MgCl2-supported Ziegler-Natta Catalysts Research Articles

The effects of a new aminosilane external donor on propylene polymerization with MgCl2-supported Ziegler-Natta catalyst

The effects of a new aminosilane external donor on propylene polymerization with MgCl2-supported Ziegler-Natta catalyst

Mechanistic study on hydrogen effects in propylene polymerization with MgCl2-supported Ziegler-Natta catalysts: New evidences from active center counting and reaction kinetics

Propylene polymerizations catalyzed by MgCl2/Di/TiCl4-AlEt3/siloxane in the absence and presence of hydrogen were studied by determining the time-dependent changes of catalytic activity, number of active centers and active center distribution. Three types of active centers with high, medium and very low stereoselectivity were differentiated by fractionating the polymer into three parts. Chain propagation rate constant of each type of active centers was estimated by linear fitting/extrapolating the time-dependent change of apparent propagation rate constant. The centers with high (Ci*), medium (Cm*) and low (Ca*) stereoselectivity responded to addition of hydrogen differently. Adding hydrogen caused marked increase of Ci*, but its propagation rate constant (kpi) was evidently reduced. In contrast, numbers of Cm* and Ca* were only slightly affected, but their propagation rate constants were evidently enhanced by hydrogen. The different hydrogen responses of Ci* with those of Cm* and Ca* were explained by two kinds of hydrogen activation mechanisms.

Porous Organic Polymers-Supported Zeigler-Natta Catalysts for Preparing Highly Isotactic Polypropylene with Broad Molecular Weight Distribution.

Porous organic polymers (POPs) have attracted much attention in numerous areas, including catalysis, adsorption and separation. Herein, POP supported Ziegler-Natta catalysts were designed for preparation of isotactic polypropylene (iPP). The POPs-based Ziegler-Natta catalysts exhibited the characteristic of broad molecular weight distribution (MWD > 11) with or without adding an extra internal electron donor. The added internal electron donor 3-methyl-5-tert-butyl-1,2-phenylene dibenzoate (ID-2) used in cat-2 showed good propylene polymerization activity of 15.3 × 106 g·PP/mol·Ti·h, high stereoregularity with 98.2% of isotacticity index and broad molecular weight distribution (MWD) of 12.3. Compared to the MgCl2-supported Ziegler-Natta catalysts (cat-4) with the same ID-2, cat-2 showed higher chain stereoregularity for propylene polymerization. As seen in the TREF results, the elution peak of PP-2 (124.0 °C, 91.7%) is 1.5 °C higher than the isotactic fraction from PP-4 (122.5 °C, 87.2%), and even 1.2 °C higher than PP-5 prepared from ID-3 with the characteristics of high stereoregularity. Moreover, the pentad methyl sequence mmmm of PP-2 (93.0%) from cat-2 is 0.5% higher than that of PP-4 from cat-4. XPS analysis revealed that the minute difference in binding energy of Ti, Mg, C and O atoms exist between the inorganic MgCl2 and the organic polymer based Z-N catalysts. The plausible interaction mechanism of active sites of Mg and Ti with the functional groups in the POP support and the added ID was proposed, which could be explained by their high stereoregularity and the broad molecular weight distribution of the POP-based Z-N catalysts.

Effect of preparation condition in one-pot synthesis of MgCl2-supported Ziegler-Natta catalyst on ethylene-1-octene copolymerization

MgCl2-supported Ziegler-Natta catalyst was prepared through one-pot synthesis. The effect of feeding conditions of 2-ethyl-1-hexanol (EHA) and Mg/Al molar ratio in feed were investigated in the one-pot synthesis. The longer EHA feeding time resulted in a higher concentration of Ti of the final catalyst, and consequently a higher activity in the ethylene polymerization. Moreover, a larger concentration of EHA also resulted in higher activity for ethylene polymerization. A lower Mg/Al molar ratio in feed (M−1) caused a smaller Ti content with a lower Ti binding energy in the final catalyst. However, M−1 catalyst showed very high activity in ethylene-1-octene copolymerizations. It also showed higher molecular weight capability as it produced a higher molecular weight copolymer.

