Strong activation of MgCl2‐supported Ziegler‐Natta catalysts by treatments with BCl3: Evidence and application of the “cluster” model of active sites

Heterogeneous Ziegler‐Natta precatalysts (with phthalate as internal donor) were modified by treatments with BCl₃ (2 h in heptane; T = 20–90 °C; B/Ti = 0.1–5) before their use in the polymerization of propylene to modify the active sites distribution. If performed on previously “detitanated” precatalysts, the treatment leads to a strong increase of productivity (up to one order of magnitude) without drastic modifications of polypropylenes properties (tacticity, molecular weight distribution). In addition, these findings are in good agreement with the hypothesis of a “cluster” organization of active sites allowing to rationalize activation by BCl₃ by formation of heteronuclear B‐Ti clusters. The activation method was also applied to unmodified precatalysts and gave a significant gain of productivity. The simple and versatile activation process can also be performed under mild conditions (low T and low [BCl₃]). © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5784–5791, 2009     

Modifications of the active sites distribution in the Ziegler‐Natta polymerization of propylene using Lewis acids

Heterogeneous Ziegler‐Natta precatalysts (with phthalate as internal donor) were modified by treatments with various Lewis acids (MCl_n with M = Ga, Sn, Si, and Sb and n = 3, 4, or 5) before their use in the polymerization of propylene. If performed on previously “detitanated” precatalysts, treatments with SnCl₄ and SiCl₄ lead to a slight activation but especially to an increase of the tacticity whereas GaCl₃ and SbCl₅ treatments deactivate the catalyst. The modification method applied to conventional unmodified precatalysts gave similar trends. A significant increase of tacticity (and/or of T_m) and a narrowing of the molecular weight distribution were observed in the case of SnCl₄ and SiCl₄ treatments. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2631–2635, 2010     

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     

Exploring pathways to reduce the distribution of active sites in the Ziegler–Natta polymerization of propylene

Chemical treatments of classical supported Ziegler–Natta precatalysts were conducted by using additional bulky ligands to attempt to narrow and homogenize the active sites distribution in propylene polymerization. Additions of monodentate ligands such as bis(trimethylsilyl)amide, cyclopentadienyl derivates or triphenylsilanol were seen to slow down the polymerization without modifying the distribute properties of polypropylenes. In the case of multidentate ligands (porphines or biquinolines), in addition to the poisoning of active sites, an extraction of titanium from the catalyst surface is observed. A decrease of both melting point and isotacticity (II%) of polymers using these compounds suggest that the most isospecific titanium sites are first extracted from the MgCl₂‐surface. The narrowing of the molecular weight distribution confirms that the highly isospecific sites are the most active sites, producing the higher molecular weight polymers. Moreover, this study shows that the distributed properties of polymers are due to the chemical diversity of the active sites with various steric and electronic environments at the catalyst surface and not to mass transfer limitations. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3941–3948, 2007     

Similarities and Differences of the Active Sites in Basic and Advanced MgCl2‐Supported Ziegler‐Natta Propylene Polymerization Catalysts

Though preparation procedures of heterogeneous Ziegler‐Natta catalysts for propylene polymerization are sophisticated, it is uncertain whether the nature of the active sites is similar or different for different preparation procedures. In this study, the effects of preparation procedures on the nature of the active sites were investigated by stopped‐flow polymerization in combination with microstructure analysis of polymers. Both basic and advanced types of catalysts showed the same two kinds of isospecific active site, which indicated little influence of the preparation method on the active site structure. On the contrary, the ratios of the two kinds of isospecific sites were not the same, resulting in variation of average polymer properties.

     

Mimicking Ziegler‐Natta Catalysts in Homogeneous Phase, 1. C2‐Symmetric Octahedral Zr(IV) Complexes with Tetradentate [ONNO]‐Type Ligands

Results of propene polymerization in the presence of two known octahedral C₂‐symmetric Zr complexes bearing tetradentate [ONNO]‐type ligands are reported for the first time. Depending on the steric hindrance at the active metal, isotactic site‐controlled or weakly syndiotactic chain‐end‐controlled polymers were obtained, in both cases via highly regioselective 1,2 (primary) monomer insertion. In this respect, the complexes mimic the behavior of the active Ti species on the surface of the heterogeneous Ziegler‐Natta catalysts of which they might represent good structural models.     

