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991.
Judit E. Puskas Sam W. P. Chan Kimberley B. McAuley Sohel Shaikh Gabor Kaszas 《Journal of polymer science. Part A, Polymer chemistry》2005,43(22):5394-5413
This article is a critical analysis of kinetic dataavailable on carbocationic polymerizations. A survey of published propagation rate constant (kp) data revealed several orders of magnitude differences. In this article, an explanation of this apparent discrepancy is offered with a case study involving the carbocationic polymerization of 2,4,6‐trimethylstyrene (TMS). With the polymerization mechanism originally proposed for this system, kp = 1.35 × 104 L mol?1 s?1 was extracted from experimental data with the Predici polyreaction package. The alternative mechanism yielded kp = 1.01 × 107 L mol?1 s?1, close to that predicted by Mayr's Linear Free Energy Relationship (LFER). We propose that true rate constants can only be obtained from direct competition experiments or from kinetic interpretation based on independently proven mechanisms. The second part of this review discusses critical analysis of the temperature and concentration dependence of various living IB systems. Comparison of the temperature dependence in systems initiated with 2‐ chloro‐2,4, 4‐ trimethylpentane (TMPCl)/TiCl4 from various laboratories yielded of ΔH ~?25 and ?34.5 kJ/mol for high and low TMPCl/TiCl4 ratios, respectively. Aromatic (cumyl‐type) initiators show ΔH ~ ?40 kJ/mol, whereas H2O/TiCl4 in the presence of the strong electron‐ pair donor dimethylacetamide gave ΔH = ?12 kJ/mol. The significant differences indicate different underlying mechanisms with complex elementary reactions. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5394–5413, 2005 相似文献
992.
993.
Sh. Sh. Nabiev I. I. Ostroukhova N. V. Revina L. P. Sukhanov 《Russian Chemical Bulletin》1998,47(3):417-422
The Raman spectra of ClOF2
+ cation in solutions of anhydrous HF were studied. In the ClOF2
+HF2
− and ClOF2
+BF4
−−HF systems, this cation exists as a pyramidal structure (C
s
symmetry), while in the ClOF2
+AuF6
−−HF system, it exists as a planar structure (C
2v
symmetry). Based on nonempirical calculations by the Hartree-Fock-Roothaan method, an explanation for the dependence of the
structure of the ClOF2
+ cation on the nature of the anion was proposed. For the Cl−O bond vibrations, the correlation functions of vibrational and
rotational relaxations were calculated, and the characteristic times of these processes were determined. The main contribution
to the formation of the band contours corresponding to the above-mentioned modes is made by the vibrational dephasing.
Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 3, pp. 432–437, March, 1998. 相似文献
994.
利用激光光解装置检测了C60-地塞米松(C60-DE)的苯溶液在355 nm激光照射下产生的激发三重态, 3C60-DE*出现四个吸收峰, 分别位于700、440、350 和310 nm. 在330 nm处观察到了它的漂白吸收最大值, 这与其基态吸收最大值相对应. 3C60-DE*能够将能量转移给O2分子而淬灭. 与3C60*相比, 3C60-DE*的三重态鄄三重态(T-T)淬灭速率常数减小(3C60*为(5.03±1.31)×109 L·mol-1·s-1, 3C60-DE*为(3.53±0.87)×109 L·mol-1·s-1), 而寿命增加了(3C60*为(12.0±2.6) μs, 3C60-DE*为(18.0±3.3) μs), 这可能是C60分子上连接了地塞米松分子后减小了C60球之间碰撞的几率所致. 相似文献
995.
996.
本工作采用溴加成法、酚解法、FTIR法及臭氧裂解法分别测定了五种不饱和蒸气压下聚合的PVC样品(u-PVC)和五种商品PVC样品(s-PVC)的总双键、总不稳定氯、孤立双键和内部双键的含量.通过研究结构缺陷和PVC的平均分子量及脱HCl速率的相互关系,揭示了不饱和总双键值,总不稳定氯和孤立双键含量彼此的相关性是建立在它们分别与1/M_n的相关性基础之上,从而得出了这三种定量值测得的主要都是端基烯丙基氯结构.根据三者对脱HCl速率的良好线性相关性,首次提出了端基烯丙基氯结构在HCl催化作用下异构化成内部烯丙基氯从而成为脱HCl速率主要原因的机理. 相似文献
997.
998.
Laura Masgrau Àngels González-Lafont José M. Lluch 《Theoretical chemistry accounts》2002,108(1):38-40
Variational transition state theory including tunneling corrections (as implemented in Polyrate 8.7) and using multilevel
energy calculations at the MCCM-CCSD(T)-1sc level for the CH4 + OH reaction and at the MCCM-CCSD(T)-2m level for the CD4 + OH process, reproduces very well the experimental rate constants. However, no single methodology was found that reproduces
equally well the experimental rate constants for both title reactions.
Received: 24 March 2002 / Accepted: 11 April 2002 / Published online: 4 July 2002 相似文献
999.
A novel carrier of ultradispersed diamond black powder (UDDBP) was used to support metallocene catalyst. Al2O3 was also used as carrier in order to compare with UDDBP. Supported catalysts for ethylene polymerization were synthesized by two different reaction methods. One way was direct immobilization of the metallocene on the support, the other was adsorption of MAO onto the support followed by addition of the metallocene. Four supported catalysts Cp2ZrCl2/UDDBP, Cp2ZrCl2/Al2O3, Cp2ZrCl2/MAO/UDDBP and Cp2ZrCl2/Al2O3/MAO were obtained. The content of the zirconium in the supported catalyst was determined by UV spectroscopy. The activity of the ethylene polymerization catalyzed by supported catalyst was investigated. The influence of Al/Zr molar ratio and polymerization temperature on the activity was discussed. The polymerization rate was also observed. 相似文献
1000.
Successive band-splitting transitions occur in the one-dimensional map xi+1=g(xi),i=0, 1, 2,... withg(x)=x, (0 x 1/2) –x +, (1/2 <x 1) as the parameter is changed from 2 to 1. The transition point fromN (=2n) bands to 2Nbands is given by=(2)1/N (n=0, 1,2,...). The time-correlation function
i=xix0/(x0)2,xi xi–xi is studied in terms of the eigenvalues and eigenfunctions of the Frobenius-Perron operator of the map. It is shown that, near the transition point=2,
i–[(10–42)/17] i,0-[(102-8)/51]i,1 + [(7 + 42)/17](–1)ie–yi, where2(–2) is the damping constant and vanishes at=2, representing the critical slowing-down. This critical phenomenon is in strong contrast to the topologically invariant quantities, such as the Lyapunov exponent, which do not exhibit any anomaly at=2. The asymptotic expression for
i has been obtained by deriving an analytic form of
i for a sequence of which accumulates to 2 from the above. Near the transition point=(2)1/N, the damping constant of
i fori N is given by
N=2(N-2)/N. Numerical calculation is also carried out for arbitrary a and is shown to be consistent with the analytic results. 相似文献