bluuebunny/crossref_metadata_2025_split huggingface dataset:
Porous Superabsorbent Hydrogel Composites: Synthesis, Morphology and Swelling Rate
Authors: Kabiri Kourosh , Zohuriaan‐Mehr Mahammad J. 
Published: June 2004

AbstractSummary: Novel porous hydrogel composites with very high swelling capacity and enhanced rate of water absorption were synthesized in aqueous media at room temperature under normal atmospheric conditions. The porosity was induced through either foaming conducted in the course of polymerization or non‐solvent dewatering of the as‐synthesized gels. Kaolin was incorporated as an inorganic component in the polymerization process. The foaming technique was used to form porosity using three systems of different porogens (porosity generators), i.e. sodium bicarbonate, acetone and their combination. The as‐synthesized gels were dried through oven drying and non‐solvent dewatering. Morphology and swelling rate of the superabsorbent hydrogel composites (SHCs) were studied versus either the porogen system or the drying method. It was found that the simultaneous polymerization‐foaming technique had great influence on the improvement of porosity, morphology of the porous structure and the rate of water absorption. It was also shown that the drying procedure had remarkable influence on preserving the preformed porosity. Methanol as a dewatering solvent produced SHCs with higher porosity and swelling rate in comparison with the porosity of the hydrogels dewatered in acetone. Our invented methodology including simultaneous polymerization and foam formation using dual‐porogen system and the subsequent methanol‐dewatering approach was found to be the most efficient, highly practical, and cost‐effective route for preparing improved superabsorbing hydrogel materials.
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Ternary Blends to Improve Heat Distortion Temperature and Rheological Properties of PVC
Authors: de Zarraga Arantxa , Villanueva Sara , Muñoz Maria Eugenia , Obeso Rafael , Peña Juan José , Pascual Belén , Santamaría Anton 
Published: June 2004

AbstractSummary: Binary and ternary blends of PVC mixed with α‐methylstyrene/acrylonitrile‐butadiene‐styrene copolymer (AMS‐ABS) and ethylene/vinyl acetate/carbon monoxide terpolymer (EVA‐CO) are investigated, with the aim to obtain a new PVC based material with an improved heat distortion temperature and good processability. Dynamic Mechanical Thermal Analysis (DMTA) reveals that ternary PVC/AMS‐ABS/EVA‐CO blends exhibit two glass transition temperatures: the lower Tg corresponds to a PVC/EVA‐CO phase and the higher one to a PVC/AMS‐ABS phase. An analysis of PVC respective interactions with AMS‐ABS and EVA‐CO leads to assert that the distribution of PVC in the ternary PVC/AMS‐ABS/EVA‐CO system is basically controlled by the binary immiscible blend composition, taken as Φ AMS‐ABS/Φ EVA‐CO ratio. The inclusion of AMS‐ABS and EVA‐CO to form ternary blends based on PVC, allows to improve heat distortion temperature (owed to the presence of AMS‐ABS), maintaining a low viscosity in the molten state, due to the plasticizing effect of EVA‐CO.Viscosity function obtained at T = 170 °C from extrusion capillary measurements.magnified imageViscosity function obtained at T = 170 °C from extrusion capillary measurements.
Isobornyl Methacrylate as a Reactive Solvent of Polyethylene
Authors: Schnell Matthias , Borrajo Julio , Williams Roberto J. J. , Wolf Bernhard A. 
Published: June 2004

