Aluminum Alkyl Cocatalysts Research Articles

Heteroleptic triphenylacetate-tetramethylaluminate complexes of lanthanum and neodymium. Coordination of La3+ with the π-system as the reason for the low catalytic activity of lanthanum complexes

Triphenylacetate-tetramethylaluminate complexes [Ln(Ph3CCOO)2Al(CH3)4]2 (Ln = La, Nd) have been synthesized and structurally characterized. Their reaction with THF leads to the formation of the solvent-separated ion pairs [(Ph3CCOO)2Ln(THF)5]+[(Al(CH3)4]−. The tendency of the La3+ cation to interact with the π-electron system of phenyl rings has been revealed by molecular structure analysis for La-Al and Nd – Al tetramethylaluminate complexes in combination with the study of butadiene and isoprene polymerization by using catalyst system based on these complexes and aluminum alkyl cocatalysts. Coordination of Ln3+ cation by the π-electron conjugated system makes it difficult to form the catalytically active intermediates and decreases the catalytic activity.

Ethylene polymerization by Cr(III) salts on acidic carriers

Chromium tris-2,4-pentanedionate becomes quite active as an ethylene polymerization catalyst when sublimed onto an acidic high-porosity carrier and in the presence of aluminum alkyl cocatalyst. The resultant catalyst produces a broad MW distribution of mostly linear polyethylene. Because the MW distribution reflects the active site distribution, it has been studied in this paper as the catalyst was probed by changing the organic ligand, the acidic carrier, the cocatalyst and reactor additives. The MW distribution was quite responsive to changes in the carrier and the cocatalyst, but not to changes in the organic ligand.These responses were then compared to those of a commercial relative, the Phillips chromium (VI) oxide catalyst system anchored on the same supports. Many common behaviors were observed, and also some contrasting ones. For example, the incorporation of 1-hexene, and the resulting branch placement within the MW distribution were identical between the two catalysts. On the other hand, the role of the support calcination temperature as a MW regulator was greatly reduced from the experimental catalyst, but the response to reactor H2 was much more pronounced than that of Phillips catalysts. Thus, the experimental catalyst offers many of the same features of commercial Cr catalysts, but with some environmental and operational advantages too.

Influence of the aluminum alkyl co-catalyst type in Ziegler-Natta ethene polymerization on the formation of active sites, polymerization rate, and molecular weight

The influence of the concentration of the co-catalysts triethylaluminium (TEAL), tri-iso-butylaluminium (TIBAL), tri-n-octylaluminium on the polymerization rate for standard Ziegler-Natta catalyst systems was studied. By comparing the influence of monomeric TIBAL with TEAL co-catalyst on the polymerization activity, the effect of TEAL dimerization was described. The use of the Eley-Ridealadsorption model instead of Langmuir-Hinselwood model is proposed for the absorption of monomeric aluminiumalkyl species and for the formation of active centers C*. It is further proposed that steric hindrance from different co-catalysts, which results in a higher molecular weight (MW) of polymers, is caused by active centers with reduced space for chain transfer reactions.

Switching from ethylene polymerization to nonselective oligomerization over a homogeneous model catalyst: A triphenylsiloxy complex of chromium(VI)

SiO2-supported silyl chromate catalyst (UCC S-2 catalyst) is an important heterogeneous catalyst for the commercial production of polyethylene (PE). However, it has not been fully investigated during the past several decades. A large obstacle has been the lack of homogeneous model systems for this chromium-based heterogeneous catalyst. In this work, bis(triphenylsilyl)chromate (BC), as a homogeneous model for UCC S-2 catalyst, was studied for its polymerization behavior with aluminum alkyl cocatalysts, namely, triisobutylaluminum (TIBA) and methylaluminoxane (MAO). Moreover, activation of BC was investigated by NMR and ESR spectroscopy, radical trapping on fullerene, and MALDI-TOF mass spectroscopy. In the case of TIBA, only an activity of ethylene polymerization were observed; In the case of MAO, BC exhibits an interesting transformation from ethylene polymerization (Al/Cr ≤ 200) to nonselective oligomerization (Al/Cr ≥ 500) with an increase of Al/Cr molar ratio. The obtained PE display a switching from a unimodal molecular weight distribution (MWD) (Al/Cr ≤ 100) to a bimodal MWD (Al/Cr ≥ 200), and the obtained oligomers are main even-numbered linear α-olefins (C6–C16). The butyl radical and [Cr(η6-arene)2]+ cation was identified during activation of BC with TIBA, and the [Cr(η6-arene)2]+ cation was proved to be inactive for ethylene polymerization by temperature-dependent ESR experiments.

