MgCl2-supported Ziegler-Natta Research Articles

Alkynyl Ether Labeling: A Selective and Efficient Approach to Count Active Sites of Olefin Polymerization Catalysts

Accurately measuring molecular kinetics of catalytic olefin polymerization has been a challenging objective. Many methods have been proposed in the literature, but all of them have drawback(s). In this paper, we introduce a labeling method to count active sites employing methyl propargyl ether (MPE) as the quench-labeling agent. It is commercially available, does not react with Al-alkyl species, and has a labeling efficiency close to 100%. The labeling reaction was evidenced by a mechanistic study on the reaction between the model system Cp2ZrMe2/MAO (Cp = cyclopentadienyl) and MPE that it may occur through a coordination–insertion mechanism without noticeable multiple insertions. The method was benchmarked by studying a MgCl2-supported Ziegler–Natta catalyst in 1-hexene polymerization. The fraction of the active transition metal χ* is found to be <1%. It rises in the very first few seconds and reaches a plateau within 10 s in the absence of precontact. With precontact, χ* remains constant over polymer yield from 0.3 up to 5.3 g(PH) g(cat)−1, demonstrating that χ* does not necessarily grow during polymerization. Longer precontact resulted in decreased χ* and decreased average rate constant of propagation <kp> which is in the order of 102 M–1 s–1.

Comparative Study on Kinetics of Ethylene and Propylene Polymerizations with Supported Ziegler⁻Natta Catalyst: Catalyst Fragmentation Promoted by Polymer Crystalline Lamellae.

The kinetic behaviors of ethylene and propylene polymerizations with the same MgCl2-supported Ziegler–Natta (Z–N) catalyst containing an internal electron donor were compared. Changes of polymerization activity and active center concentration ([C*]) with time in the first 10 min were determined. Activity of ethylene polymerization was only 25% of that of propylene, and the polymerization rate (Rp) quickly decayed with time (tp) in the former system, in contrast to stable Rp in the latter. The ethylene system showed a very low [C*]/[Ti] ratio (<0.6%), in contrast to a much higher [C*]/[Ti] ratio (1.5%–4.9%) in propylene polymerization. The two systems showed noticeably different morphologies of the nascent polymer/catalyst particles, with the PP/catalyst particles being more compact and homogeneous than the PE/catalyst particles. The different kinetic behaviors of the two systems were explained by faster and more sufficient catalyst fragmentation in propylene polymerization than the ethylene system. The smaller lamellar thickness (<20 nm) in nascent polypropylene compared with the size of nanopores (15–25 nm) in the catalyst was considered the key factor for efficient catalyst fragmentation in propylene polymerization, as the PP lamellae may grow inside the nanopores and break up the catalyst particles.

Silyl diol ester as a new selectivity control agent in MgCl2-supported Ziegler-Natta systems for propylene polymerization: catalyst structure and polymer properties.

In this study, bis(benzoyloxy)dimethylsilane (SDE) was developed as a non-phthalate selectivity control agent (internal donor (ID) and external donor (ED)) in MgCl2-supported Ziegler–Natta (ZN) systems. The impact of SDE as an ID was investigated in regards to chemical composition, morphology, adsorption behavior (using FTIR spectroscopy, WAXD and SEM studies) and the propylene polymerization performance of the catalyst. The results of the adsorption behavior of SDE revealed that SDE, while able to stabilize the electronically unsaturated surfaces of MgCl2 [(104) and (110)], has a rather high tendency to be absorbed on strongly acidic Mg ions at the corners of these crystals. The catalyst with optimum SDE content as an ID afforded a system with reasonable activity and isotacticity (even in the absence of ED) along with broader molecular weight distribution (PDI: 6.2–10) and greater flexural modulus than phthalate-based ZN systems. SDE was also used as a new ED in most widely used fourth- and fifth-generation catalyst systems. The results revealed that in fifth-generation catalyst systems, there was an increase in hydrogen response (>40%) along with an increase of activity without any change in the thermal properties of the final polymer in comparison to a conventional ED (alkoxy silane).