AbstractSummary: Solutions containing 15 wt.‐% of a low‐molar‐mass polyethylene (PE) in isobornyl methacrylate (IBoMA), containing 0, 5 or 10 wt.‐% of 1,4 butanediol dimethacrylate (BDDMA) as crosslinker, were polymerized using either benzoyl peroxide (BPO), at 80 °C, or dicumyl peroxide (DCPO), with a thermal cycle attaining 150 °C, as initiators. Phase separation of an amorphous PE‐rich phase took place when carrying out the reaction at temperatures higher than the PE melting temperature. Partial crystallization of PE was observed when cooling to room temperature. Depending on the initial amount of BDDMA, the fraction of PE that was phase separated varied between 57 and 66% of the initial amount, with crystalline fractions in the range of 15 to 42%. The use of IBoMA as a reactive solvent of PE has two main advantages over other reactive solvents reported in the literature: a) it has a very low vapor pressure, and b) its free‐radical polymerization gives a polymer with a relatively high glass transition temperature.Part of the cloud‐point curve for IBoMa, PIBoMA and PE solutions at 80 °C.magnified imagePart of the cloud‐point curve for IBoMa, PIBoMA and PE solutions at 80 °C.
Recent data on the number of active centers and propagation rate constants in olefin polymerization with supported ZN catalysts
Authors: Zakharov V.A. , Bukatov G.D. , Barabanov A.A. 
Published: June 2004

AbstractData on the number of active centers (Cp) and propagation rate constants (Kp) have been obtained by means of polymerization quenching with 14CO of propylene and ethylene polymerization with supported titanium‐magnesium catalysts (TMC) with different composition. In the case of propylene polymerization the Cp and Kp values have been measured separately for isospecific, aspecific and low stereospecific centers. Effects of MgCl2 support, internal and external donors are discussed on the basis of data obtained.Data on the strong effect of diffusion limitation at ethylene polymerization with number of TMC have been obtained and a set of methods have been used to exclude this effect. Data on Cp and Kp values at ethylene polymerization with low stereospecific and highly stereospecific catalysts are presented.
Comparison of silyl chromate and chromium oxide based olefin polymerization catalysts
Authors: Cann Kevin , Apecetche Maria , Zhang Minghui 
Published: June 2004

AbstractA comparison of the ethylene polymerization performance conducted with an oxo and a triphenylsilyl chromate catalyst on silica was performed. The oxo catalyst has higher activity and better comonomer response. The silylchromate catalyst has a much longer induction time and made a much broader molecular weight distribution polymer compared to the oxo analogue. Performance similar to silylchromate on silica was observed when triphenylsilanol (TPS) was added to the oxo chromium catalyst. The oxo catalyst was converted to the silyl chromate catalyst by ligand substitution. Analysis of the catalyst components when TPS was added to the oxo chrome analogue showed that bis triphenylsilyl chromate can form and be removed from the support.
D/A‐metallocenes: the new dimension in catalyst design
Authors: Starzewski Aleksander Ostoja 
Published: June 2004

AbstractD/A metallocenes constitute a novel unique class of catalysts, in that they are able to express structural information within the elementary steps of the catalytic cycle out of bridged as well as out of unbridged states due to the coexistence of donor (D) and acceptor (A) atoms in complex sandwich structures. Highly polarized transannular bridging interactions as well as Lewis basic and Lewis acidic functionalities in unbridged states are the prerequisites which define a modular highly flexible catalyst system with outstanding options to tailor materials and processes.
Microstructural characteristics and thermal properties of ansa‐zirconocene catalyzed copolymers of propene with higher α‐olefins
Authors: Sacchi Maria Carmela , Fortini Fabrizio , Losio Simona , Tritto Incoronata , Costa Giovanna , Stagnaro Paola , Tincul Ioan , Wahner Udo M. 
Published: June 2004

