Crystallization analysis fractionation of ethene/1-hexene copolymers made with the MAO-activated dual-site (1,2,4-Me3Cp)2ZrCl2 and (Me5Cp)2ZrCl2 system

Different poly(ethene-co-1-hexene) samples with varying amounts of 1-hexene were characterized by crystallization analysis fractionation (Crystaf). The samples were synthesized with (1,2,4-Me₃Cp)₂ZrCl₂, (Me₅Cp)₂ZrCl₂, and a mixture of these two catalysts in a 1:1 molar ratio. In addition, preparative Crystaf was used to fractionate some of the samples made with the catalyst mixture into 1-hexene-rich and 1-hexene-poor fractions. These fractions were characterized by Crystaf, differential scanning calorimetry (DSC) and gel permeation chromatography (GPC), and compared with copolymers made under similar conditions using the individual catalysts. Both (1,2,4-Me₃Cp)₂ZrCl₂ and (Me₅Cp)₂ZrCl₂ produced copolymers with unimodal distribution of short chain branches (SCBD), as expected for single-site catalysts. The catalyst mixture produced copolymers with bimodal SCBDs when 0.38 mol/l or higher concentrations of 1-hexene were used. The high temperature peak results from crystallization of polymer chains with few comonomer units, and these are attributed to (Me₅Cp)₂ZrCl₂. The low temperature peak results from crystallization of polymer chains made by (1,2,4-Me₃Cp)₂ZrCl₂, and these chains contain many comonomer units. Direct evidence for relative activity enhancement of the (Me₅Cp)₂ZrCl₂ catalyst in the dual-site system was observed.     

Synthesis and characterization of alternating silane copolymers: (Me2Si?R2Si?Me2Si)n

Bimodal molecular weight distributions of alternating copolymers of the type (Me₂Si–R₂Si–SiMe₂) _n , where R=hexyl and butyl, were obtained by polymerization of BrSiMe₂–R₂Si–Me₂SiBr, using the Wurtz coupling method. Analysis of the samples by pyrolysis GC MS and solution²⁹Si NMR indicated that some randomization occurred, due to cleavage of the original Si–Si bonds in the monomer. The extent of randomization was significantly greater in the high molecular weight fractions. Based on the nature of the rings from the py MGC/MS traces and the number on nonads found in the²⁹Si spectra, two types of randomization processes have been proposed, involving backbiting followed by (1) ring expulsion or (2) redistribution.     

A kinetic analysis on the gas phase polymerization of ethylene over polymer supported (CH3)2Si[Ind]2ZrCl2 catalyst

Kinetics of the gas phase polymerization of ethylene over polymer supported (CH₃)₂Si[Ind]₂ZrCl₂ catalyst is examined. It requires a two-site model to adequately describe the experimental results. The active sites seem to follow a first-order deactivation. The number of sites activated is dependent upon the cocatalyst (MAO) concentration level as well as temperature. It was observed that the decrease of activity at higher temperatures resulted from a strong dependency of the first-order decay rate constant on temperature. On the other hand, the propagation rate constants of two active sites show similar dependency on temperature.     

Ethylene and Propylene Polymerization Using In Situ Supported Me2Si(Ind)2ZrCl2 Catalyst: Experimental and Theoretical Study

Summary: Me₂Si(Ind)₂ZrCl₂ was in situ immobilized onto SMAO and used for ethylene and propylene polymerization in the presence of TEA or TIBA as cocatalyst. The catalytic system Me₂Si(Ind)₂ZrCl₂/SMAO exhibited different behavior depending on the amount and nature of the alkylaluminum employed and on the monomer type. The catalyst activity was nearly 0.4 kg polymer · g cat^?1 · h^?1 with both cocatalysts for propylene polymerization. Similar activities were observed for ethylene polymerization in the presence of TIBA. When ethylene was polymerized using TEA at an Al/Zr molar ratio of 250, the activity was 10 times higher. Polyethylenes made by in situ supported or homogeneous catalyst systems had practically the same melting point (T_m). On the other hand, poly(propylenes) made using in situ supported catalyst systems had a slightly lower T_m than poly(propylenes) made using homogeneous catalyst systems. The nature and amount of the alkylaluminum also influenced the molar mass. The poly(propylene) molar mass was higher when TIBA was the cocatalyst. The opposite behavior was observed for the polyethylenes. Concerning the alkylaluminum concentration, the molar mass of the polymers decreased as the amount of TEA increased. In the presence of TIBA, the polyethylene's molar mass was almost the same, independent of the alkylaluminum concentration, and the poly(propylene) molar mass increased with increasing amounts of cocatalyst. The deconvolution of the GPC curves showed 2 peaks for the homogeneous system and 3 peaks for the heterogeneous in situ supported system. The only exception was observed when TEA was used at an Al/Zr molar ratio of 500, where the best fit was obtained with 2 peaks. Based on the GPC deconvolution results and on the theoretical modeling, a proposal for the active site structure was made.