The atomic defects on the (104) and (110) surfaces of the MgCl2-supported Ziegler–Natta catalyst: a periodic DFT study

This work presents a DFT study on the effects of atomic defects in the MgCl2-supported Ziegler–Natta catalyst. The adsorption behaviours of TiCl4 and internal donors on the ideal and defective MgCl2(104) and (110) surfaces were investigated.

Effect of external electron donor on stereoselective Copolymerization of isoprene and butadiene with MgCl2-Supported Ziegler-Natta catalyst

External electron donors (ED) play significant roles in regulating stereoselectivity and catalytic activity of MgCl2-supported (TiCl4/MgCl2) type Ziegler-Natta catalysts. Investigating the contribution of ED on copolymerization of isoprene (Ip) and butadiene (Bd) with TiCl4/MgCl2 Ziegler-Natta catalysts is little reported. Herein, we reported the effects of three alkoxysilanes including dimethyldimethoxysilane (DMDMS), diphenyldimethoxysilane (DPDMS), phenyltrimethoxysilane (PTMS) added as ED on trans-1,4 stereoselective copolymerization of Ip and Bd using TiCl4/MgCl2-triisobutylaluminium (Al(i-Bu)3) catalyst system. Copolymers generated by TiCl4/MgCl2 catalyst in the absence of ED featured the microstructure transformation from cis-1,4 and trans-1,4 mixed microstructure on initial stage to trans-1,4 dominated microstructure and the molecular weight (Mw) evolution from low Mw to high Mw, indicating the transformation from low trans-1,4 stereoselective active center C*-1 to high trans-1,4 stereoselective active center C*-2. Appropriate introduction of alkoxysilane donors improved the polymerization activity, trans-1,4 unit content and molecular weight of copolymer in the order of PTMS > DPDMS > DMDMS, while excess alkoxysilane donors caused the decrease in that of polymerization behaviors. Increasing polymerization temperature also promoted the enhance in catalytic activity and trans-1,4 stereoseletivity of TiCl4/MgCl2 catalyst in the presence of ED. The equilibrium between deactivation and activation effect strongly depended on alkoxysilane structure, concentration and temperature. Plausible mechanisms for stereoselective transformation of active centers in Ip and Bd copolymerization using MgCl2-supported type Ziegler-Natta catalyst were proposed in terms of special roles of ED.

Spectroscopic Signature and Structure of the Active Sites in Ziegler-Natta Polymerization Catalysts Revealed by Electron Paramagnetic Resonance.

Despite decades of extensive studies, the atomic-scale structure of the active sites in heterogeneous Ziegler-Natta (ZN) catalysts, one of the most important processes of the chemical industry, remains elusive and a matter of debate. In the present work, the structure of active sites of ZN catalysts in the absence of ethylene, referred to as dormant active sites, is elucidated from magnetic resonance experiments carried out on samples reacted with increasing amounts of BCl3 so as to enhance the concentration of active sites and observe clear spectroscopic signatures. Using electron paramagnetic resonance (EPR) and NMR spectroscopies, in particular 2D HYSCORE experiments complemented by density functional theory (DFT) calculations, we show that the activated ZN catalysts contain bimetallic alkyl-Ti(III),Al species whose amount is directly linked to the polymerization activity of MgCl2-supported Ziegler-Natta catalysts. This connects those spectroscopic signatures to the active species formed in the presence of ethylene and enables us to propose an ethylene polymerization mechanism on the observed bimetallic alkyl-Ti(III),Al species based on DFT computations.

Polyethylene/crystalline ethylene-propylene copolymer/amorphous ethylene-propylene copolymer in-reactor alloys synthesized by periodic switching polymerization process: An excellent toughener for polypropylene

