Supported Titanium-magnesium Catalysts Research Articles

Heterogeneity of Active Sites in the Polymer Chain Transfer Reactions at Olefin Polymerization over Multisite Supported Ziegler-Natta Catalysts.

In this review, we summarize and discuss our experimental data published in a number of papers on the transfer reactions of polymer chains in the polymerization of ethylene, propylene, and hexene-1, and the copolymerization of ethylene with α-olefins over multisite supported titanium-magnesium catalysts (TMC). Three groups of transfer reactions are discussed in the review: (1) transfer reactions with AlEt3 cocatalyst, (2) transfer reactions with hydrogen, and (3) transfer reactions with participation of α-olefins in the case of ethylene copolymerization with α-olefins. We have found polymerization conditions where it is possible to observe heterogeneity of active sites of TMC for all three groups of the indicated reactions. It is shown that (1) the transfer reaction with AlEt3 proceeds with higher reactivity on the active sites that produce polymers with low molecular weight; (2) the transfer reaction with hydrogen, in the case of α-olefin polymerization and copolymerization of ethylene with α-olefins, proceeds with higher reactivity on the active sites which produce polymers with high molecular weight; (3) the transfer reaction with α-olefins proceed with higher reactivity on the active sites that produce high molecular weight polymers.

Copolymerization of Ethylene with Alpha-Olefins over Supported Titanium–Magnesium Catalysts Containing Titanium Compounds in Different Oxidation and Coordination States

Data were obtained on the copolymerization of ethylene with α-olefins over supported titanium–magnesium catalysts (TMC) prepared on the same magnesium dichloride support but differing in the composition and oxidation state of titanium. The copolymerization kinetics of ethylene with 1-hexene over TMC of different compositions were studied. Data on the composition of the produced ethylene–1-hexene copolymers, their molecular weight distribution, thermophysical characteristics, and branching distribution were presented. The constants of ethylene–1-hexene copolymerization over catalysts with different compositions were calculated. The TMC containing only Ti(II) compounds as the active component exhibited increased copolymerizing ability compared to the conventional TiCl4/MgCl2 catalyst containing Ti(III) compounds as the active component. In addition, TMC with Ti(II) as an active component produces copolymers with a more uniform branching distribution. It was shown that the TMC containing isolated Ti(II) ions could be used to produce X-ray amorphous ethylene-propylene elastomers with a high yield.

Polymerization of Hexene-1 and Propylene over Supported Titanium–Magnesium Catalyst: Comparative Data on the Polymerization Kinetics and Molecular Weight Characteristics of Polymers

Data are presented on the great differences of the kinetics of hexene-1 and propylene polymerization over the same supported titanium-magnesium catalyst, as well as molecular weight and molecular weight distribution of the polymers produced. It is found that the composition of cocatalysts (AlEt3 or Al(i-Bu)3 greatly affects the kinetics of hexene-1 polymerization and molecular weight distribution of polyhexene, contrary to data obtained at propylene polymerization. The presence of hydrogen at hexene-1 polymerization leads to a much higher increase of activity in comparison with propylene polymerization. Possible reasons for these differences are discussed on the basis of experimental results.

Effect of 1-hexene on the propylene polymerization with supported phthalate-free titanium–magnesium catalyst

The polymerization of propylene with small amounts of 1-hexene in the presence of supported titanium–magnesium catalyst containing two internal non-phthalate donors was accomplished. Polypropylene modification with small amounts of 1-hexene affects the thermophysical properties and molecular weight characteristics of the polymers. The activation mechanism of the reaction is described; the introduction of small amounts of 1-hexene causes an increase in activity by a factor of 1.5.