The discovery and progress of MgCl2‐supported TiCl4 catalysts

Polyolefins represented by polyethylene (PE) and polypropylene (PP) are indispensable materials in our daily lives. TiCl₃ catalysts, established by Ziegler and Natta in the 1950s, led to the births of the polyolefin industries. However, the activities and stereospecificities of the TiCl₃ catalysts were so low that steps for removing catalyst residues and low stereoregular PP were needed in the production of PE and PP. Our discovery of MgCl₂‐supported TiCl₄ catalysts led to more than 100 times higher activities and extremely high stereospecificities, which enabled us to dispense with the steps for the removals, meaning the process innovation. Furthermore, they narrowed the molecular weight and composition distributions of PE and PP, enabling us to control the polymer structures precisely and create such new products as very low density PE or heat‐sealable film at low temperature. The typical example of the product innovations by the combination of the high stereospecificity and the narrowed composition distribution is high‐performance impact copolymer used for an automobile bumper that used to be made of metal. These process and product innovations established these polyolefin industries. The latest MgCl₂‐supported TiCl₄ catalyst is very close to perfect control of isotactic PP structure and is expected to bring about further innovations. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1–8, 2004     

Multicenter nature of titanium‐based Ziegler–Natta catalysts: Comparison of ethylene and propylene polymerization reactions

This article discusses the similarities and differences between active centers in propylene and ethylene polymerization reactions over the same Ti‐based catalysts. These correlations were examined by comparing the polymerization kinetics of both monomers over two different Ti‐based catalyst systems, δ‐TiCl₃‐AlEt₃ and TiCl₄/DBP/MgCl₂‐AlEt₃/PhSi(OEt)₃, by comparing the molecular weight distributions of respective polymers, in consecutive ethylene/propylene and propylene/ethylene homopolymerization reactions, and by examining the IR spectra of “impact‐resistant” polypropylene (a mixture of isotactic polypropylene and an ethylene/propylene copolymer). The results of these experiments indicated that Ti‐based catalysts contain two families of active centers. The centers of the first family, which are relatively unstable kinetically, are capable of polymerizing and copolymerizing all olefins. This family includes from four to six populations of centers that differ in their stereospecificity, average molecular weights of polymer molecules they produce, and in the values of reactivity ratios in olefin copolymerization reactions. The centers of the second family (two populations of centers) efficiently polymerize only ethylene. They do not homopolymerize α‐olefins and, if used in ethylene/α‐olefin copolymerization reactions, incorporate α‐olefin molecules very poorly. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1745–1758, 2003     

New Synthesis Method Using Magnesium Alkoxides as Carrier Materials for Ziegler‐Natta Catalysts with Spherical Morphology

In this study, novel carrier materials were synthesized by addition of metal dihalide compounds in the synthesis reaction of magnesium diethoxide using metallic magnesium, ethanol and iodine. Poly(propylene) polymerizations were then investigated with the MgCl₂‐supported TiCl₄ catalysts using these carrier materials. As results, magnesium diethoxide with extremely large particle sizes and spherical shapes were obtained and the angles of repose of PP particles obtained by using their catalysts as a flowability index showed high values. Furthermore, in order to confirm key points for excellent catalyst performance from detailed characterizations, SEM images, compositions and WAXS were measured.

     

Synthesis of a TiCl4 Ziegler‐Natta Catalyst Supported on Spherical MgCl2 · nEtOH for the Polymerization of Ethylene and Propylene

Experiments designed to prepare Ziegler‐Natta catalysts with controlled particle morphology are reported. Different dealcoholation processes are used on the adduct MgCl₂ · nEtOH to prepare the catalysts: either thermal treatment or chemical dealcoholation employing different substances such as titanium tetrachloride, triethylaluminium, dichlorodimethylsilane, and chlorotrimethylsilane. In addition, dichlorodimethylsilane dealcoholation is also performed after thermal treatment. SEM analysis of adducts, supports, and catalysts is carried out. The obtained catalysts are characterized through impregnated titanium content evaluation. The polyethylenes and poly(propylene)s obtained employing the so prepared catalysts show spherical morphology when examined by optical microscopy.

