Ethylene Hexene Research Articles

Copolymerization of Ethylene with Alpha-Olefins over Supported Titanium–Magnesium Catalysts Containing Titanium Compounds in Different Oxidation and Coordination States

Data were obtained on the copolymerization of ethylene with α-olefins over supported titanium–magnesium catalysts (TMC) prepared on the same magnesium dichloride support but differing in the composition and oxidation state of titanium. The copolymerization kinetics of ethylene with 1-hexene over TMC of different compositions were studied. Data on the composition of the produced ethylene–1-hexene copolymers, their molecular weight distribution, thermophysical characteristics, and branching distribution were presented. The constants of ethylene–1-hexene copolymerization over catalysts with different compositions were calculated. The TMC containing only Ti(II) compounds as the active component exhibited increased copolymerizing ability compared to the conventional TiCl4/MgCl2 catalyst containing Ti(III) compounds as the active component. In addition, TMC with Ti(II) as an active component produces copolymers with a more uniform branching distribution. It was shown that the TMC containing isolated Ti(II) ions could be used to produce X-ray amorphous ethylene-propylene elastomers with a high yield.

Very Sensitive 13C NMR Method for the Detection and Quantification of Long-Chain Branches in Ethylene–Hexene Linear Low-Density Polyethylene

Very Sensitive ¹³C NMR Method for the Detection and Quantification of Long-Chain Branches in Ethylene–Hexene Linear Low-Density Polyethylene

Long-Chain Branch Detection and Quantification in Ethylene–Hexene LLDPE with 13C NMR

Ethylene–hexene linear low-density polyethylene (LLDPE) is widely used in the packaging industry due to its outstanding mechanical properties. Some LLDPEs contain long-chain branching (LCB), which ...

Structural, Rheological, and Mechanical Properties of Binary Compounds Based on High-Density Polyethylene and Linear Low-Density Polyethylene

Rheological, molecular, thermal, and mechanical properties of binary blends based on bimodal high-density polyethylene (HDPE; ethylene–1-hexene copolymer) and unimodal linear low-density polyethylene (LLDPE; ethylene–1-butene–1-hexene terpolymer) were studied. Six blends with different HDPE/LLDPE weight ratios, 80/20, 70/30, 60/40, 50/50, 40/60, and 20/80, were prepared by blending in a double-worm extruder. The correlation of the rheological properties of the blends, analyzed by the capillary and rotation methods, with the melt flow index and molecular characteristics was demonstrated. LLDPE with increased flowability (MFI = 3 g/10 min) as a polymer additive is an effective plasticizer allowing control of the melt viscosity in a wide range and choice of the optimum target value of the melt flow index of the blend. The activation energy of the melt flow was calculated, and the relationship between this quantity and the content of short-chain branchings in polyethylene blends was revealed. The blends obtained are homogeneous, but an increase in the content of LLDPE in HDPE leads to a change in the polyethylene structure, which is confirmed by a decrease in the crystallinity and in the lamella thickness. The strength values at the yield point and at break decrease with an increase in the fraction of LLDPE in HDPE. Changes in the properties of these blends in their processing into a film by flat-die extrusion were studied.

Ethylene/1-Hexene Copolymerization with Modified Ziegler-Natta Catalyst

Siloxane compounds were treated with the compounds containing internal donors, such as aromatic ester, phosphate, or 1,3-diether compounds, and the resulting intermediates were further reacted with TiCl4 to form the modified Ziegler-Natta catalysts for ethylene/1-hexene(E-H) copolymerization. Using the modified Ziegler-Natta catalysts, the effect of the internal donors and the molar ratio of alcohol to magnesium on the catalyst performance was investigated by the orthogonal experiments. The synthetic method of the catalysts was also optimized by choosing the proper way to mix the internal donors. The branch degree, sequence structure, molecular weight, and molecular weight distribution of the copolymer products were determined by Fourier transform infrared spectroscopy(FTIR), carbon-13 nuclear magnetic resonance(13C NMR) spectrometry and gel permeation chromatography(GPC). Under the optimum conditions, the catalytic system with tetraethyl orthosilicate and 2,2-dimethoxypropane as the internal donors exhibited the best performance with an activity of 926.74 g copolymer/(g Cat·h−1). The branch degree, 1-hexene content, and molecular weight distribution of the resulting copolymers reached up to 40 branches/1000C, 5.99% and 5.8, respectively.

