Krystyna Czaja

Studies of Intermolecular Heterogeneity Distribution in Ethylene/1-hexene Copolymers Using DSC Method

The investigation of the intermolecular composition distribution of an ethylene/1-hexene copolymers using DSC method has been carried out. The known methods: step crystallization (SC) and successive self-nucleation/annealing (SSA) have been adapted for this purpose, and particularly, the optimal condition of the process have been chosen to enable the best fractional crystallization of the copolymer. The method has been applied for fractionation of two ethylene/1-hexenecopolymers synthesized with supported vanadium and zirconocene catalysts and having similar concentrations of 1-hexene. Although metallocene catalysts are known from their more homogeneous structure of active sites in comparison to multi-site Ziegler–Natta catalysts, the copolymers obtained over both catalytic systems gave DSC curves resolved into several peaks but with different melting points. Using the Thomson–Gibbs equation, comparable average lamellar thickness of the separated peaks has been calculated. The amounts of copolymer fraction with defined lamellar thickness have been determined. It was obtained that the copolymer produced from the metallocene system contains a thinner and more homogeneous lamella thickness than that obtained with Ziegler–Natta vanadium catalyst supported on the same carrier.

Two‐step polymerization of propylene over MgCl2‐supported titanium catalyst

The prepolymerization effect on propylene polymerization in the presence of a MgCl 2 -supported titanium catalyst was studied. The catalyst was pre-activated by polymerizing a small amount of propylene in the presence of Et 3 Al and cyclohexylmethyldimethoxysilane under mild conditions, and then it was used in a second step of propylene polymerization. Comparative studies of the polymerization process involving the investigated catalyst and its unmodified counterpart showed the rate-enhancement effect of prepolymerization and the same molecular weights, MWDs and isotacticities of the polymers obtained. Concentrations of active sites and propagation rate as well as transfer rate constants were found from detailed kinetic studies. The findings revealed that the activating effect of catalyst prepolymerization resulted from the increased concentration of active sites. This in turn is the result of the slow and uniform fragmentation of initial catalyst agglomerates with the prepolymer created under mild conditions. The prepolymerization process causes no change in the reactivity of active sites since the chain propagation and chain transfer rates are the same for single-step and two-step polymerization reactions; similarly, the properties of the polymers produced are unchanged.

Effect of hydrogen on the ethylene polymerization process over Ziegler–Natta catalysts supported on MgCl2(THF)2. I. Studies of the chain-transfer reaction

The effect of hydrogen on the molecular weight of polyethylene obtained over vanadium catalysts (based on VCl4 and VOCl3) supported on MgCl2(THF)2 was studied and the results were compared to those obtained for similar titanium catalysts. It was confirmed that the dependencies of the transfer reaction on the hydrogen concentration are a half-order in all investigated systems. However, the transition metal of the catalytic site affects the ratio of the transfer rate with hydrogen to the propagation rate (ktr,H/kp) and the results showed that hydrogen is a more effective agent of polyethylene molecular weight control in vanadium-based systems as compared to the titanium catalyst.

Ethylene/1-olefin copolymerization behaviour of vanadium and titanium complexes bearing salen-type ligand

Ethylene/1-olefin copolymerization using vanadium and titanium complexes bearing tetradentate [O,N,N,O]-type ligand and EtAlCl2 or MAO as a cocatalyst is carried out. In the presence of the vanadium complex activated with EtAlCl2 is observed (a) negative “comonomer effect”, (b) high comonomer incorporation and narrow chemical composition distribution (CCD), (c) unexpected copolymer microstructure, and (d) increased molecular weight of copolymers when compared with the homopolymer. In contrast, titanium catalyst gives copolymers with lower 1-olefin content and broad CCD. Supported complexes show higher activity, lower 1-olefins incorporation and give copolymers with ultra high molecular weights.

Effect of immobilization of titanocene catalyst in aralkyl imidazolium chloroaluminate media on performance of biphasic ethylene polymerization and polyethylene properties

Abstract1-(2-Phenylethyl)-3-methylimidazolium and 1-benzyl-3-methylimidazolium chloroaluminates, [Ph-C2mim][AlCl4] and [Bzlmim][AlCl4], were applied as media of the Cp2TiCl2 catalyst for biphasic ethylene polymerization. The studied aralkyl ionic liquids ensure greater stability of the catalyst at higher temperatures and more regular morphology of the produced polyethylene than analogous 1-n-alkyl-3-methylimidazolium chloroaluminates. The alkylaluminium compound participates in the termination reaction of the polymer chain. The catalyst is stable and enables recycling of the ionic liquid phase in the consecutive polymerization reactions. The [Ph-C2mim][AlCl4] ionic liquid and AlEt2Cl alkylaluminium compound turned out to be the most suitable for the biphasic process. The influence of the kind of ionic liquid, alkylaluminium compound (AlEt2Cl and AlEtCl2), activator/catalyst molar ratio, reaction temperature, reaction time and catalyst recycling on the polymerization performance as well as polyethylene properties such as molecular weight (Mw), polydispersity, melting temperature, crystallinity degree, bulk density and particle size is presented.

