Ethylene polymerization reactions with Ziegler–Natta catalysts. I. Ethylene polymerization kinetics and kinetic mechanism

Kinetics of ethylene homopolymerization reactions and ethylene/1-hexene copolymerization reactions using a supported Ziegler–Natta catalyst was carried out over a broad range of reaction conditions. The kinetic data were analyzed using a concept of multicenter catalysis with different centers that respond differently to changes in reaction parameters. The catalyst contains five types of active centers that differ in the molecular weights of material they produce and in their copolymerization ability. In ethylene homopolymerization reactions, each active center has a high reaction order with respect to ethylene concentration, close to the second order. In ethylene/α-olefin copolymerization reactions, the centers that have poor copolymerization ability retain this high reaction order, whereas the centers that have good copolymerization ability change the reaction order to the first order. Hydrogen depresses activity of each type of center in the homopolymerization reactions in a reversible manner; however, the centers that copolymerize ethylene and α-olefins well are not depressed if an α-olefin is present in the reaction medium. Introduction of an α-olefin significantly increases activity of those centers, which are effective in copolymerizing it with ethylene but does not affect the centers that copolymerize ethylene and α-olefins poorly. To explain these kinetic features, a new reaction scheme is proposed. It is based on a hypothesis that the Ti—C₂H₅ bond in active centers has low reactivity due to the equilibrium formation of a Ti—C₂H₅ species with the H atom in the methyl group β-agostically coordinated to the Ti atom in an active center. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4255–4272, 1999     

Ethylene polymerization reactions with Ziegler–Natta catalysts. II. Ethylene polymerization reactions in the presence of deuterium

Ethylene polymerization reactions with many Ziegler–Natta catalysts exhibit several features which differentiate them from polymerization reactions of α-olefins: a relatively low ethylene reactivity, higher polymerization rates in the presence of α-olefins, a high reaction order with respect to ethylene concentration, and strong reversible rate depression in the presence of hydrogen. A detailed kinetic analysis of ethylene polymerization reactions (see ref. 1 ) provided the basis for a new reaction scheme which explains all these features by postulating the equilibrium formation of a Ti C₂H₅ species with the H atom in the methyl group β-agostically coordinated to the Ti atom in an active center. This mechanism predicts that the β-agostically stabilized Ti C₂H₅ groups can decompose in the β-hydride elimination reaction with expulsion of ethylene and the formation of a Ti H bond even in the absence of hydrogen in the reaction medium. If D₂ is used as a chain transfer agent instead of H₂, the mechanism predicts the formation of deuterated ethylene molecules, which copolymerize with protioethylene. To prove this prediction, several ethylene homopolymerization reactions were carried out with a supported Ziegler–Natta titanium-based catalyst in the presence of large amounts of D₂. Analysis of gaseous reaction products and polymers confirmed the formation of several types of deuterated ethylene molecules and protio/deuterioethylene copolymers, respectively. In contrast, a metallocene catalyst, Cp₂ZrCl₂ MAO, does not exhibit these kinetic features. In the presence of deuterium, it produces only DCH₂ CH₂ (CH₂ CH₂)_x CH₂ CH₂D molecules. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4273–4280, 1999     

Monomer-isomerization polymerization. XXXVIII. Monomer-isomerization copolymerizations of styrene and 2-Butene with Ziegler–Natta catalyst

Monomer-isomerization copolymerizations of styrene (St) and cis-2-butene (c2B) with TiCl₃-(C₂H₅)₃Al catalyst were studied. St and c2B were found to undergo a new type of monomer-isomerization copolymerization, i.e., only isomerization of 2B to 1-butene ( 1B ) took place to give a copolymer consisting of St and 1B units. The apparent copolymerization parameters were determined to be r_st = 16.0 and r_c2b = 0.003. The parameters were changed by the addition of NiCl₂ (r_St = 8.4, r_c2b = 0.05). The copolymers containing the major amount of St units were produced easily through monomer-isomerization copolymerization of St and 2B. © 1995 John Wiley & Sons, Inc.     