AbstractCopolymers of propene and several higher α‐olefins, 1‐butene, 1‐pentene, 1‐hexene, 1‐heptene, 1‐octene, and 4‐methyl‐1‐pentene (4M1P), have been prepared with two different metallocene catalysts, rac‐Et(Ind)2ZrCl2 (EI) and rac‐Me2Si(2‐MeBenz‐[e]Ind)2ZrCl2 (MBI), and comprehensively characterized by 13C NMR spectroscopy. All copolymers produced with EI have comparable isotacticities as well as similar stereosequences. The copolymers produced with MBI significantly differ both from the reference propene homopolymer and among themselves: in fact, they are characterized by rather low isotacticities and the degree of tacticity is strongly influenced by comonomer type and concentration. A detailed 13C NMR analysis revealed that these copolymers are constituted by highly isotactic and atactic sequences of various lengths. The similar thermal behavior, in terms of transition temperatures, for the homologues of the series obtained with the two different catalysts show that the capability of crystallization of these systems is not directly connected to the overall microstructural properties (e.g. the isotacticity index) but can be accounted for by the details of the microstructure.
Relative reactivity enhancement of (Me5Cp)2ZrCl2 in mixture with (1,2,4‐Me3Cp)2ZrCl2 during ethene/1‐hexene copolymerization
Authors: Bruaseth Ingvild , Rytter Erling 
Published: June 2004

AbstractDual‐site ethene/1‐hexene copolymerizations with MAO‐activated (1,2,4‐Me3Cp)2ZrCl2 and (Me5Cp)2ZrCl2 catalysts were performed. Copolymers with narrow molecular weight distributions and bimodal short chain branching distributions could be produced. The combined catalyst system demonstrates a number of discrepancies from an expected average behavior of the individual sites. Dual‐site (1,2,4‐Me3Cp)2ZrCl2/(Me5Cp)2ZrCl2 systems produce copolymers with lower incorporation than expected. Clear evidences for relative activity enhancement of the (Me5Cp)2ZrCl2 catalyst in the mixture were observed in melting endotherms and Crystaf profiles. Molecular weights obtained by the mixture were higher than for any of the individual catalysts. A similar effect is observed for a dual‐site system of the (1,2,4‐Me3Cp)2ZrCl2 catalyst together with the Me4Si2(Me4Cp)2ZrCl2 catalyst as an alternative to (Me5Cp)2ZrCl2.
Sterically demanding bis(2‐silylindenyl)zirconium(IV) dichlorides as polymerisation catalyst precursors
Authors: Hannisdal Andreas , Möller Andreas C. , Rytter Erling , Blom Richard 
Published: June 2004

AbstractA set of different 1‐ and 2‐silyl‐substituted zirconocene dichloride/MAO catalyst systems was investigated with respect to their performance in ethene/1‐hexene copolymerisations. In‐depth studies of bis(2‐dimethylsilylindenyl) zirconium(IV) dichloride (1) revealed a multi‐site behaviour, illustrating sensitivity to the reaction temperature and the comonomer mole fraction. Surprisingly, an upper limit is observed for the latter, leading to complete catalyst inhibition. Analysis of the chain termination processes implies the possibility of a predominant, although in general less favourable, β‐hydride elimination route under certain polymerisation conditions.
From zirconium to titanium: the effect of the metal in propylene polymerisation using fluxional unbridged bicyclic catalysts
Authors: Polo Eleonora , Losio Simona , Fortini Fabrizio , Locatelli Paolo , Sacchi Maria Carmela 
Published: June 2004

AbstractCatalytic systems based on unbridged substituted indenyl systems are becoming of interest in the production of elastomeric polypropylene. A full understanding of the structural features necessary to control this kind of behaviour has not yet been achieved, since relatively slight changes in the molecular architecture can lead to polymers with remarkably different properties.We report here our recent findings regarding the study of bicyclic zirconium and titanium complexes as fluxional catalysts in propylene polymerisation. Most of them have been synthesised according to a synthetic procedure that allowed us to prepare a series of complexes in which the ring fused to the cyclopentadienyl moiety is saturated and of different sizes, thus introducing a flexibility parameter that can be finely tuned.The results obtained show that the stereoselectivity induced by this class of catalysts strongly depends both on the structure of the ligand and on the nature of metal atom (Zr vs. Ti). The titanium‐based catalysts yield polypropylenes with new and interesting microstructures, in particular when an higher stability is achieved through a careful choice of the substitution pattern of the ligands.
On the ethylene‐norbornene copolymerization mechanism
Authors: Tritto Incoronata , Boggioni Laura , Sacchi Maria Carmela , Locatelli Paolo , Ferro Dino R. 
Published: June 2004

