Catalyst Cp Research Articles

EPDM Synthesis by the Ziegler Catalyst Cp*TiMe3/B(C6F5)3

The utilization of the Ziegler catalyst Cp*TiMe3/B(C6F5)3 for the terpolymerization of ethylene, propylene and 5-ethylidene-2-norbornene to give EPDM is investigated, and the major factors affecting yields, molecular weights, molecular weight distributions and compositional distributions of the EPDM polymers are assessed. High molecular weight polymers with narrow molecular weight distributions are obtained at ˜18°C.

Structure and performance of the solid methylalumoxane at temperatures 20–250°C: Experimental and DFT calculation study

The structure of solid methylalumoxane (MAO) and its behavior in the temperature interval from 20 to 250°C have been studied using IR spectroscopy in diffusion reflection mode (DRIFT) and mass-spectrometric (MS) methods. It has been shown that three-dimensional MAO molecule may entrap water molecule in its volume (absorption bands (a.b.) 3550 cm −1 in the IR-spectrum). Using a density functional theory (DFT) quantum-chemical method, the three-dimensional molecular model of MAO with composition (Al(CH 3)O) 12, comprising water molecule in its structure, has been calculated. The release of the water molecule from the structure of MAO molecule is accompanied by the protolysis of AlCH 3 bonds and methane evolving. This reaction seems to be responsible for the observed evolving of methane on the MAO aging. As the temperature of MAO heating increases from 20 to 150°C, the process of methane evolving intensifies. The heating of the solid MAO samples is also accompanied by the release of trimethylaluminum (TMA) in significant amount (0.18 mole AlMe 3/mole Al MAO at 100°C). Obviously, the released TMA was strongly associated with the MAO in the initial sample. The data on the activity in the ethylene polymerization of catalyst Cp 2ZrCl 2/MAO prepared with the use of the MAO samples dried at temperatures 20–250°C are presented as well.

Copolymerization of 1-hexene and ethylene catalyzed by the Ziegler catalyst Cp*TiMe3/B(C6F5)3

The Ziegler–Natta system Cp*TiMe3/B(C6F5)3 catalyzed the copolymerization of ethylene and 1-hexene in toluene into materials that were characterized by 1H and 13C{1H} NMR spectroscopy, differential scanning calorimetry, and gel permeation chromatography. The effects of temperature and ethylene/1-hexene and olefin/catalyst ratios on catalyst activities and copolymer molecular weights and molecular weight distributions were studied; the ethylene proportions varied from less than 5% to 85% or more. In addition, significant amounts of 1-hexene were incorporated into the growing polymer chain in a 2,1-fashion; consequently, conventional 13C NMR analytical methodologies for deducing monomer proportions and dispersions and polymer microstructures, based on a low 1,2-incorporation of α-olefin, did not work very well. A soluble (in toluene at ambient temperature) but very high molecular weight (weight-average molecular weight ∼ 8 × 105, weight-average molecular weight/number-average molecular weight = 1.8) rubbery copolymer that formed at −78 °C exhibited a predominantly alternating microstructure. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3966–3976, 2000

Organolanthanide-Catalyzed Cyclization/Silylation of Nitrogen-Containing Enynes

Cyclization/silylation reactions of nitrogen-containing enynes catalyzed by the complexes Cp*2LnMe·THF (Ln = Y, Lu) were investigated. Utilizing standard conditions previously developed for carbocyclic systems, the Cp*2YMe·THF complex suffered from poor reactivity at room temperature with nitrogen α to the alkyne. This was overcome in the current studies either by slow catalyst addition or by using the smaller lutetium catalyst Cp*2LuMe·THF. The use of Cp*2LuMe·THF allowed the preparation of various nitrogen-containing ring systems in excellent yields with good to excellent diastereoselectivities at room temperature. The results of this study highlight the ability to tune the reactivity of an organolanthanide complex by changing the metal center.

Borane-functionalized oxide supports: development of active supported metallocene catalysts at low aluminoxane loading

Treatment of hydroxylated silica or alumina with tris(perfluorophenyl)borane ( 1), bis(perfluorophenyl)borane ( 2) or bis(perfluorophenyl)boron chloride ( 3), provides borane-functionalized supports of variable composition, as revealed by in situ monitoring by 19 F NMR spectroscopy and/or elemental analysis. These chemically treated supports can be impregnated with Cp 2ZrMe 2 to provide supported catalysts for ethylene polymerization. Although some of these supported catalysts are active for ethylene polymerization in the presence of alkylaluminum compounds (e.g., TMA, TIBAL), all of these catalysts are more efficiently activated in the presence of small quantities of methyl aluminoxane, even at very low Al:Zr ratios of 10:1. The polymer properties are quite similar to those produced using the soluble catalyst Cp 2ZrMe 2/B(C 6F 5) 3, again in the presence of MAO at low loading. A variety of experiments suggest that minimal leaching of the metallocene complex from the support occurs under the conditions studied; in particular, production of polyethylene with high bulk densities (>0.2 g/cm 3) and little reactor fouling, even at elevated temperature in toluene slurry, is observed.

