Modeling of isospecific Ti sites in MgCl2 supported heterogeneous Ziegler-Natta catalysts

New models for the steric environment of Ti isospecific polymerization sites for poly(propylene) on MgCl₂ microcrystals are proposed. They directly involve a donor molecule in order to obtain isospecific activable Ti atoms otherwise belonging to isolated adsorbed TiCl₄ molecules or Ti₂Cl₈ dimers which are lacking of the required chirality for stereocontrol. The donor molecules able to attain at best this effect keep to some peculiar conformational rules settled by the authors in a previous theoretical-correlative study on highly active Lewis bases. The new 1,3-dimethoxypropane series suggested by the authors and recently patented by Montell has been examined in detail. Essentially three different types of closeness between Ti atoms and donor molecules can take place, in which different moieties of the diether compound help to build the ‘right’ steric environment in the site's neighbouring. In the three proposed models S1, S2, S3 the stereocontrol is attained through, respectively, one of the methoxy moieties, one of the methyls, and one of the central carbon atom substituents. New hypotheses on the role of Lewis bases in the preparation of isospecific heterogeneous Ziegler-Natta catalysts are discussed.     

Experimental and calculated vibrational spectra and structure of Ziegler-Natta catalyst precursor: 50/1 comilled MgCl2-TiCl4

The vibrational Infrared and Raman Spectra of a MgCl₂-TiCl₄ Ziegler-Natta catalyst precursor with a 50/1 MgCl₂/TiCl₄ ratio have been recorded. The Raman spectrum of this catalyst precursor, in the range 50-500 cm⁻¹, shows clear scattering lines which can be assigned to the complex MgCl₂-TiCl₄, well separated from those of the initial species. Analogous, but less clear signals can be found in the infrared spectrum. Vibrational symmetry analysis and quantum chemical calculations of suitable models of MgCl₂-TiCl₄ complex have been made for the interpretation of the experimentally recorded spectra. The observed spectroscopic signals can be explained in terms of the existence of only one type of MgCl₂-TiCl₄ complex where the TiCl₄ molecules are complexed on the MgCl₂ along the (110) lateral cuts in a local C_2v symmetry with the Ti atoms in an octahedral coordination.     

Adsorption of TiCl4 and electron donor on defective MgCl2 surfaces and propylene polymerization over Ziegler-Natta catalyst: A DFT study

The formations of defective MgCl₂ surfaces, and subsequent adsorption of Ti species and electron donor, as well as propylene polymerization over the Ziegler-Natta catalyst have been investigated using density functional theory (DFT) method. Twelve possible support models of regular and defective MgCl₂ (110) and (100) surfaces were built. The individual adsorptions of titanium chlorides as mononuclear or dinuclear, and ethyl benzoate (EB) as electron donor, on these models were evaluated. The analysis of energies presented the cases of EB adsorption were generally more stable than titanium chlorides on both surfaces. Thus, EB as internal electron donor mainly prevented TiCl₄ from coordinating on the MgCl₂ surfaces where mostly non-stereospecific active sites could be formed. Exceptionally, A5 the site model with terminal Cl-vacancy on the MgCl₂ support, presented stronger adsorption of TiCl₄ than that of EB on (110) surface. Since the TiCl₄ and ethyl benzoate (EB) would compete to adsorb on the support surface, it seems reasonable to assume that TiCl₄ might predominately occupy this site, which can act as the most plausible active site for propylene polymerization. The first insertion of propylene monomer into the A5 active site model showed that it exhibited good regioselectivity but poor stereospecificity in the absence of electron donor.     

