Copolymerization of ethylene and propene with a TiCl4/MgCl2-Al(C2H5)3 catalyst system using a stopped-flow method

Copolymerization of ethylene with a small amount of propene was conducted with a TiCl₄/MgCl₂-Al(C₂H₅)₃ catalyst system using a stopped-flow method. With an increase in the polymerization time from 0,035 to 0,145 s, the copolymer yield increases linearly but the number-average molecular weight of the copolymer showed a tendency of saturation. The rate constants of the propagation reaction as well as the concentrations of active sites were estimated using these data. On the other hand, the ¹H NMR spectra of copolymer did not display the resonances due to C?C double bonds which should be formed by chain transfer with monomer and/or by β-hydrogen elimination. It was also found that the crystallinity of the copolymer decreases to a great extent upon incorporation of even a very small quantity of propylene units. From these results, it was concluded that the apparent rate of copolymerization is not controlled by monomer diffusion through the polymer films. Rather the transfer reaction with Al(C₂H₅)₃ might predominantly take place during the copolymerization.     

Aspects of hydrogen activation in propene polymerization using MgCl2/TiCl4/diether catalysts

¹³C NMR analysis of the chain-end distribution of poly(propylenes) prepared using the highly active catalyst system MgCl₂/TiCl₄/diether—AlEt₃ has revealed particularly high proportions of butyl chain-ends in polymers prepared at relatively low hydrogen pressures. This indicates that the high sensitivity of this type of catalyst to hydrogen, both with respect to catalyst activity and polymer molecular weight, can largely be ascribed to chain transfer following regioirregular (2,1-) insertion, such an insertion leading to a species having low activity in chain propagation. Isotactic stereoregularity increases with increasing hydrogen pressure, indicating that a stereoirregular insertion may also slow down the chain propagation, again leading to chain transfer and resulting in the conversion of a potential chain defect into an isobutyl chain-end. Analysis of highly isotactic polymer fractions isolated via temperature rising elution fractionation revealed the presence of both butyl and isobutyl chain-ends, indicating that even the most highly stereo-specific sites in MgCl₂-supported catalysts are not totally regiospecific.     

EXAFS study of supported TiCl4/MgCl2 catalyst

Extended X-ray absorption fine structure analysis (EXAFS) has been used to study supported TiCl₄/MgCl₂ catalysts containing 1.8 wt.-% Ti. Adsorbed TiCl₄ was found to exist on the MgCl₂ surface as a dimeric complex. The possible structures of dimeric TiCl₄ complexes on the (100) face of MgCl₂ are discussed.     

Copolymerization of ethylene and propene with soluble TiCl3 catalyst systems

For random copolymerization of ethylene and propene, the catalytic activity was studied of various catalyst systems consisting of TiCl₃ prepared by reduction of TiCl₄ with hydrogen. The extent of reduction of TiCl₄ markedly affects the activity of the catalyst. Furthermore, the combination of solvent and donor affects the activity, and 1,2-dichloroethane and dibutyl ether (DBE) was found to be the most favourable combination. Metal halides (MX_n) such as VCl₄ and VOCl₃ enhance the activity of the TiCl₃ catalyst. Concerning the co-catalyst, triisobutylaluminium results in the highest activity. As a whole, both the catalytic activity of these catalyst systems and the structure of the resulting copolymer are largely dependent on the solubility of the catalyst system. The copolymer obtained with a soluble TiCl₃ · DBE · Al(isobutyl)₃ system is highly isotactic in the PPP triad and has a lower content of inversion than a copolymer prepared with a conventional VOCl₃ catalyst system. Moreover, the copolymer obtained with the above TiCl₃ catalyst has superior physical properties, especially tensile properties, in comparison with the copolymer obtained with the conventional VOCl₃.     

Polymerization of olefins with the TiCl4/SiO2 catalyst system modified with MgCl2

MgCl₂/TiCl₄/SiO₂ catalysts were prepared by treating TiCl₄/SiO₂ with MgCl₂ · 2THF (THF: tetrahydrofuran) at different Mg/Ti ratios. The catalysts were analysed by X-ray, electron spin resonance (ESR), atomic absorption spectrometry to determine the crystal structure, oxidation state of titanium and also contents of Ti, Al and Mg. Homo- and copolymerizations of ethylene and propene were conducted with them using Al(i-C₄H₉)₃ as a cocatalyst. The results of polymerization were correlated with the analytical data of the catalyst.     

