Ethylene polymerization, catalyzed

The stoichiometry of this conversion is in accordance with a carbene starting structure. An alternating alkyUdene/metallacyclobutane mechanism [102, 131-133], which has precedent in the ethylene polymerization catalyzed by a Ta(III) neopentilydene complex [134], has been proposed where the chro-miiun alkyUdenes undergo [2-1-2] cycloaddition to give chromacyclobutane intermediates (mechanism III in Scheme 7). 

Although polymerization plays an important role in today s organometallic catalysis, almost no reports exist about the application of NHCs in this reaction. Ethylene polymerization catalyzed by a chromium-NHC complex was reported but the efficiency of this catalyst is only moderate. After activation with MAO the... 

Scheme 4 Initiation, propagation, chain-transfer, and chain-walking reactions in ethylene polymerization catalyzed with group 10 a-diimine catalysts.

Chien, J. C. W. Kinetics of ethylene polymerization catalyzed by bis(cyclo-pentadienyl)-titanium dichloride-dimethylaluminium chloride. J. Amer. Chem. Soc. 81, 86 (1959). 

Table 10 Ethylene polymerizations catalyzed by the phosphine-borane zirconocenes 161a-d...

Write the steps in the mechanism of ethylene polymerization catalyzed by a free radical. 

We will start by discussing the calculations on ethylene polymerization. In this case, the issues are mostly polymer length and the formation of linear or branched polymers. The first DFT/MM applications on this topic were published near simultaneously in 1997 and 1998 by the groups of Ziegler [36] and Morokuma [37, 38] on the reaction of ethylene polymerization catalyzed by cationic diimine Ni(II) complexes of the type (ArN=C(R)-C(R)=NAr)-Ni-CFl3+. Shortly before this, these species (one of them is... 

Table 3 Data for the ethylene polymerization catalyzed by complex 5a [32]...

The efficiencies of CO and C02 to inhibit the polymerization are far from being equivalent. A detailed study 85-87) shows that C02 is much less efficient than CO in the propylene polymerization catalyzed by the TiCl3/AlEt2Cl system and the former is entirely inert in the ethylene polymerization catalyzed by the Cp2TiEtCl/AlEtCl2 system. On the other hand, C02 is much more effective in the ethylene polymerization... 

Density functional calculations on the chain initiation reaction shown in Scheme 136 for the ethylene polymerization catalyzed by bis-alkoxo titanium complexes have been studied. Activation barriers of 6.4kcal mol-1 are found for the titanium sulfur-bridged catalysts with higher insertion barriers of 10-15 kcalmol-1 for the GH2-bridged catalysts. 

Ethylene polymerization catalyzed by the well-characterized homogeneous lanthanide complexes [M(C5H R)2 ] 2 been studied. 

Conventional MAO has very low solubility in aliphatic solvents as well as poor storage stability in solution, which considerably limits its utility. Other more soluble and commonly used aluminoxanes are ethylaluminoxane and isobutylaluminoxane, which are synthesized by the partial hydrolysis of triethyl-aluminum (TEA) and triisobutylaluminum (TIBA), respectively. However, these alkylaluminoxanes do not perform as well as MAO in metallocene-mediated olefin polymerization. " It was reported, however, that tetrakis(isooctyl) alumoxane [(i-octyl)2—O—Al-(i-octyl)2], prepared by reaction of Al(i-octyl)3 with 0.5 equiv of water, exhibits remarkable cocatalytic activity, comparable to or even greater than that obtained with MAO, for ethylene polymerization catalyzed by racemic an5a-bis(indenyl)-type zir-conocene dichlorides. Furthermore, commercial modified methylaluminoxanes (MMAO) available from... 

The catalytic activity of this partially CeFs-substi-tuted MAO is enhanced by 3.7- to 7-fold for ethylene polymerization catalyzed by Cp2ZrCl2 at Al Zr = 1600 1 and [MeA10] M(C6F5)3 (M = B, Al) = 200 1, compared to cocatalytic results with untreated MAO under identical reaction conditions. Flowever, it is also reported that heavier CeFs substitution (increasing amounts of M(CeF5)3 vs MAO) reduces the favorable effects of the CeFs substitution. 

Table 7. Product Molecular Weight for Ethylene Polymerizations Catalyzed by a Series of Co(PBI)Cl2 Complexes ...

The major goal of this study was to understand the factors controlling polyolefin branching and the relationship between the catalyst structure, temperature, pressure and the polyolefin topology. The DFT calculations were carried out for the elementary reactions in the polymerization of ethylene and propylene catalyzed by Pd-based diimine catalysts [13c,d] and the ethylene polymerization catalyzed by the Ni-anilinotropone catalyst [28]. The polymer growth in these processes was modeled by a stochastic approach [27-29]. Further, the model simulations were performed by systematically changing insertion barriers to model the influence of catalyst, beyond the diimine systems [29]. 

Fig. 9 Energetics (in kcal/mol) of the chain growth and chain isomerization reactions in the ethylene polymerization catalyzed hy the Pd-diimine complex 7 (see Scheme 2). The experimental and theoretical (in parentheses) values are presented. The calculated energies for the chain isomerization reactions (marked with ) were obtained from the model catalyst 1...

Fig. 11 The pressure and temperature dependence of the average number of branches in the ethylene polymerization catalyzed by the Pd-based diimine catalyst 7 obtained from the stochastic simulations based on experimental energetics (left). An influence of pressure on the polymer topology (right) in each case a chain of 1000 carbon atoms is shown and the main chain of the polymer is projected along the horizontal axis...

Kinetic data on olefin polymerization by polymer-immobilized zirconocene are scarce. It is generally accepted that homogeneous metallocene catalysts contain uniform active sites however, if they are immobilized on a polymer support, the MWD polymer production becomes broader compared with a homogeneous catalyst [103]. Kinetic analysis of gas-phase ethylene polymerization catalyzed by (CH3)2[Ind]2ZrCl2 bound at a hydroxylated copolymer of styrene with divinylbenzene and previously activated with MAO (0.17 wt.% Zr) has been carried out [104]. The influence of temperature (333 to 353 K), ethylene partial pressure (2 to 6 atm) and MAO level (molar ratio of MAO to zirconium from 2600 to 10,700) were studied. The activity of the catalyst in the gas-phase process changed from 5 to 32 kg PE (g of Zr atm h)It is possible that there are two types of active site. They are stable to temperature and deactivated by the same mechanism. A first-order reaction takes place. The propagation rate constants of two active sites show a similar dependence on temperature. 

Paolucci G, Bortoluzzi M, Napoli M, Longo P, Bertolasi V. The role of the ionic radius in the ethylene polymerization catalyzed by new group 3 and lanthanide scorpionate complexes. Mol Catal A. 2010 317 54-60. 

Wang S, Liu D, Huang R, Zhang Y, Mao B (2006) Studies on the activation and polymerization mechanism of ethylene polymerization catalyzed by bis(imino)pyridyl iron(II) precatalyst with alkylaluminum. J Mol Catal A Chem 245(1-2) 122-131. doi 10.1016/j.molcata.2005.09.023... 

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