MgCl2-supported Ziegler-Natta Catalysts Research Articles

Influences of silane/ether composite external donors on propylene polymerization with MgCl2-supported Ziegler-Natta catalyst in the presence of hydrogen

AbstractIn this work, the effects of two silane type external donors, Ph2Si(OCH3)2 (DPDMS) and dicyclopentyldimethoxysilane (DCPDMS) on propylene slurry polymerization in the presence of hydrogen with a MgCl2/TiCl4/DIBP - Al(C2H5)3 catalyst system were investigated, and the tacticity distribution and molecular weight distribution (MWD) of polypropylene (PP) were further regulated by using silane/ether (ROCH3, R = n-Bu, i-C8H17) composite external donors. The tacticity distribution of PP, as revealed by successive self-nucleation and annealing (SSA) thermo analysis of the polymer, is markedly changed by the external donors. DCPDMS shows much stronger effects than DPDMS. Addition of the ether as an auxiliary external donor in MgCl2/TiCl4/DIBP-TEA/silane (DPDMS or DCPDMS) lead to further increase in catalytic activity and isotacticity of PP, and changes in tacticity distribution. By using the composite external donors, PP with both higher isotacticity index, lower molecular weight or narrower molecular weight distribution was prepared at high activity. Effects of the external donors on the active center distribution were discussed based on the results of deconvoluting the MWD curve with multiple Flory “most-probable” distributions, and possible mechanism of the external donor effects was proposed.

A periodic hybrid DFT approach (including dispersion) to MgCl2-supported Ziegler–Natta catalysts – 1: TiCl4 adsorption on MgCl2 crystal surfaces

The adsorption of TiCl4 on the surfaces of MgCl2 crystals has been investigated by means of state-of-the-art periodic hybrid DFT methods, as the first step of a comprehensive study aiming to elucidate the structure of the active species in industrial MgCl2-supported Ziegler–Natta catalysts for ethene and propene polymerization. A first distinctive feature of the approach was the thorough evaluation of dispersion forces, crucial because the binding of TiCl4 on MgCl2 surfaces turned out to be essentially dispersion-driven. Also important was a careful investigation of the effects of different choices on basis set and density functional (DF) on the quantitative aspects of the results; this allowed us to trace the unusually large disagreement in the previous literature and identify unambiguous trends. In particular, three full sets of calculations were run adopting the B3LYP(-D), PBE0(-D) and M06 approximations; to the best of our knowledge, the last represents the first case of M06 functional implementation in a periodic code (CRYSTAL) of widespread use. The results consistently indicated that the adsorption of TiCl4 on well-formed MgCl2 crystals under conditions relevant for catalysis can only occur on MgCl2(110) or equivalent lateral faces, whereas the interaction with MgCl2(104) – for decades claimed as the most important surface in stereoselective catalysts – is too weak for the formation of stable adducts. The implications of these findings for catalysis are discussed.

Propylene polymerization over MgCl2-supported TiCl4 catalysts bearing different amounts of a diether internal electron donor: Extrapolation to the role of internal electron donor on active site

In this work, the composition of MgCl2-supported Ziegler-Natta (ZN) catalysts bearing different amounts of 9,9-bis(methoxymethyl)fluorine (BMMF) and the propylene polymerization over these catalysts are investigated. Based on the experimental results, the models of active sites suitable for ZN catalysts with/without internal donor BMMF are established, and they can be described as follows: atactic site (I)—isolated TiCl4 monomeric species on the (110) lateral cut (around which there is no complexes), weakly isospecific site (II)—semi-isolated and surrounded TiCl4 monomeric species on the (110) lateral cut (in the vicinity of which BMMF or other TiCl4 is adsorbed), and highly isospecific site (III)—dimeric TiCl4 species (Ti2Cl8) on the (104) lateral cut. Meanwhile, the mechanism underlying the role of BMMF on active sites is revealed (i) convert atactic sites (I) to weakly isospecific site (II) by occupying either or both of L1 or/and L2 vacancies around site; (ii) improve the isotacticity of weakly isospecific site (II) in donor-free ZN catalyst by adsorbing at L2 vacancy or/and replacing the Cl for TiCl4 at L1; and (iii) replace highly isospecific site (III). In addition, these roles of BMMF are successively achieved according to the amount of BMMF added. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Effects of ethylene as comonomer on the active center distribution of 1-hexene polymerization with MgCl 2-supported Ziegler–Natta catalysts