Propylene Polymerization over Titanium-Magnesium Catalyst: Effect of Hydrogen on the Content of Active Centers with Different Stereospecificity and Their Reactivity in Propagation Reaction

This paper presents data on the number of active centers (CPi) with different stereospecificity and the values of propagation rate constant (kPi) for these centers in the propylene polymerization over supported titanium-magnesium catalyst in the presence or absence of hydrogen. The indicated data were obtained by quenching the polymerization with radioactive carbon monoxide (14CO) with subsequent fractionation of the produced polymers by the TREF method into separate fractions differing in stereospecificity and estimation of the CPi and kPi values, which determine the formation of these fractions. The propagation rate constants calculated for active centers with different stereospecificity increase in the following order: kP(aspecific) ≈ kP(low isospecific) < kP(isospecific) < kP(highly isospecific).

1-Hexene Polymerization on a Highly Active Titanium–Magnesium Catalyst

The review summarizes the results obtained by the authors in a study of the polymerization of 1‑hexene on a supported titanium–magnesium catalyst. The effects of the composition of a complex titanium–magnesium catalytic system, the composition of a reaction medium, and the conditions of polymerization on the activity of the catalyst and the molecular weight, molecular-weight distribution, and isotacticity of the resulting polyhexene were considered. The main factors (the composition of a catalytic system and the polymerization conditions) that allow one to control the catalyst activity and the molecular structure of polyhexene over wide ranges were determined. Specific features of the polymerization of 1-hexene in comparison with the polymerization of ethylene and propylene on titanium–magnesium catalysts are discussed.

Kinetic Study of Chain Transfer Reactions upon 1-Hexene Polymerization on Highly Active Supported Titanium–Magnesium Catalysts

Data on the effect the concentrations of the monomer, triethylaluminium, and hydrogen on the molecular weight of polyhexene upon 1-hexene polymerization over supported titanium–magnesium catalyst are obtained. The ratios between propagation rate constant and the rate constants of the reactions of polymer chain transfer with monomer, triethylaluminium (AlEt3), triisobutylaluminium, and hydrogen are calculated. The data allow us to estimate the effect of individual reactions of chain transfer on the polymer’s molecular weight under different polymerization conditions. The data help to select polymerization conditions for obtaining polyhexene of a desired molecular weight. The heterogeneity of active centers is observed in the reaction of chain transfer with hydrogen when using AlEt3 as co-catalyst, which alters the polymer’s polydispersity when the concentration of hydrogen is varied in the reaction mixture. The molecular weight distribution is analyzed via resolution into Flory components for polyhexene characterized by different polydispersity.

Synthesis of Polypropylene in the Liquid Monomer in the Presence of a Titanium—Magnesium Catalyst: Effect of Various Internal Donors

The influence of supported titanium-magnesium catalysts with various internal and external donors on the propylene polymerization in the liquid monomer medium and the characteristics of the polypropylene formed were studied. The following internal donors were used: dibutyl phthalate, diisobutyl phthalate, 9,9′-bis(methoxymethyl) fluorene, and diethyl 2,3-diisopropylsuccinate. The catalysts studied allow synthesis of polypropylene with high isotacticity (>96%) and different molecular-mass distribution (Mw/Mn from 3.3 to 6.3). The influence of external donors (alicyclic, amine) in combination with phthalate and nonphthalate electron-donor compounds on the stereospecificity and activity of the catalysts and on their sensitivity to hydrogen was studied. The optimum catalytic systems for preparing polypropylene for various purposes can be found by varying pairs of internal and external donors.

Polypropylene synthesis in liquid monomer with titanium–magnesium catalyst: effect of different alkoxysilanes as external donors

The effect of a set of methoxy- and ethoxysilanes as external donors (ED) on propylene polymerization in liquid monomer with the supported titanium–magnesium catalyst (TMC) and on properties of the produced polypropylene (PP) are studied. Addition of the studied donors to the polymerization system significantly increases the PP isotacticity compared to polymerization over TMC without ED (up to 92–98% vs. 66%, respectively). It is found that activity and stereospecificity of the catalytic system, as well as the molecular weight of the produced PP, decline as the number and size of alkoxy groups increase and the size (branching) of alkyl substituents decreases. Bulky substituents in alkoxysilanes have a positive effect on activity and stereospecificity of TMC. However, the double bond in the substituent moiety reduces the catalyst activity. Nitrogen atom in the substituent (with the same alkoxy groups) increases isotacticity and crystallinity of PP, its flexural modulus and strength characteristics. Varying electron donors with different number and size of alkoxy groups and different substituents (aliphatic, aromatic, alicyclic, amino, and vinyl) at silicon atom allows one to control the catalyst activity and isotacticity, as well as the molecular and thermal characteristics and impact strength properties of PP.

