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891.
Michael J. Monteiro 《Journal of polymer science. Part A, Polymer chemistry》2005,43(15):3189-3204
Living radical polymerization has allowed complex polymer architectures to be synthesized in bulk, solution, and water. The most versatile of these techniques is reversible addition–fragmentation chain transfer (RAFT), which allows a wide range of functional and nonfunctional polymers to be made with predictable molecular weight distributions (MWDs), ranging from very narrow to quite broad. The great complexity of the RAFT mechanism and how the kinetic parameters affect the rate of polymerization and MWD are not obvious. Therefore, the aim of this article is to provide useful insights into the important kinetic parameters that control the rate of polymerization and the evolution of the MWD with conversion. We discuss how a change in the chain‐transfer constant can affect the evolution of the MWD. It is shown how we can, in principle, use only one RAFT agent to obtain a polymer with any MWD. Retardation and inhibition are discussed in terms of (1) the leaving R group reactivity and (2) the intermediate radical termination model versus the slow fragmentation model. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3189–3204, 2005 相似文献
892.
893.
The stability constants and the partial molal volume and isentropic partial molal compressibility changes of complex formation between cations and crown ethers in water at 25°C are presented. The cations involved are Na+, K+, Rb+, Cs+, Ca2+, and Ba2+, and the crown ethers are 12-crown-4, 15-crown-5, and 18-crown-6. Values of V of complex formation have been discussed in terms of two simple models, one based on the scaled particle theory, and the others on the Drude-Nernst continuum model. The results indicate that the charge of the potassium cation in 18-crown-6 is especially well screened from the water. On this basis hydration numbers of complexed cations have been calculated. This shows that the size of the cation compared to the crown ether hole is important for the contacts between complexed cations and water. 相似文献
894.
环多肽晶体的浮动电荷极化力场模拟 总被引:2,自引:0,他引:2
利用原子键电负性均衡结合分子力场方法(ABEEM/MM)对五种环多肽晶体进行了研究. 与传统力场相比, 该方法中的静电势包含了分子内和分子间的静电极化作用, 以及分子内电荷转移影响, 同时加入了化学键等非原子中心电荷位点, 合理地体现了分子中的电荷分布. 相对其他极化力场模型, 具有计算量较小的特点. 该模型下计算得到的环多肽分子单元相对实验测得的结构的原子位置、氢键长度和二面角的均方根偏差分别为0.009 nm、0.013 nm和5.16°, 能够很好地重复实验结果. 总体上, 其结果优于或相当于其他力场模型, 适用于对实际蛋白质体系的模拟和研究. 相似文献
895.
Fabienne Poncin-Epaillard Barbara Pomepui Jean-Claude Brosse 《Journal of polymer science. Part A, Polymer chemistry》1993,31(11):2671-2680
Polyolefins and their model molecules, some n-alcanes, have been modified by a tetrafluoromethane microwave plasma. The chosen molecules are high-density polyethylene (HDPE) and hexatriacontane (HTC), low-density polyethylene (LDPE) and paraffin, and polycaprolactone (PCL) and octadecyl octadecanoate (ODO). It has been found, except for paraffin, that the model surfaces have the same behavior as the corresponding polymers. Plasma modification is described as the sum of two mechanisms: degradation and fluorination. These reactions seem to be competitive and parallel. Degradation and fluorination rates are dependent on treatment time and are practically independent on substrate position. A domain of fluorination exists near the edge of plasma, whatever the substrate in or outside plasma. © 1993 John Wiley & Sons, Inc. 相似文献
896.
用X_α方法对CH_4在Ni(Ⅲ)表面吸附解离反应的理论研究马晨生,马理,杨忠志(吉林大学理论化学研究所,长春,130023)关键词CH_4/Ni(Ⅲ)吸附体系,原子簇模型,CH_4/Mi_7,MS-X_α-SCF方法甲烷在Ni表面的催化重整反应在工业和国... 相似文献
897.
