排序方式: 共有37条查询结果,搜索用时 15 毫秒
1.
Abstract From 3-hydroxymethyl-3-methyl oxetane, we synthesized three oxetane monomers with allylic, diethoxysilane or triethoxysilane functions. These monomers allowed us to prepare different polydimethylsiloxanes bearing oxetane groups by two routes. One is hydrosilylation of allylic monomer on hydrogeno polydimethylsiloxane, with different percentages of Si-H bonds. The second route is a condensation of α,ω-dihydroxy polydimethylsiloxane with di and tri ethoxysilane monomers. By using a photocalorimeter, we studied the kinetic of cationic photopolymerization of a polydimethylsiloxane bearing 7.4% oxetane functions. Influence of temperature in the range 35 to 125°C showed a maximum conversion for 80°C due to an increase in transfer and termination reactions. 相似文献
2.
Polymerization of styrene initiated by triflic acid in CH2Cl2 solution was reexamined, using a new stopped-flow device working in high purity conditions over a wide temperature range. Monomer and styryl cation were followed simultaneously through their respective absorbances at 290 and 340 nm. Initiation is very rapid, and cations concentration reaches a plateau the duration of which is depending on temperature. In our conditions (I0 = 0.5 − 9.10−3M, M0/I0 = 1 to 20), cations concentration is so low at room temperature that it is almost unmeasurable. At −65°C, it is 100 times higher, remains constant for several seconds and complete termination takes place within a minute or more. Such a profile of cation evolution agrees with an equilibrium situation between initiation and a much more temperature-dependent backward deprotonation. Apparent initial rate of initiation is first order with respect to monomer, but the order with respect to initiator was found very high and variable with temperature (from 4.5 at −65°C to 3 at −20°C). This supports the presence, even if they are in low concentration, of acid high agregates, the reactivity of which increases with size. A first order monomer consumption is observed during the plateau, which leads to kp values ranging from 103 at −65°C to 9.104 M−1.s−1 at −10°C (Ep# = 43 kJ.mol−1). The disappearance of cations, which follows the plateau, slows down and becomes unimolecular when monomer consumption is complete, and kt values range from 6.10−2s−1 at −65°C to 1.2s−1 at −23°C (Et# = 33 kJ.mol−1). 相似文献
3.
Won Jei Cho Claude Bunel Ernest Marchal 《Journal of polymer science. Part A, Polymer chemistry》1980,18(6):1995-2000
Improved methods for the synthesis of 4-halogeno 1-vinylnaphthalenes are proposed. Reactivity ratios are determined for the following systems: 4-fluoro-1-vinyl naphthalene-styrene, 4-chloro-1-vinyl naphthalene-styrene, and 4-bromo-1-vinyl naphthalene-styrene. The determinations are performed at various specific temperatures and the relative activation parameters are obtained. It appears that the reactivity of 1-vinyl naphthalene is drastically decreased when substituted by a halogen. In the case of 1-vinyl naphthalene the selection is enthalpically controlled. 相似文献
4.
A photopolymerization reaction involving 2-hydroxyethylmethacrylate (HEMA), a hydroxytelechelic polybutadiene (HTPB) and 2,2-dimethyl-2-hydroxyacetophenone (Darocur 1173) as photoinitiator has been investigated by Real Time UV and Real Time IR spectroscopies. For this system, RTUV analyses are completely opposite to the expected curves, namely a decrease in the sample absorbance (photobleaching phenomenon). This apparent contradiction has been explained by a scattering phenomenon inside the sample, which can be linked to the formation of sub-structures that can create local fluctuations of refractive index during the polymerization. A four-flux theory, which takes into account the polymer absorption and scattering properties, has allowed us to describe the UV light intensity variation inside the sample. 相似文献
5.
6.
The preparation of aminocomplexes of Co(III) containing d-ribose, l-sorbose or d-glucosamine as second ligand is described. Their spectral features and optical activity are reported. Formation in solution of complex species with other chosen carbohydrates gives ground for discussion as to the origin and sign of the observed Cotton effects. 相似文献
7.