Insight into the Synthesis Process of an Industrial Ziegler–Natta Catalyst

In Ziegler–Natta catalysis, the catalyst particle size has a strong influence not only on catalyst performance but also on the morphology and particle size distribution of the final polymer particles. Fundamental insight into the catalyst particle formation process is therefore of industrial importance when addressing specific requirements in the final products. In the present work, we fully characterize a single-step catalyst preparation process, which comprises a reactive precipitation of a MgCl2-supported Ziegler–Natta catalyst, through decomposition of the hetero-bimetallic complex, Mg(OR)2·Ti(OR)4, by addition of ethyl aluminum dichloride (EADC). We track the evolution of both of the concentrations of the metals (Mg, Ti, Al) as well as Cl in the liquid phase and the size of the formed catalyst particles. It is observed that the liquid-phase composition is governed by the EADC feed rate under fully Cl-starved conditions. The process can be divided into two stages: The first stage is dominated by the p...

Enhancing stereoselectivity of propylene polymerization with MgCl2-supported Ziegler-Natta catalysts by electron donor: Strong effects of titanium dispersion state

Propylene polymerization with two TiCl4/MgCl2 catalysts (Cat-1: Ti 0.1%; Cat-2: Ti 1.0%) showed different responses to (cyclo-C5H9)2Si(OCH3)2 (De) addition. Cat-1 was deactivated more than Cat-2 by De. Amount of three groups of active centers producing atactic, medium isotactic and isotactic polypropylene chains were all reduced by De addition, but those of Cat-1 were reduced more. Reactivity of the isospecific active centers (kpi) was enhanced by De, but kpi of Cat-2 was changed more sensitively than Cat-1. The lower stereoselectivity of Cat-1 than Cat-2 are attributable to weaker De adsorption on its surface Mg, and the more sparsely dispersed Ti species on Cat-1 is responsible for weak De adsorption.

Mechanistic study on comonomer effect in ethylene/1-hexene copolymerization with TiCl4/MgCl2 model Ziegler-Natta catalysts

Two MgCl2-supported Ziegler-Natta model catalysts were prepared by contacting activated MgCl2 with deficient or excess amount of TiCl4. Ethylene/1-hexene copolymerization with the catalysts was conducted under different 1-hexene feed, and active center concentration of the reaction system was determined by quench-labeling method using thiophene-2-carbonyl chloride as the quencher. The catalytic activity was only moderately enhanced (increment <50%) by adding 1-hexene in ethylene polymerization with the model catalyst (Cat-1) of 0.1 wt% Ti content, and the active center ratio ([C*]/[Ti]) was only slightly increased (from 64 to 72 mol%). The comonomer activation effect in Cat-1 catalyzed copolymerization was mainly attributed to increase of apparent propagation rate constant by the comonomer, which can be explained by reduction of mass transfer barrier in the polymer/catalyst particles because of larger monomer diffusion coefficients in the copolymer phase than in the more crystalline polyethylene phase. In contrast, the activity was nearly tripled by adding 1-hexene when the catalyst of 1.47% Ti content (Cat-2) was used, meanwhile its [C*]/[Ti] was also tripled (from 15 to 49 mol%). The strong comonomer activation effect in Cat-2 catalyzed copolymerization was caused by marked increase of active center concentration. In this case, fragmentation of the polymer/catalyst particles was intensified by adding the comonomer, and more active centers that were originally inaccessible to the cocatalyst and monomer were exposed through the particle fragmentation. According to the results of this work, the comonomer effect in conventional supported Ziegler-Natta catalysts can be explained by the combination of physical factor (increase in diffusion coefficient) and chemical factor (exposure of inaccessible active center precursors through particle fragmentation).

