Ethylene–hexene copolymerization by heterogeneous and homogeneous Ziegler–Natta catalysts and the “comonomer” effect

Homopolymerization of ethylene and 1-hexene and their copolymerizations were compared to investigate the influence of α-olefin on the enhancement of ethylene polymerization rate (R_p), which is often referred to as the “comonomer” effect. With the two homogeneous Ziegler–Natta catalysts, Et[Ind]₂ZrCl₂/MAO and (π-C₅H₅)₂ZrCl₂/MAO (MAO = methylaluminoxane), hexene causes reduction of R_p—in other words a negative “comonomer” effect. In the case of the high activity MgCl₂ supported TiCl₃ catalysts there is a slight positive “comonomer” effect; the R_p increases by 25 to 70% with the addition of 15 mol % of hexene. The “comonomer” effects in there catalyst systems are much smaller than that observed for the classical TiCl₃ catalyst. © 1993 John Wiley & Sons, Inc.     

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     

A new route to atactic polypropylene: The second life of premetallocene homogeneous polymerization catalyst

Amorphous atactic polypropylene (PP) with an average molecular weight of 50,000–100,000 is produced by polymerizing propylene with a ternary Ti(Oiso‐Pr)₄ ‐ AlEt₂Cl/MgBu₂ catalyst at 30–50 °С. Main advantages of this catalyst compared with other catalysts capable of nearly exclusively producing atactic PP (such as some heterogeneous Ziegler‐Natta, metallocene and postmetallocene catalysts) are high activity, low cost and the ease of use: the catalyst is prepared in situ from three commercially available compounds readily soluble in aliphatic and aromatic hydrocarbons. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2124–2131     

Polymer-supported Ziegler–Natta catalyst for the polymerization of isoprene

A polymer-supported Ziegler–Natta catalyst, polystyrene-TiCl₄AlEt₂Cl (PS–TiCl₄AlEt₂Cl), was synthesized by reaction of polystyrene–TiCl₄ complex (PS–TiCl₄) with AlEt₂Cl. This catalyst showed the same, or lightly greater catalytic activity to the unsupported Ziegler–Natta catalyst for polymerization of isoprene. It also has much greater storability, and can be reused and regenerated. Its overall catalytic yield for isoprene polymerization is ca. 20 kg polyisoprene/gTi. The polymerization rate depends on catalyst titanium concentration, mole ratio of Al/Ti, monomer concentration, and temperature. The kinetic equation of this polymerization is: R_p = k[M]^0.30[Ti]^0.41[Al]^1.28, and the apparent activation energy ΔE_act = 14.5 kJ/Mol, and the frequency factor A_p = 33 L/(mol s). The mechanism of the isoprene polymerization catalyzed by the polymer-supported catalyst is also described. © 1993 John Wiley & Sons, Inc.     

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     

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     

Polymerization mechanism for in situ supported metallocene catalysts

The polymerization of ethylene was carried out with a novel in situ supported metallocene catalyst that eliminated the need for a supporting step before polymerization. In the absence of trimethyl aluminum (TMA), in situ supported Et[Ind]₂ZrCl₂ was not active, but the addition of TMA during polymerization activated the catalyst. Et[Ind]₂Zr(CH₃)₂ was active even in the absence of TMA, whereas the addition of TMA during polymerization enhanced the catalytic activity. The polymerization‐rate profiles of the in situ supported metallocene catalysts did not show rate decay as a function of time. A polymerization mechanism for the in situ supported metallocene catalysts is proposed for this behavior. During polymerization, the in situ supported metallocene catalysts may deactivate, but homogeneous metallocene species present in the reactor may form new active sites and compensate for deactivated sites. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 462–468, 2000     

AlR2Cl/MgR2 combinations as universal cocatalysts for Ziegler–Natta,metallocene, and post‐metallocene catalysts