Shifting From Ziegler–Natta to Philips‐Type Catalyst? A Simple and Safe Access to Reduced Titanium Systems for Ethylene Polymerization

Silica-supported titanium(IV) chloride is readily reduced by Mashima and co-workers' reagent (1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene) to afford materials active in ethylene polymerisation without need of aluminum alkyl cocatalyst.

A Triphenylsiloxy Complex of Chromium(II) as a Switchable Catalyst for Ethylene Polymerization and Nonselective Oligomerization

A novel dinuclear oxygen-bridged chromium(II) complex of [(Ph3SiO)Cr·(THF)]2(μ-OSiPh3)2 (1) was successfully synthesized and structurally characterized. The crystal structure of complex 1 shows that the chromium centers adopt a tetrahedrally distorted square-planar coordination geometry. Complex 1 was tested for ethylene polymerization upon activation with and without aluminum alkyl cocatalysts. No ethylene polymerization activity was observed in the absence of aluminum alkyl cocatalysts; however, in the presence of methylaluminoxane (MAO), complex 1 demonstrates an unexpected transformation from ethylene polymerization to ethylene oligomerization, with the molar ratio of Al to Cr increasing from 50 to 1000. The activation of complex 1 with MAO was investigated by ESR and 29Si NMR spectroscopy. It was found that the Ph3SiO− groups could be transferred from the chromium centers of complex 1 to the aluminum of MAO.

Copolymerization of ethylene and cyclopentene with the Phillips CrOx/SiO2 catalyst in the presence of an aluminum alkyl cocatalyst

AbstractThe Phillips CrOx/SiO2 catalyst is an important industrial catalyst for ethylene polymerization. However, understanding of the state of active sites and chain propagation mechanisms concerning the Phillips catalyst is still waiting for conclusive evidence. In this work, the Phillips CrOx/SiO2 catalyst, having been calcined, was used for investigating the copolymerization of ethylene and cyclopentene in the presence of triethylaluminum as a cocatalyst for the first time. The microstructures of the polymers were investigated with 13C‐NMR and gel permeation chromatography methods. Because of the absence of internal double bond (CC) in the copolymer main chain, the ring‐opening metathesis polymerization of cyclopentene was excluded during the copolymerization stage of ethylene and cyclopentene. Also, the 1,2‐insertion and 1,3‐insertion of cyclopentene into the polyethylene main chain were confirmed. This evidence strongly implies that CrC species may not be the active sites for chain propagation; instead, the CrC active site model under the Cossee–Arlman chain propagation mechanism may be responsible for the chain propagation during the normal polymerization period. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

FINE-TUNING CATALYSTS

ELECTRON-DONOR COMPONENTS of commercial Ziegler-Natta polymerization catalysts dictate the structure of the catalyst’s active site, according to a new study ( J. Catal. 2008, 257 , 81). The study provides new avenues for probing details of catalytic reaction mechanisms and may lead to more active and chemically selective catalysts for tailoring molecular properties of polymers. Ziegler-Natta catalysts drive commercial poly-olefin production. They consist of titanium tetrachloride supported on magnesium dichloride crystallites and include aluminum alkyl cocatalysts and organic electron-donor compounds. Although the well-studied catalytic materials mediate polyethylene and polypropylene production on an industrial scale, some aspects of the catalytic system remain poorly understood. For example, researchers have speculated that the electron-donor compounds may play a role in enhancing polymerization by controlling the geometry of MgCl 2 crystals and the orientations of their external surfaces. That proposal...

Notable Effects of Aluminum Alkyls and Solvents for Highly Efficient Ethylene (Co)polymerizations Catalyzed by (Arylimido)- (aryloxo)vanadium Complexes

The effects of both Al cocatalyst and solvent on catalytic activity in the ethylene polymerization by the (arylmido)(aryloxo)vanadium(V) complex, VCl2(N-2,6-Me 2 C 6 H 3 )(O-2,6-Me 2 C 6 H 3 ) (1), have been explored in detail. The activity of 5.84 × 10 5 kgPE/molV-h (TOF 2.08 × 10 7 h -1 ) has been achieved by 1/EtAlCl 2 catalyst in CH 2 Cl 2 at 0°C, and the activity in toluene increased in the order: i-Bu 2 AlCl > EtAlCl 2 > Me 2 AlCl > Et 2 AlCl > Et 2 Al(OEt), AlEt 3 , AlMe 3 (negligible activities). Both aluminum alkyl cocatalyst and solvent also affected the catalytic activity and the norbornene (NBE) incorporation in the ethylene/NBE copolymerization using complex 1, whereas the NBE contents were not strongly affected by the kind of aryl oxide ligand in VCl2(N-2,6-Me 2 C 6 H 3 )(OAr) [OAr=O-2,6-Me 2 C 6 H 3 (1), O-2,6-i-Pr 2 C 6 H 3 (2), O-2,6-Ph 2 C 6 H 3 (3)].