Synthesis of Perylene-Tagged Internal and External Electron Donors for Magnesium Dichloride Supported Ziegler–Natta Catalysts

We report on the synthesis of three perylene-tagged electron donors representing three major types – phthalates, diethers, and alkoxysilanes – which are of importance for the subsequent studies of MgCl2-supported Ziegler–Natta catalysts by means of laser scanning confocal fluorescence microscopy. The obtained products were unambiguously characterized, including by X-ray crystal structure analysis; their photophysical properties (absorption and emission spectra) were investigated as well. Additionally, a reliable and convenient protocol for the multigram synthesis of the required starting material – 3-bromo­perylene (PerBr) – was developed. The key step of this method was synthesis of trialkylsilyl-substituted perylenes, which were further separated by means of flash chromatography followed by conversion of the isolated 3-trialkylsilyl-substituted product to PerBr.

Probing Interactions between Electron Donors and the Support in MgCl2-Supported Ziegler-Natta Catalysts.

Olefin polymerization using Ziegler–Natta catalysts (ZNCs) is an important industrial process. Despite this, fundamental insight into the inner working mechanisms of these catalysts remains scarce. Here, we focus on the low-γ nuclei 25Mg and 35Cl for an in-depth solid-state NMR and density functional theory (DFT) study of the catalyst’s MgCl2 support in binary adducts prepared by ball-milling. Besides the bare MgCl2 support and a MgCl2–TiCl4 adduct, samples containing donors that are part of the families of 2,2-dialkyl-1,3-dimethoxypropanes and phthalates used in fourth- and fifth-generation ZNCs are studied. DFT calculations indicate that the quadrupolar coupling parameters of the chlorines differ significantly between bulk and surface sites. As a result, the NMR visibility of the chlorine sites correlates with the particle size except for the adduct with 2,2-dimethyl-1,3-dimethoxypropane donor. The DFT calculations furthermore show that the surface sites are fairly insensitive to binding of different donor molecules, making it difficult to identify specific binding motives. The surface sites with large 35Cl NMR line widths can be observed using high radio frequency field strengths. For the 2,2-dimethyl-1,3-dimethoxypropane donor, we observe additional surface sites with intermediately high quadrupolar couplings, suggesting a different surface structure for this particular adduct compared to the other systems. For 25Mg, pronounced effects of donor binding on the quadrupole interaction parameters are observed, both computationally and experimentally. Again the adduct with the 2,2-dimethyl-1,3-dimethoxypropane donor shows a different behavior of the surface sites compared to the other adducts, which display more asymmetric coordinations of the surface Mg sites. Identifying specific binding motives by comparing 25Mg NMR results to DFT calculations also proves to be difficult, however. This is attributed to the existence of many defect structures caused by the ball-milling process. The existence of such defect structures both at the surface and in the interior of the MgCl2 particles is corroborated by NMR relaxation studies. Finally, we performed heteronuclear correlation experiments, which reveal interactions between the support and Mg–OH surface groups, but do not provide indications for donor–surface interactions.

Synthesis of spherical polyethylene/poly(1-butene) reactor blends with two-stage sequence polymerization technology

In this paper, a series of polyethylene (PE)/poly(1-butene) (PB) reactor blends (PE/PB reactor blends) with excellent spherical particle morphology were firstly prepared by MgCl2-supported Ziegler-Natta catalyst with sequential two-stage polymerization technology. The polyethylene particles formed in the first stage ethylene polymerization can be used as micro-reactors to initiate the bulk polymerization of 1-butene. The spherical reactor blends as novel thermoplastic polymers were obtained within reactor. The composition of the reactor blends was studied by solvent fractionation. The particle morphology, chain structures, physical and mechanical properties of the reactor blends and their fractions were further characterized by scanning electron microscopy (SEM), gel permeation chromatography (GPC), 13-carbon nuclear magnetic resonance (13C NMR), differential scanning calorimeter (DSC) and tensile tester. The reactor blends are mainly composed of high density polyethylene (HDPE, fraction C), stereoblock isotactic poly(1-butene) (iPB) and a small amount of poly(ethylene-b-1-butene) block copolymers (fraction B), moderate isotactic poly(1-butene) and poly(ethylene-co-1-butene) random copolymers (fraction A). The HDPE incorporation in the first stage not only resolves the reaction engineering problems, but also contributes to the higher isotactic stereoregularity of 1-butene polymerization. The reactor blends present higher tensile strength and lower permanent deformation with the incorporation of PE component.