The gas-chromatographic method is used to study the interaction of water and methanol molecules with active hydrophilic centres
existing at the surface of thermally exfoliated graphite and graphitized thermal carbon black. The concentration of carboxyl
and phenol hydroxyl groups at the surface of these sorbents is determined, and heats of adsorption of the studied molecules
are shown to be
and 28−25 kJ/mol, respectively. It is also shown that adsorption of water at the hydrophilic centres at lowest relative pressure
values takes place with formation of clusters consisting of n = 2 water molecules. 相似文献
898.
Luc M. H. Koymans Nico P. E. Vermeulen Allard Baarslag Gabriëlle M. Donné-Op den Kelder 《Journal of computer-aided molecular design》1993,7(3):281-289
Summary A homology model building study of cytochrome P450 2D6 has been carried out based on the crystal structure of cytochrome P450 101. The primary sequences of P450 101 and P450 2D6 were aligned by making use of an automated alignment procedure. This alignment was adjusted manually by matching -helices (C, D, G, I, J, K and L) and -sheets (3/4) of P450 101 that are proposed to be conserved in membrane-bound P450s (Ouzounis and Melvin [Eur. J. Biochem., 198 (1991) 307]) to the corresponding regions in the primary amino acid sequence of P450 2D6. Furthermore, -helices B, B and F were found to be conserved in P450 2D6. No significant homology between the remaining regions of P450 101 and P450 2D6 could be found and these regions were therefore deleted. A 3D model of P450 2D6 was constructed by copying the coordinates of the residues from the crystal structure of P450 101 to the corresponding residues in P450 2D6. The regions without a significant homology with P450 101 were not incorporated into the model. After energy-minimization of the resulting 3D model of P450 2D6, possible active site residues were identified by fitting the substrates debrisoquine and dextrometorphan into the proposed active site. Both substrates could be positioned into a planar pocket near the heme region formed by residues Val370, Pro371, Leu372, Trp316, and part of the oxygen binding site of P450 2D6. Furthermore, the carboxylate group of either Asp100 or Asp301 was identified as a possible candidate for the proposed interaction with basic nitrogen atom(s) of the substrates. These findings are in accordance with a recently published predictive model for substrates of P450 2D6 [Koymans et al., Chem. Res. Toxicol., 5 (1992) 211]. 相似文献
899.
John L. Spouge 《Journal of statistical physics》1983,31(2):363-378
This paper gives the equilibrium distribution of polymer sizes for Flory'sA
g
RB
f–g model of polymerization. In this model, the polymers are composed of structural units withg functional groups of the typeA and (f-g) functional groups of the typeB. Reaction is subject to three conditions: (1) Functional groups of the typeA react only with those of typeB, and vice versa. (2) Intramolecular reactions do not occur [and therefore only branched-chain (noncyclic) polymers and formed]. (3) Subject to conditions (1) and (2), all functional groups are equally reactive. The derivation employs Stockmayer's statistical mechanical method (first used on Flory'sRA
f model), coupled with a recursion giving the number of distinct polymers which may be assembled fromk units of theA
g
RB
f–g type. We also give distributions for a limiting case of theA
g
RB
f–g model, the so-calledA
g
RB
model. This paper completes the solution of the Smoluchowski coagulation equation (monodisperse case) for the kernelsa
ij =A + B(i +j)+ Cij. The proof will be given in another publication. 相似文献
900.
Jin Chen Lin-xi Zhang A-gen Xia 《高分子科学》2006,(1):13-19
It is important to know the rate of intra-molecular contact formation in proteins in order to understand how proteins fold clearly. Here we investigate the rate of intra-molecular contact formation in short two-dimensional compact polymer chains by calculating the probability distribution p(r) of end-to-end distance r using the enumeration calculation method and HP model on two-dimensional square lattice. The probability distribution of end-to-end distance p(r) of short two-dimensional compact polymers chains may consist of two parts, i.e. p(r) = p1(r) p2(r), where p1(r) and p2(r) are different for small r. The rate of contact formation decreases monotonically with the number of bonds N, and the rate approximately conforms to the scaling relation of k(N) ∝ N-α. Here the value of α increases with the contact radius a and it also depends on the percentage of H (hydrophobic) residues in the sequences of compact chains and the energy parameters of εHH, εHP and εPP . Some comparisons of theoretical predictions with experimental results are also made. This investigation may help us to understand the protein folding. 相似文献