Temperature effect of TiCl4 impregnation of a recrystallized MgCl2 support on propene polymerization

The MgCl₂ support for a propene polymerization catalyst was prepared by the recrystallization method using ethanol as a solvent and ethyl benzoate as an internal electron donor. The recrystallized MgCl₂ was impregnated with TiCl₄ at various temperatures, and the effect of impregnation temperature on the catalytic activity and isotactic index (I. I.) of polypropene (PP) was investigated. It was found that ethanol and titanium chloride ethoxide (TCE) in the catalyst play an important role in the catalytic activity and I. I. The support treated with TiCl₄ at high temperatures showed good performance but large activity reduction during polymerization. The enhanced performance is mainly due to activity Ti species containing small amounts of TCE. The large activity reduction is also mainly due to small amounts of ethanol and TCE, yielding small amounts of atactic sites with low Lewis acidity.     

Copolymerization of propene and butadiene with MgCl2 supported titanium catalytic systems

Upon copolymerization of propene and butadiene, using the recent generation of high mileage stereospecific MgCl₂ supported TiCl₄AIR₃ catalytic systems, it is possible to incorporate a small proportion of butadiene units in polypropene, keeping reasonable, although strongly reduced, activity. The isospecificity of the catalysts for the propene polymerization is not affected, and even may be improved upon butadiene incorporation. The butadiene is incorporated in blocks of trans-1,4 units. The amount of incorporation is dependent on the temperature, on the presence and then on the nature and concentration of a silane compound as an external Lewis base, and also on the nature of the alkylaluminium cocatalyst. The silane causes a small amount of 1,2-butadiene units to be incorporated. The molecular weights of the polymers are strongly decreased but the crystallinity is only slightly affected.     

Copolymerization of styrene and 1-butene with a novel MgCl2-supported Ti catalyst

Copolymerization of styrene and 1-butene was first conducted at 50°C with a novel Ti catalyst by using Al(iBu)₃ (iBu = isobutyl) as cocatalyst. The Ti catalyst is composed of MgCl₂/TiCl₄/PhCOOEt/Ph₂SiCl₂/SiCl₄/NdCl_x(OR)_y/Al(iBu)₃ (assigned as SN-1 catalyst), which gave mixtures of poly(1-butene), polystyrene, and a copolymer of both monomers. The catalytic efficiency is 1 200 - 4 000 g_p/(g_Ti · h). The monomer reactivity ratios were estimated to be r_s = 0.47 and r_b = 29.1. The polymers produced were fractionated successively, and a styrene-1-butene copolymer was obtained which was characterized in detail.     

Highly isospecific SiO2/MgCl2 bisupported catalyst for propene polymerization

The propene polymerization with SiO₂/MgCl₂/TiCl₄/D (D = dibutyl phthalate) (SMTD) bisupported catalyst was studied, MgCl₂/TiCl₄/D (MTD) catalyst was used for comparison. It was found that the Mg content of the catalyst has a remarkable effect on the catalyst activity. The molecular weight distribution (MWD) of polypropene (PP) obtained was similar for both the SMTD and the MTD catalyst. In contrast, the kinetic curves of propene polymerization are largely different in both cases. With the SMTD catalyst the polymerization rate increases with time, while for MTD mainly a decay-type curve is obtained. The microstructure of PP obtained with SMTD is highly isotactic in comparison to the PP obtained with MTD catalyst, although the ester content in SMTD catalyst is much lower than in MTD. The morphology of the polymer particles obtained with SMTD catalyst is mainly controlled by the morphology of the SiO₂ used for preparation of the catalyst. Thus, some template effect occurred during polymerization with this catalytic system.     

