8 research outputs found
Thermodynamics of Titanium and Vanadium Reduction in Non-Aqueous Environment Calculated at Various Levels of Theory
Reduction of titanium and vanadium compounds is a process
accompanying
the activation of coordinative olefin polymerization catalysts. Four
density functional theory (DFT) functionals, coupled cluster with
single, double, and perturbative triple excitations method CCSDÂ(T)
as well as complete active-space second-order perturbation theory
method CASPT2 with a complete active-space self-consistent field CASSCF
reference wave function were applied to investigate the thermodynamics
of titanium and vanadium reduction. The performance of these theoretical
methods was assessed and compared with experimental values. The calculations
indicate that vanadiumÂ(IV) chloride is more easily reduced by trimethylaluminum
than the corresponding titanium compound; the energies of reaction
calculated at the CCSDÂ(T) level are equal â57.21 and â33.10
kcal/mol, respectively. The calculations deal with the redox reactions
of metal chlorides in the gas phase, rather than solvated ions in
the aqueous solution. This approach may be more appropriate for olefin
polymerization, usually carried out in nonpolar solvents
Impact of Organoaluminum Compounds on Phenoxyimine Ligands in Coordinative Olefin Polymerization. A Theoretical Study
The
reduction of the phenoxyimine moiety in three individual speciesîžnamely
free ligand, aluminum complex, and titanium complexîžwith aluminum
alkyls and aluminum hydride has been studied by means of DFT. It was
demonstrated that
the free phenoxyimine ligand in an equimolar mixture with trimethylaluminum
does not undergo reduction. Instead, experimentally observed formation
of the six-membered cyclic aluminumâphenoxyimine complex, useful
in the ring-opening polymerization of lactones, takes place as the
kinetically and thermodynamically favored process. However, it is
anticipated that a 2-fold excess of the aluminum compound, especially
aluminum hydride, acting on the resulting cyclic complex can convert
the imine to the aluminum-subsituted amine functionality easily with
an energetic barrier of approximately 10 kcal/mol. Finally, the propensity
of the imine moiety in the titanium-based precursor of the coordinative
olefin polymerization toward reduction with organoaluminum compounds
is revealed and the mechanism of this reaction is also suggested
Activity and Thermal Stability of Cobalt(II)-Based Olefin Polymerization Catalysts Adorned with Sterically Hindered Dibenzocycloheptyl Groups
Five examples of unsymmetrical 2-(2,4-bis(dibenzocycloheptyl)-6-methylphenyl- imino)ethyl)-6-(1-(arylyimino)ethyl)pyridine derivatives (aryl = 2,6-Me2C6H3 in L1; 2,6-Et2C6H3 in L2; 2,6-i-Pr2C6H3 in L3; 2,4,6-Me3C6H2 in L4 and 2,6-Et2-4-MeC6H2 in L5) were prepared and characterized. Treatment with CoCl2 offered the corresponding cobalt precatalysts Co1−Co5, which were characterized by FT-IR and NMR spectroscopy as well as elemental analysis. The molecular structures of Co3 and Co4 determined by single crystal X-ray diffraction revealed distorted square pyramidal geometries with τ5 values of 0.052−0.215. Activated with either MAO or MMAO, the precatalysts displayed high activities in ethylene polymerization, where Co1 with the least bulky substituents exhibited a peak activity of 1.00 × 107 g PE mol−1 (Co) h−1 at 60 °C. With MAO as a cocatalyst, the activity was reduced only by one order of magnitude at 90 °C, which implies thermally stable active sites. The polymerization product was highly linear polyethylene with vinyl end groups. Co3 with the most sterically hindered active sites was capable of generating polyethylene of high molecular weight, reaching 6.46 × 105 g mol−1. Furthermore, high melting point and unimodal molecular weight distribution were observed in the resulting polyethylene. It must be stressed that the thermal stability of the catalyst and the molecular weight of the obtained polyethylene attain the highest values reported for the unsymmetrical 2,6-bis(imino)pyridylcobalt (II) chloride precatalysts
Characterization of a Double Metal Cyanide (DMC)-Type Catalyst in the Polyoxypropylation Process: Effects of Catalyst Concentration
The
alkaline catalysts commonly applied to alkoxylation are characterized
by a limited spectrum of activity caused by an irreversible termination
of the polyether chains. The presented results show that double metal
cyanide (DMC) catalysts reduce or eliminate the aforementioned adverse
rearrangement of hydroxyl groups. Moreover, DMC catalysts indicate
high activity at low concentrations (ppm range), as expressed by high
polymerization rates. It was demonstrated that decreased concentrations
of DMC catalyst irreversibly influence its reactivity and the dispersity
of the obtained products, as exemplified by the production and determination
of selected polyoxypropylenediols at different concentrations of the
catalyst. Because of their unique advantages, the DMC catalysts are
a very attractive alternative to conventional alkaline catalysts for
the polyaddition of oxiranes. The phenomenon was discussed and explained
by an alteration of reaction rate coefficients at subsequent polyaddition
stages