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971.
972.
Jungahn Kim Keon Hyeong Kim Jae Cheol Cho Soonjong Kwak Kwang Ung Kim Won Ho Jo Hye Sung Yoon Dong Soo Lim 《Journal of polymer science. Part A, Polymer chemistry》1998,36(11):1733-1741
The catalytic activities in syndiospecific polymerization of styrene in hydrocarbon using homogeneous Ti-based catalysts in the presence of methylaluminoxane (MAO) were investigated through UV/visible spectroscopic analysis. A strong UV absorption band of CpTiCl3, itself, incipiently appeared at λmax = 400 nm in toluene, followed by a bathochromic shift with its remarkable decrease by the addition of MAO. The absorption band intensity at λmax = 400 nm arising from delocalization of π-electrons on the cyclopentadienyl ring decreased by methylation in the presence of MAO with regard to the mechanism for production of an active center (“cation-like”), for example, the change of the ionic nature. The intensity decrease at λmax = 400 nm was suppressed over 2000 of the [Al]/[Ti] ratio. In the case of Ti(OC4H9)4 having a σ-ligand, new and broad UV absorption bands were developed at λmax = 340 nm and 410 nm in the presence of MAO in contrast with the CpTiCl3/MAO system. Comparison between the relative absorption intensities at λmax = 340 nm and 410 nm led to the determination of a maximum catalytic activity of Ti(OC4H9)4 in the presence of MAO related to the polymerization yield. The maximum polymerization yield was observed with regard to the relative maximum value of the absorption intensity at λmax = 410 nm with the [Al]/[Ti] ratio (500). From observation for polymorphism of the final products via differential scanning calorimetric analysis (DSC), the thermally unstable β-form seemed to be produced by the CpTiCl3/MAO system independent of the MAO concentration, the Ti(OC4H9)4/MAO system produced a thermally stable α-form in the low MAO concentration (up to 100 of the [Al]/[Ti] ratio), and a mixture of α- and/or β-forms over 200 of the [Al]/[Ti] ratio under our experimental conditions. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1733–1741, 1998 相似文献
973.
Mathew D. Penrose 《Random Structures and Algorithms》1998,12(1):63-82
The N-cube is a graph with 2N vertices and N2N−1 edges. Suppose independent uniform random edge weights are assigned and let T be the spanning tree of minimal (total) weight. Then the weight of T is asymptotic to N−12N ∑∞i=1 i−3 as N→∞. Asymptotics are also given for the local structure of T and for the distribution of its kth largest edge weight, k fixed. © 1998 John Wiley & Sons, Inc. Random Struct. Alg., 12, 63–82, 1998 相似文献
974.
975.
We study binary search trees constructed from Weyl sequences {nθ}, n≥1, where θ is an irrational and {·} denotes “mod 1.” We explore various properties of the structure of these trees, and relate them to the continued fraction expansion of θ. If Hn is the height of the tree with n nodes when θ is chosen at random and uniformly on [0, 1], then we show that in probability, Hn∼(12/π2)log n log log n. © 1998 John Wiley & Sons, Inc. Random Struct. Alg., 12, 271–295, 1998 相似文献
976.
977.
Jens A. Griepentrog 《Mathematische Nachrichten》2002,243(1):19-42
In this paper linear elliptic boundary value problems of second order with non‐smooth data L∞‐coefficients, sets with Lipschitz boundary, regular sets, non‐homogeneous mixed boundary conditions) are considered. It will be shown that such boundary value problems generate isomorphisms between certain Sobolev‐Campanato spaces of functions and functionals, respectively. 相似文献
978.
979.
980.