Combinations of dialkylaluminum chlorides and dialkylmagnesium compounds, when used at molar [AlR₂Cl]:[MgR₂] ratios ≥ 2, act as universal cocatalysts for all three presently known types of alkene polymerization catalysts—Ziegler–Natta, metallocene, and post‐metallocene. When these cocatalysts are used with supported Ti‐based Ziegler–Natta catalysts, they produce catalyst systems which are 1.5–2 times more active than the systems utilizing AlR₃ compounds as cocatalysts. Combinations of AlR₂Cl/MgR₂ cocatalysts and various metallocene complexes produce single‐center catalyst systems similar to those formed in the presence of MAO. The same cocatalysts activate numerous post‐metallocene Ti complexes containing bidentate ligands of a different nature and produce multicenter systems of very high activity. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3271–3285, 2009     

ESR studies on oxidation state of titanocene and zirconocene catalysts

An on‐line electron spin resonance (ESR) technique was applied to investigate the oxidation states of the metallocene catalysts CpTiCl₃, CpZrCl₃, Cp₂TiCl₂, and Cp₂ZrCl₂. These metallocene catalysts were activated by modified methylaluminoxane (MMAO). It was found that the titanocene catalysts (CpTiCl₃ and Cp₂TiCl₂) were readily reduced to the trivalent state while the zirconocene catalysts (CpZrCl₃ and Cp₂ZrCl₂) were quite stable with respect to reduction. The concentrations of the trivalent species Ti(III) and Zr(III) showed linear relationships with the concentrations of metallocene catalyst precursors. However, their slopes were always smaller than unity indicating the existence of bimetallic interactions of the active sites. The ESR detectable Ti(III) and Zr(III) concentrations initially increased with the MAO/catalyst ratio and then leveled off after an 800–1000 Al/catalyst molar ratio. The deactivation processes were followed as a function of aging time over a range of temperature (25–100°C). The decay curves strongly depended on aging temperature with higher temperature giving faster decay rates. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1465–1472, 1999     

Dialkylzinc Compounds as Chain Transfer Agents in Ethylene and Propylene Polymerizations Catalyzed by Metallocene Catalysts

Dialkylzinc compounds (ZnR₂) with the alkyl groups of different steric hindrance were used as chain transfer agents in ethylene and propylene polymerizations catalyzed by two conventional metallocene catalysts including rac-Et(Ind)₂ZrCl₂ and rac-Me₂Si[2-Me-4-Ph-Ind]₂ZrCl₂. In general, catalyst activities for ethylene polymerizations are barely affected by chain transfer agents, regardless of the R type; however, there are significant activity reductions in propylene polymerizations when the R in ZnR₂ is less hindered, and as R becomes bulkier, catalyst activities are gradually restored. ZnR₂ and metallocene catalyst active site tend to form a reversible and catalytically inactive complex, thus the geometry congested ZnR₂ would reduce complex formation tendency and hence decreased its negative effect on catalyst activities.     

Polymer-supported Ziegler–Natta catalysts. I. A preliminary study of catalyst synthesis

Polymer-supported Ziegler–Natta catalysts based on various polymer carriers were synthesized by different methods, including (1) loading TiCl₄ directly onto the polymer supports; (2) loading TiCl₄ onto the polymer supports modified by magnesium chloride (MgCl₂); (3) loading TiCl₄ onto the polymer supports modified by Grignard reagent (RMgCl); and (4) loading TiCl₄ onto the polymer supports modified by magnesium alkyls (MgR₂). The activity and kinetic features of the catalysts for ethylene polymerization were examined. Among the combinations tested, the best was found to be TiCl₄/n-Bu₂Mg.Et₃Al/poly(ethylene-co-acrylic acid) (92:8), which produced a catalyst of very high activity for ethylene polymerization. © 1994 John Wiley & Sons, Inc.     

Dialkylzinc Compounds as Chain Transfer Agents in Ethylene and Propylene Polymerizations Catalyzed by Metallocene Catalysts

Dialkylzinc compounds (ZnR₂) with the alkyl groups of different steric hindrance were used as chain transfer agents in ethylene and propylene polymerizations catalyzed by two conventional metallocene catalysts including rac-Et(Ind)₂ZrCl₂ and rac-Me₂Si[2-Me-4-Ph-Ind]₂ZrCl₂. In general, catalyst activities for ethylene polymerizations are barely affected by chain transfer agents, regardless of the R type; however, there are significant activity reductions in propylene polymerizations when the R in ZnR₂ is less hindered, and as R becomes bulkier, catalyst activities are gradually restored. ZnR₂ and metallocene catalyst active sites tend to form a reversible and catalytically inactive complex, thus, the geometry congested ZnR₂ would reduce complex formation tendency and hence, decrease its negative effect on catalyst activities.     