AbstractResults of our studies on polymerization kinetics and tests of copolymerization statistical models of ethylene‐norbornene (E‐N) copolymers obtained on the basis of microstructures determined by 13C NMR analysis are reported. Ethylene‐norbornene (E‐N) copolymers were synthesized by catalytic systems composed of racemic isospecific metallocenes, i‐Pr[(3Pri‐Cp)(Flu)]ZrCl2 or a constrained geometry catalyst (CGC) and methylaluminoxane.Polymerization kinetics revealed that E‐N copolymerization is quasi living under standard polymerization conditions. Calculations of the number of active sites and of chain propagation and chain transfer turnover frequencies indicate that the metal is mainly in the Mt‐N* state, while the Mt‐E* state contributes more to transfer and propagation rates.The first‐order and the second‐order Markov statistics have been tested by using the complete tetrad distribution obtained from 13C NMR analysis of copolymer microstructures. The root‐mean‐square deviations between experimental and calculated tetrads demonstrate that penultimate (second‐order Markov) effects play a decisive role in E‐N copolymerizations. Results show clues for more complex effects depending on the catalyst geometry in copolymers obtained at high N/E feed ratios.Comonomer concentration was shown to have a strong influence on copolymer microstructure and copolymer properties.The copolymer microstructure of alternating isotactic copolymers obtained with i‐Pr[(3Pri‐Cp)(Flu)]ZrCl2 have been described at pentad level. Second‐order Markov statistics better describes also the microstrucure of these copolymers.
Living random copolymerization of ethene and norbornene using ansa‐fluorenylamidodimethyltitanium complex
Authors: Hasan Tariqul , Shiono Takeshi , Ikeda Tomiki 
Published: June 2004

AbstractEthene‐norbornene copolymerizations were conducted by (t‐BuNMe2SiFlu)TiMe2 (1) activated with dried methylaluminoxane (MAO) (free from Me3Al) at 0°C and 20°C under atmospheric pressure of ethene at various norbornene concentrations. At 0°C and high norbornene concentration, the molecular weight of the copolymer increased linearly against polymerization time while keeping narrow molecular‐weight distribution. The 13C NMR spectrum revealed that random copolymer with 53 mol‐% of norbornene was obtained. The glass transition temperature was above 150°C.
New organic supports for metallocene catalysts applied in olefin polymerizations
Authors: Klapper Markus , Jang Yong‐Jun , Bieber Kirsten , Nemnich Tanja , Nenov Nikolay , Müllen Klaus 
Published: June 2004

AbstractNano‐sized latex particles as organic supports for metallocenes applied in olefin polymerizations are introduced. The particles are functionalized with nucleophilic surfaces such as polyethylenoxide (PEO), polypropyleneoxide (PPO) or pyridine units allowing an immobilization of the metallocene catalysts via a non‐covalent immobilization process. The latices are obtained by emulsion or miniemulsion polymerization with styrene, divinylbenzene as the crosslinker, and either PEO or PPO functionalized styrene or 4‐vinylpyridine for surface functionalization. The supported catalysts, e.g. [Me2Si(2MeBenzInd)2ZrCl2/MAO] on PPO containing latices or Cp2ZrMe2/([Ph3C][B(C6F5)4]) on pyridine functionalized materials were tested in ethylene polymerizations. Remarkably, high activities and excellent product morphologies were obtained. The influence of the degree of surface functionalization on activity and productivity was investigated. Furthermore, the fragmentation of the catalyst was studied by electron microscopy using bismuth‐labeled latex particles or by fluorescence and confocal fluorescence microscopy using dye‐labeled supports.Finally, a self‐immobilizing catalyst/monomer system is presented. It is demonstrated that by using PEO‐functionalized olefins, the metallocenes were immobilized on the monomers. Subjecting these mixtures to an ethylene copolymerization, again high activities and productivities as well as polyolefin beads with high bulk densities are observed, indicating that an extra supporting process for controlling the product size and shape of the polyolefins is not necessary for these monomers.
Heterogenization of metalorganic catalysts of olefin polymerization and evaluation of active site non‐uniformity
Authors: Novokshonova Lyudmila , Kovaleva Natalia , Meshkova Irina , Ushakova Tatiana , Krasheninnikov Vadim , Ladygina Tatiana , Leipunskii Ilia , Zhigach Alexey , Kuskov Michail 
Published: June 2004