Reaction between carbon monoxide and a Ti-polyethylene bond with a MgCl2-supported TiCl4 catalyst system

The reaction between carbon monoxide and the Ti-polyethylene bond was investigated by using a stopped-flow polymerization method which can provide quasi-living polymers with very low molecular weights. Ethylene polymerization was conducted with a typical MgCl₂-supported TiCl₄ catalyst combined with triisobutylaluminium (Al(i-C₄H₉)₃) or Al(CH₃)₃. The quasi-living polymers were introduced into either the heptane solution saturated with carbon monoxide or the heptane solution containing ¹³C-enriched carbon monoxide at ?78°C. The mixture was gradually heated up to room temperature. During this procedure, a small portion of the reaction mixture was taken out at ?78°C, 0°C and at room temperature. The polymers produced were analyzed by means of Fourier-transform infrared spectroscopy, gel-permeation chromatography, proton nuclear magnetic resonance (¹H NMR) and ¹³C NMR spectroscopy. The analytical data show that carbon monoxide is predominantly incorporated into the polymers as a ketonic carbonyl.     

Effect of hydrogen on propene polymerization with solvay-type TiCl3/Cp2 TiMe2 catalyst

Polymerization of propene with a highly isospecific Solvay-type TiCl₃/Cp₂TiMe₂ catalyst in the presence and absence of hydrogen was investigated. It was found that hydrogen exerts almost no effect on the molecular weight and the stereoregularity of the resulting polypropylene. Furthermore, the H₂/D₂ exchange reaction hardly proceeds over the catalyst. To discuss this unusual behavior, plausible mechanistic explanations on the basis of the structure of active species are proposed.     

Ethylene polymerization on a high activity supported catalyst: MgCl2 (THF)2/TiCl4/AlEt2Cl

A magnesium/electron donor complex was used as support for the synthesis of a titanium catalyst. By interaction of TiCl₄ with MgCl₂(THF)₂ a highly active and stable catalyst for ethylene polymerization was obtained. The catalytic activity of the MgCl₂(THF)₂/TiCl₄/AlEt₂Cl system in the polymerization of ethylene was investigated as a function of Mg/Ti and Al/Ti mole ratios as well as the donor concentration in the system. Some kinetic studies of ethylene polymerization in the presence of this catalyst allowed us to determine the concentration of active centres (ca. 60 mol-% of Ti), rate constants of elementary reactions (k_p ≈ 92 dm³/(mol · s)) and kinetic equations. From these results it was possible to find further evidence for the role of the donor ligands in titanium catalysts supported on magnesium dichloride in the presence of Al-alkyls as cocatalysts.     

Ziegler-Natta oligomerization of 1-alkenes: a catalyst's “fingerprint”, 1. Hydrooligomerization of propene in the presence of a highly isospecific MgCl2-supported catalyst

The stereochemistry of propene polyinsertion catalyzed by a highly isospecific MgCl₂-supported Ziegler-Natta system was investigated by means of the gas chromatographic/mass spectrometric (GC-MS) characterization of the saturated oligomers (hydrooligomers) formed in the presence of H₂ at high pressure (as a chain transfer agent). It was shown that the hydrooligomer distribution is a true catalyst “fingerprint”, from which, in case of systems with (prevailingly) a single type of active centres, both the regio- and the stereospecificity of polyinsertion can be quantitatively evaluated.     

Copolymerization of ethylene and styrene with a MgCl2‐supported CpTiCl3 catalyst

Copolymerization of ethylene and styrene was carried out with CpTiCl₃/MgCl₂‐PMAO as a catalyst at various temperatures and comonomer concentrations. The present catalyst system produces a pseudorandom copolymer of ethylene and styrene beside syndiotactic poly(styrene) (sPS) and poly(ethylene) (PE). The copolymers were obtained at temperature ⪈60°C, indicating the active species promoting the copolymerization being formed at elevated temperatures. On the other hand, styrene incorporation in the copolymer increases progressively with the increase of styrene concentration.     