In order to obtain softer polyolefins applicable as toughener for polypropylene (PP), a series of in-reactor alloys composed of polyethylene, crystalline ethylene-propylene copolymer (cEP) and amorphous ethylene-propylene copolymer (aEP) were synthesized by sequential ethylene polymerization followed by ethylene-propylene copolymerization in periodic switching polymerization process (PSPP) using a spherical MgCl2-supported Ziegler-Natta catalyst. In the PSPP step, ethylene-propylene copolymerization was repeatedly switched between two different monomer feed ratios: one with high propylene/ethylene ratio to form aEP component, and the other with low propylene/ethylene ratio to form cEP component. In the synthesized PE/cEP/aEP in-reactor alloys, aEP accounted for 40~56 wt%, and cEP accounted for 18~30 wt% of the alloy. With aEP content rising above 48 wt%, the alloys showed rather low modulus, and no yielding appeared in their stress–strain curve, implying that they were a sort of elastomeric material. By blending the PE/cEP/aEP alloys with isotactic PP, their PP-toughening performances were evaluated and compared with a commercial EPDM. Impact strength of PP toughened by the alloy was significantly higher than that toughened by EPDM at the same blend ratio. Cryo-fractured surface of PP/alloy blends showed that the alloy particles were uniformly dispersed in PP matrix, and showed a core–shell type morphology, in which the core domain was composed of polyethylene, and aEP constituted the shell domain in between the core and the PP matrix. Segmented EP copolymer chains (s-EP) located inside the aEP domains formed thin filaments bridging the core and the PP matrix, which are believed to act as compatibilizer that significantly enhanced interfacial adhesion between the PP matrix and the dispersed phase.

Preparation of MgCl2-Supported Ziegler-Natta Catalysts via New Surfactants Emulsion for Propylene Polymerization

This article describes a MgCl2-supported Ziegler-Natta catalyst for propylene polymerization prepared via an in situ emulsion technique with new surfactants. The effect of preparation conditions such as the TiCl4/toluene molar ratio, TiCl4-contacting temperature, amount of phthaloyl dichloride, stirring rate, mixing time of TiCl4/toluene with the Mg complex, and the n-butyl chloride loading was investigated in detail. Scanning electron microscopy and laser particle size analyzer measurements showed that the catalyst particles exhibit a perfectly spherical shape, narrow particle size distribution, and low fine powder content. Energy-dispersive X-ray spectrometry indicated that the Ti, Mg, and Cl elements were evenly distributed throughout the particles. Powder X-ray diffraction measurements indicated the presence of δ-MgCl2 in the catalyst, and FTIR and GC-MS studies confirmed the presence of in situ formed di(ethylhexyl)phthalate (DEHP). Bulk polymerization of propylene using the catalyst was studied, and it was found that the catalyst displayed high activity, high bulk density with high stereospecificity, and excellent hydrogen sensitivity. The polymer produced by the catalyst has a narrow particle size distribution with a low fine powder content.

The influences of alkylaluminium as cocatalyst on butene-1 polymerization with MgCl2-supported TiCl4 Ziegler-Natta catalysts

The influences of alkylaluminium compounds including triethylaluminium (TEA), triisobutylaluminium (TIBA) and diethylaluminium chloride (DEAC) on the catalyst activity, isotacticity and weight-average molecular weight (Mw) of the products in butene-1 (Bt-1) polymerizations with MgCl2-supported TiCl4 Ziegler-Natta catalysts were investigated. The catalytic activity of polybutene-1 (PB) increased in the order of TEA > TIBA > DEAC, while the Mw of PB synthesized with TEA was the lowest when compared with that with other alkylaluminium compounds. Solvent extracted fractionation combining with differential scanning calorimetry (DSC), gel permeation chromatography (GPC) and carbon-13 nuclear magnetic resonance spectrometry (13C-NMR) were used to conduct composition analysis of PB. Fractions including atactic polybutene-1 (aPB), high isotactic polybutene-1 with high Mw (HiPB) and medium high isotactic polybutene-1 with medium high Mw (MHiPB) were isolated for the first time from the PB. Using TIBA resulted more HiPB fractions with higher Mw. A minor incorporation of TIBA or DEAC in TEA increased the HiPB fractions of PB. The contribution mechanism of alkylaluminium compound in TiCl4/MgCl2 Ziegler-Natta catalyst was proposed to explain the influence of alkylaluminium compound with different structures on the active species and polymer components in PB.