Preparation of Octadecylamine-Graphene/MgCl₂-Supported Ziegler-Natta Catalyst and Its Application in Ethylene Polymerization.

A facile coagglomeration method for preparing a long alkyl chain modified graphene oxide (MGO)/MgCl2-supported Ti-based Ziegler-Natta catalyst was reported. The effects of MGO on the catalyst morphology and activity for ethylene polymerization were examined. The resultant polyethylene (PE)/MGO nanocomposites exhibited a layered morphology, with the MGO fillers being well dispersed and exhibiting strong interfacial adhesion to the PE matrix. The thermal stability and mechanical properties of the PE were significantly enhanced with the introduction of a small amount of the MGO filler. Thus, this work provides a facile approach to the production of high-performance PE.

Trans-1,4- stereospecific polymerization of isoprene with MgCl2-supported Ziegler-Natta catalyst I. Initial polymerization kinetic and polymerization mechanism

In order to understand the trans-1,4 stereospecific polymerization mechanism of conjugated dienes with the heterogeneous Ziegler-Natta catalyst, isoprene polymerization kinetics at different polymerization temperature in the initial polymerization stage and the microstructures of polymers and its fractions were studied in this work. Both polymerization temperature and time influenced the polymerization behavior greatly. The nascent polyisoprene (30s) with unusual high cis-1,4-units was observed and then the trans-1,4/cis-1,4 ratio of the polymers increased obviously to finally trans-1,4-dominated (96 mol%) with the increase in polymerization time. Fractions of the products based on the different crystallizability at different temperature were isolated and characterized by 1H-NMR, 13C-NMR, GPC and DSC. The nascent polyisoprene under 30s polymerization time was mainly composed by fractions A and G with 30–60 mol% trans-1,4-units and 2–26 mol % trans-cis dyad linkages. Active species with low stereoregularity may contribute to theses fractions. While fractions D and E with high trans-1,4 unit (>96 mol%) from active species with high stereoregularity were growing to the predominated components with increased conversion. At least two kinds of active species with different stereospecificity and lifetime were proposed to explain the isoprene polymerization with heterogeneous TiCl4/MgCl2-Al(i-Bu)3 catalyst. Effects of temperature on cis/trans-1,4 structure were discussed. Hopely, this research would provide a further understanding about trans-1,4 stereospecific polymerization of conjugated dienes with heterogeneous Ziegler-Natta type catalysts.

Kinetics and mechanism of ethylene polymerization with TiCl4/MgCl2 model catalysts: Effects of titanium content

Two MgCl2-supported Ziegler–Natta catalysts were prepared by loading TiCl4 onto activated δ-MgCl2 supports, which contained 0.1 wt% (Cat-1) and 1.0 wt% (Cat-2) of titanium (Ti), respectively. Cat-1 has a slightly larger specific surface area (88.1 m2/g) and nearly the same pore volume (0.45 cm3/g) as the δ-MgCl2 support, but the catalyst with 1.0% Ti (Cat-2) has smaller specific surface area (66.3 m2/g) and pore volume (0.40 cm3/g). The kinetics and active center distribution of ethylene polymerization with the catalysts were compared. Cat-1 showed much higher activity per gram of titanium than Cat-2, which can be attributed mainly to the much higher [C∗]/[Ti] value of Cat-1 (60%) than of Cat-2 (12%). In both polymerization systems, the [C∗]/[Ti] value increased with time, but the Cat-2 system showed a much larger [C∗]/[Ti] increase than the Cat-1 system. Active center distribution of the catalysts disclosed by deconvolution of the molecular weight distribution into multiple Flory components was evidently different in the initial stage (0–200 s). The Cat-1 system produced more polymer with low molecular weight than the Cat-2 system, and the Flory components of low molecular weight decayed faster than those with high molecular weight. Influence of Ti content on number and structure of active centers formed on clustered Ti species is discussed with reconsideration of the current models of catalyst structure.