Concentration fluctuations in the early stages of LLPS and partial dissolution of melt-memory in broadly distributed ethylene copolymers

The mechanism of liquid-liquid phase separation (LLPS) is investigated in a random ethylene 1-hexene copolymer (E/H) with a broad comonomer content distribution. SANS measurements were first carried out using the original resin with deuterium added via direct isotope exchange to preserve the original branching distribution. Upon lowering the melt temperature, the upshift of SANS profile observed only below a critical scattering vector is consistent with the signature of nucleation and growth during LLPS in the metastable region. Differences in branching distribution between the original broad resin and a broad multi-components blend of narrow E/H copolymers studied previously, result in slower LLPS dynamics in the former that has a more continuous change of comonomer content between molecules. Time-resolved light scattering and light transmittance measurements in the metastable region of the broad resin, reveal the nucleation and growth stages of LLPS. Dissolution of crystalline melt-memory, associated with an inversion of the crystallization rate with decreasing the temperature of the initial melt, is found to occur mainly within the LLPS induction period and is associated with concentration fluctuations prior to the LLPS nucleation step. The typical sea-island morphology associated with LLPS via nucleation and growth is observed under bright-field optical microscopy after one-day annealing in the metastable region.

SANS Evidence of Liquid–Liquid Phase Separation Leading to Inversion of Crystallization Rate of Broadly Distributed Random Ethylene Copolymers

Aiming to understand the inversion of crystallization kinetics observed by DSC, detailed SANS investigations of the melt structure of a broadly distributed ethylene–1-hexene copolymer have been undertaken in a wide range of temperatures that were reached either by heating the solid or cooling from the homogeneous melt state. In both cases, the observed SANS signal transitions from a scattering cross section consistent with a homogeneous melt state (high temperature range) to an intensity that in the low Q region displays the characteristics of the Porod region for particles dispersed in a homogeneous matrix (low melt temperature range). The latter structure is consistent with demixing of the highly branched molecules and corroborates the postulated liquid–liquid phase separation (LLPS) as an explanation for the peculiar crystallization kinetics observed by DSC. The solution temperature is found at 160 °C by heating the solid and at 150 °C when cooling from the one-phase melt, thus denoting the effect of c...

Single-Site Organozirconium Catalyst Embedded in a Metal-Organic Framework.

A structurally well-defined mesoporous Hf-based metal-organic framework (Hf-NU-1000) is employed as a well-defined scaffold for a highly electrophilic single-site d(0) Zr-benzyl catalytic center. This new material Hf-NU-1000-ZrBn is fully characterized by a variety of spectroscopic techniques and DFT computation. Hf-NU-1000-ZrBn is found to be a promising single-component catalyst (i.e., not requiring a catalyst/activator) for ethylene and stereoregular 1-hexene polymerization.

Effects of supported metallocene catalyst active center multiplicity on antioxidant-stabilized ethylene homo- and copolymers