Polymer-supported Ziegler–Natta catalysts. II. Ethylene homo- and copolymerization with TiCl4/MgR2/poly(ethylene-co-acrylic acid) catalyst

A novel polymer-supported titanium-based catalyst shows high activity and nondecaying kinetic profiles for ethylene polymerization. The presence of 1-hexene comonomer drastically increases the catalyst activity, exhibiting a similarity to the MgCl₂-supported catalysts. However, the nondecaying kinetic profiles of copolymerization distinguish this catalyst from the latter. Infrared analysis indicates that the transition metals were immobilized on the polymer support via functional groups. The effects of polymerization conditions on catalyst activity have been assessed. Characterization of the resulting polymer product by means of ¹³C-NMR, DSC, and SEM demonstrates a branch-free structure with high melting point, high crystallinity, and high molecular weight for the ethylene homopolymer. The reactivity ratios of ethylene-1-hexene copolymerization are evaluated from ¹³C-NMR analysis data. © 1994 John Wiley & Sons, Inc.     

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.     

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.     

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     

Multi‐center nature of heterogeneous Ziegler–Natta catalysts: TREF confirmation

A new approach to detailed Tref analysis of ethylene/α‐olefin copolymers prepared with multi‐center polymerization catalysts is developed. It is based on resolution of complex Tref curves into elemental components described with the Lorentz distribution function. This approach was applied to the study of a series of ethylene/1‐butene copolymers prepared with a supported Ti‐based catalyst. The analysis showed that the copolymers, which, on average, contain from 6.5 to 3.5 mol % of 1‐butene, consist of seven discrete components with different compositions, ranging from a completely amorphous material with a 1‐butene content of > 15–20 mol %, to two highly crystalline components with 1‐butene contents < 1 mol %. A comparison of these Tref results with the data on the molecular weight distribution of the copolymers (based on resolution of their GPC curves) shows that Tref and GPC data provide complimentary information on the properties of active centers in the catalysts in terms of the molecular weights of the material they produce and their ability to copolymerize α‐olefins with ethylene. Tref analysis of copolymers produced at different reaction times showed that the active centers responsible for the formation of various Tref components differ in the rates of their formation and in stability. The centers that produce copolymer molecules with a high 1‐butene content are formed rapidly but decay rapidly as well whereas the centers producing copolymer molecules with a low 1‐butene content are formed more slowly but are more stable. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4351–4362, 2005     

Ethylene–styrene thermoplastic copolymers prepared with supported Ziegler–Natta catalysts

The magnesium chloride supported Ziegler–Natta catalysts are able to copolymerize ethylene and styrene in conditions of high activity. Different parameters, including catalyst type, monomer ratio, temperature and Lewis bases, allow variation of the activity and reactivity ratio. The incorporation of styrene in the copolymer remains always rather limited in moles. The products obtained contain less than 20% styrene in weight, and seem very similar in structure to linear low-density polyethylenes (LLDPE) obtained with the same catalyst: the melting temperature is only about 5°C lower than that of pure polyethylene. The polymer can be fractionated by solvents in a similar manner to LLDPE, and contains a styrene-enriched fraction, but homopolystyrene production has never been observed.     

Polymerization of ethylene using a high-activity Ziegler–Natta catalyst. II. Molecular weight regulation

This article describes studies on the variables that regulate the molecular weight in ethylene polymerization using a highly active Ziegler–Natta catalyst with hydrogen for molecular weight control. The dependence of the degree of polymerization on the concentration of catalyst, cocatalyst, monomer, partial pressure of hydrogen, and temperature has been established. The rate constant for chain transfer with cocatalyst has been evaluated. © 1993 John Wiley & Sons, Inc.     

Peculiarities of Ethylene Polymerization Reactions with Heterogeneous Ziegler–Natta Catalysts: Kinetic Analysis

The previously developed kinetic scheme for olefin polymerization reactions with heterogeneous Ziegler–Natta catalysts states that the catalysts have several types of active centers which have different activities, different stabilities, produce different types of polymer materials, and respond differently to reaction conditions. In the case of ethylene polymerization reactions, each type of center exhibits an unusual chemical feature: a growing polymer chain containing one ethylene unit, Ti—C₂H₅, is unusually stable and can decompose with the formation of the Ti—H bond. This paper examines quantitative kinetic ramifications of this chemical mechanism. Modeling of the complex kinetics scheme described in the Scheme demonstrates that it correctly and quantitatively predicts three most significant peculiarities of ethylene polymerization reactions, the high reaction order with respect to the ethylene concentration, reversible poisoning with hydrogen, and activation in the presence of α‐olefins.     