Polymerization Kinetics of Propylene with the MgCl2-Supported Ziegler-Natta Catalysts—Active Centers with Different Tacticity and Fragmentation of the Catalyst

The catalytic activity and stereospecificity of olefin polymerization by using heterogeneous TiCl4/MgCl2 Ziegler-Natta (Z-N) catalysts are determined by the structure and nature of active centers, which are mysterious and fairly controversial. In this work, the propylene polymerization kinetics under different polymerization temperatures by using Z-N catalysts were investigated through monitoring the concentration of active centers [C*] with different tacticity. SEM was applied to characterize the catalyst morphologies and growing polypropylene (PP) particles. The lamellar thickness and crystallizability of PP obtained under different polymerization conditions were analyzed by DSC and SAXS. The PP fractions and active centers with different tacticity were obtained with solvent extraction fractionation method. The catalytic activity, active centers with different tacticity and propagation rate constant kp, fragmentation of the catalyst, crystalline structure of PP are correlated with temperature and time for propylene polymerizations. The polymerization temperature and time show complex influences on the propylene polymerization. The higher polymerization temperature (60 °C) resulted higher activity, kp and lower [C*], and the isotactic active centers Ci* as the majority ones producing the highest isotactic polypropylene (iPP) components showed much higher kp when compared with the active centers with lower stereoselectivity. Appropriate polymerization time provided full fragmentation of the catalyst and minimum diffusion limitation. This work aims to elucidate the formation and evolution of active centers with different tacticity under different polymerization temperature and time and its relations with the fragmentation of the PP/catalyst particles, and provide the solutions to the improvement of catalyst activity and isotacticity of PP.

Monitoring the Kinetics of Internal Donor Clean-up from Ziegler–Natta Catalytic Surfaces: An Integrated Experimental and Computational Study

Surface modification of MgCl2-supported Ziegler–Natta catalysts (ZNCs) by means of organic Lewis bases, either used as precatalyst components (“internal donors”, ID) or in combination with the AlEt3 cocatalyst (“external donors”, ED), is key for achieving a high stereoselectivity in propene polymerization. In fourth-generation systems, which are the working horses of this important catalyst class, the ID is an (ortho-)­dialkyl phthalate; under polymerization conditions, this reacts with AlEt3 and must be replaced by an ED (typically an alkoxysilane) in order to obtain the desired performance. In a previous study, we investigated the molecular kinetics of the reaction between dibutyl phthalate and AlEt3 in solution by means of integrated experimental and computational protocols. A similar approach has now been applied to monitor the progress of the reaction for the complete catalyst system. Compared with solution (ΔH# ≈ 15 kcal mol–1; ΔS# ≈ −28 cal mol–1 K–1), the activation parameters in heterogeneous phase (ΔH# ≈ 10 kcal mol–1; ΔS# ≈ −49 cal mol–1 K–1) indicate that phthalate reduction is less activated, but suffers from an augmented entropic penalty. This suggests that the ID reacts with AlEt3 while still on the MgCl2 surface, rather than in the liquid phase after desorption, and that the surface is not innocent. Whether or not an alkoxysilane ED was present in the system turned out to be immaterial on reaction kinetics. High-level DFT calculations on a well-established MgCl2/dibutyphthalate model cluster reproduced the experimental data with a remarkably good agreement (ΔH# ≈ 10 kcal mol–1; ΔS# ≈ −46 cal mol–1 K–1). Experiments and calculations agree on the first ethyl transfer to the reacting ester group representing the rate-determining step, in solution as well in heterogeneous phase. In both cases, a four-membered transition state (TS) appears to be involved; in heterogeneous phase, though, the ester carbonyl is extra-activated by the interaction with a Lewis-acidic surface Mg center, rather than a second AlEt3 molecule. In our opinion, the interest of the proposed approach goes beyond the present study; indeed, further applications can be proposed for the design and engineering of novel ZNCs with tailored behaviors.