Binary metallocene supported catalyst for propylene polymerization

The binary silica supported catalyst system comprising the Cp₂ZrCl₂ and SiMe₂(Ind)₂ZrCl₂ metallocene compounds was prepared with different immobilization methods and evaluated at different propylene polymerization conditions. The performance results of the homogeneous isolated catalysts and also the homogeneous catalyst mixture were also included for comparison. High activities were obtained with the supported systems and the molecular weight of the produced polypropylene was invariably higher than that obtained using the homogeneous precursor.     

Study of the compositional heterogeneity of ethylene–hexene-1 copolymers by thermal fractionation technique by means of differential scanning calorimetry

The successive self-nucleation/annealing technique (SSA) by differential scanning calorimetry has been applied to study the heterogeneity of ethylene–hexene-1 copolymers produced with supported catalytic systems of different compositions: highly active supported Ziegler–Natta (Z–N) catalysts—a titanium–magnesium catalyst TiCl₄/MgCl₂ (TMC) and a vanadium–magnesium catalyst VCl₄/MgCl₂ (VMC), a supported zirconocene catalyst Me₂Si(Ind)₂ZrCl₂/SiO₂ (MAO), and a chromium-oxide catalyst CrO₃/SiO₂. Comparative data by SSA technique with the same temperature program were obtained for copolymers differed by MWD from narrow to very broad (Mw/Mn = 2.4–54) and short chain branching distribution from narrow (zirconocene catalyst) to very broad (TMC and chromium oxide catalysts). It is demonstrated that copolymers produced with the zirconocene catalyst have the narrowest melting range and do not contain thick lamellae. The widest lamella thickness distribution has been found for a copolymer produced with the chromium-oxide catalyst. Copolymers produced with the supported Z–N catalysts are ranked in the middle with a more narrow lamella thickness distribution for copolymer prepared with VMC as compared with the one produced with TMC. The SSA results are compared with the data on copolymer fractionation by TREF. It is shown that these methods give a good correlation for copolymers with narrow short-chain branching distribution produced with the supported zirconocene catalyst. In the case of copolymers produced with TMC, TREF yields a higher content of the high-branched fractions.     

Preparation of hindered piperidine and its copolymerization with propylene over a supported high activity ziegler–natta catalyst

A new polymerizable stabilizer 4-(hex-5-enyl)-2,2,6,6-tetramethylpiperidine is prepared. This sterically hindered piperidine was copolymerized with propylene over a fourth generation TiCl₄/MgCl₂ Ziegler–Natta catalyst, using Al(C₂H₅)₃ as cocatalyst and diphenyldimethoxysilane, DMS, as external electron donor. The copolymer exhibited high thermo-oxidative stability even after exhaustive extraction with n-heptane.     

Novel spherical Ziegler–Natta catalyst for polymerization and copolymerization. I. Spherical MgCl2 support

Spherical MgCl₂ adducts used as supports for a Ziegler–Natta catalyst for olefin polymerization were prepared by the general precipitation method. The influence of MgCl₂/EtOH (mol/mol) and the dispersion speeds on the particle size (PS) and particle size distribution (PSD) were investigated. It was found that the former played a trivial role in controlling the PS and PSD, and the latter was the key factor. In particular, the influence of ethanol on the crystal structure was further examined, with consideration given to the performance of the supported Ziegler–Natta catalyst. It was believed that the reactions between MgCl₂ and ethanol had a controlling effect on the destruction of the original anhydrous MgCl₂, which was the key point in the preparation of suitable supports for the latest generation Ziegler–Natta catalyst. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3112–3119, 2002     