     

Effects of external donors and hydrogen concentration on oligomer formation and chain end distribution in propylene polymerization with Ziegler‐Natta catalysts

The effect of type and concentration of external donor and hydrogen concentration on oligomer formation and chain end distribution were studied. Bulk polymerization of propylene was carried out with two different Ziegler‐Natta catalysts at 70 °C, one a novel self‐supported catalyst (A) and the other a conventional MgCl₂‐supported catalyst (B) with triethyl aluminum as cocatalyst. The external donors used were dicyclopentyl dimethoxy silane (DCP) and cyclohexylmethyl dimethoxy silane (CHM). The oligomer amount was shown to be strongly dependent on the molecular weight of the polymer. Catalyst A gave approximately 50 % lower oligomer content than catalyst B due to narrower molecular weight distribution in case of catalyst A. More n‐Bu‐terminated chain ends were found for catalyst A indicating more frequent 2,1 insertions. Catalyst A also gave more vinylidene‐terminated oligomers, suggesting that chain transfer to monomer, responsible for the vinylidene chain ends, was a more important chain termination mechanism for this catalyst, especially at low hydrogen concentration. Low site selectivity, due to low external donor concentration or use of a weak external donor (CHM), was also found to increase formation of vinylidene‐terminated oligomers. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 351–358, 2010     

Propylene polymerization reactions with supported Ziegler–Natta catalysts: Observing polymer material produced by a single active center

Medium‐ and high‐resolution SEM analysis of several Ti‐based MgCl₂‐supported Ziegler–Natta catalysts and isotactic polypropylene produced with them is carried out. Each catalyst particle, 35–55 μ in size, produces one polymer particle with an average size of 1.5–2 mm, which replicates the shape of the catalyst particle. Polymer particles contain two distinct morphological features. The larger of them are globules with D_av ～400 nm; from 1 to 2 × 10¹¹ globules per particle. Each globule represents the combined polymer output of a single active center. The globules consist of ～2500 microglobules with an average size of ～20 nm. The microglobules contain several folded polymer molecules; they are the smallest thermodynamically stable macromolecular ensembles in propylene polymerization reactions. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 3832–3841     

Ziegler‐Natta/Metallocene Hybrid Catalyst for Ethylene Polymerization

A Ziegler‐Natta/metallocene hybrid catalyst was produced and utilized in the polymerization of ethylene with the aim of producing bimodal polyethylene. The MgCl₂ adduct was prepared by a melt quenching method and Cp₂ZrCl₂ and TiCl₄ catalysts were loaded, respectively, after treating the surface with TiBAl. The polymerization kinetics involved an induction period, followed by fragmentation and expansion of particles. SEM micrographs revealed that the spherical morphology was retained through the initial mild reaction conditions of induction period. The polymers produced showed bimodal molecular weight distribution patterns, suggesting that both components of the hybrid catalyst were active over the support.

     

Mathematical Modeling of the Microstructure of Poly(propylene) Made with Ziegler‐Natta Catalysts in the Presence of Electron Donors

Two different modeling techniques, the method of moments and Monte Carlo simulation, were compared for propylene polymerization with coordination catalysts including a new mechanistic step, site transformation by electron donors. We used the models to show how the molecular weight and tacticity distributions of several poly(propylene) chain populations were affected by changing the concentration of hydrogen, electron donor, and propylene in the reactor, under steady‐state or dynamic operating conditions. The Monte Carlo simulation describes the molecular weight and tacticity distributions for the whole polymer and chain populations with distinct microstructural characteristics. We have also applied the Monte Carlo model to simulate the pentad sequence distributions and its equivalent ¹³C NMR spectra.

     

Hydrogen effects in propylene polymerization reactions with titanium‐based Ziegler–Natta catalysts. I. Chemical mechanism of catalyst activation