1-Hexene homopolymerization and 1-hexene–ethylene copolymerization with TiCl 4/MgCl 2–Al(C 2H 5) 3 catalyst were compared to investigate the effect of ethylene on the distribution of active centers. The polymerizations were quenched with cinnamoyl chloride, and the number of active centers ([C*]/[Ti]) was determined by measuring the cinnamoyl group labeled on the propagation chains. Both polymer samples were fractionated into 9–10 fractions according to molecular weight, and [C*]/[Ti] in each fraction was also determined. Adding small amount of ethylene in 1-hexene polymerization markedly increased the number of active centers that produce low molecular weight polymer. This phenomenon agrees with the mechanism suggesting the presence of Ti−CH(CH 3)(CH 2) 3CH 3 type dormant sites and their activation by ethylene. The k p value of the newly emerging active centers in the copolymerization system is much lower than that of the homopolymerization system. In the copolymerization system, the active centers producing polymer chains of the second highest molecular weight and isotacticity show the highest k p value, while those producing polymer of the highest molecular weight and isotacticity show only the second highest k p. On the other hand, the active centers producing polymer with lower–middle chain length show the lowest ethylene incorporation rate. These results disclose differences of catalytic properties between the multiple active center types and correlations between their different kinetic parameters, which may lead to new understanding of the active centers and polymerization mechanism.

Effects of Ti oxidation state on ethylene, 1-hexene comonomer polymerization by MgCl2-supported Ziegler–Natta catalysts

In this study, the influences of the Ti oxidation state on the catalytic properties of MgCl2-supported Ziegler–Natta catalysts in ethylene homo- and co-polymerization with 1-hexene were investigated. Three catalysts having different Ti oxidation states were synthesized by milling TiCl4, TiCl3, or TiCl2 together with MgCl2. With these catalysts having different Ti oxidation states, the polymerization conditions such as the Al concentration, temperature, and 1-hexene concentration were varied to figure out their catalytic abilities in ethylene homo- and co-polymerization. The Ti oxidation state affected the catalyst activity largely, having unique dependences on the polymerization conditions. A higher oxidation state led to a higher activity, slightly larger comonomer incorporation, and lower molecular weight as well as its narrower distribution. However, rough characteristics of copolymers were similar among the different Ti oxidation states.

Ethylene Polymerization Using Improved Polyethylene Catalyst

The study concerns the use of MgCl 2-supported high-activity Ziegler-Natta catalysts for the polymerization of ethylene. In particular, two types of catalysts were investigated, which were N-catalyst (BRICI) and improved polyethylene catalyst. The effects of catalyst structure on kinetic behavior were examined. The distribution of active centers in these catalysts was investigated by energy dispersive analysis by X-rays (EDAX), and morphologies of catalyst particles and polymer products were examined by scanning electron microscope (SEM). Hydrogen response and copolymerization performance were investigated and compared with the two catalysts. The results were correlated with the kinetic behavior of the two catalysts and appropriate models for polymer particle growth were presented. The improved polyethylene catalyst showed higher activity, better hydrogen response and copolymerization performance.

Role of dispersion state of ti species in deactivation of MgCl2-supported Ziegler-Natta catalysts

The deactivation behaviors of TiCl3/MgCl2 model catalysts with molecularly dispersed TiCl3 were investigated to clarify the role of the dispersion state of the Ti species in the deactivation of MgCl2-supported Ziegler-Natta (ZN) catalysts for propylene and ethylene polymerization. The propylene and ethylene polymerization activities of clustered Ti species supported on MgCl2 were approximately one tenth of those of isolated Ti species, which is indicative of the significance of the aggregation of Ti species in the deactivation of ZN catalysts for olefin polymerization. Moreover, the time-course depression of monomer consumption for the clustered Ti species was slower than that for the isolated Ti species in propylene polymerization, whereas the dispersion state barely affected the depression rate in ethylene polymerization. The reduction rate of the Ti species was concluded to be important for the time-course depression of propylene consumption, whereas ethylene polymerization was insensitive to the oxidation state of the Ti species.