Copolymerization of ethylene with higher linear α-olefins—reactivity ratios

Copolymerization of ethylene with mixtures of linear α-olefins C₆–C₃₆ in the presence of two heterogeneous Ziegler–Natta catalysts, δ-TiCl₃–AlEt₃ and TiCl₄/MgCl₂–AlEt₃, at 90°C was studied by the GC method, and reactivity ratios for all paris ethylene–α-olefin were estimated from the data on olefin consumption in the reactions. In the case of the δ-TiCl₃–AlEt₃ system, the r₂ value decreases from ca. 0.05 for 1-decene to ca. 0.02 for α-C₂₂H₄₄ and then remains approximately constant. This change is similar to the dependence of the modified steric parameter E_S^C of the olefin alkyl group on the size of the alkyl group. In the case of the supported TiCl₄/MgCl₂–AlEt₃ system a similar variation of r₂ with the length of the alkyl group were observed but the absolute values of r₂ were six to ten times lower than those for the first catalytic system.     

Superactive and stereospecific catalysts. III. Definitive identification of active sites by electron paramagnetic resonance

Superactive Ziegler–Natta catalysts have been prepared from a soluble MgCl₂·2-ethyl hexanol adduct in the presence of organic esters through reactions with TiCl₄ and activated with AlEt₃/phenyltriethoxy-silane. Electron paramagnetic spectra (EPR) were used to elucidate the nature and amount of those Ti⁺³ ions not bridged to another Ti⁺³ ion; the chlorine bridged Ti⁺³ ions are EPR silent. The EPR spectra were attributed to two rhombic Ti⁺³ sites with principal values for the g-tensors (1.967, 1.949, 1.915; and 1.979, 1.935, 1.887). The total amount of the EPR species, obtained by double integration of the EPR spectra, is in close agreement with the concentration of isospecific catalytic sites determined by radiotagging. This suggests that the nonspecific sites are EPR silent. When o-phthalic ester was present during the catalyst synthesis, there appears an EPR signal at the free electron g-value. This signal was attributed to a Ti⁺³ phthalate species with resonance stabilization and spin delocalization; it is absent in the catalysts made in the presence of monoesters such as ethyl benzoate. The effects of monomer, O₂, H₂O, and I₂ on the EPR spectra were investigated. The changes in the EPR spectral intensity and the total Ti⁺³ ions, the latter determined by redox titrations during a polymerization or catalyst aging, are described. The results were extensively compared with those observed for supported Ziegler–Natta catalyst prepared with crystalline MgCl₂.     

In‐Situ and Ex‐Situ Microscopic Study of Gas Phase Propylene Polymerization over a High Activity TiCl4‐MgCl2 Heterogeneous Ziegler‐Natta Catalyst

In‐situ gas phase poly(propylene) (PP) formation over a high activity TiCl₄‐MgCl₂‐supported Ziegler‐Natta catalyst has been studied by video microscopy combined with ex‐situ light microscopy, SEM, high‐resolution TEM, and STEM/PEELS/EDX for the first time. In‐situ observation revealed rapid formation of poly(propylene) beads 9–10 μm in size (< 1/30 s) as well as growth of significant amounts of polymer within local regions. Catalyst particles containing 2–5 nm‐sized MgCl₂ crystalline domains are subjected to transformations during catalysis that form PP/catalyst aggregated structures of 30–50 μm in size.     

Synthesis of polypropylene graft copolymers by the combination of a polypropylene copolymer containing pendant vinylbenzene groups and atom transfer radical polymerization

This article discusses a facile and inexpensive reaction process for preparing polypropylene‐based graft copolymers containing an isotactic polypropylene (i‐PP) main chain and several functional polymer side chains. The chemistry involves an i‐PP polymer precursor containing several pendant vinylbenzene groups, which is prepared through the Ziegler–Natta copolymerization of propylene and 1,4‐divinylbenzene mediated by an isospecific MgCl₂‐supported TiCl₄ catalyst. The selective monoenchainment of 1,4‐divinylbenzene comonomers results in pendant vinylbenzene groups quantitatively transformed into benzyl halides by hydrochlorination. In the presence of CuCl/pentamethyldiethylenetriamine, the in situ formed, multifunctional, polymeric atom transfer radical polymerization initiators carry out graft‐from polymerization through controlled radical polymerization. Some i‐PP‐based graft copolymers, including poly(propylene‐g‐methyl methacrylate) and poly(propylene‐g‐styrene), have been prepared with controlled compositions. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 429–437, 2005