High mileage Ziegler-catalysts: excellent tools for polyethylene production

High mileage titanium based Ziegler-catalyst, aluminum alkyl cocatalyst systems are widely used in industry today to produce a broad spectrum of polyethylene grades. The performance of these catalyst systems is excellent because they are well adapted to the technical processes and able to produce an interesting product portfolio. It will be reported how these catalyst systems work, how they are formed, and how they can be utilized in technical processes. These catalyst systems can be regarded as a benchmark for further catalyst developments in industry. Their introduction in industrial processes represents a technological breakthrough.

A combined experimental far-infrared and computational density functional approach applied to aluminum-alkyl cocatalysts

The structure of aluminiummethyl(chloride) complexes is investigated by experimental far-infrared spectroscopy and by density functional theory calculations. The vibrational frequencies and relative intensities are reproduced well using the B3-LYP based density functional method (DFT). The computed spectra allow for a unique identification of the configuration of the aluminiummethyl(chloride) complexes. The results show that far-IR spectra may be accurately predicted for medium-sized molecules using DFT calculations with moderate basis sets size.

Structure and Stability of Aluminum Alkyl Cocatalysts in Ziegler−Natta Catalysis

The structure of organoaluminum(halide) complexes, used as cocatalysts in Ziegler−Natta catalysis, is investigated by experimental far-infrared spectroscopy and by quantum simulations. Experimental...

Olefin Polymerizations and Copolymerizations with Aluminum Alkyl-Cocatalyst Systems. IV. The Polymerization of Styrene

Abstract The mechanism of cationic styrene polymerization with the AlEt2Cl/RCl (R = alkyl or aryl) catalyst-cocatalyst system has been investigated. Polymerization initiation, i.e., cocatalyst efficiency, is apparently determined by the relative stability and/or concentration of the initiating carbonium ions provided by the cocatalyst RCl. Whereas n-butyl, isopropyl, and sec-butyl chlorides exhibit low cocatalyst efficiencies because of low ion concentrations, triphenyl methyl chloride is a poor cocatalyst because the stability of the derivative triphenyl methyl ion is much higher than that of the propagating styryl ion. Alkyl halides that give ions of intermediate stability are efficient cocatalysts. Isobutyl chloride and benzyl chloride seem to be exceptions and the reasons for this are discussed. A general simple scheme of the polymerization mechanism is proposed.

Olefin Polymerizations and Copolymerizations with Aluminum Alkyl-Cocatalyst Systems. V. The Molecular Weight and Molecular Weight Distribution of Polystyrene

Abstract The polymerization of styrene with the cationic initiator system AlR2Cl/RCl was investigated further. Earlier conclusions [1] were corroborated and expanded by an analysis of the number and weight average molecular weights. Alkyl halides with low (high) R‒Cl bond dissociation energies are efficient (poor) coinitiators of the polymerization of styrene. In the presence of efficient RCl coinitiators, the molecular weights strongly decrease and the molecular weight distributions strongly increase with increasing conversions (Fig. 1). With poor cointiators, only low conversions are obtained even in the presence of large amounts of AlEt2Cl. The data are discussed in terms of a previously proposed mechanism [1].

Olefin polymerizations and copolymerizations with aluminum alkyl–cocatalyst systems. I. Cocatalysis with brönsted acid systems

AbstractCertain aluminum alkyls and aluminum dialkyl halides in conjunction with suitable simple cocatalysts generating proton or carbonium ion exhibit high polymerization activity and produce high molecular weight polyolefins, diolefins and/or copolymers at comparatively high temperatures. In this paper we report our investigations with AlEt2Cl–Brönsted acid systems used for polymerizing isobutylene and copolymerizing isobutene‐isoprene mixtures for high molecular weight products. The cocatalyst is not specific in these reactions. Highly active systems can be obtained with HCl, HBr, CCl3COOH, etc. Evidently proton donors in general (Brönsted acids) may function as cocatalysts in this system. The sequence of polymerization initiation activity of various Brönsted acids in conjunction with AlEt2Cl is HCl, HBr ≫ HF, H2O > CCl3COOH ≫ CH3OH > CH3COCH3. The effect of temperature on the molecular weights was studied. In contrast to linear plots of log molecular weight versus 1/T, obtained for polyisobutylene or isobutylene‐isoprene copolymers with conventional Friedel‐Crafts halides in the −30 to −100°C range, this Arrhenius plot showed a marked maximum near −70°C for polymers prepared with AlEt2Cl–HCl catalyst systems. Features of this polymerization reaction are discussed.