)=1.5). By a prolonged reaction with the oxidant, the oligomeric product was repolymerized to produce PPO essentially identical to the starting material, making the oligomer useful as a reusable resource. During the depolymerization reaction, an intermediate phenoxyl radical was observed by ESR spectroscopy. Kinetic analysis showed that the rate of the oxidation of PPO was about 10 times higher than that of DMP. These results show that a monomeric phenoxyl radical attacks the polymeric phenoxyl to induce the redistribution via a quinone ketal intermediate, leading to the substantial decrease in the molecular weight of PPO, which is much faster than the chain growth. 相似文献
2.
《Journal of polymer science. Part A, Polymer chemistry》2018,56(16):1868-1877
Self‐immolative polymers (SIPs) undergo depolymerization in response to the cleavage of stimuli‐responsive end‐caps from their termini. Some classes of SIPs, including polycarbamates, have depolymerization rates that depend on environmental factors such as solvent and pH. In previous work, hydrophobic SIPs have been incorporated into amphiphilic block copolymers and used to prepare nanoassemblies. However, stimuli‐responsive hydrophilic blocks have not previously been incorporated. In this work, we synthesized amphiphilic copolymers composed of a hydrophobic polycarbamate SIP block and a hydrophilic poly(2‐(dimethylamino)ethyl methacrylate) (PDMAEMA) block connected by a UV light‐responsive linker end‐cap. It was hypothesized that after assembly of the block copolymers into nanoparticles, chain collapse of the PDMAEMA above its lower critical solution temperature (LCST) might change the environment of the SIP block, thereby altering its depolymerization rate. Self‐assembly of the block copolymers was performed, and the depolymerization of the resulting assemblies was studied by fluorescence spectroscopy, dynamic light scattering, and NMR spectroscopy. At 20 °C, the system exhibited a selective response to the UV light. At 65 °C, above the LCST of PDMAEMA, the systems underwent more rapid depolymerization, suggesting that the increase in rate arising from the higher temperature dominated over environmental effects arising from chain collapse. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1868–1877 相似文献
3.
Dr. Stephan Enthaler 《Angewandte Chemie (International ed. in English)》2014,53(10):2716-2721
Polymers occupy an important role in our current society. Besides their great success, an issue is the accumulation of huge amounts of end‐of‐life polymers. Currently, the waste management is based primarily on landfills, thermal recycling, and downcycling. Notably, only a small portion of end‐of‐life materials is recycled by depolymerization, which refers to the creation of synthetic precursors that can be polymerized to new polymers to close the cycle. Widely used polymers in modern times are silicones (polysiloxanes), the intrinsic properties of which make their depolymerization demanding; only a few high‐temperature or less environmentally friendly processes have been reported. In this regard, we have established an efficient low‐temperature protocol for the depolymerization of silicones with benzoyl fluoride in the presence of cheap zinc salts as precatalysts to yield defined products. Notably, the products can be useful synthetic precursors for the preparation of new polymers, so that an overall recycling process is feasible. 相似文献
4.
Catalytic Ethanolysis of Kraft Lignin into High‐Value Small‐Molecular Chemicals over a Nanostructured α‐Molybdenum Carbide Catalyst 下载免费PDF全文
Rui Ma Wenyue Hao Xiaolei Ma Ye Tian Prof. Yongdan Li 《Angewandte Chemie (International ed. in English)》2014,53(28):7310-7315
We report the complete ethanolysis of Kraft lignin over an α‐MoC1?x/AC catalyst in pure ethanol at 280 °C to give high‐value chemicals of low molecular weight with a maximum overall yield of the 25 most abundant liquid products (LP25) of 1.64 g per gram of lignin. The LP25 products consisted of C6–C10 esters, alcohols, arenes, phenols, and benzyl alcohols with an overall heating value of 36.5 MJ kg?1. C6 alcohols and C8 esters predominated and accounted for 82 wt % of the LP25 products. No oligomers or char were formed in the process. With our catalyst, ethanol is the only effective solvent for the reaction. Supercritical ethanol on its own degrades Kraft lignin into a mixture of small molecules and molecular fragments of intermediate size with molecular weights in the range 700–1400, differing in steps of 58 units, which is the weight of the branched‐chain linkage C3H6O in lignin. Hydrogen was found to have a negative effect on the formation of the low‐molecular‐weight products. 相似文献
5.
6.