Kinetic Studies of Chain-Transfer Reactions in Polymerization of Hexene-1 over Highly Active Supported Titanium-Magnesium Catalysts

The influence of concentrations of the monomer, triethylaluminum, and hydrogen on the molecular mass of polyhexene obtained by polymerization of hexene-1 over a supported titanium-magnesium catalyst was studied. Ratios of rate constants of the transfer of polymer chain with the monomer, triethylaluminum (AlEt3) and hydrogen to the rate constant of the chain-growth were calculated.The data obtained allowed the contribution of individual reactions of chain transfer to molecular mass of the polymer to be estimated at various polymerization conditions and the polymerization conditions to be chosen deliberately in order to synthesize polyhexene with the required molecular mass. The discovered inhomogeneity of the centers active to the chain transfer with hydrogen in the presence of the AlEt3 co-catalyst caused changes in the polymer polydispersion upon changes in the hydrogen concentration in the reaction medium. Curves of the polyhexene molecular mass distribution in polyhexene samples with different polydispersion were analyzed using their resolution into Flory components.

Kinetic features of ethylene copolymerization with 1-hexene over titanium-magnesium Ziegler–Natta catalysts: Effect of comonomer on the number of active centers and the propagation rate constant

We present data on the number of active centers (CP) and the propagation rate constant (kP) upon copolymerization of ethylene and 1-hexene over supported titanium–magnesium catalysts of different compositions obtained by polymerization quenching with radioactive carbon monoxide (14CO). The rise in ethylene/1-hexene copolymerization rate as compared to the rate of homopolymerization (the comonomer effect) is found to be related to the increase in the calculated propagation rate constant. Meanwhile, the increased 1-hexene concentration results in a higher calculated kP value. Addition of 1-hexene during ethylene polymerization also increases the polymerization rate by raising the calculated kP value. These findings demonstrate that the comonomer effect is caused by the increase in monomer concentration in the copolymer layer that is formed on a catalyst particle and is characterized by reduced crystallinity as compared to that of homopolymer. The decline in the copolymerization rate with time is caused by reduction of the number of active centers.

Regulation of Molecular Weight Characteristics and Microtacticity of Polyhexene Produced over Highly Active Supported Titanium–Magnesium Catalysts

AbstractThe data on the effect of polymerization temperature of 1‐hexene within the 30–70 °C range in the presence of a highly active supported titanium–magnesium catalyst on molecular weight characteristics and microtacticity of polyhexene, with cocatalyst composition being additionally varied (AlEt3 or Al(i‐Bu)3), in the absence and presence of an external stereoregulating electron‐donating compound and hydrogen, are reported. Polymerization conditions, making it possible to specifically regulate molecular weight and molecular weight distribution of polyhexene over a broad range ((Mw = 7 × 104–2.2 × 106 g mol−1; Mw/Mn = 3.7–33) and regulate isotacticity of polyhexene (content of mmmm pentads from 56% to 96%), while retaining high catalyst activity, are determined.