Molar mass distribution deconvolution of polyethylene prepared with the system Me₂Si(Ind)₂ZrCl₂/SMAO/TIBA with 500 mol/mol of alkylaluminum as cocatalyst.     

A QM/MM study of the ethylene and styrene insertion process into the ion pair [Me2Si(C5Me4)(NBu)Ti(CH2CH2CH3)][μ-Me-Al(Me)2-(AlOMe)6Me]

Ethylene and styrene insertion into the metal-alkyl bond of [Me₂Si(C₅Me₄)(N^tBu)Ti (CH₂CH₂CH₃)]⁺[μ-Me-Al(Me)₂-(AlOMe)₆Me]⁻ species has been investigated using a QM/MM approach. Validation of the B3LYP//B3LYP/UFF theoretical model was performed by comparing some results with full QM calculations. Ion pairs which contain a bounded trimethylaluminium molecule give rise to active species, whereas direct coordination of the MAO cage to the catalyst leads to dormant species for polymerisation. Ion pair formation and dissociation for both ion-pair complexes have been performed. In addition, monomer insertions into the active ion-pair species have been studied. The monomer coordination step results to be endothermic in contrast to the values obtained for the ‘naked’ cation. The energetic insertion barriers starting from the π-complexes are similar to those obtained for the ‘naked’ cationic species. The net effect of the cocatalyst is to increase the coordination barriers at a similar amount for two-monomer insertions. Some implications of the cocatalyst effect in ethylene/styrene copolymerisation are discussed.     

Polymerization of 1,3-butadiene with VO(P204)2 and VO(P507)2 activated by alkylaluminum

The oxovanadium phosphonates (VO(P₂₀₄)₂ and VO(P₅₀₇)₂) activated by various alkylaluminums (AlR₃, R = Et, i-Bu, n-Oct; HAlR₂, R = Et, i-Bu) were examined in butadiene (Bd) polymerization. Both VO(P₂₀₄)₂ and VO(P₅₀₇)₂ showed higher activity than those of classical vanadium-based catalysts (e.g. VOCl₃, V(acac)₃). Among the examined catalysts, the VO(P₂₀₄)₂/Al(Oct)₃ system (I) revealed the highest catalytic activity, giving the poly(Bd) bearing M_n of 3.76 × 10⁴ g/mol, and M_w/M_n ratio of 2.9, when the [Al]/[V] molar ratio was 4.0 at 40 °C. The polymerization rate for I is of the first order with respect to the concentration of monomer. High thermal stability of I was found, since a fairly good catalytic activity was achieved even at 70 °C (polymer yield > 33%); the M_n value and M_w/M_n ratio were independent of polymerization temperature in the range of 40-70 °C. By IR and DSC, the poly(Bd)s obtained had high 1,2-unit content (>65%) with atactic configuration. The 1,2-unit content of the polymers obtained by I was nearly unchanged, regardless of variation of reaction conditions, i.e. [Al]/[V], ageing time, and reaction temperature, indicating the high stability of stereospecificity of the active sites.     

Preparation, cross-linking and ceramization of AHPCS/Cp2ZrCl2 hybrid precursors for SiC/ZrC/C composites

SiC/ZrC/C composites were prepared via pyrolysis of a polymeric precursor, namely AHPCS/Cp₂ZrCl₂ hybrid precursor prepared by the blend of allylhydridopolycarbosilane (AHPCS) and bis(cyclopentadienyl) zirconium dichloride (Cp₂ZrCl₂). The cross-linking and polymer-to-ceramic conversion of as-synthesized AHPCS/Cp₂ZrCl₂ were characterized by means of FTIR, ¹³C NMR, TGA, EDS, Raman spectroscopy and XRD. It is suggested that dehydrocoupling, hydrosilylation and dehydrochlorication are involved in the cross-linking of the hybrid precursor, which is responsible for a relatively high ceramic yield of 75.5% at 1200 °C. The polymer-to-ceramic conversion is complete at 900 °C, and it gives an amorphous ceramic. Further heating at 1350 °C induces partial crystallization, and then the characteristic peaks of β-SiC and cubic ZrC appear at 1600 °C. The effect of the composition of the hybrid precursor is also studied in the work.     