Density Functional Calculations for Electronic and Steric Effects of Ethyl Benzoate on Various Ti Species in MgCl2-Supported Ziegler-Natta Catalysts

Possible coadsorption states of Ti mononuclear species and ethyl benzoate (EB) and their interaction on MgCl₂ (110) and (100) surfaces were investigated with periodic density functional calculations in order to obtain the microscopic understanding about how EB affects the steric and electronic natures of the Ti species. EB was unlikely to be attached to the TiCl₄ species on both the MgCl₂ (110) and (100) surfaces. The coadsorption of EB at Mg²⁺ ions near the Ti species was as favorable as the separate adsorption, which implied the random placement of these adsorbants in the final catalyst. The charge redistributions upon coadsorption among the Ti species, EB and the support were found to be dependent on the surface structures: the electron density of the Ti speceies was rather decreased by the coadsorption on the (100) surface, while that of the Ti species was enhanced due to the support-mediated electron transfer from EB on the (110) surface. It was suggested that the presence of EB close to the Ti species should generate donor-related active sites selectively on the (110) surface.     

State of various stereoregulating electron-donating compounds in titanium-magnesium catalysts for propylene polymerization: A diffuse reflectance IR spectroscopic study

Propylene polymerization on TiCl₄/donor/MgCl₂ (donor = ethyl benzoate, dibutyl phthalate, diisobutyl phthalate, diethyl 2,3-diisopropylsuccinate) supported catalysts is considered. The states of the donors in the catalysts have been investigated by diffuse reflectance IR spectroscopy. Data characterizing the distribution of the donors and the active component (TiCl₄) on the support surface have been obtained. Molecular weight distribution data for polypropylene are presented. The molecular weight distribution of polypropylene depends on the location of the donor and TiCl₄ molecules.     

Reductive Formation of Isospecific Ti Dinuclear Species on a MgCl2 (110) Surface in Heterogeneous Ziegler‐Natta Catalysts

Dinucleation of TiCl₄ on the MgCl₂ (100) surface has been conventionally believed as the origin of the stereospecificity of heterogeneous Ziegler‐Natta catalysts for propylene polymerization, while the MgCl₂ (110) surface has been regarded as non‐stereoselective in the absence of organic donors. Based on periodic density functional calculations, we propose a new isospecific Ti dinuclear species on the MgCl₂ (110) surface, which is formed as a result of reduction of Ti from 4+ to 3+. The new species closely resembles the dinuclear species on the (100) surface, but two Ti ions are obliquely stacked along the (110) surface through Cl bridges. The results address the importance of the reduction and re‐distribution of surface Ti species after contacting with cocatalysts in considering the origin of the stereospecificity.

     

Synthesis and X-Ray Crystal Structure of the TiMgCl6(CH3COOC2H5)4 Adduct

The molecular and crystal structure of TiMgCl₆(CH₃COOC₂H₅)₄, obtained by reacting TiCl₄ with a solution of MgCl₂ in dry CH₃COOC₂H₅, have been determined by x-ray diffraction. The structure was solved by direct and Fourier methods and refined by least-squares techniques to R = 0.052 for 2722 independent observed reflections. Unit-cell dimensions are a = 17.122(7), b = 9.833(3), c = 9.646(3) Å, α = 111.10(7)°, β = 107.22(6)°, γ = 103.11(6)° with Z = 2 for P1 . The titanium(IV) atom is octahedrally coordinated by six chlorine atoms (Ti? Cl_t = 2.293(2) Å, Ti? Cl_b = 2.480(2) Å) and magnesium by two chlorine atoms (Mg? Cl_b = 2.528(2) Å) and the carbonyl oxygen atoms of the four CH₃COOC₂H₅ residues (Mg? O = 2.038(5) Å). The octahedra share an edge by a double chlorine bridge between the magnesium and titanium atoms. Changes in the configurations and dimensions of the free acceptor and donor molecules on adduct formation are discussed. One of the ethylacetate residues shows positional disorder, eventually with Bonding through its ethereal oxygen.     