Comparative Study of Distribution of Active Sites According to Their Stereospecificity in Propylene Polymerization over the Traditional TiCl3 and Supported Titanium–Magnesium Catalysts with Different Composition

AbstractThe preparative‐temperature rising elution fractionation method is used to obtain comparative data on contents of fractions with different microtacticities for polypropylene (PP) samples prepared using three catalytic systems: the traditional Ziegler–Natta (Z–N) catalyst δ‐TiCl3 and two types of supported titanium–magnesium catalysts: the “donor‐free” TiCl4/MgCl2 catalyst and TiCl4/MgCl2·nDBP catalyst (DBP – dibutylphthalate used as an internal donor) at polymerization with the same cocatalyst (AlEt3) in the absence and presence of an external donor (propyltrimetoxy silane). The separated individual PP fractions are also studied by gel permeation chromatography (molecular weight and molecular weight distribution) and differential scanning calorimetry. The results demonstrate general regularities and differences in the formation of active sites having different isospecificities for the traditional TiCl3‐based Z–N catalyst and highly active supported titanium–magnesium catalysts.

1‐Hexene Polymerization over Supported Titanium–Magnesium Catalyst: The Effect of Composition of the Catalytic System and Polymerization Conditions on Temperature Dependence of the Polymerization Rate

AbstractThe effect of temperature on the rate of 1‐hexene polymerization over supported titanium–magnesium catalyst of composition TiCl4/D1/MgCl2 + AlR3/D2 (D1 is dibutyl phthalate, D2 is propyltrimethoxysilane, and AlR3 is an organoaluminum cocatalyst) is studied. The unusual data that the polymer rate decreases when temperature is increased from 30 to 70 °C are obtained. The 1‐hexene polymerization rate and the pattern of changes in polymerization rate with temperature depend on a combination of factors such as cocatalyst (AlEt3 or Al(i‐Bu)3) and presence/absence of hydrogen and an external donor in the reaction mixture. These factors differ in their effects on catalytic activity at different polymerization temperatures, so the temperature coefficient (Eeff) values calculated using the Arrhenius dependence of the polymerization rate on polymerization temperature vary greatly. The “normal” Arrhenius plot where polymerization rate increases with temperature is observed only for polymerization with the Al(i‐Bu)3 cocatalyst in the presence of hydrogen and without an external donor. Formation of high‐molecular‐weight polyhexene at low polymerization temperatures results in catalyst particle fragmentation, which may additionally contribute to the increase in polymerization rate as polymerization temperature is reduced.

Kinetic features of short-time polymerization of isoprene in the presence of supported titanium–magnesium catalyst

Short-time polymerization of isoprene under the action of supported titanium–magnesium catalyst is carried out. The pulsation mixing of the reagent flows and the unit design features allow one to reduce the average residence time of reagents in the reaction zone and to study the first 0.7 s of the isoprene polymerization. It is found that, very early in polymerization, the propagation of polyisoprene macromolecules proceeds on the surface of the primary aggregates of catalyst particles characteristic of a high trans-1,4 specificity via the “living” mechanism with a high rate. Furthermore, the fragmentation of the initial aggregates of the catalyst particles occurs, which results in formation of new polymerization centers, a decrease in the average molecular masses of polyisoprene, and a broadening of the polymer MMD. The results are explained by the existence of a range of the kinetic continuity of the rapid initiation stage and several subsequent stages of macromolecule propagation, followed by a significant decrease in the chain propagation rate constant compared to the initiation constant.

Effect of the structure of titanium–magnesium catalysts on the morphology of polyethylene produced

ABSTRACTThe structure and formation of polyethylene (PE) particles on supported titanium–magnesium catalysts having different structural characteristics (sizes of microcrystallites, mesopores, and subparticles) were studied for the first time. Scanning electron microscopy was used to identify structural elements of the polymer particles formed over such catalysts and to reveal morphological changes in the growing polymer particles when the yield was increased from approximately 0.2 g PE/g catalyst to approximately 13 kg PE/g catalyst. A relationship was found between structural characteristics of the porous catalyst particles, morphology of the nascent polymer particles, and bulk density of the polymer powder. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 2298–2308

Dynamic mechanical analysis of ethylene/1‐hexene copolymers: The effect of the catalyst type on the short‐chain branching distribution and properties of the amorphous and crystalline phases