Determination and tracing of active and dormant propagation chains in 1-hexene polymerization with supported Ziegler-Natta catalyst

Propagation chains with 1,2 and 2,1 last-inserted monomer unit in 1-hexene polymerization with MgCl2-supported Ziegler-Natta catalysts were quench-labeled by thiophene-2-carbonyl chloride, and the two kinds of 2-thenoyl-labeled chain ends were determined by 1H NMR analysis. The number of labeled chains was noticeably smaller than the number of dead chains formed by chain transfer with alkylaluminum cocatalyst, and grew with time more slowly than the latter, showing that side reactions between the quencher and Al-polymeryl moieties are negligible. The active sites with 2,1 last-inserted monomer were halted in dormant state, which lead to their gradual accumulation in the 30 s polymerization process. The catalyst containing phthalate type internal donor has higher percentage of dormant sites as compared with the catalyst containing diol ester. The combination of acyl chloride quench-labeling and 1H NMR analysis forms a reliable and easily accessible method of tracing and characterizing active sites in transition metal catalyzed olefin polymerization.

Fundamental aspects of heterogeneous Ziegler–Natta olefin polymerization catalysis: an experimental and computational overview

Using an internal/external donor containing MgCl2-supported Ziegler–Natta catalyst is one of the important processes for polyolefin production.

Activity Enhancement of MgCl2-supported Ziegler-Natta Catalysts by Lewis-acid Pre-treatment for Ethylene Polymerization

Ziegler-Natta precatalysts were synthetized from Lewis-base-modified-MgCl2 supports and treated by various Lewis acids, prior to activation by triethylaluminum, in order to increase their activity in ethylene polymerization. BCl3 provided the highest increase in activity. Interestingly, polymerization results showed no substantial modification of polymer properties, which is consistent with that Lewis acid only promotes the creation of new active sites, after activation by TEA, possessing very similar features to the original ones achievable with conventional precatalysts (i.e. without Lewis-acid treatment).

Deactivation Effect Caused by Catalyst-Cocatalyst Pre-contact in Propylene Polymerization with MgCl2-supported Ziegler-Natta Catalyst

Propylene slurry polymerization with a MgCl2-supported Ziegler-Natta catalyst containing internal electron donor was conducted after different durations of pre-contact of the catalyst with triethylaluminum cocatalyst. The number of active centers ([C*]/[Ti]) was determined by quenching the polymerization with 2-thiophenecarbonyl chloride and measuring sulfur content in the polymer. The pre-contact treatment caused selective deactivation of a part of active centers with low stereoselectivity and much lower activity in the initial stage of polymerization as compared with the polymerization run without the pre-contact stage. The active center concentration and polymerization activity decreased with prolonging of the pre-contact stage. The proportion of stereoselective active centers was increased by prolonging the pre-contact stage, so the isotacticity of produced polypropylene was enhanced. Release of active centers through catalyst particle fragmentation was significantly retarded, and the polymerization rate curve changed from decay type to induction type by the precontact treatment. In the induction period both non-stereoselective and stereoselective active centers were released and activated, resulting in gradual reduction of the polymer’s isotacticity in the first 5–10 min of polymerization. Selective deactivation of non-stereoselective active centers also took place in propylene polymerization using the catalyst without pre-contacting with the cocatalyst. In this case, the polymerization rate decayed with time after a short induction period of 2–5 min. Over reduction of the active center precursors with low stereoselectivity by triethylaluminum was considered as the reason for their deactivation during the pre-contact or the polymerization processes.

Effect of alkylaluminium on the regio- and stereoselectivity in copolymerization of isoprene and butadiene using TiCl4/MgCl2 type Ziegler-Natta catalyst