Laurence Lecamp Philippe Lebaudy Boulos Youssef Claude Bunel 《Macromolecular Symposia》1999,148(1):77-86
Photoinitiated polymerization of multifunctional monomers is an usual method to prepare highly crosslinked networks which have a wide variety of applications. This method leads to high reaction rates and the resulting exothermic effect of this reaction can be the cause of defects in the final material. The heterogeneities alter greatly the physical properties of ultimate products, particularly the optical ones, what causes problems in the design of thick and optically perfect materials. The knowledge of the conversion profile and the temperature distribution within the material during the photopolymerization is useful for the process optimization. Unfortunately, these parameters cannot be measured during the process. Thus, we decided to simulate them. Firstly, the necessary parameters (like conversion and reaction rate) were measured on thin material in isothermal conditions by photocalorimetry. Secondly, these kinetic data were used in a computational calculation to obtain the conversion profile and the temperature distribution within dimethacrylate thick material. The calculated temperature and conversion-time curves are in good agreement with the experimental curves determined under the same conditions. 相似文献
8.
Photocalorimetry (DSC) and real time infrared (RTIR) spectroscopy are the two usual methods used to follow photopolymerization reactions. Kinetics obtained by DSC on thin samples and kinetics obtained by real time near-infrared (RT-NIR) spectroscopy on thick samples are not the same. The heat release during the photopolymerization induced a high temperature increase in thick samples because of the absence of temperature control. We think that this temperature offset is the main cause of this kinetic difference. In this paper, we want to verify this assumption. To reach this aim, conversion and temperature evolutions versus time were numerically simulated for a sample placed in the thermal conditions of NIR analysis by using experimental kinetic data obtained by photocalorimetry in isothermal mode. The boundary conditions were determined such as the simulated evolution of the sample temperature is the same as the experimental one. At last, conversion curves simulated to obtain these temperature profiles were compared to experimental results obtained by RT-NIR. 相似文献
9.
Evidence of CuI/CuII Redox Process by X‐ray Absorption and EPR Spectroscopy: Direct Synthesis of Dihydrofurans from β‐Ketocarbonyl Derivatives and Olefins 下载免费PDF全文
Hong Yi Zhixiong Liao Guanghui Zhang Guoting Zhang Chao Fan Xu Zhang Prof. Dr. Emilio E. Bunel Dr. Chih‐Wen Pao Prof. Dr. Jyh‐Fu Lee Prof. Dr. Aiwen Lei 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(52):18925-18929
The CuI/CuII and CuI/CuIII catalytic cycles have been subject to intense debate in the field of copper‐catalyzed oxidative coupling reactions. A mechanistic study on the CuI/CuII redox process, by X‐ray absorption (XAS) and electron paramagnetic resonance (EPR) spectroscopies, has elucidated the reduction mechanism of CuII to CuI by 1,3‐diketone and detailed investigation revealed that the halide ion is important for the reduction process. The oxidative nature of the thereby‐formed CuI has also been studied by XAS and EPR spectroscopy. This mechanistic information is applicable to the copper‐catalyzed oxidative cyclization of β‐ketocarbonyl derivatives to dihydrofurans. This protocol provides an ideal route to highly substituted dihydrofuran rings from easily available 1,3‐dicarbonyls and olefins. 相似文献
10.
Discovery of Highly Selective Alkyne Semihydrogenation Catalysts Based on First‐Row Transition‐Metallated Porous Organic Polymers 下载免费PDF全文
Dr. Kristine K. Tanabe Dr. Magali S. Ferrandon Dr. Nathan A. Siladke Dr. Steven J. Kraft Guanghui Zhang Dr. Jens Niklas Dr. Oleg G. Poluektov Dr. Susan J. Lopykinski Dr. Emilio E. Bunel Dr. Theodore R. Krause Dr. Jeffrey T. Miller Dr. Adam S. Hock Dr. SonBinh T. Nguyen 《Angewandte Chemie (International ed. in English)》2014,53(45):12055-12058
Five different first‐row transition metal precursors (VIII, CrIII, MnII, CoII, NiII) were successfully incorporated into a catechol porous organic polymer (POP) and characterized using ATR‐IR and XAS analysis. The resulting metallated POPs were then evaluated for catalytic alkyne hydrogenation using high‐throughput screening techniques. All POPs were unexpectedly found to be active and selective catalysts for alkyne semihydrogenation. Three of the metallated POPs (V, Cr, Mn) are the first of their kind to be active single‐site hydrogenation catalysts. These results highlight the advantages of using a POP platform to develop new catalysts which are otherwise difficult to achieve through traditional heterogeneous and homogeneous routes. 相似文献