Spectroscopic Evidences for TiCl4/Donor Complexes on the Surface of MgCl2-Supported Ziegler–Natta Catalysts

In this work, we have addressed the question of the location of electron donors (we chose ethylbenzoate, EB, for practical reasons) on the surface of MgCl2-supported Ziegler–Natta catalysts by syne...

1-Hexene polymerization with supported Ziegler-Natta catalyst: Correlation between catalyst particle fragmentation and active center distribution

1-Hexene was polymerized with a commercial MgCl2-supported Ziegler-Natta catalyst using triethylaluminum as cocatalyst at different monomer/catalyst mass ratio (mhexene/mcat = 5, 15, 30), and the number of active centers ([C*]/[Ti]) was determined by quench-labeling the catalyst with 2-thiophenecarbonyl chloride and monitoring sulfur content of the polymer. Changes of polymerization rate (Rp) and propagation rate constant (kp) with time were also determined. [C*]/[Ti] was found to increase with time for 3 − 5 folds in the first 10 min of polymerization, but the maximum [C*]/[Ti] value reached after the induction period markedly enhanced with increase of monomer/catalyst mass ratio. Gradual fragmentation of the catalyst particles in parallel with the [C*]/[Ti] increase was observed by SEM analysis. It means that a large proportion of active center precursors are inaccessible to cocatalyst and monomer at the beginning of reaction, and fragmentation of the particles by hydraulic force of the growing polymer chains leads to exposure and activation of these precursors. Raising the monomer concentration can provide larger hydraulic force to expose those active site precursors that are more tightly buried inside the catalyst particle. The polymer molecular weight distribution and related active center distribution was found to shift with increase of the monomer/catalyst ratio, meaning that there are different types of buried active center precursors, and their exposure requires different extent of particle fragmentation.

The coordinative state of aluminium alkyls in Ziegler-Natta catalysts.

The most commonly used cocatalyst species in Ziegler-Natta catalysts are aluminium alkyls. In this study we aim to find the interaction between aluminium centres of these activators and other components in the ZNC system. Initially we look at binary systems of Al-alkyl/MgCl2 and ternary systems of Al-alkyl/MgCl2/TiCl4, followed by donor containing systems. The aluminium alkyls prove to be very reactive species and only in the case of trimethylaluminium the alkyl is strongly present in the sample. This species appears to convert, however, over time. 1H NMR proves to be an efficient method to detect the presence of the Al-alkyl species. The use of high magnetic field strengths and 27Al MQMAS NMR alleviates signal overlap and gives insight in the dominant line broadening mechanisms thus providing an in-depth view of the cocatalyst. Various Al species with different coordinations can be identified in the samples. The heterogeneity of the samples turns out to have a larger effect on the 27Al NMR spectra than the quadrupolar interaction, which argues against the presence of highly distorted sites with mixed coordinations. Nevertheless for the samples indicating the presence of alkyls in the 1H NMR spectra, we observe an aluminium site at 97 ppm in the 27Al spectra that might be coordinated to an organic group.

MgCl₂-Supported Titanium Ziegler-Natta Catalyst Using Carbon Dioxide-Based Poly(propylene ether carbonate) Diols as Internal Electron Donor for 1-Butene Polymerization.

MgCl2-supported titanium Ziegler-Natta catalyst containing CO2-based poly(propylene ether carbonate) diols as a potential internal electron donor (IED) was synthesized and employed for 1-butene polymerization. When compared with the Ziegler-Natta catalyst using poly(polypropylene glycol) as IED, the catalyst prepared with poly(propylene ether carbonate) diols showed good particle morphology, higher activity and stereoselectivity. The results suggested that existence of the carbonate group within the structure of poly(propylene ether carbonate) diols truly plays an important role in improving the performance of the catalyst for the 1-butene polymerization.