Alkylaluminums as one of the most important components of the heterogeneous MgCl2-supported (TiCl4/MgCl2) type Ziegler-Natta catalysts, play significant roles in yielding active species with varied regio- and stereoselectivity. Studying the contribution of alkylaluminums on the conjugated diene isoprene (Ip) and butadiene (Bd) copolymerizations with TiCl4/MgCl2 - alkylaluminum catalysts system is little reported. In this work, effects of various alkylaluminums including triethylaluminium (TEA), triisobutylaluminium (TiBA), diisobutylaluminium hydride (DiBAH) and diethylaluminum chloride (DEAC) on the copolymerization behaviors of Ip and Bd using the TiCl4/MgCl2 - alkylaluminum catalysts system, such as catalytic activity, active center numbers, butadiene incorporation, trans-1,4 stereoselectivity, molecular weight Mw as well as molecular weight distribution Mw/Mn of copolymers were first investigated. TEA with smaller size ethyl groups showed higher copolymerization activity, higher reducing ability and then yielding more active center numbers ([C*]/[Ti]) when compared with TiBA, DiBAH and DEAC. Microstructure of copolymers gradually transformed from the cis-1,4/trans-1,4 mixed structure in the initial polymerization period to the finally high trans-1,4 structure, indicating a transformation of the active center nature during the early polymerization stage, which can be accelerated effectively through the addition of DEAC into the TiCl4/MgCl2-TEA catalyst system. TEA with smaller size ethyl group showed higher transformation rate from the mixed structures of trans-1,4 and cis-1,4 to highly trans-1,4 structure. The active center numbers of the copolymerization systems increased gradually with increasing time until reaching a stable level. The Mw/Mn of copolymers showed interesting evolution with increasing polymerization time from broad distribution with low to medium molecular weight fractions in the initial polymerization stage to relatively narrow distribution with high molecular weight fractions. The contribution of alkylaluminium compounds on the structure of active species which determining the regio- and stereoselectivity of Ip and Bd copolymerization using heterogeneous TiCl4/MgCl2 type catalysts were discussed in details

Alkynyl Ether Labeling: A Selective and Efficient Approach to Count Active Sites of Olefin Polymerization Catalysts

Accurately measuring molecular kinetics of catalytic olefin polymerization has been a challenging objective. Many methods have been proposed in the literature, but all of them have drawback(s). In this paper, we introduce a labeling method to count active sites employing methyl propargyl ether (MPE) as the quench-labeling agent. It is commercially available, does not react with Al-alkyl species, and has a labeling efficiency close to 100%. The labeling reaction was evidenced by a mechanistic study on the reaction between the model system Cp2ZrMe2/MAO (Cp = cyclopentadienyl) and MPE that it may occur through a coordination–insertion mechanism without noticeable multiple insertions. The method was benchmarked by studying a MgCl2-supported Ziegler–Natta catalyst in 1-hexene polymerization. The fraction of the active transition metal χ* is found to be <1%. It rises in the very first few seconds and reaches a plateau within 10 s in the absence of precontact. With precontact, χ* remains constant over polymer yield from 0.3 up to 5.3 g(PH) g(cat)−1, demonstrating that χ* does not necessarily grow during polymerization. Longer precontact resulted in decreased χ* and decreased average rate constant of propagation <kp> which is in the order of 102 M–1 s–1.

Insight into the Synthesis Process of an Industrial Ziegler–Natta Catalyst

In Ziegler–Natta catalysis, the catalyst particle size has a strong influence not only on catalyst performance but also on the morphology and particle size distribution of the final polymer particles. Fundamental insight into the catalyst particle formation process is therefore of industrial importance when addressing specific requirements in the final products. In the present work, we fully characterize a single-step catalyst preparation process, which comprises a reactive precipitation of a MgCl2-supported Ziegler–Natta catalyst, through decomposition of the hetero-bimetallic complex, Mg(OR)2·Ti(OR)4, by addition of ethyl aluminum dichloride (EADC). We track the evolution of both of the concentrations of the metals (Mg, Ti, Al) as well as Cl in the liquid phase and the size of the formed catalyst particles. It is observed that the liquid-phase composition is governed by the EADC feed rate under fully Cl-starved conditions. The process can be divided into two stages: The first stage is dominated by the p...

Enhancing stereoselectivity of propylene polymerization with MgCl2-supported Ziegler-Natta catalysts by electron donor: Strong effects of titanium dispersion state

Propylene polymerization with two TiCl4/MgCl2 catalysts (Cat-1: Ti 0.1%; Cat-2: Ti 1.0%) showed different responses to (cyclo-C5H9)2Si(OCH3)2 (De) addition. Cat-1 was deactivated more than Cat-2 by De. Amount of three groups of active centers producing atactic, medium isotactic and isotactic polypropylene chains were all reduced by De addition, but those of Cat-1 were reduced more. Reactivity of the isospecific active centers (kpi) was enhanced by De, but kpi of Cat-2 was changed more sensitively than Cat-1. The lower stereoselectivity of Cat-1 than Cat-2 are attributable to weaker De adsorption on its surface Mg, and the more sparsely dispersed Ti species on Cat-1 is responsible for weak De adsorption.