Effects of molecular dispersion state of surface Ti species on ethylene-propylene copolymerization with TiCl3-based Ziegler-Natta model catalyst

The dispersion state of surface Ti species is one of the most important factors affecting the polymerization properties of MgCl2-supported heterogeneous Ziegler-Natta catalysts. In this paper, ethylene-propylene copolymerization was carried out using a novel Ziegler-Natta model catalyst with “isolated” and “clustered” TiCl3 molecules on MgCl2. At a lower ethylene/propylene feed ratio, the dispersion state of the Ti species strongly affected both the composition and sequence distribution of the copolymers. The “isolated” Ti species gave a relatively random copolymer, and clustering of Ti species enhanced the bulkiness. The addition of an external donor basically deteriorated the random copolymerization character of the “isolated Ti species but the degree of deterioration depended on the type of donors. On the other hand, the balance between the monomer feed and consumption dominated the copolymerization performance at a higher ethylene/propylene feed ratio to obscure the effects of the dispersion state.

Preparation of MgCl 2-supported Ziegler–Natta catalyst systems with new electron donors

In the present work, the effect of new internal and external electron donors for Ziegler–Natta systems of the type MgCl 2/DI/TiCl 4/DE/AlR 3 was compared with the conventional use of n-butyl phthalate and dimethoxy-diphenylsilane as electron donors. Catalyst systems employing various internal and external donors were evaluated in propylene polymerization. The polymers were characterized by infrared absorption spectroscopy, differential scanning calorimetry and heptane extraction. The prepared catalysts with the new internal donors produced polypropylene with isotactic index up to 98% under the polymerization conditions employed.

Effect of dealcoholation of support in MgCl2-supported Ziegler–Natta catalysts on catalyst activity and polypropylene powder morphology

Spherical support was produced using melt quenching method. Scanning electron microscopy (SEM) images showed that support particles have a rough surface morphology with small pore sizes. The size of primary particles determined to be about 50 nm. Produced particles were dealcoholated using hot nitrogen flow. In this study, the dealcoholation apparatus was similar to a fluidized bed reactor. It consists of three main zones: heating zone, bed zone, and expansion zone. This apparatus is capable to dealcoholate and elutriate support particles at the same time. Results showed a significant increase in specific pore volume and surface area of support particles during the dealcoholation. According to the obtained results, two mechanisms were proposed to explain effect of the dealcoholation on the morphology of support particles. Three dealcoholated samples of different alcohol contents were used in the catalyst preparation. Results showed that by decrease in alcohol content of support particles, specific surface area, and pore volume of catalysts increased. Moreover, titanium content of final catalysts increased slightly. Polymerization of propylene was carried out using obtained catalysts in slurry phase. An increase in catalyst activity and enhancement of polypropylene powder morphology were observed as the alcohol content of support particles decreased.

Effect of internal electron donor on copolymerization of ethylene and 1-hexene catalyzed by supported Ziegler- Natta catalysts

AbstractThe four kinds of TiCl4/MgCl2 type supported Ziegler-Natta catalysts, Cat-1, Cat-2, Cat-3 and Cat-4, were prepared by reaction of MgCl2-ethanol adduct with TiCl4. Cat-1 contains no internal electron donor (ID), Cat-2 contains diethyl phthalate (DEP) as ID, Cat-3 contains anisole as ID, and Cat-4 contains both DEP and anisole as ID. Ethylene copolymerizations with 1-hexene by these catalysts were compared. Influences of the internal electron donor on catalyst activity, copolymer composition, composition distribution, molecular weight and molecular weight distribution were investigated. Cat-3 showed the largest ability of incorporating 1-hexene into copolymer chains among the four catalysts. Each copolymer sample was fractionated into two fractions: fraction soluble in boiling nheptane (HS) and fraction insoluble in boiling n-heptane (HI). 1-Hexene content of the HS fraction is 6~10 times of that in the HI fractions. Amount of the HS fraction of copolymer by Cat-2 and Cat-4 was much lower than that of Cat-1 and Cat-3. Introduction of internal donor to the catalysts, especially diethyl phthalate (DEP), increased the blockiness of HI fractions. The molecular weight of both fractions increased when there is ID in the catalyst. Addition of anisole reduced the difference in molecular weight between HS fraction and HI fraction, but addition of PA enlarged the difference. The results are discussed based on a mechanistic model of MgCl2-supported Ziegler-Natta catalysts.