Effects of Various Amounts of New Hepta-Ether as the Internal Donor on the Polymerization of Propylene with and without the External Donor

The new hepta-ether compound as the internal donor was synthesized using the Williamson reaction of dipentaerythritol with sodium hydride as the strong base and iodomethane as the alkyl halide. The hepta-ether compound was characterized by NMR, FTIR, and GC techniques. The MgCl2-supported catalysts incorporated with different amounts of hepta-ether compound as the internal donor and without the internal donor were synthesized and characterized. The propylene polymerization was carried out using these catalysts in the presence of triethylaluminum as a co-catalyst and hydrogen as a chain transfer agent, with and without the external donor. The effect of a new internal donor on propylene polymerization using prepared MgCl2-supported Ziegler-Natta catalysts was investigated.

Synthesis of polyethylene/poly(ethylene-co-propylene) in-reactor alloys by periodic switching polymerization process: Effects of switching frequency on polymer structure and properties

In this work, a series of polyethylene/poly(ethylene-co-propylene) in-reactor alloys (PE/EPR) were synthesized with MgCl2-supported Ziegler-Natta catalyst by a three-stage reaction process including slurry ethylene homopolymerization, gas-phase ethylene/propylene copolymerization and alternating homo- and copolymerization (PSPP stage), and the effects of switching frequency (SF) on their structure and properties were studied. The alloy was composed of random ethylene/propylene copolymer (EPR), segmented ethylene/propylene copolymer (EPS) and polyethylene (PE). Increasing SF in the PSPP stage led to significant changes in the composition, chain structure, mechanical properties and phase morphology of the alloys. By increasing SF from 1 to 30, the amount of EPR fraction slightly decreased then increased, but the EPS fraction significantly increased in a monotonous way. Increasing SF led to formation of EPS with longer PE sequence and higher crystallinity. The alloys showed much higher toughness at low temperature (-50 °C) than polyethylene. The alloy prepared at SF=6 showed better toughness-stiffness balance than those prepared under other conditions. The PE/EPR alloys showed typical sea-islands phase structure, with the EPR domains being the dispersion phase. The alloy prepared at SF = 6 showed EPR phases with the smallest size and the most uniform spacial distribution in the PE matrix. The observed improvements in alloys mechanical properties are explained by the enhancement of phase compatibility with increase in SF.

Preparation of graphene/MgCl2-supported Ti-based Ziegler-Natta catalysts by the coagglomeration method and their application in ethylene polymerization

We report a facile coagglomeration method for preparing graphene (G)/MgCl2-supported Ti-based Ziegler-Natta catalysts. The effects of graphene feed ratio on catalyst morphology and ethylene polymerization behavior were examined. The synthesized catalyst exhibited very high activity for ethylene polymerization. The resultant polyethylene (PE)/G nanocomposites showed a layered morphology, and the graphene fillers were well dispersed in the PE matrix. In addition, the thermal stability and mechanical properties of PE were significantly enhanced with the introduction of a very small amount of G fillers (0.05 wt%). This work provides a facile approach to the production of high-performance PE.

Preparation and properties of PE/MoS2 nanocomposites with an exfoliated-MoS2/MgCl2-supported Ziegler-Natta catalyst via an in situ polymerization

Here, we report the first example of preparation of polyethylene (PE)/exfoliated-MoS2 nanocomposites through in-situ Ziegler-Natta polymerization. The exfoliated-MoS2/MgCl2-supported Ti-based Ziegler–Natta catalyst was synthesized via a facile coagglomeration method. The effects of MoS2 on the catalyst morphology and the ethylene polymerization behavior were examined. The resultant PE/MoS2 nanocomposites had a flake shape morphology, and the MoS2 fillers were well dispersed throughout the entire PE matrix. In addition, the thermal stability and mechanical properties of the PE base material were significantly enhanced by the introduction of a very small amount of the MoS2 filler (0.08wt%). Therefore, this work provides a facile method to produce of high-performance PE.