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991.
Yury V. Kissin Laura A. Rishina 《Journal of polymer science. Part A, Polymer chemistry》2002,40(9):1353-1365
The hydrogen activation effect in propylene polymerization reactions with Ti‐based Ziegler–Natta catalysts is usually explained by hydrogenolysis of dormant active centers formed after secondary insertion of a propylene molecule into the growing polymer chain. This article proposes a different mechanism for the hydrogen activation effect due to hydrogenolysis of the Ti? iso‐C3H7 group. This group can be formed in two reactions: (1) after secondary propylene insertion into the Ti? H bond (which is generated after β‐hydrogen elimination in the growing polymer chain or after chain transfer with hydrogen), and (2) in the chain transfer with propylene if a propylene molecule is coordinated to the Ti atom in the secondary orientation. The Ti? CH(CH3)2 species is relatively stable, possibly because of the β‐agostic interaction between the H atom of one of its CH3 groups and the Ti atom. The validity of this mechanism was demonstrated in a gas chromatography study of oligomers formed in ethylene/α‐olefin copolymerization reactions with δ‐TiCl3/AlEt3 and TiCl4/dibutyl phthalate/MgCl2–AlEt3 catalysts. A quantitative analysis of gas chromatography data for ethylene/propylene co‐oligomers showed that the probability of secondary propylene insertion into the Ti? H bond was only 3–4 times lower than the probability of primary insertion. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1353–1365, 2002 相似文献
992.
The present study is to examine the photo‐stabilization effect of an antioxidant on the photo‐oxidation of ethylene‐propylene‐diene monomer (EPDM)–clay nanocomposites. During the preparation of EPDM–clay nanocomposites via melt processing antioxidants are usually incorporated along with clay, which allows phenolic antioxidant molecules to get adsorbed onto acidic clay platelets and their interaction with metallic impurities reduces the stabilizing efficiency of the antioxidant. The nanocomposites were obtained by solution dispersion followed by melt compounding of EPDM and organophilic montmorillonite (OMMT). The samples were characterized by conventional tools such as X‐ray diffraction (XRD), Fourier Transform Infra Red (FT‐IR) spectroscopy, and thermo‐gravimetric analysis (TGA). It was found, upon photo‐irradiation (λ > 290 nm) studies by following the changes in functional groups and surface morphology, that photo‐degradation was lowered by the antioxidant and the efficiency of the antioxidant could be improved by initial incorporation of antioxidant in the EPDM matrix. In EPDM–clay nanocomposites, a stabilizing activity of the antioxidant was observed above some threshold concentration of the antioxidant. The relationship between the nanoclay reinforcement and stabilizing efficiency in terms of photo‐oxidation and surface morphology for their applicability are discussed. The methodology adopted for this study is also justified through our observation. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
993.
Torvald Vestberg Peter Denifl Carl‐Eric Wilén 《Journal of polymer science. Part A, Polymer chemistry》2013,51(9):2040-2048
High impact polypropylene was produced in a two‐reactor polymerization process operating in series using two different Ziegler‐Natta catalysts (referred to as catalysts A and B) that had been prepared by Sirius emulsion technology in the absence and presence of SiO2 nanoparticles, respectively. The homo polypropylene matrix was produced in liquid bulk and the ethylene/propylene rubber in gas phase under industrial conditions. Catalyst B was prepared with the same emulsion technology as catalyst A, except that SiO2 nanoparticles (average particles size 80 nm) were added during catalyst preparation. Scanning electron microscopy studies showed that the nanoparticles were fairly evenly distributed within catalyst B particles, although there was some agglomeration. It was shown that the nanoparticles in catalyst B increased the internal porosity in the homo polypropylene matrix particles and this enabled a significant increase in the rubber content. Maximum rubber content, before running into stickiness problems, was approximately 25 wt % for catalyst A without nanoparticles, whereas the maximum rubber content for catalyst B was almost doubled to 45 wt % due to the beneficial transformation of the internal catalyst morphology by the nanoparticles. In addition, it was also found that the reaction was not mass transfer limited during the ethylene/propylene rubber polymerization stage, even at very high rubber contents where all pores and cavities were filled with rubber. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013 相似文献
994.
Hideyuki Kaneko Junji Saito Nobuo Kawahara Shingo Matsuo Tomoaki Matsugi Norio Kashiwa 《Journal of polymer science. Part A, Polymer chemistry》2009,47(3):812-823
A new method to prepare the polypropylene (PP) macroinitiator for controlled radical polymerization was described. Bromination of terminally-unsaturated PP was carried out by using N-bromosuccinimide and 2,2′-azobis(isobutyronitrile) to give a brominated PP (PP-Br), that has allylic bromide moieties at or near the chain ends. Thus, the obtained PP-Br was successfully used as a macroinitiator for radical polymerization of styrene, methyl methacrylate, and n-butyl acrylate using a copper catalyst system. From 1H NMR analysis, it was confirmed that the chain extension polymerization was certainly initiated from allylic bromide moieties with high efficiency, leading to the PP-based block copolymers linking the polar segment. From differential scanning calorimetry, it was observed that peak melting temperature of block copolymers was higher than that of PP-Br and the obtained PP-PS block copolymers with different compositions of each segment demonstrated the unique morphological features due to the microphase separation between both segments. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 812–823, 2009 相似文献
995.