Nature of activating effect of two‐step polymerization of propylene

The prepolymerization effect on propylene polymerization in the presence of a TiCl 3 -based catalyst, modified by di-n-buthyl ether, was studied. The influence of prepolymerization on the electron spin resonance spectra and morphology of the catalyst, as well as the properties and the morphology of both prepolymer and regular polymerization products, was investigated. The polymer morphology was evaluated through scanning electron microscopy, polymer bulk density, and particle size distribution. Some evidence of the enhancement effect of prepolymerization on the catalyst activity and stereospecificity was obtained. No influence from prepolymerization was observed on molecular weight and its distribution, melting point, and crystallinity of polypropylene. These findings, when discussed in connection with the morphology results of the catalyst and prepolymer, showed that the prepolymerization performed at mild reaction conditions prevents fast and extensive fragmentation of the original catalyst agglomerates. The more controlled breakup of the catalyst particles in the course of slowed growth of prepolymer exposes the occluded catalyst fragments with uniform size and prevents their reagglomeration. Resulting from the above, catalyst homogeneity, catalyst activity, and polymer morphology are improved.

Study of thermal properties of polyethylene and polypropylene nanocomposites with long alkyl chain-substituted POSS fillers

Abstract The effect of incorporation of octakis({alkyl}dimethylsiloxy)octasilsesquioxanes molecules with n-octyl, n-octadecyl and 4-methyl-hexyl substituents on thermal properties of polypropylene (PP), low-density polyethylene (LDPE) and high-density polyethylene (HDPE) was investigated. Thermal properties of those composite materials were evaluated by means of the differential scanning calorimetry (DSC) and thermogravimetric analysis (TG) methods. The type and mass% content of POSS nanofillers influenced the crystallization and melting properties as well as thermal stability of the obtained polyolefin nanocomposites. The incorporated POSS particles—acting as nucleating agents—improved the crystallization process of those polyolefin materials. The POSS nanofiller with n-octyl substituents turned out the most effective nucleating agent. The addition of POSS nanofiller particles into the polyolefin matrix affected the melting behavior of the nanocomposites obtained, decreasing their melting temperatures. Thermal stability under nitrogen as well as in air atmosphere was most significantly enhanced for polyolefin nanocomposites contained the POSS with n-octadecyl substituents. That may result from improved compatibility of the POSS structure with long n-alkyl chain substituents at the silicon–oxygen core. The uniform dispersion of the long n-alkyl chain-substituted POSS was confirmed by SEM analysis.

Effect of hydrogen on the ethylene polymerization process over Ziegler–Natta catalysts supported on MgCl2(THF)2. II. Kinetic studies

This article reports on a study of the effects of hydrogen on the activity of vanadium and titanium catalysts supported on MgCl 2 (THF) 2 in ethylene polymerization. It was found that hydrogen did not change the stable nature of the active sites and the polydispersity index of the polyethylene obtained. The propagation rate, expressed as k p , was found to be independent of the presence and concentration of hydrogen, indicating that this reacting agent does not modify the reactivity of the active sites. However, the presence of hydrogen in the polymerization medium is responsible for partial deactivation of the active sites just before polymerization is initiated.

High crystallinity polyethylene obtained in biphasic polymerization using pyridinium chloroaluminate ionic liquid

A series of N-n-alkylpyridinium chloroaluminate ionic liquids [Cn-py][AlCl4] (where n-alkyl = n-butyl, n-hexyl, and n-octyl) was applied as a medium of the Cp2TiCl2 catalyst, activated by AlEtCl2 or AlEt2Cl, to evaluate the influence of the studied ionic liquids on the performance of the biphasic ionic liquid/hexane ethylene polymerization and the properties of the produced polyethylene (PE). The best results were obtained using N-n-butylpyridinium chloroaluminate. The polyethylene obtained in the biphasic polymerization have the high crystallinity, which was confirmed by DSC, WAXS and PALS methods, as well as the bulk density comparable to commercial HDPE. These unique properties results from the biphasic mode of the ethylene polymerization where the ionic liquid is used to immobilize the catalyst, and polyethylene is produced in the ionic liquids phase.

Polymerization of ethylene by oxide-supported titanium halide catalyst: kinetic model with a deactivation of active species

Abstract The effect of the calcination temperature of alumina, which was then used as a support for a titanium halide catalyst [TiCl4/Et2AlCl], on the catalyst activity in ethylene polymerization was investigated. α-Al2O3 was found to make a more advantageous catalyst support as compared to γ-Al2O3 despite the fact that the former offered a clearly lower specific surface area and its content of surface OH groups was inferior. The ethylene polymerization in the presence of the catalytic system on different alumina supports was investigated on the basis of a proposed kinetic model, taking into consideration the deactivation of active sites in the process. The improved activity was found to result from the better utilisation of adsorbed titanium chloride in the formation of active sites (increase of [C 0 ∗ ] ) and from the higher stability of said sites (lower kt), while their chemical nature underwent no significant change (kp and ktr remain unchanged).