A silica-supported bis(n-butylcyclopentadienyl) zirconium dichloride [( n BuCp)2ZrCl2] catalyst was synthesized. This was used to prepare an ethylene homopolymer and an ethylene–1-hexene copolymer. The active center multiplicity of this catalyst was modeled by deconvoluting the copolymer molecular mass distribution and chemical composition distribution. Five different active site types were predicted, which matched the successive self-nucleation and annealing temperature peaks. The thermo-oxidative melt stability, with and without Irganox 1010 and Irgafos 168, of the above polyethylenes was investigated using nonisothermal differential scanning calorimetric (DSC) experiments at 150 °C. This is a temperature that ensures complete melting of the samples and avoids the diffusivity of oxygen to interfere into polyethylene crystallinity and its thermo-oxidative melt degradation. The oxidation parameters such as onset oxidation temperature, induction period, protection factor, and S-factor were determined by combining theoretical modeling with the DSC experiments. Subsequently, these findings were discussed considering catalyst active center multiplicity and polymer microstructure, particularly average ethylene sequence length. Several insightful results, which have not been reported earlier in the literature, were obtained. The antioxidant effect, for each polymer, varied as (Irganox + Irgafos) ≈ Irganox > Irgafos > Neat polymer. The as-synthesized homopolymer turned out to be almost twice as stable as the corresponding copolymer. The antioxidant(s) in the copolymer showed higher antioxidant effectiveness (AEX) than those in the homopolymer. Irganox exhibited more AEX than Irgafos. To the best of our knowledge, such findings have not been reported earlier in the literature. However, mixed with Irganox or Irgafos, their melt oxidation stability was comparable. The homopolymer, as per the calculated S-factor, showed Irganox–Irgafos synergistic effect five times that of the copolymer. This illustrates how the transition in backbone structure, from exceedingly high to low ethylene sequence length, influences antioxidant synergistic performance. Finally, this study shows a DSC-aided approach that can elucidate the effect of polyethylene structural backbone on its thermo-oxidative melt degradation as well as antioxidant synergism in a facile fashion.

Tandem Action of SNS-Cr and CGC-Ti in Preparation of Ethylene-1-Hexene Copolymers from Ethylene Feedstock

Ethylene–1-hexene copolymer materials, ranging from semicrystalline linear low-density poly­ethylene (LLDPE) to completely amorphous polyethylenes (PEs), are prepared from ethylene alone in a single reactor by the tandem polymerization of bis(2-dodecylsulfanyl-ethyl)amine–CrCl3 (SNS–Cr) and (N-tert-butylamido) (tetramethylcyclopentadienyl)–titanium dichloride (CGC–Ti) at 75 °C and under atmospheric pressure. The polymerization activities are on the order of 105–106 g (mol Ti)−1 h−1. 1-Hexene incorporation in the resulting copolymers can be adjusted by varying the Cr–Ti molar ratio and/or applying a short period of pre-trimerization. Copolymers with high 1-hexene incorporation up to 15 mol% are obtained. Few vinyl and vinylene chain ends are detected by 13C NMR, suggesting that 1-hexene does not act as a chain-transfer agent. Narrow molecular-weight distributions with polydispersities from 1.9 to 2.6 are obtained, characteristic of a single-active-site nature of the catalyst system.

The evolutionary development of chain microstructure during tandem polymerization of ethylene: A Monte Carlo simulation study

Employing a Kinetic Monte Carlo (KMC) simulation algorithm, tandem polymerization of ethylene is thoroughly investigated. To do this, the evolution of ethylene copolymerization is evaluated in terms of ethylene consumption and 1-hexene accumulation. The results obtained are in good agreement with the existing experimental kinetic data. It is revealed that the pre-trimerization time is a key factor in controlling the architecture of copolymer chains. Moreover, the precise computation of instantaneous and cumulative comonomer contents along with the chemical composition distribution of simulated copolymers provides a comprehensive image of the growing chains during tandem polymerization. A well-established crystallization fractionation (CRYSTAF) analysis mathematical model is applied to delve into the molecular architecture and the crystallization fractionation behavior of the produced polyolefins.

Effects of supported (nBuCp)2ZrCl2 catalyst active center multiplicity on crystallization kinetics of ethylene homo- and copolymers

Two different supported zirconocene, that is, bis(n-butylcyclopentadienyl) zirconium dichloride (nBuCp)2ZrCl2, catalysts were synthesized. Each catalyst was used to prepare one ethylene homopolymer and one ethylene–1-hexene copolymer. Catalyst active center multiplicity and polymer crystallization kinetics were modeled. Five separate active center types were predicted, which matched the successive self-nucleation and annealing (SSA) peak temperatures. The predicted crystallinity well matched the differential scanning calorimetric (DSC) values for a single Avrami–Erofeev index, which ranged between 2 and 3 for the polymers experimented. The estimated apparent crystallization activation energy Ea did not vary with cooling rates, relative crystallinity α, and crystallization time or temperature. Therefore, the concept of variable/instantaneous activation energy was not found to hold. Ea linearly increased with the weight average lamellar thickness Lwav DSC-GT; and for each homopolymer, it exceeded that of the corresponding copolymer. Higher Ea, hence slower crystallization, was identified as a pre-requisite to attain higher crystallinity. Crystallization parameters were correlated to polymer backbone parameters, which are influenced by catalyst active center multiplicity.