Synthesis of polypropylene-co-p-methylstyrene copolymers by metallocene and Ziegler–Natta catalysts

This paper discusses the copolymerization reaction of propylene and p-methylstyrene (p-MS) via four of the best-known isospecific catalysts, including two homogeneous metallocene catalysts, namely, {SiMe₂[2-Me-4-Ph(Ind)]₂}ZrCl₂ and Et(Ind)₂ZrCl₂, and two heterogeneous Ziegler–Natta catalysts, namely, MgCl₂/TiCl₄/electron donor (ED)/AlEt₃ and TiCl₃. AA/Et₂AlCl. By comparing the experimental results, metallocene catalysts show no advantage over Ziegler–Natta catalysts. The combination of steric jamming during the consective insertion of 2,1-inserted p-MS and 1,2-inserted propylene (k₂₁ reaction) and the lack of p-MS homopolymerization (k₂₂ reaction) in the metallocene coordination mechanism drastically reduces catalyst activity and polymer molecular weight. On the other hand, the Ziegler–Natta heterogeneous catalyst proceeding with 1,2-specific insertion manner for both monomers shows no retardation because of the p-MS comonomer. Specifically, the supported MgCl₂/TiCl₄/ED/AlEt₃ catalyst, which contains an internal ED, produces copolymers with high molecular weight, high melting point, and no p-MS homopolymer. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2795–2802, 1999     

Ethylene polymerization reactions with multicenter Ziegler‐Natta catalysts—Manipulation of active center distribution

This article describes ethylene/1‐hexene copolymerization reactions with a supported titanium‐based, multicenter Ziegler‐Natta catalyst. The catalyst was modified by pretreating its solid precursor with AlEt₂Cl and with similar organoaluminum chlorides, Al₂Et₃Cl₃, AlEtCl₂, and AlMe₂Cl. Testing of the untreated and the pretreated catalysts in copolymerization reactions under standard reaction conditions demonstrated that the modifying agents produce two changes in the catalyst. First, the pretreatment significantly reduces the reactivity of active centers that produce high molecular weight, highly crystalline copolymer components with a low 1‐hexene content. Second, the pretreatment noticeably increases the reactivity of active centers that produce low molecular weight copolymer components with a high 1‐hexene content. The first effect is caused by Lewis acid‐base interactions of the modifiers with the active centers, whereas the second (activating) effect is due to the removal of catalyst poisons (organosilicon compounds generated in the process of the catalyst synthesis) by AlEt₂Cl. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 4219–4229, 2010     

Polymerization of propylene by metallocene and Ziegler–Natta mixed catalytic systems. Study of polypropylene properties

Propylene was polymerized with metallocene and Ziegler–Natta mixed-catalytic systems to obtain reactor blends of metallocene and Ziegler–Natta-derived propylene homopolymers. The two catalytic systems are able to act jointly, providing individual polymers with different melting and crystallization temperatures. Compatibility between the components of the mixed-catalytic systems and the influence of the components on the polymerization process and on the properties of the reactor blends were studied. Thermal, mechanical, viscoelastic, rheological, and optical properties of the blends were tested and compared with those of conventional polypropylene grades. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 3063–3072, 1998     

Polymerization of propylene by mixed Ziegler–Natta and metallocene catalysts

The polymerization of propylene using a mixture of racemic metallocenes and Ziegler–Natta catalysts was investigated. The polypropylene was obtained as a mixture of a fine powder and beads, with the powder being absorbed partially on the beads. The relative amount of the powder can be controlled by the concentration of the metallocene. The compatibility between the components of the mixed catalytic systems and the effect of the components on the rate of polymerization and on the properties of the new polymers were studied. The metallocene system dramatically affects the melt viscosity, isotacticity and molecular weight of the polymers. The two catalytic systems are able to act jointly, producing different polymers, for which separate melting and crystallization temperatures are obtained. © 1998 John Wiley & Sons, Ltd.     

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     

Effects of Alterations to Ziegler–Natta Catalysts on Kinetics and Comonomer (1‐Butene) Incorporation

Various MgCl₂‐supported Ziegler–Natta (ZN) catalysts are synthesized with the intention to influence polymerization performance and 1‐butene incorporation in an ethylene copolymer. Modifications are introduced during different steps in the synthesis process, namely support preparation, titanation, and catalyst workup. While multiple different effects are observed upon modification, heat treatment during titanation shows the greatest impact. Increasing the heat‐treatment temperature increases polymerization activity. More importantly, the 1‐butene distribution can be shifted toward a more homogeneous profile. The amount of 1‐butene incorporated is similar to both for short‐ and for very long‐chain molecules. This behavior has so far been known only from metallocene‐based polyethylene and suggests that active sites are distributed more homogeneously in the ZN catalyst.

     

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     

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