The hydrogen activation effect in propylene polymerization reactions with Ti‐based Ziegler–Natta catalysts is usually explained by hydrogenolysis of dormant active centers formed after secondary insertion of a propylene molecule into the growing polymer chain. This article proposes a different mechanism for the hydrogen activation effect due to hydrogenolysis of the Ti? iso‐C₃H₇ group. This group can be formed in two reactions: (1) after secondary propylene insertion into the Ti? H bond (which is generated after β‐hydrogen elimination in the growing polymer chain or after chain transfer with hydrogen), and (2) in the chain transfer with propylene if a propylene molecule is coordinated to the Ti atom in the secondary orientation. The Ti? CH(CH₃)₂ species is relatively stable, possibly because of the β‐agostic interaction between the H atom of one of its CH₃ groups and the Ti atom. The validity of this mechanism was demonstrated in a gas chromatography study of oligomers formed in ethylene/α‐olefin copolymerization reactions with δ‐TiCl₃/AlEt₃ and TiCl₄/dibutyl phthalate/MgCl₂–AlEt₃ catalysts. A quantitative analysis of gas chromatography data for ethylene/propylene co‐oligomers showed that the probability of secondary propylene insertion into the Ti? H bond was only 3–4 times lower than the probability of primary insertion. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1353–1365, 2002     

Determination of the Catalytic Sites for Ziegler‐Natta Homo‐Polymerization from GPC Data

Deconvolution of the MWD of a polymer produced by multi‐site catalysts into independent Flory modes is the first step in modeling the polymerization process. A new deconvolution procedure for GPC data is developed that does not require an a priori assumption concerning the nature of the discrete distribution and can be used with a continuous distribution. The MWD measured via GPC is a linear function of the individual catalytic sites, but it is numerically ill‐conditioned, preventing direct inversion of the GPC data. Tikhonov regularization has been developed to uniquely invert the MWD. Applying the regularizing method to a polyethylene produced via a Ziegler‐Natta catalyst, seven discrete sites were found, and the kinetic constant ratios were determined for each of these sites.

     

Organic nanoparticles as fragmentable support for Ziegler–Natta catalysts

A fragmentable support material for Ziegler–Natta catalysts is presented based on micrometer‐sized aggregates of polystyrene nanoparticles. Hydroxyl anchoring groups are introduced by copolymerization of hydroxymethylstyrene in emulsion process to immobilize the catalysts. The catalytic activity in ethylene slurry polymerizations is found to be directly correlated to the hydroxyl group content of the supports. Furthermore, the fragmentation behavior of dye‐labeled support aggregates into the initial nanoparticles is demonstrated using laser scanning confocal fluorescence microscopy as a nondestructive method. These supported catalysts fulfill two important design criteria, high fragmentability and high catalyst loading, and produce high‐density polyethylene with medium molecular weight distributions (MWDs = 3–4). These values lie between those obtained using single‐site metallocene‐based (narrow MWD < 3) or inorganic supported multi‐site Ziegler–Natta‐based (broad MWD = 4–12) polymerizations without the need of blending. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 15–22     

Propylene Polymerization Performance of Isolated and Aggregated Ti Species Studied Using a Well‐Designed TiCl3/MgCl2 Ziegler‐Natta Model Catalyst

In propylene polymerization with MgCl₂‐supported Ziegler‐Natta catalysts, it is known that the reduction of TiCl₄ with alkylaluminum generates Ti³⁺ active species, and at the same time, leads to the growth of TiCl_x aggregates. In this study, the aggregation states of the Ti species were controlled by altering the Ti content in a TiCl₃/MgCl₂ model catalyst prepared from a TiCl₃ · 3C₅H₅N complex. It is discovered that all the Ti species become isolated mononuclear with a highly aspecific feature below 0.1 wt.‐% of the Ti content, and that the isolated aspecific Ti species are more efficiently converted into highly isospecific ones by the addition of donors than active sites in aggregated Ti species.

     

Hydrogen effects in propylene polymerization reactions with titanium‐based Ziegler–Natta catalysts. II. Mechanism of the chain‐transfer reaction

Hydrogen is a very effective chain‐transfer agent in propylene polymerization reactions with Ti‐based Ziegler–Natta catalysts. However, measurements of the hydrogen concentration effect on the molecular weight of polypropylene prepared with a supported TiCl₄/dibutyl phthalate/MgCl₂ catalyst show a peculiar effect: hydrogen efficiency in the chain transfer significantly decreases with concentration, and at very high concentrations, hydrogen no longer affects the molecular weight of polypropylene. A detailed analysis of kinetic features of chain‐transfer reactions for different types of active centers in the catalyst suggests that chain transfer with hydrogen is not merely the hydrogenolysis reaction of the Ti? C bond in an active center but proceeds with the participation of a coordinated propylene molecule. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1899–1911, 2002