Silica-supported (n BuCp)2 ZrCl2 : effect of catalyst active center distribution on ethylene-1-hexene copolymerization

Metallocenes are a modern innovation in polyolefin catalysis research. Therefore, two supported metallocene catalysts—silica/MAO/(nBuCp)2ZrCl2 (Catalyst 1) and silica/nBuSnCl3/MAO/(nBuCp)2ZrCl2 (Catalyst 2), where MAO is methylaluminoxane—were synthesized, and subsequently used to prepare, without separate feeding of MAO, ethylene–1-hexene Copolymer 1 and Copolymer 2, respectively. Fouling-free copolymerization, catalyst kinetic stability and production of free-flowing polymer particles (replicating the catalyst particle size distribution) confirmed the occurrence of heterogeneous catalysis. The catalyst active center distribution was modeled by deconvoluting the measured molecular weight distribution and copolymer composition distribution. Five different active center types were predicted for each catalyst, which was corroborated by successive self-nucleation and annealing experiments, as well as by an extended X-ray absorption fine structure spectroscopy report published in the literature. Hence, metallocenes impregnated particularly on an MAO-pretreated support may be rightly envisioned to comprise an ensemble of isolated single sites that have varying coordination environments. This study shows how the active center distribution and the design of supported MAO anions affect copolymerization activity, polymerization mechanism and the resulting polymer microstructures. Catalyst 2 showed less copolymerization activity than Catalyst 1. Strong chain transfer and positive co-monomer effect—both by 1-hexene—were common. Each copolymer demonstrated vinyl, vinylidene and trans-vinylene end groups, and compositional heterogeneity. All these findings were explained, as appropriate, considering the modeled active center distribution, MAO cage structure repeat units, proposed catalyst surface chemistry, segregation effects and the literature that concerns and supports this study. While doing so, new insights were obtained. Additionally, future research, along the direction of the present work, is recommended. © 2013 Society of Chemical Industry

Polymer chain conformation of copolymers with different monomer size: 13C NMR spectroscopy and MALS study

Copolymer composition, distribution and molecular size of the comonomer influence the final properties of polymer materials. Such influence can be followed from the effect on the chain conformation in solution determined from the scaling law between radius of gyration and molecular weight.In this work, several copolymers, with similar comonomer content (mol%) and distribution but with different comonomer size, have been studied. Ethylene/propylene (EP), ethylene/1-hexene (EH), ethylene/4-methyl-1-pentene (EMP) and propylene/1-butene (PB) with a comonomer content up to 20mol% have been analyzed.Characterization was carried out by nuclear magnetic resonance (13C NMR spectroscopy) and by gel permeation chromatography with a multi-angle light scattering (GPC-MALS). Obtained copolymers present a relatively homogeneous comonomer sequence distribution along the chain, but 13C NMR spectroscopy results revealed a more homogeneous comonomer chain distribution for EP copolymers, probably due to the monomer size.MALS data were adequately treated with algorithm for self data reduction. No significant effect of overall composition was found and the MALS results were averaged for each kind of copolymers. Linear dependence of log (Rg) vs. log (n) according to regular scaling law was adequate for EP and PB samples along the whole analyzed range. However, a marked nonlinearity was found for EH and EMP samples and, consequently, a unique linear regression of log (Rg) vs. log (n) was not recommended. A new method of regression including two different contributions was developed in this work and different values for the shape parameter (q) was obtained in function of molecular weight range.