Copolymerization behavior of Cp2ZrCl2/MAO and [(CO)5WC(Me)OZrCp2Cl]/MAO: a comparative study on ethylene/1-pentene copolymers

Ethylene/1-pentene copolymers were synthesized using Cp₂ZrCl₂(1)/MAO and [(CO)₅WC(Me)OZr(Cp)₂Cl](2)/MAO catalyst systems. The copolymers were characterized by SEC, DSC, FTIR and ¹³C NMR spectroscopy. The copolymers synthesized with [(CO)₅WC(Me)OZr(Cp)₂Cl](2)/MAO had higher average molecular weights and broader polydispersities compared to those produced with Cp₂ZrCl₂(1)/MAO. The chemical heterogeneity was investigated by SEC-FTIR and fractionation techniques. All copolymers showed a higher incorporation of the 1-pentene in the low molecular weight fraction as revealed by SEC-FTIR. Crystallization analysis fractionation (CRYSTAF) showed a broad chemical composition distribution (CCD) for all the copolymers synthesized with these two catalyst systems. Selected copolymers were also analyzed using an automated preparative molecular weight fractionation.     

Dynamic Model of the Homopolymerization of Propylene with the Me2Si(2-Me-Ind)2ZrCl Catalyst: The Effect of Reaction Variables

The effect of four variables—temperature, propylene pressure, Al/Zr ratio, and catalyst concentration—on catalyst productivity and molecular weight of the polymer synthesized with Me₂Si(2-Me-Ind)₂ZrCl₂ was studied. A mathematical model was developed based on the method of the moments that fit the experimental data. A special effect was seen with respect to the co-catalyst (MAO) on productivity, and therefore it was necessary to add deactivation-reactivation mechanisms associated with MAO to the model. With respect of the molecular weight of the polymer, a bimolecular transfer reaction associated with the active center was added to fit the experimentally observed data.     

Novel Supramolecular Square Grid-Like Structure: Synthesis and Characterization of [(Me3Sn)2CrO4]

Reactions of K₂Cr₂O₇ and K₂CrO₄ with Me₃SnCl yielded [(Me₃Sn)₂CrO₄] (1) and [(Me₃Sn)₂CrO₄(Me₃SnOH)] (2), respectively, which were characterized by elemental analysis, ¹H-NMR spectroscopy, and an X-ray diffraction study. Compound 1 shows a novel square grid-like structure consisting of CrO₄ ^2– and Me₃Sn⁺ ions, whereas 2 exhibits a three-dimensional structure composed of CrO₄ ^2–, Me₃Sn⁺, and [(Me₃Sn)₂SnOH]⁺ ions.     

Study of the polymerization of 1-octadecene with different rnetallocene catalysts

Summary 1-Octadecene (C18) was polymerized by using different metallocene catalysts. The rac-Et(Ind)₂ZrC1₂/MAO (I) and rac-Me₂Si(Ind)₂ZrC1₂/MAO (III) presented the highest activity as compared with ra-Et(2-Me-Ind)₂ZrCl₂/MAO (II) and Ph₂C(Flu)(Cp)ZrC1₂/MAO (IV) catalysts. Catalyst IV produced polymers with highest molecular weights. The microstructure of the polymers was determined by ¹³C-NMR spectroscopy. Catalyst systems I, II and III produced isotactic polymers while catalyst IV produced polymers with mainly syndiotactic structures but with large amount of stereoregular error. Received: 21 June 2002/Revised version: 4 November 2002/ Accepted: 4 November 2002     

Copolymerization of propylene and norbornene with different metallocene catalysts

Propylene and norbornene were copolymerized by metallocene/MAO catalysts. The organometallic compounds rac-[Me₂C(Ind)₂]ZrCl₂ (1) and [Me₂C(Cp)(Flu)]ZrCl₂ (2), [Ph₂C(Cp)(2,7-di^tBuFlu)]ZrCl₂ (3) and [Me₂Si(3-^tBuCp)(N^tBu)]TiCl₂ (4) were used to catalyze polymerization series, in which the influence of the molar fraction of norbornene in the feed and of the polymerization temperature were investigated in detail. The obtained polymers, which exhibit a wide range of properties with glass transition temperatures above 200 °C, were characterized by ¹³C NMR spectroscopy, differential scanning calorimetry and gel permeation chromatography techniques.In this article, the emphasis is placed on the copolymerization behaviour of the catalysts and the properties of the obtained polymers, while other articles concentrate on NMR investigations of propylene/norbornene copolymers.     