AbstractHeterogenized activators ‐ “support‐H2O/AlR3” (where R=Me, iBu, support=montmorillonite, zeolite), synthesized directly on the support, form with metallocenes metal alkyl complexes highly active in olefin polymerization without the use of commercial methylaluminoxane (MAO). It was shown by the method of temperature programmed desorption with the application of mass‐spectrometry (TPD‐MS) that the aluminumorganic compound in support‐H2O/AlR3 is in general similar to the structure of commercial MAO. The heterogenization of Zr‐cenes on support‐H2O/AlR3 is accompanied by the appearance of the energy non‐uniformity of active sites. The activation energy of thermal destruction of active Zr‐C bonds in the active sites of prepared catalysts changes in the range from 25 to 32 kcal/mol.
Fluorenyl based syndiotactic specific metallocene catalysts structural features, origin of syndiospecificity
Authors: Razavi Abbas , Bellia Vincenzo , De Brauwer Yves , Hortmann Kai , Peters Liliane , Sirole Sabine , Van Belle Stephan , Thewalt Ulf 
Published: June 2004

AbstractThe stereochemistry of propylene insertion/propagation reactions with a variety of Cs symmetric fluorenyl‐ containing single site catalysts is discussed. Our recent results indicate that independent of the chemical composition of the ancillary ligand fragments, or nature of the transition metal, active sites with local Cs symmetry and enantiotopic coordination positions behave syndioselectively in the general context of chain migratory insertion mechanism. Perfect bilateral symmetry neither exists nor is required in these processes. In this context the mechanism of syndiospecific polymerization is revisited by taking into account the structural characteristics and catalytic behavior of the original metallocene based (η5‐C5H4‐CMe2‐η5‐C13H8) MCl2/ MAO; M = Zr (1), Hf (2) catalyst systems and new syndiotactic specific systems including (η5‐C5H4‐CPh2‐η5‐3,6‐di‐tBut‐C13H6)ZrCl2 (3), η1,η5‐(μMe2Si)(3,6‐di‐tBut‐Flu)(t‐ButN)MCl2/ MAO; M =Ti (4), Zr (5) and η1,η5‐(μMe2Si)(2,7‐di‐tBut‐Flu)(t‐ButN)MCl2/ MAO; M = Ti (6), Zr (7).
Observation and identification of the catalytically active species of bis(phenoxy‐imine) group 4 transition metal complexes for olefin polymerization using 1H NMR spectroscopy
Authors: Makio Haruyuki , Fujita Terunori 
Published: June 2004