A qualitative model for polymerisation of propylene with a MgCl2-supported TiCl4 Ziegler-Natta catalyst

A qualitative model for polymerisation of propylene with a MgCl₂-supported TiCl₄ Ziegler-Natta catalyst is developed. A series of polymerisation with increasing external electron donor/Ti ratio is performed and the polymers are analysed with GPC, ¹³C NMR and DSC. The model explains the trends in isotacticity and isotactic sequence length based on an equilibrium reaction of the electron donor, which is either coordinated next to an active site or extracted to the solution by the cocatalyst AlR₃. Different rates for propagation and termination reactions when the donor is present or absent are used to explain the trends in activity and molar mass. The possibilities for a quantitative model and parameter estimation are discussed.     

Catalyseurs Ziegler-Natta supportés sur MgCl2 pour la polymérisation stéréorégulée du propène. III. Systèmes catalytiques à hautes performances comprenant un solide ternaire: MgCl2, ester aromatique,TiCl4

An examination of the precatalyst which contains three compounds (MgCl₂–TiCl₄-aromatic ester) uncovered numberous properties that apparently are common to all precatalysts and have already been observed in the simpler systems: a continous decrease in the polymerization rate during polymerization, characterized by a deactivation index that does not depend on the precatalyst but only on the cocatalyst; isotacTiClty control by the [AI]/[aromatic ester] ratio in the cocatalytic solution; and fast and reversible control of kinetics and tacTiClty by the same ratio. The precatalyst prepared by impregnation of the aromatic ester in MgCl₂ or MgCl₂–TiCl₄ presents moderate or no improvement when compared with the simpler MgCl₂–TiCl₄ catalysts. The yellow precatalyst prepared by milling MgCl₂ with the aromatic ester and impregnating with TiCl₄ are the only products that provide high activity and isotactic index above 95% simultaneously, as revealed by the patent literature. Interpretation of the role played by the electron donor, based on infrared studies, are proposed: in the precatalyst it controls fixation of TiCl₄ on MgCl₂; in the cocatalytic solution it regulates the isospecificity of the active site by contact with the alkylaluminium-aromatic ester complex and slows polymerization. Free electron donor gradually poisons the active centers.     

Ethene–propene copolymerization with the MgCl2-supported dichlorobis(4,4,4-trifluoro-1-phenyl- 1,3-butanedionato)titanium catalyst activated by an ordinary trialkylaluminum

The Ti(BFA)₂Cl₂/MgCl₂–Al(C₂H₅)₃ catalyst (BFA = 4,4,4-trifluoro-1-phenyl-1,3-butanedione) modified by DIPDMS (diisopropyldimethoxysilane), which had been proved to yield an extremely high isotactic polypropene in high selectivity, was tested for the copolymerization of ethene with propene. The analysis of resulting copolymers by CFC (cross fractionation chromatography) indicated the formation of a small quantity of ethene-rich copolymers as a byproduct, suggesting that the catalyst possesses not only Ti(III) species but a small portion of Ti(II) species. Whereas, the same catalyst without being modified by an external donor selectively yielded propene-rich random copolymers resulting from Ti(III) species in high yields. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 2735–2740, 1998     

Hydrogen Effects for Propylene Polymerization with Ultra Low TiCl3 Loading MgCl2-Supported Catalyst

Hydrogen effects for propylene polymerization were investigated with ultra low TiCl₃ loading MgCl₂-supported catalysts in which the electric states of Ti species can be almost uniform. Hydrogen did not affect the catalyst activity, while the efficiency of hydrogen as a chain transfer agent was found to depend on the Ti content of the catalyst and the stereospecificity of the polymerization sites: Hydrogen was effective for isospecific sites independent of Ti contents, but inert for aspecific sites only at the extremely low Ti content. These results were explained within the island model, where isospecific sites may be located in the islands with other Ti species in their surroundings acting, as a steric hindrance for isospecific polymerization and as hydrogen dissociation sites after deactivation. Most of the aspecific sites should be isolated only at the extremely low Ti content. These isolated sites have no other Ti species in their surroundings, i.e. no hydrogen dissociation sites, and are inert to hydrogen.     