G. N. Pirogova N. M. Panich R. I. Korosteleva Yu. V. Voronin N. N. Popova 《Russian Chemical Bulletin》2000,49(9):1536-1539
The catalytic properties of supported cobaltites MCo2O4 (M=Cu, Mn, Zn, Mg) in the oxidation of CO, C3H6, and ethylbenzene and reduction of nitrogen oxides were investigated. The catalytic activity depends on the calcination temperature
and the nature of the cation. The regularities of formation and the state of the surface of the catalysts were studied by
IR spectra and diffuse reflectance spectra in the UV and visible regions.
Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1547–1550, September, 2000. 相似文献
996.
Joo B. P. Soares Siripon Anantawaraskul 《Journal of Polymer Science.Polymer Physics》2005,43(13):1557-1570
Crystallization analysis fractionation (CRYSTAF) is an analytical technique for determining the distribution of chain crystallizabilities of semicrystalline polymers. After only approximately a decade since it was developed, CRYSTAF has become one of the most important characterization techniques in polyolefin characterization laboratories because it provides fast and crucial information required for the proper understanding of polymerization mechanisms and structure–property relationships. In the polyolefin industry, it has been established as an indispensable tool for product development and product quality monitoring. This highlight article covers basic operation procedures, applications, and theoretical aspects of polymer fractionation with CRYSTAF. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1557–1570, 2005 相似文献
997.
A. Kulbickas J. Tamuliene L. Rasteniene M. Franckevicius R. Vaisnoras M. Marcos J.L. Serrano B. Jaskorzynska L. Wosinski 《Photonics and Nanostructures》2007,5(4):178-183
Optical study of two families of poly(propylene imine) (PPI) dendrimers (G = 1–5) are reported. The enlarging of the chain number (generations) of the PPI dendrimer leads to the spectra shift to IR region in solvent until 8 and 19 nm for both families, respectively. The theoretical modelling of the dendrimer structure was performed. The geometric characterization of dendrimer structure demonstrates that the preferable free space for encapsulation is periphery of the PPI dendrimer. 相似文献
998.
Thomas Gross Gerhard Kühn Wolfgang E. S. Unger 《Surface and interface analysis : SIA》2009,41(5):445-448
The X‐ray‐induced sample damage during mono XPS analysis of an oxygen‐plasma‐oxidized and subsequently wet‐chemically reduced poly(propylene) film was investigated as a showcase for plasma‐modified or plasma‐deposited samples. By doing this, the degradation index approach as introduced by Beamson and Briggs in the Scienta ESCA300 high‐resolution XPS database of organic polymers has been adopted. As to be expected, the sample degrades by loosing oxygen as revealed by observation of decreasing O/C and C OR/Csum ratios. However, the X‐ray degradation indices are definitely higher than those of conventional reference polymers. Moreover, the C OR/Csum degradation index is significantly higher in comparison with one obtained for the O/C ratio. In that context, there is no difference between the plasma sample and a conventional poly(vinyl alcohol) polymer. It is concluded that for reliable quantitative surface chemical analysis, the quality of spectra in terms of acquisition times must be optimized aimed to a minimization of X‐ray degradation. Finally, it is proposed to describe the photon flux of an X‐ray gun in an XPS experiment, which defines the degradation rate at the end, by using the sample current simply measured with a carefully grounded sputter‐cleaned reference silver sample. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
999.
Francisco J. Medellín‐Rodríguez Manuel Mata‐Padilla Saul Sánchez‐Valdes Sofia Vega‐Díaz Olga Dávalos‐Montoya 《Journal of Polymer Science.Polymer Physics》2008,46(20):2188-2200
The complex melting behavior of isotactic polypropylene, after isothermal crystallization, was studied within the context of step‐like melting mechanisms which were previously proposed for high temperature polymers. The morphological characteristics of the melting process were also studied as a function of molecular weight, and close similarities were observed with respect to high temperature polymers. Positive birefringence crystals of low molecular weight samples developed double melting behavior in three steps. The first melting step was assigned to continuous melting of secondary crosshatch reversing lamellae, together with recrystallization of the remaining isothermal crystals. In the second melting step (first melting endotherm), crystals tended to lose their original coarse negative birefringence due to melting of secondary reversing branching. This effect rendered new, finer texture, but still negative birefringence crystals. In the third melting step (second melting endotherm), there was a combination of melting of two crystal populations, one consisting of the remaining fraction of reversing primary crystals, and the other consisting of nonreversing primary crystals. A crosshatch secondary branching model was therefore proposed to explain the overall results. Mixed birefringence spherulites of high molecular weight samples displayed similar, although proportional, behavior under identical crystallization and melting conditions corroborating the proposed melting mechanism. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2188–2200, 2008 相似文献
1000.