On the Mechanism of Polypropene Growth over MgCl2/TiCl4 Catalyst Systems

A morphological investigation was carried out on spherical catalysts based on MgCl₂‐supported TiCl₄ and related nascent polymer particles, including both homopolypropene and polypropene‐based multiphase copolymers. Transmission and scanning electron microscopy show that both catalyst and polymer grains display a dual morphological texture consisting, respectively, of microparticles and subparticles (catalyst) and microglobules and subglobules (polymer). The morphology of sequential, multiphase copolymers indicates that the second monomer, either ethylene or 1‐butene, or monomer mixture, ethylene/propene, polymerizes around the pre‐formed subglobules of the homopolypropene matrix. Based on the experimental results, a model/mechanism of polypropene growth has been proposed that entails the features of both a dual grain and a polymeric flow system: the monomer polymerizes at the surface of catalyst microparticles forming a polymer shell (microglobule) around each of them; polymer microglobules form larger agglomerates (subglobules) which, as polymerization goes on, tend to behave as individual polymeric flow units: catalyst microparticles undergo further fragmentation and tend to be convected from the bulk to the surface of polymer subglobules, where they sustain the reaction.     

Effect of the external donors on the polymerization of 4-methyl-1-pentene with high activity MgCl2/TiCl4 catalytic system

In order to find out correlations between the structure of an external donor and the obtained polymer, the effects of different external donors on the activity and stereospecificity of the MgCl₂/TiCl₄ catalytic system in the bulk polymerization of 4-methyl-1-pentene (4MP) were carefully studied. Different silane compounds of the structure R_nSi(OR')_4-n (where: n = 1–3, R = alkyl/phenyl, R = alkyl) and Al(i-Bu)₃ (TIBA) were used as external donors and cocatalyst, respectively. The effect of the donor/TIBA mole ratio on the activity and stereospecificity of the catalytic system was studied. Some major effects were observed for the three different external donors, namely, Me₃Si(OMe), Me₂Si(OMe)₂, and Me₃Si(OBu)₃, in the 4MP polymerization process. It was observed that the effect of the external silane donor on the polymerization strongly depends upon the size of the alkoxy and hydrocarbon (alkyl/phenyl) groups which are attached to the silicon atom. A selective deactivation of the non-stereospecific centers, as well as a transformation of the non-stereospecific into isospecific centers, is assumed to occur. On the basis of the obtained results and literature data available for the propene polymerization, the concept of structural conformity between the ligand-surrounded active center and the monomer molecule was carried forward.     

Copolymerization of propene and higher α-olefins with the metallocene catalyst Et[Ind]2HfCl2/methylaluminoxane

Copolymerizations of propene with higher α-olefins including 1-butene, 1-hexene, 1-octene, 1-dodecene and 1-hexadecene were carried out with an isospecific metallocene catalyst (Et[Ind]₂HfCl₂/methylaluminoxane) at 30°C in toluene. ¹³C NMR analysis showed that all products obtained are random copolymers (r₁ · r₂ ～ 1). The reactivity of the higher α-olefins in the copolymerization is surprisingly high and decreases only slightly with increasing length of the olefin. The incorporation rates of the comonomers in this study were found to be much higher than those obtained by the use of heterogeneous Ziegler-Natta catalysts. Hence, copolymers with every desired composition and α-olefin homopolymers can be prepared. The molecular weight of the copolymers is reduced with rising comonomer content. Melting points and glass transition temperatures studied by means of differential scanning calorimetry show a decrease with rising comonomer content and increasing length of the α-olefin.     

New synthesis of a highly active δ-MgCl2 for MgCl2/TiCl4/AlEt3 catalytic systems

A good supporting material for Ziegler-Natta type catalysts was obtained by reaction between powdered metallic magnesium and 1-chloro-n-C_mH_2m+1 (m = 3–9). This reaction affords a highly disordered form of MgCl₂ which was characterized by FT-IR spectroscopy and powder X-ray diffraction (XRD) analysis. This MgCl₂ form shows a crystallographic disorder much higher than that exhibited by the products obtained following the commercial activation procedures of α-MgCl₂, either mechanical or chemical. As shown by the XRD spectra, the synthetized MgCl₂ is characterized by the typical structure of the δ-form. Therefore, this MgCl₂ form, in view of its highly disordered structure, appears as a promising material for the preparation of active supported Ziegler-Natta type catalysts. Thus, by titanation of the obtained δ-MgCl₂ we have prepared some supported catalysts which have been tested in the slurry propene polymerizations, showing high activity (136000–138000 gPP/gTi) and good stereoselectivity (82–88% I.I.).