The role of electron donors on lateral surfaces of MgCl 2-supported Ziegler–Natta catalysts: Observation by AFM and SEM

An active model for a Ziegler–Natta propylene polymerization catalyst has been prepared by spin-coating of a MgCl 2/diether donor solution in ethanol on a flat silicon wafer, followed by crystal growth by Ostwald ripening to give well-defined MgCl 2⋅donor⋅ nEtOH crystallites. The growth of the crystallites on the flat wafer indicates the formation of only 120° edge angles in the presence of the diether donor, suggesting the preference for the formation of one particular crystallite face for the MgCl 2. In contrast, the presence of diisobutyl phthalate or ethyl benzoate leads to the formation of both 120° and 90° edge angles, indicating the formation of more than one face for the MgCl 2. Subsequent treatment with TiCl 4 generates the catalyst and in propylene polymerization polymer forms on the lateral face of each crystallite. This work represents the first visualization of the effect of an internal donor in controlling the crystallite faces of magnesium chloride formed during a Ziegler–Natta catalyst preparation.

Effects of propylene prepolymerization on ethylene/1‐hexene and ethylene/1‐octene copolymerization with an immobilized metallocene catalyst

It has been known for some time that the activity of a heterogeneous catalyst for ethylene polymerization can be increased if a prepolymerization is carried out first in the presence of propylene or another a-olefin. Recently, it has been demonstrated that the increased polymerization activity can be attributed to a significant lowering of the monomer diffusion barrier in ethylene homopolymerization. The aforementioned studies were based on the use of MgCl2-supported Ziegler–Natta catalysts. However, monomer diffusion limitation in ethylene polymerization is also a frequently encountered phenomenon in ethylene polymerization with immobilized single-center catalysts and provides the most likely explanation for the increase in activity when ethylene polymerization is carried out in the presence of an a-olefin comonomer. The presence of the comonomer reduces the crystallinity of the polymer, leading to increased monomer sorption and easier monomer mass transport through the growing polymer particle, and facilitates catalyst fragmentation. However, compositional heterogeneity in the resulting copolymer may still occur, even in the case of a single-center catalyst. Fink et al. ascribed the compositional distribution of ethylene/1hexene copolymers prepared with a SiO2/methylaluminoxane (MAO)/zirconocene system to the formation, during the initial stages of polymerization, of a copolymer envelope around the catalyst particle. Easier diffusion of the smaller monomer, ethylene, with respect to 1-hexene was proposed to lead to a polymer particle comprising an ethylene-rich center surrounded by an outer layer of the copolymer, thus giving a broad overall chemical composition distribution (CCD). In recent ethylene/1-hexene copolymerization studies carried out with a catalyst comprising a zirconocene and MAO immobilized on a silica support, we found that the CCD of the copolymers became broader as the polymerization time was increased as a result of the gradual formation of a relatively high molecular weight, ethylene-rich fraction. Again, this is a strong indication that significant monomer diffusion effects can play a role not only in ethylene homopolymerization but also in copolymerization. Taking into account that monomer diffusion effects may be reduced if a prepolymerization of the catalyst is carried out first with a Correspondence to: J. C. Chadwick (E-mail: j.c.chadwick@ polymers.nl)

Study on Ethylene-α-Olefin Copolymerization Catalyzed by the MgCl2-Supported and Low Ti-Loading Ziegler–Natta Catalyst

Ethylene-α-olefins copolymers were synthesized with MgCl2-supported and low Ti-loading Ziegler-Natta catalyst prepared using the one-pot balling method. The effects of the type of comonomer, the concentration of comonomer, and polymerization temperature on the catalytic activity, kinetic behavior, molecular weight, and its distribution of the resultant copolymer were studied. It was found that the resultant copolymers have a wide molecular weight distribution. The formation mechanism of polymer with broadened molecular-weight distribution was also discussed using the method of fitting the molecular weight distribution (MWD) curves with multiple Flory-Schulz functions.