Active center determinations on MgCl2‐supported fourth‐ and fifth‐generation Ziegler–Natta catalysts for propylene polymerization

Active center determinations on different Ziegler–Natta polypropylene catalysts, comprising MgCl₂, TiCl₄, and either a diether or a phthalate ester as internal donor, have been carried out by quenching propylene polymerization with tritiated ethanol, followed by radiochemical analysis of the resulting polymers. The purpose of this study was to determine the factors contributing to the high activities of the catalyst system MgCl₂/TiCl₄/diether—AlEt₃. Active center contents (C*) in the range 2–8% (of total Ti present) were measured and a strong correlation between catalyst activity and active center content was found, indicating that the high activity of the diether‐containing catalysts is due to an increased proportion of active centers rather than to a difference in propagation rate coefficients. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1635–1647, 2006     

Propylene polymerization with catalysts containing divalent titanium

The behavior in propylene polymerization of divalent titanium compounds of type [η⁶-areneTiAl₂Cl₈], both as such and supported on activated MgCl₂, has been studied and compared to that of the simple catalyst MgCl₂/TiCl₄. Triethylaluminium was used as cocatalyst. The Ti–arene complexes were active both in the presence and in the absence of hydrogen, in contrast to earlier reports that divalent titanium species are active for ethylene but not for propylene polymerization. ¹³C-NMR analysis of low molecular weight polymer fractions indicated that the hydrogen activation effect observed for the MgCl₂-supported catalysts should be ascribed to reactivation of 2,1-inserted (“dormant”) sites via chain transfer, rather than to (re)generation of active trivalent Ti via oxidative addition of hydrogen to divalent species. Decay in activity during polymerization was observed with both catalysts, indicating that for MgCl₂/TiCl₄ catalysts decay is not necessarily due to overreduction of Ti to the divalent state during polymerization. In ethylene polymerization both catalysts exhibited an acceleration rather than a decay profile. It is suggested that the observed decay in activity during propylene polymerization may be due to the formation of clustered species that are too hindered for propylene but that allow ethylene polymerization. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 2645–2652, 1997     

The activation of MgCl2-supported ziegler-natta catalysts—II: Correlation between activity and structural disorder

The properties have been examined for Ziegler-Natta catalysts comprising TiCl₄ (Ti contents 2.0, 3.4 and 4.2%) supported on MgCl₂ activated by dry ball-milling up to 250 hr. The samples have been investigated by measurements of catalytic activity in polymerization, of surface area and of some structural parameters related to the crystal disorder introduced by ball-milling. It is shown that the TiCl₄ content affects the activation; a relationship has been found between the crystal disorder of the MgCl₂ support and the catalytic activity.     

烷氧基氯化钛载体型催化体系用于丙烯聚合反应的研究——Ⅲ.不同烷氧基团对催化剂性能的影响

我们研究了烷氧基氯化钛Ti(OR)_nCl_(4-n)负载于MgCl_2-EB(EB为苯甲酸乙酯)做为丙烯定向聚合反应的催化剂。本文报告不同种类及不同数目的-OR基团对催化剂性能的影响。结果表明,它们能以高活性催化丙烯定向聚合并得到高等规度聚合物,其中以n-C_6H_(13)-OTiCl_3活性最好。由于空间位阻的缘故,-OR基团数目增多,对催化活性和等规度均不利。在钛活性组分中引入烷氧基团,聚合物分子量明显降低。     

Amalgamation of ziegler-natta and metallocene catalysts

The MgCl₂ supported half titanocenes and Ti(4, 4, 4-trifluoro-1-phenyl-1, 3-butanedionato)₂Cl₂ catalysts were synthesized and applied to propene polymerization. Without supporting on MgCl₂, those complexes displayed almost no activity even using methylaluminoxane (MAO) as cocatalyst. When supported on MgCl₂, on the other hand, the resulting catalysts could be activated by ordinary alkylaluminums to yield polypropene in fairly high yields. The catalyst isospecificity was markedly improved by the addition of a suitable Lewis base.     