AbstractSolution structures of bis(phenoxy‐imine) group 4 transition metal complexes (FI Catalysts) were investigated using 1H NMR spectroscopy. At least two isomers exist in equilibrium for FI Catalysts precursors, bis[N‐(3‐tert‐butylsalicylidene)anilinato]zirconium(IV) dichloride (1), and bis[N‐(3,5‐dicumylsalicylidene)anilinato]zirconium(IV) dichloride (2), while bis[N‐(3‐tert‐butylsalicylidene)‐2,3,4,5,6‐pentafluoroanilinato]titanium(IV) dichloride (3) exhibits only one isomer under the conditions examined. Upon activation with MAO, all FI Catalysts (1‐3) generate two species at ambient temperature judging from some key signals in the 1H NMR. When temperature is raised (up to 75°C), one species (1a‐3a) converts irreversibly to the other species (1b‐3b). The resulting species, 1b‐3b, are stereochemically rigid, in contrast to precursors 1 and 2. Species 3b, derived from a living FI Catalyst, exhibited virtually no reactivity toward olefin insertion. The imine protons of species 1b‐3b are temperature and solvent polarity sensitive. Two possibilities are proposed for the assignment of species 1b‐3b, i) heterobinuclear complexes of group 4 metal and alkylaluminum with methyl and/or chlorine as bridging groups and ii) phenoxy‐imine ligated aluminum complexes whose ligands are transferred from the group 4 metal. The latter is more probable from the separate synthesis of LAlMe2 (L: phenoxy‐imine ligand). When 3 was activated with MAO in the presence of olefins, a new imine signal was observed. This species (3c for ethylene and 3d for propylene) is thermally more robust than 3a toward transformation to 3b and assignable to the living propagating species.
Polymerization of norbornene with Co(II) complexes
Authors: Pelascini Frédéric , Peruch Frédéric , Lutz Pierre Joseph , Wesolek Marcel , Kress Jacky 
Published: June 2004

AbstractThe norbornene polymerization was studied in the presence of 6 pyridine bis(imine) cobalt(II) complexes activated with methylaluminoxane (MAO). Norbornene was also polymerized with CoCl2 associated to MAO. All these catalytic systems generate an addition polymerization of norbornene, yielding fully saturated polymers. It was shown that the polymerization yield and the molar masses are highly dependant on several reaction parameters (monomer concentration, [Al]/[Co] ratio, polymerization temperature and time) and the frame of the ligand.
Experimental and calculated vibrational spectra and structure of Ziegler‐Natta catalyst precursor: 50/1 comilled MgCl2‐TiCl4
Authors: Brambilla Luigi , Zerbi Giuseppe , Nascetti Stefano , Piemontesi Fabrizio , Morini Giampiero 
Published: June 2004

AbstractThe vibrational Infrared and Raman Spectra of a MgCl2‐TiCl4 Ziegler‐Natta catalyst precursor with a 50/1 MgCl2/TiCl4 ratio have been recorded. The Raman spectrum of this catalyst precursor, in the range 50‐500 cm−1, shows clear scattering lines which can be assigned to the complex MgCl2‐TiCl4, well separated from those of the initial species. Analogous, but less clear signals can be found in the infrared spectrum. Vibrational symmetry analysis and quantum chemical calculations of suitable models of MgCl2‐TiCl4 complex have been made for the interpretation of the experimentally recorded spectra. The observed spectroscopic signals can be explained in terms of the existence of only one type of MgCl2‐TiCl4 complex where the TiCl4 molecules are complexed on the MgCl2 along the (110) lateral cuts in a local C2v symmetry with the Ti atoms in an octahedral coordination.
Defects distribution of metallocene and MgCl2‐supported Ziegler‐Natta isotactic poly(propylenes) as revealed by fractionation and crystallization behaviors
Authors: Alamo Rufina G. 
Published: June 2004

AbstractThe inter and intramolecular distribution of defects of poly(propylenes) of the Ziegler‐Natta (ZN) and metallocene (M) types, with matched molar masses and overall defect concentrations, are inferred from the crystallization and polymorphic behavior of their narrow molecular mass fractions. The fractions obtained from the M‐iPP display a range in molecular masses but the same concentration of defects and provide direct evidence of the uniform intermolecular defect distribution and the “single site” nature of the catalyst. The stereodefects of the ZN‐iPP fractions are more concentrated in the low molecular mass fractions, corroborating a broad interchain distribution of the nonisotactic content. In addition, the invariance of the linear growth rates among the ZN fractions and very low contents of the gamma polymorph, developed even by the most defected ZN fraction, are consistent with a stereo blocky intramolecular distribution of defects in the ZN‐iPP molecules. In contrast to the linear growth rates, which are sensitive to the defect microstructure, the overall crystallization rates correlate with nucleation density and not necessarily with the iPP chain microstructure.