Potential Catalyst Deactivation Reaction in Homogeneous Ziegler−Natta Polymerization of Olefins: Formation of an Allyl Intermediate

We investigate the possibility of premature deactivation of d0 transition metal Ziegler−Natta metallocene catalysts Cp‘2MR+ (Cp‘ = any cyclopentadienyl derivative) used in homogeneous olefin polymerization. Density functional calculations show that β-hydride elimination from the β-agostic precursor likely leads to formation of an allyl dihydrogen complex. This complex is prone to eject the dihydrogen ligand and form a “torpid” Cp‘2M (allyl)+ complex that can be slow to restart chain propagation, depending on the steric hindrance imposed on the metal by the Cp‘ moiety. Our theoretical observations are in line with previous experimental evidence for such a reaction.

Relative Rates of Hydrosilylation of Representative Alkenes and Alkynes by Cp*2YMe·THF and [CpTMS2YMe]2

Reactivity in the hydrosilylation of alkenes and alkynes catalyzed by the organoyttrium catalysts Cp(2)YMe.THF and [Cp(TMS)(2)YMe](2) is generally determined by the steric environment of the substrate. Alkynes and conjugated alkenes show an increased reaction rate because of electronic effects; the magnitude of this increase is highly substrate dependent. The electron rich pyrrole system is particularly reactive, especially with the sterically open [Cp(TMS)(2)YMe](2) precatalyst. For nonconjugated substrates, Cp(2)YMe.THF is generally a more selective catalyst than [Cp(TMS)(2)YMe](2).

Side chain effects on the thermal properties of ethylene copolymers prepared over metallocene catalyst

Abstract From a study of thylene copolymerization with different comonomers (propylene, 1-butene, 1-hexene, 1-octene) over metallocene catalyst (Cp 2 ZrCl 2 ), the catalytic activity shows an enhancement of the polymerization rate in the presence of comonomers. The thermal properties of the produced copolymers depend on the kind of comonomer and the composition of comonomer in the produced copolymers prepared from copolymerization. Especially, the ethylene copolymers having high comonomer composition from the metallocene catalyst showed that the melting point T m 1 is higher than T m 2 for the polyethylenes from Ziegler-Natta catalyst but not for the ethylene copolymers having low comonomer composition. From differential scanning calorimetry (DSC) analysis, the change of melting temperature with scanning time may be dependent upon the type and composition of comonomer used in copolymerization.

Influence of rotation between agostic structures on ethene interaction with a zirconocene polymerization site

Density-functional theory has been used to study the first few steps of ethene polymerization with the single-site catalyst Cp 2ZrCH 3 + (biscyclopentadienyl methyl zirconium cation). Insertion of the first monomer yields a γ-agostic product Cp 2ZrC 3H 7 + which can transform into a more stable β-agostic conformation with an activation barrier of 11 kJ mol −1. The interaction of the β-agostic structure with a second monomer leads to chain termination (front-side attack) or insertion (back-side attack) with barriers of about 35–40 kJ mol −1. A more likely step appears to be insertion directly into the γ-agostic Cp 2ZrC 3H 7 +, to form the γ-agostic product Cp 2ZiC 5H 11 + with a barrier of only 11 kJ mol −1.

Effect of polymerization temperature on the polymerization of ethylene with dicyclopentadienylzirconiumdichloride/ methylalumoxane catalyst

The effect of polymerization temperature on catalyst activity and polymer properties in the polymerization of ethylene was studied. The catalyst system was a homogeneous metallocene catalyst Cp 2ZrCl 2 with methylaluminoxane (MAO) as a cocatalyst. The polymerizations were carried out in n-heptane at temperatures from −20 to 90 °C and the polymer properties determined were molecular weight, molecular weight distribution, melting point, crystallinity, and density. The catalyst activity increased 2.3-fold between 70 °C and 80 °C, evidently due to the improved solubility of polyethylene in n-heptane at elevated temperatures. Above 30 °C the molecular weight began to decrease significantly, causing the crystallinity and density to increase and the melting point to decrease. The molecular weight distribution of the polyethylene was tailored by using stepwise polymerization procedure. The MWDs varied from 2.4 to 20.2 when the polymerization was performed at two different temperatures.

Polymerization of phenylacetylene by hafnocene dichloride/R xAlCl 3− x catalytic systems

The polymerization of phenylacetylene was investigated using the homogeneous catalysts Cp 2HfCl 2/R x AlCl 3− x . The reaction was performed in hydrocarbon and chlorinated hydrocarbon solvents at 0–60°. In all cases, the polymers were coloured, soluble, polyconjugated and paramagnetic; they were characterized by i.r., 1H-NMR, u.v.-visible and ESR spectroscopy.

Isotopic exchange between H2 and D2 by the Rideal-Eley mechanism

The rate of the reaction, H2+D2 → 2HD, is very rapid at −160 °C on the catalyst Cp′2Th-(CH3)2/dehydroxylated Al2O3, activated in H2+D2 and believed to contain supported Cp′2ThH+(Cp′ = η5-pentamethylcyclopentadienyl). Even at −195 °C, the turnover frequency is greater than 0.1 sec−1 at\(P_{H_2 + D_2 } = 1\) atm. Strong evidence is adduced that the exchange proceeds by a Rideal-Eley process, i.e. by a four-center transition state involving Th-H+D-D.