Probing the Coordinative Unsaturation and Local Environment of Ti3+ Sites in an Activated High‐Yield Ziegler–Natta Catalyst

The typical activation of a fourth generation Ziegler–Natta catalyst TiCl₄/MgCl₂/phthalate with triethyl aluminum generates Ti³⁺ centers that are investigated by multi‐frequency continuous wave and pulse EPR methods. Two families of isolated, molecule‐like Ti³⁺ species have been identified. A comparison of the experimentally derived g tensors and ^35,37Cl hyperfine and nuclear‐quadrupole tensors with DFT‐computed values suggests that the dominant EPR‐active Ti³⁺ species is located on MgCl₂(110) surfaces (or equivalent MgCl₂ terminations with tetra‐coordinated Mg). O₂ reactivity tests show that a fraction of these Ti sites is chemically accessible, an important result in view of the search for the true catalyst active site in olefin polymerization.     

Car‐parrinello molecular dynamics investigation of active surfaces and Ti catalytic sites in Ziegler‐Natta heterogeneous catalysis

A first principles molecular dynamics study of various active surfaces and Ti catalytic sites is presented. We first review some of our recent results about MgCl₂ lateral cuts acting as a support for Ti catalytic sites and inspect the structure and stability of the standard binuclear model. We find that catalytic adducts TiCl₄ and Ti₂Cl₆, which are the standard precursors of the active catalytic system, can bind more or less efficiently to the support according to the morphology of the active surfaces. Furthermore, the activation and polymerization phases are shown to be critical in determining whether or not a particular site is stable and suitable to carry out the polymerization process. We also address the problem of regioselectivity. Finally, we have attempted for the first time a first principles study of the role of a typical donor phthalate elucidating its binding properties on the different substrate cuts and its behavior as a poisoning agent for the Corradini mononuclear active site. These studies are very preliminary.     

Propene polymerization with the MgCl2-supported dichlorobis(β-diketonato)titanium catalysts activated by ordinary alkylaluminums

Several kinds of dichlorobis(β-diketonato)titanium complexes, i.e., Ti(ace-tylacetonato)₂Cl₂, Ti(1-benzoylacetonato)₂Cl₂, Ti(2,2,6,6-tetramethyl-3,5-heptanedionato)₂Cl₂ and Ti(4,4,4-trifluoro-1-phenyl-1,3-butanedionato)₂Cl₂, were synthesized and the corresponding MgCl₂-supported catalysts were prepared by impregnation method. The test of them for propene polymerization revealed that those MgCl₂-supported catalysts could be activated not only by methylaluminoxane (MAO) but also by ordinary alkylaluminums as well. The effect of typical Lewis bases on the catalyst performance was investigated in some detail, which indicated that organic silanes are most effective for the improvement of isospecificity of those catalysts. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 129–135, 1998     

Comparative characterization of MgCl2/ethyl benzoate/TiCl4 and MgCl2/2,2,6,6 tetramethylpiperidine/TiCl4 Ziegler-Natta precatalysts

In this article we present the results of the preparation and characterization of two Ziegler–Natta precatalysts: MgCl₂/Ethyl benzoate (EB)/TiCl₄ and MgCl₂/2,2,6,6 tetramethylpiperidine (TMPiP)/TiCl₄ by means of FTIR, X-ray diffraction, SEM, BET surface area measurements, and other techniques applied at different steps of their preparation procedures. The precatalysts were prepared by impregnating with TiCl₄ a given amount of MgCl₂, which was previously ball-milled with the electron donor chosen. Prior to impregnation, the ball-milled material presented different surface compounds depending on the electron donor used: [(MgCl₂)₂] · 2EB, MgCl₂ · EB, or a salt of the amine. The solid milled with EB is more homogeneous than the one milled with the TMPiP. Titanium is better retained in the solid milled with EB. This precatalyst has better morphological properties and larger BET surface area. By means of FTIR, we found evidences that an adequate surface structure for the formation of stereospecific sites in MgCl₂/TMPiP/TiCl₄ was formed. © 1994 John Wiley & Sons, Inc.