Low isotactic polypropylene synthesized with a MgCl 2/AlCl 3-supported Ziegler catalyst

A MgCl 2/AlCl 3-supported Ziegler–Natta catalyst is prepared and low isotactic polypropylene is synthesized using this catalyst in the paper. The results indicate that the polymerization activity of the MgCl 2/AlCl 3-supported Ziegler–Natta catalyst is much higher than that of the MgCl 2-supported Ziegler–Natta catalyst. And the attenuation curves of the polymerization rate of different Al/Ti molar ratios suggest the double metal active center is more stable than the single metal active center. Also the effects of the AlCl 3/MgCl 2 molar ratio, polymerization temperature and external electron donor on the polymerization activity and isotactic index of the products are studied in detail.

Study of Propylene Polymerization Catalyzed by Spherical MgCl2-Supported Ziegler-Natta Catalyst System: Preparation of Spherical Support

A novel route to prepare the MgCl2–EtOH adduct support was developed. Many factors which effect the morphology of the support were studied in detail. It was found that the stirring speed of dispersion and the molar ratio of EtOH/MgCl2 had an important influence on the support particle size and particle size distribution. Through varying the stirring speed or changing the EtOH/MgCl2 ratio, MgCl2 adduct supports with different particle size and particle size distribution could be obtained.

Study on the distribution of active centers in novel low Ti-loading MgCl2-supported Ziegler-Natta catalyst.

Novel MgCl2-supported Ziegler-Natta (Z-N) catalysts prepared using a new one-pot ball milling method can effectively control the amounts of Ti-loading in the catalysts. Complex GPC data on polypropylene synthesized by these novel catalysts were analyzed using the method of fitting the molecular weight distribution (MWD) curves with a multiple Flory-Schulz function. It was found that multiple active centers exist in these novel catalysts. Detailed study of the effects of the Ti-loadings in the catalysts on the distribution of the active centers showed that the Ti-loadings in the novel MgCl2-supported Z-N catalysts might affect the proportion of each type of active centers; and might be the main factor responsible for the effect of the Ti-loadings on the microstructure, the molecular weight and molecular weight distribution width of the resultant polymer, the catalytic activity and polymerization kinetics.

Study on the distribution of active centers in novel low Ti-loading MgCl2-supported Ziegler-Natta catalyst

Study on the distribution of active centers in novel low Ti-loading MgCl2-supported Ziegler-Natta catalyst

Precise arguments on the distribution of stereospecific active sites on MgCl2‐supported ziegler‐natta catalysts

AbstractThe stereospecific nature of active sites on various MgCl2‐supported Ziegler‐Natta catalysts was investigated by stopped‐flow technique combined with temperature rising elution fractionation (TREF) method. A modified three‐sites model with precise description of the stereospecific nature of various types of active sites stemmed from surface titanium species, Al‐alkyl compounds, Mg‐compounds and electron donors has been proposed. It was demonstrated that the isospecificity of active sites strongly depends on the bulkiness of the ligands situated at the two most important ligand positions for construction of asymmetry and chirality of the active sites with steric hindrance. In general, there may exist both monometallic and bimetallic sites in heterogeneous Ziegler‐Natta catalyst system. The kinds of active titanium species with different chemical structures on this catalyst system should be limited, whereas, the non‐discrete distribution of isospecificity of active sites could be considered to generate from the numerous types of steric and electronic effects from the surroundings of active titanium species as well as large number of reversible and dynamic transformation reactions simultaneously occurred on the heterogeneous catalyst surface.

Novel MgCl2-supported catalyst containing diol dibenzoate donor for propylene polymerization

A novel MgCl2-supported Ziegler–Natta catalyst containing 2-isoamyl-2-isopropyl-1,3-propandiol dibenzoate donor for propylene polymerization was studied, and compared with catalysts with different internal donors, such as benzoate, phthalate and diether, in polymerization dynamic behavior, activity, hydrogen response and stereospecificity. It was found that there are two highly isospecific active sites in the catalyst with dibenzoate internal donor system and the effections of temperature are different on the performances of the two highly isospecific sites.