Effects of Supported (nBuCp)2ZrCl2Catalyst Active-Center Distribution on Ethylene–1-Hexene Copolymer Backbone Heterogeneity and Thermal Behaviors

Two catalysts, denoted as catalyst 1 [silica/MAO/(nBuCp)2ZrCl2] and catalyst 2 [silica/nBuSnCl3/MAO/(nBuCp)2ZrCl2] were synthesized and subsequently used to prepare, without separate feeding of methylaluminoxane (MAO), ethylene homopolymer 1 and homopolymer 2, respectively, and ethylene–1-hexene copolymer 1 and copolymer 2, respectively. Gel permeation chromatography (GPC), Crystaf, differential scanning calorimetry (DSC) [conventional and successive self-nucleation and annealing (SSA)], and 13C nuclear magnetic resonance (NMR) polymer characterization results were used, as appropriate, to model the catalyst active-center distribution, ethylene sequence (equilibrium crystal) distribution, and lamellar thickness distribution (both continuous and discrete). Five different types of active centers were predicted in each catalyst, as corroborated by the SSA experiments and complemented by an extended X-ray absorption fine structure (EXAFS) report published in the literature. 13C NMR spectroscopy also supported this active-center multiplicity. Models combined with experiments effectively illustrated how and why the active-center distribution and the variance in the design of the supported MAO anion, having different electronic and steric effects and coordination environments, influence the concerned copolymerization mechanism and polymer properties, including inter- and intrachain compositional heterogeneity and thermal behaviors. Copolymerization occurred according to the first-order Markovian terminal model, producing fairly random copolymers with minor skewedness toward blocky character. For each copolymer, the theoretical most probable ethylene sequences, nE MPDSC-GT and nE MPNMR-Flory, as well as the weight-average lamellar thicknesses, Lwav DSC–GT and Lwav SSA DSC, were found to be comparable. To the best of our knowledge, such a match has not previously been reported. The percentage crystallinities of the homo- and copolymers increased linearly as a function of LMPDSC-GT. This indicates that the homo- and copolymer chains folded excluding the butyl branch. The results of the present study will contribute to developing future supported metallocene catalysts that will be useful in the synthesis of new grades of ethylene−α-olefin linear low-density polyethylenes (LLDPEs).

Synthesis, Structural Characterization, and Olefin Polymerization Behavior of Vanadium(III) Complexes Bearing Tridentate Schiff Base Ligands

Vanadium(III) complexes bearing tridentate salicylaldiminato ligands (2a−f) [OC6H4CH═NL]VCl2(THF) (L = CH2CH2OMe, 2a; CH2CH2NMe2, 2b; CH2C5H4N, 2c; 8-C9H6N (quinoline), 2d; 2-MeSC6H4, 2e; 2-Ph2PC6H4, 2f) and tridentate β-enaminoketonato ligands [OC6H8CH═N-2-Ph2PC6H4]VCl2(THF) (2g) and [O(Ph)C═CHCH═N-2-Ph2PC6H4]VCl2(THF) (2h) were prepared from VCl3(THF)3 by treating with 1.0 equiv of the deprotonated ligands in tetrahydrofuran (THF). These complexes were characterized by FTIR and mass spectrometry as well as elemental analysis. Structures of complexes 2e, 2f, and 2h were further confirmed by X-ray crystallographic analysis. These complexes were investigated as catalysts for olefin polymerization in the presence of organoaluminum compounds. On activation with Et2AlCl, complexes 2a−h exhibited high catalytic activities toward ethylene polymerization (up to 20.64 kg PE/mmolV·h·bar) even at high temperature, suggesting these catalysts possess high thermal stability. Moreover, high molecular weight polymers with unimodal molecular weight distribution can be obtained, indicating the single site behavior of these catalysts. The copolymerizations of ethylene and norbornene or 1-hexene with catalysts 2a−h were also explored in the presence of Et2AlCl, which led to high molecular weight poly(ethylene-co-1-hexene)s (Mw up to 138 000) and poly(ethylene-co-norbornene)s (Mw up to 164 000). Catalytic activity, comonomer incorporation, and polymer molecular weight can be controlled in a wide range by the variation of catalyst structure and the reaction parameters such as Al/V molar ratio, comonomer feed concentration, and polymerization reaction temperature.