ABSTRACTDynamic mechanical analysis was used to study ethylene/1‐hexene copolymers with different compositions, molecular weight distributions, and profiles of short‐chain branching (SCB) versus molecular weight. These copolymers were produced over a highly active supported titanium–magnesium catalyst (TMC), a highly active supported vanadium–magnesium catalysts (VMC), and a supported zirconocene catalyst. A higher fraction of the crystalline phase in the copolymers prepared with VMC was shown to result in higher elastic modulus values. β relaxation was found to be sensitive to the SCB distribution versus the molecular weight. The copolymers prepared with the zirconocene catalyst and VMC were characterized by more uniform SCB distributions and higher temperatures of β relaxation compared to the copolymers prepared with TMC. The mobility of the polymer chains at room temperature in the amorphous phase obtained by the spin‐probe method rose with increasing branch content in the copolymers and was not sensitive to different SCB distribution profiles. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44638.

Kinetic Features of Hexene-1 Polymerization over Supported Titanium-Magnesium Catalyst

The data on the effect of cocatalyst composition and polymerization conditions on activity and MWD of polyhexene (PH) produced over a high-activity titanium-magnesium catalyst are obtained. The cocatalyst has an extremely strong effect on the MWD of polyhexene. When triisobutylaluminum is used as a cocatalyst, PH with high Mw and narrow MWD is produced. The use of TEA decreases the Mw of PH and broadens the MWD. It is found the ratio between the rate constant of chain transfer to TEA (ktrAl) and the propagation rate constant (kp) is two orders of magnitude higher than the ktrAl/kp value calculated for polymerization of ethylene and propylene. We attributed this effect to formation of polyhexene in the solution, as opposed to polymerization of ethylene and propylene in slurry polymerization. The addition of hydrogen at polymerization of hexene-1 significantly increases activity and stereospecificity.

Formation of Isolated Titanium(III) Ions as Active Sites of Supported Titanium–Magnesium Catalysts for Polymerization of Olefins

Electron spin resonance (ESR) studies reveal a high yield of isolated Ti3+ compounds (30–70% of the total titanium content) that form in superactive supported titanium–magnesium catalysts (TMCs) of composition TiCl4/MgCl2 with a low titanium content (≤0.1 wt%) after their interaction with different aluminum trialkyls (AlR3). These catalysts strongly differ from conventional TMCs with titanium contents of 1–3 wt%, which produce a set of isolated or associated Ti(II) and Ti(III) species after the interaction with the AlR3. A correlation is found between the content of the isolated Ti(III) ions and the activity in ethylene polymerization. According to temperature rising elution fractionation (TREF) data, the catalysts with a low titanium content (isolated Ti(III) ions) mainly produce polypropylene with a very high atactic‐fraction content. Organic products formed on the interaction of AlEt3 and AlMe3 with TMC are identified. The data obtained allow a scheme of Ti3+ and Ti2+ compounds formed in these catalysts to be proposed. image

A comparative study of ethylene and propylene polymerization over titanium–magnesium catalysts of different composition

ABSTRACTNew data on the molecular weight characteristics of polypropylene (PP) and polyethylene (PE) were obtained from the polymerization over supported titanium–magnesium catalysts differing in their compositions (presence and absence of internal and external donors). Internal and external donors were found to affect the molecular weight of polymers in a different manner for ethylene and propylene polymerization. The introduction of the internal donor increases the molecular weight of PP and does not affect the molecular weight of PE. The effect of external donor introduced to catalytic system on the polymer molecular weight depends on catalyst composition: for a catalyst without internal donor, the introduction of the external donor increases the molecular weight of PP and does not affect that of PE. In the case of catalyst with the internal donor, the introduction of the external donor increases the molecular weight of PP and substantially decreases that of PE. The data on polymerization degree of the polymers produced under conditions when chain transfer with hydrogen was the dominant reaction were used to calculate the values for ethylene polymerization over the catalysts of different composition. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40658.