Study on the copolymerization of propylene with norbornene using metallocene catalysts

The copolymerization of propylene with norbornene, using the metallocene catalysts Me₂Si(2-Me-Ind)₂ZrCl₂ and Ph₂C(Flu)(Cp)ZrCl₂ was evaluated. The presence of norbornene decreases the polymerization activity in both systems. In the Me₂Si(2-Me-Ind)₂ZrCl₂ system, the decrease is ca. 80?% using 0.5?mL of norbornene while the addition of 2?mL reduces the activity by 97?%. The molecular weight of the materials decreases between 27,000 and 37,000?g/mol in the presence of norbornene. The Ph₂C(Flu)(Cp)ZrCl₂/MAO system has the same tendency, but the norbornene has a lesser effect on the activity, with 2?mL of comonomer reducing the activity ca. 80?%. The molecular weight decreased significantly with this system as well. The elongation at break of some of the materials was 80 times higher than the homopolymer and the Young’s modulus slightly superior to the homopolymer. This indicates that it is possible to generate materials that keep the properties of a syndiotactic polypropylene but with good elastomeric properties.     

Copolymerization of norbornene and ethene with homogenous zirconocenes/methylaluminoxane catalysts

The norbornene/ethene copolymerization was investigated by using two C _S-symmetric ([Me₂C(Fluo)(Cp)]ZrCl₂ III, [Ph₂C(Fluo)(Cp)]ZrCl₂ IB) and two C ₂-symmetric ([Me₂Si(Ind)₂]ZrCl₂ I, [Ph₂Si(Ind)₂]ZrCl₂ II) catalysts with methylaluminoxane (MAO) as cocatalyst. This investigation focussed not only on the different polymerization behavior, like catalyst activity, but also considers the material properties of the synthesized copolymers. It was found, that the C _S-symimetric catalysts are very well suitable to yield amorphous copolymers with glass transition temperatures above 180°C and molecular weights >100.000 g/mol. These copolymers could be used as potential starting materials for optical discs and fibers.     

NiP^O and [Cp2ZrCl2/MAO] as a versatile dual-function catalyst system for in situ polymerization of ethylene to linear low-density polyethylene (LLDPE)

[Ni(Ph₂PCHCOPh)(Ph)(PPh₃)] (NiP^O) and Cp₂ZrCl₂/methylaluminoxane (MAO), well known as ethylene oligomerization and polymerization catalysts, respectively, are combined to produce LLDPE by in situ polymerization. Melting temperature (T_m), degree of crystallinity (χ_c), intrinsic viscosity, average molecular weight and ¹³C NMR analysis of copolymers confirm the insertion of α-olefins into the polyethylene chain. Variations in α-olefin concentration and ethylene pressure during the polymerization stage lead to changes in the degree of branching. The resulting copolymers have χ_c and T_m in the 25.8–65.2% and 114–132 °C ranges, respectively. Experimental results show the versatility of the dual-function catalyst.     

Synthesis and electrical properties of the (PVA)0.7(KI)0.3·xH2SO4 (0 ≤ x ≤ 5) polymer electrolytes and their performance in a primary Zn/MnO2 battery

Solid acid polymer electrolytes (SAPE) were synthesised using polyvinyl alcohol, potassium iodide and sulphuric acid in different molar ratios by solution cast technique. The temperature dependent nature of electrical conductivity and the impedance of the polymer electrolytes were determined along with the associated activation energy. The electrical conductivity at room temperature was found to be strongly depended on the amorphous nature of the polymers and H₂SO₄ concentration. The ac (100 Hz to 10 MHz) and dc conductivities of the polymer electrolytes with different H₂SO₄ concentrations were analyzed. A maximum dc conductivity of 1.05 × 10⁻³ S cm⁻¹ has been achieved at ambient temperature for electrolytes containing 5 M H₂SO₄. The frequency and temperature dependent dielectric and electrical modulus properties of the SAPE were studied. The charge transport in the present polymer electrolyte was obtained using Wagner's polarization technique, which demonstrated the charge transport to be mainly due to ions. Using these solid acid polymer electrolytes novel Zn/SAPE/MnO₂ solid state batteries were fabricated and their discharge capacity was calculated. An open circuit voltage of 1.758 V was obtained for 5 M H₂SO₄ based Zn/SAPE/MnO₂ battery.     