Oskar Michalski Ewa Bidzińska Michał Borowski Krystyna Dyrek Paweł Olko Liliana Stolarczyk Jan Swakoń Piotr Tomasik Elżbieta Wenda 《科学通报(英文版)》2010,55(31):3556-3561
Cornstarch heated in the range of 230–280°C depolymerized into pyrodextrins characterized by two-component EPR signals of relatively stable free radicals. These thermally generated radicals could serve as efficient scavengers for free radicals generated from pyrodextrins with the 200 Gy dose of γ-radiation. The most efficient traps/scavengers were produced from cornstarch at 250–280°C. IR data indicated incorporation of the OH groups to the pyrodextrins. These groups most probably originated from the OH· radicals formed by the radiolysis of water. EPR spectra provided evidence for trapping free radicals generated by γ-irradiation and for their subsequent annihilation on contact with pyrodextrins. Water affected radical processes occurring in pyrodextrins caused by γ-irradiation. 相似文献
7.
Kazuki Fukushima Olivier Coulembier Julien M. Lecuyer Hamid A. Almegren Abdullah M. Alabdulrahman Fares D. Alsewailem Melanie A. Mcneil Philippe Dubois Robert M. Waymouth Hans W. Horn Julia E. Rice James L. Hedrick 《Journal of polymer science. Part A, Polymer chemistry》2011,49(5):1273-1281
We describe the organocatalytic depolymerization of poly(ethylene terephthalate) (PET), using a commercially available guanidine catalyst, 1,5,7‐triazabicyclo[4.4.0]dec‐5‐ene (TBD). Postconsumer PET beverage bottles were used and processed with 1.0 mol % (0.7 wt %) of TBD and excess amount of ethylene glycol (EG) at 190 °C for 3.5 hours under atmospheric pressure to give bis(2‐hydroxyethyl) terephthalate (BHET) in 78% isolated yield. The catalyst efficiency was comparable to other metal acetate/alkoxide catalysts that are commonly used for depolymerization of PET. The BHET content in the glycolysis product was subject to the reagent loading. This catalyst influenced the rate of the depolymerization as well as the effective process temperature. We also demonstrated the recycling of the catalyst and the excess EG for more than 5 cycles. Computational and experimental studies showed that both TBD and EG activate PET through hydrogen bond formation/activation to facilitate this reaction. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011 相似文献
8.
以麦秸为研究对象,解聚剂为HCl、HNO3和H3PO4,对解聚产物进行定性和定量分析,并利用动力学模型描述木糖及糠醛的产生过程。 结果表明,解聚液中的产物有葡萄糖、木糖、阿拉伯糖、纤维二糖、乙酸、糠醛、5-羟甲基糠醛。 通过引入变量(α,木糖/木聚糖的比值)利用Saeman动力学模型获得了不同温度下,木聚糖的水解速率常数、木糖的转化速率常数以及糠醛的生成速率常数。 HCl、HNO3和H3PO4解聚麦秸,木糖的生成活化能分别为55.5、46.3和59.8 kJ/mol。 结合反应温度、反应时间、反应速率以及木糖和糠醛的浓度,确定最佳解聚条件为:硝酸作解聚剂,在130 ℃下水解95 min。 相似文献
9.
Takeo Sasaki Takumi Yoneyama Shota Hashimoto Sumie Takemura Yumiko Naka 《Journal of polymer science. Part A, Polymer chemistry》2013,51(18):3873-3880
Photoinduced depolymerization of poly(olefin sulfone)s possessing photobase generators in the side‐chain was investigated. Irradiation with UV light generated base on the side‐chains and induced depolymerization based on proton abstraction on the main‐chain. The effect of the length of the spacer chain, which connects the photobase‐generating moiety to the polymer main chain on the photoinduced depolymerization, also was investigated. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 3873–3880 相似文献
10.
J. P. Geiger B. Huguenin M. Nicole D. Nandris 《Applied biochemistry and biotechnology》1986,13(2):97-110
A laccase that has been isolated previously (1) from the Basidiomycete,Rigidoporus lignosus, a white rot fungus of rubber tree, was used in the present study. When a thioglycolic lignin (TGL) was incubated in the
presence of this enzyme, pronounced changes in the UV spectrum and size distribution of the substrate were observed. Sephadex
gel filtration indicated that two types of reactions occurred: (1) A degradation of the polymer, as evidenced by the production
of low-molecular-weight material; and (2) a condensation of some of the TGL molecules, as revealed by an increase in the fractions
of higher molecular weight. 相似文献