Kinetic Study of Ethylene Polymerization Over Supported Bis(imino)pyridine Iron (II) Catalysts

Summary: The number of active centers (C_P) and propagation rate constants (k_P) for polymerization of ethylene with supported catalysts LFeCl₂/SiO₂, LFeCl₂/Al₂O₃ and LFeCl₂/MgCl₂ (L = 2,6‐(2,6‐(Me)₂C₆H₃NCMe)₂C₅H₃N), activated by an Al(i‐Bu)₃ co‐catalyst, were determined by a method of polymerization inhibition with radioactive ¹⁴CO. In contrast to homogeneous systems based on LFeCl₂, the supported catalysts are highly active and stable in ethylene polymerization at 70–80 °C. In the presence of hydrogen, the activity of the supported catalysts substantially increases (2–4 fold). The data obtained on the effect of hydrogen on the calculated C_P and k_P values suggests that for ethylene polymerization without hydrogen, the “dormant” active centers are formed in the catalytic systems. A scheme for the formation of these “dormant” centers and their reactivation in presence of hydrogen is suggested. For the investigated supported catalysts the C_P values were found to be only 2 to 4% of the total iron complex content in the catalysts. The k_P value for the catalysts prepared using different supports (SiO₂, Al₂O₃ and MgCl₂) were close (3.2 × 10⁴ to 4.5 × 10⁴ L · (mol · s)⁻¹ at 70 °C). The support composition affects neither the molecular mass (MM) nor the molecular mass distribution (MMD) of the polymers produced. The obtained C_P and k_P values and data on the polymer MM and MMD lead to conclusion that the nature of the support has almost no effect on the structure of the active centers and the distribution of their reactivity.

Effect of support on the MMD of PE produced over supported LFeCl₂ catalysts.     

Kinetic Peculiarities of Ethylene Polymerization over Homogeneous Bis(imino)pyridine Fe(II) Catalysts with Different Activators

Summary: Method of polymerization inhibition by radioactive carbon monoxide (¹⁴CO) has been used to determine the number of active centers (C_P) and propagation rate constant (k_P) for ethylene polymerization with homogeneous complex 2,6‐(2,6‐(Me)₂C₆H₃NCMe)₂C₅H₃NFeCl₂ (LFeCl₂), activated with methylalumoxane (MAO) or Al(i‐Bu)₃. With both activators the rate profile of polymerization was unstable: high activity [0.8 × 10³–1.5 × 10³ kg PE per (mol_Fe · h · atm) at 35 °C] of the initial period sharply decreases (sevenfold in 10 min). In the beginning of polymerization with the catalysts LFeCl₂/MAO and LFeCl₂/Al(i‐Bu)₃, the C_P values were found to be 8 and 41% of total Fe‐complex content in catalysts, respectively, and decreased 1.5–2‐fold in 9 min. As polymerization proceeds, the k_P value for LFeCl₂/MAO system decreases from 5 × 10⁴ to 1.5 × 10⁴ L · (mol · s)⁻¹ LFeCl₂/MAO, and for LFeCl₂/Al(i‐Bu)₃ system from 2.6 × 10⁴ to 0.82 × 10⁴ L · (mol · s)⁻¹. Data on the effect of polymerization time on polyethylene molar mass distribution are presented. Basing on the obtained results it was suggested that highly reactive, but unstable centers, dominating at short polymerization times, produce low‐molar‐mass polyethylene, while polyethylene with higher molar mass is produced by less active (low k_P) and more stable centers.

Data showing change in molar mass distribution of polyethylene with polymerization time.     

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

The 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.     

Propylene Polymerization Behavior of Fluorinated Bis(phenoxy‐imine) Ti Complexes with an MgCl2‐Based Compound (MgCl2‐Supported Ti‐Based Catalysts)

Summary: Fluorinated bis(phenoxy‐imine)Ti complexes 1 – 3 combined with MgCl₂/i‐Bu_nAl(OR)_3−n (MgCl₂‐supported catalysts) were able to polymerize propylene in a living fashion at room temperature to provide slightly to highly syndiotactic poly(propylenes) (PPs) with extremely narrow distributions of molecular weight. These represent the first examples of MAO‐ and borate‐free group 4 metal‐based living catalysts. The supported complexes 2 and 3 formed PPs with higher syndiotacticity and T_m's than the corresponding homogeneous MAO‐activation systems (e.g., 3 : rr 97%, T_m 155 °C; MAO activation: rr 93%, T_m 152 °C). The measured T_m of 155 °C represents the highest known T_m for syndiotactic PPs synthesized at room temperature.

Polymerization of propylene to poly(propylene) with supported Ti‐based catalysts.