Structural and luminescence properties of RE (RE = Eu, Tb):(MgCa)2Bi4Ti5O20 ceramics

Rare-earth ions (Eu³⁺, Tb³⁺) activated magnesium calcium bismuth titanate [(MgCa)₂Bi₄Ti₅O₂₀] ceramics were prepared by conventional solid state reaction method for their structural and luminescence properties. By using XRD patterns, the structural properties of ceramic powders have been analyzed. Emission spectrum of Eu³⁺:(MgCa)₂Bi₄Ti₅O₂₀ ceramic powder has shown strong red emission at 615 nm (⁵D₀ → ⁷F₂) with an excitation wavelength λ_exci = 393 nm and Tb³⁺:(MgCa)₂Bi₄Ti₅O₂₀ ceramic powder has shown green emission at 542 nm (⁵D₄ → ⁷F₅) with an excitation wavelength λ_exci = 376 nm. In addition, from the measurements of scanning electron microscopy (SEM), Fourier transform-infrared (FTIR) and energy dispersive X-ray analysis (EDAX) results the morphology, structure and elemental analysis of these powder ceramics have been studied.     

Rapid preparation and electrochemical behavior of carbon-coated Li3V2(PO4)3 from wet coordination

An improved solid-state coordination method namely wet coordination has been developed to synthesize carbon-coated monoclinic Li₃V₂(PO₄)₃ rapidly at a low temperature of 600 °C in 1 h. The structure of the sample was characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM) and energy dispersive analysis of X-rays (EDAX). The diffusion coefficient of the lithium ions was measured by cyclic voltammetry (CV). The electrochemical behavior of the sample exhibits a high initial discharge capacity of 130 mAh g⁻¹, which is very close to its theoretical capacity of 132 mAh g⁻¹ under 95 mA g⁻¹ (0.67 C) in the range of 3.3-4.3 V (vs. Li/Li⁺). These results suggest that wet coordination is a promising method for large-scale production of carbon-coated Li₃V₂(PO₄)₃.     

Electrocatalyst materials for fuel cells based on the polyoxometalates—K7 or H7[(P2W17O61)Fe(H2O)] and Na12 or H12[(P2W15O56)2Fe4(H2O)2]

Free acids of the iron substituted heteropoly acids (HPA), H₇[(P₂W₁₇O₆₁)Fe^III(H₂O)] (HFe1) and H₁₈[(P₂W₁₅O₅₆)₂Fe^III₂(H₂O)₂] (HFe2) were prepared from the salts K₇[(P₂W₁₇O₆₁)Fe^III(H₂O)] (KFe1) and Na₁₂[(P₂W₁₅O₅₆)₂Fe^III₄(H₂O)₂] (NaFe4), respectively. The iron-substituted HPA were adsorbed on to XC-72 carbon based GDLs to form HPA doped GDEs after water washing with HPA loadings of ca. 1 μmol. The HPA was detected throughout the GDL by EDX. Solution electrochemistry of the free acids are reported for the first time in sulfate buffer, pH 1-3. The hydrogen oxidation reaction was catalyzed by KFe1 at 0.33 V, with an exchange current density of 38 mA/cm². Moderate activity for the oxygen reduction reaction was observed for the iron substituted HPA, which was dramatically improved by selectively removing oxygen atoms from the HPA by cycling the fuel cell cathode under N₂ followed by reoxidation to give a restructured oxide catalyst. The nanostructured oxide achieved an OCV of 0.7 V with a Tafel slope of 115 mV/decade. Cycling the same catalysts in oxygen resulted in an improved catalyst/ionomer/carbon configuration with a slightly higher Tafel slope, 128 mV/decade but a respectable current density of 100 mA/cm² at 0.2 V.     

Dynamic Model of the Copolymerization of Propylene and 1-Hexene with the Me2Si(2-Me-Ind)2ZrCl2 Catalytic System: Effect of 1-Hexene Concentration

The effect of the concentration of 1-hexene on catalytic productivity, copolymer molecular weight, and comonomer incorporation was studied and modeled for the synthesis of isotactic polypropylene copolymers by using the Me₂Si(2-Me-Ind)₂ZrCl₂/MAO catalytic system. The method of moments was employed to simulate the system and to quantify the corresponding catalytic behavior. An increase in the catalytic yield, associated with the “comonomer effect,” together with a decrease in the copolymer average molecular weight, was found when the comonomer concentration was increased. In the range studied, a linear relation between comonomer incorporation and its concentration in the reactor was found. All these behaviors were simulated with a good fit between experimental and predicted values. An important change appears at the highest comonomer concentration studied, associated with modifications in the catalytic behavior, and the model presented here aids in quantifying this performance.