Precisely and Irregularly Sequenced Ethylene/1-Hexene Copolymers: A Synthesis and Thermal Study

Step-growth acyclic diene metathesis (ADMET) polymerization chemistry followed by exhaustive hydrogenation offers a new alternative in modeling ethylene/1-hexene (EH) copolymers. In contrast to chain-growth chemistry, this new approach produces well-defined, defect-free primary structures. This report describes the synthesis, characterization, and thermal behavior of ADMET-produced polyethylene materials containing either precisely or irregularly spaced butyl branches, the latter to serve as models for ethylene/1-hexene copolymers made via chain-growth chemistry. The thermal behavior of the new materials was studied using differential scanning calorimetry, and detailed NMR and IR analyses permitted the characterization of the primary structures. Properties of the here presented ethylene/1-hexene copolymers models can be varied from semicrystalline to fully amorphous by precise control of comonomer content and spacing.

Behaviour of the very-low-temperature crystallization peak of linear low-density polyethylene

The crystallization behaviour of Ziegler–Natta (ZN) and single-site catalyst based ethylene–1-butene and ethylene–1hexene copolymers with different comonomer content were studied by differential scanning calorimetry. In addition to a hightemperature crystallization peak, and for ZN copolymers in addition to a low-temperature crystallization peak, quite often a verylow-temperature crystallization peak (VLTCP) was observed at temperatures in between approximately 330 and 345 K. It was found that the VLTCP temperature decreased with increasing comonomer content and did not depend on the type of catalyst used. The fractional degree of crystallinity calculated from the VLTCP was independent of the chemical nature and content of the comonomers present as well as of the polydispersity of molar mass within the used range of magnitudes. However, crystallinity as related to the area of the VLTCP was strongly catalyst type dependent and was higher for the single-site catalyst used compared to the ZN catalyst used.

Titanium-magnesium catalysts for olefin polymerization - effect of titanium oxidation state on catalyst performance

In this work, we investigated the influence of oxidation state of titanium and the effect of ethoxy groups' introduction into titanium compounds on the catalytic properties of MgCl2 - supported catalysts [with triisobutylaluminum (TIBA) or triethylaluminum (TEA) as cocatalyst] in ethylene polymerization and ethylene 1-hexene copolymerization, and on the polymer characteristics. To prepare supported titanium-magnesium catalysts of various compositions, soluble compounds of titanium in different oxidation states [?6-benzene-Ti2+Al2Cl8, Ti3+Cl3źnDBE, and Ti4+(OEt)2Cl2] were synthesized, analyzed by ESR and 13C NMR methods, and immobilized on the highly dispersed magnesium chloride. The data were obtained on the effects produced by composition and oxidation state of titanium compounds (covering also commonly used TiCl4) on the activity, control of PE molecular weight by hydrogen, copolymerization ability, and molecular weight distribution (MWD) of polyethylene. The results obtained demonstrate that different titanium precursors comprising Ti(II), Ti(III) and Ti(IV) compounds supported on MgCl2 allow preparing the highly active catalysts for ethylene polymerization or ethylene/1-hexene copolymerization. It was found that titanium oxidation state in the initial titanium compounds had a weak effect on the molecular weight and MWD of PE, whereas the ligand environment of titanium affected this parameter more strongly.

Modeling and kinetics of tandem polymerization of ethylene catalyzed by bis(2-dodecylsulfanyl-ethyl)amine- [formula omitted] and [formula omitted

A mathematical model was developed to describe ethylene–1-hexene copolymerization with a tandem catalysis system. A series of semi-batch polymerization runs catalyzed by a trimerization catalyst bis(2-dodecylsulfanyl-ethyl)amine- CrCl 3 and a copolymerization catalyst Et ( Ind ) 2 ZrCl 2 in toluene at 74 ∘ C were carried out to verify the model. Both experimentation and modeling showed that adjusting the Cr/Zr ratio yielded various branching densities and thus melting temperatures, as well as molecular weights and polydispersities. Broad composition distributions and thus broad DSC curves were observed at high Cr/Zr ratios. Modeling results elucidated that this is due to an accumulation of 1-hexene component and to composition drifting during the copolymerization. It was also found that applying a short time period of pre-trimerization improved homogeneity in chain microstructure and minimized broadening in DSC curves.