Metallocenes activation

Most catalysts for solution processes are either completely soluble or pseudo-homogeneous all their catalyst components are introduced into the reactor as Hquids but produce soHd catalysts when combined. The early Du Pont process employed a three-component catalyst consisting of titanium tetrachloride, vanadium oxytrichloride, and triisobutjlalurninum (80,81), whereas Dow used a mixture of titanium tetrachloride and triisobutylalurninum modified with ammonia (86,87). Because processes are intrinsically suitable for the use of soluble catalysts, they were the first to accommodate highly active metallocene catalysts. Other suitable catalyst systems include heterogeneous catalysts (such as chromium-based catalysts) as well as supported and unsupported Ziegler catalysts (88—90). 

Eisch s work promoted investigation into the preparation of cationic metallocene complexes of Group 4 metals. Several preparative routes to cationic group 4 metallocene complexes are illustrated in Scheme II. Catalytic activities of some selected cationic metallocene complexes for the polymerization of a-olefins are summarized in Tables 5 and 6. The catalyst systems based on these cationic complexes are just as active as M AO-activated metallocene catalysts for the polymerization of a-olefins. 

The use of weakly coordinating and fluorinated anions such as B(C6H4F-4)4, B(C6F5)4, and MeB(C6F5)3 further enhanced the activities of Group 4 cationic complexes for the polymerization of olefins and thereby their activity reached a level comparable to those of MAO-activated metallocene catalysts. Base-free cationic metal alkyl complexes and catalytic studies on them had mainly been concerned with cationic methyl complexes, [Cp2M-Me] +. However, their thermal instability restricts the use of such systems at technically useful temperatures. The corresponding thermally more stable benzyl complexes,... 

Based on Chien s research results, Collins et al. modified the basic structure of the catalysts and also achieved elastic material [8,18,19]. In both cases the elastic properties of the polymers are justified in a block structure with isotactic and atactic sequences. In 1999 Rieger et al. presented a couple of asymmetric, highly active metallocene catalysts, e.g., the dual-side catalyst rac-[l-(9-r 5-fluorenyl)-2-(5,6-cyclo-penta-2-methyl-l-q5-indenyl)ethane]zirconium dichloride (Fig. 3). These catalysts allowed building of isolated stereoerrors in the polymer chain to control the tacticity and therefore the material properties of the polymers [9],... 

The key to highly active metallocene catalysts is the use of cocatalysts. In an activation step, the cocatalyst creates out of the metallocene a polymerization-active species. At first, methylaluminoxane (MAO) was usually used to activate metallocenes. Nowadays an alternative activation via borane and borate is becoming more and more important [20, 24, 25]. 

Finally, Brintzinger s synthesis of stereo-rigid ansa-metallocenes [18,19] provided the basis for modern high-activity metallocene catalysts and greatly increased their economic potential. However, although MAO activation and stereo-rigid structures of the new metallocene systems were extremely successful, the complexity of these catalytic systems was such that precise information about the structure of the active species remained elusive. 

Olefin Polymers Isotactic polymers of propylene and 1-butene obtained by optically active metallocene catalyst (145) have been reported to show large specific rotation in suspension ([a]D-123°, -250° for polypropylene [a]D+130° for polybutene), which was lost when the polymers were completely dissolved or heated [176,177]. The optical activity was ascribed to a helical conformation of the polymer chain with preferential screw sense. 

These were the first well-defined zirconocene catalysts capable of polymerising ethylene, propylene and higher a-olefins at high rates without the addition of any further activator. Metallocene catalysts obtained by abstraction of the Me-anion from Cp 2ZrMc2 by a strong Lewis acid, B(C6F5)3, were likewise found to exhibit high activity in ethylene and a-olefin polymerisation [30,105] ... 

Occasional regioerrors appear significantly to inhibit the polymerisation of a-olefins by methylaluminoxane-activated metallocene catalysts [114, 138, 253— 261], In order to reduce the number of secondary Zr-CH(R)-CH2 species, and therefore to accelerate the polymerisation, advantage has been taken of the chain transfer reaction with hydrogen ... 

Ethylene polymerisation with alkylaluminium-activated metallocene catalysts involves a propagation step characterised by the appearance of species alternating between an inactive ( dormant ) state and an active state in which the chain growth actually occurs [330-333] ... 

Jiingling, S., Mulhaupt, R., Stehling, U., Brintzinger, H.-H., Fischer, D. and Langhau-ser, F., The Role of Dormant Sites in Propene Polymerization using Methylalumox-ane Activated Metallocene Catalysts , Macromol. Symp., 97, 205-216 (1995). 

Kaminsky, W., Highly Active Metallocene Catalysts for Olefin Polymerisation , J. Chem. Soc., Dalton Trans., 1998, 1413-1418 (1998). 

Further evidence for the existence of cationic centers is given by the activation of metallocene catalysts for olefin polymerization by the use of anionic counterions such as tetraphenylborate (CeFIs B-, carborane (C2B9H12), or fluorinated borate. The use of (CgF5)4B counterions by Hlatky et al. (83), Sishta et al. (84), and Zambelli et al. (85) leads to highly active metallocene catalysts, which are formed by the reaction of a dealkylated zirconocene with dimethylaniliniumtetra(/us,-perfluorophenyl)borate ... 

McLaughlin ML, Cronan JM Jr, Schaller TR, Snelling RD (1990) DNA-metal binding by antitumor-active metallocene dichlorides from inductively coupled plasma spectroscopy analysis titanocene dichloride forms DNA-Cp2Ti or DNA-CpTi adducts depending on pH. J Am Chem Soc 112 8949-8952... 

The ultimate aim of the project was to see if rectification (asymmetric current-voltage responses) or gating could be achieved by use of the redox-active metallocene moieties. Not only did channel-like function prove difficult to establish but the potentials required to oxidize the Fe(II) center for example (ca. 720 mV), were beyond the limits of the experimental setup. Indeed, membrane stability at very high voltages (>200 mV) is questionable.41 With the small amounts of data obtained (Figure 14) it is difficult to substantiate whether any rectification is realized. 

Solid-state hydroxyisobutylaluminoxane co-catalysts prepared by Wu [4] were as effective in activating metallocenes in olefin polymerization as the corresponding alkyl aluminoxanes but at a lower aluminum/metal ratio. 

The active species in B(C6F5)3-activated metallocene catalysts is an ion pair, consisting of an electron-deficient cation, such as [Cp2ZrMe]+, stabilised by a weakly coordinating anion, here [MeB(C6F5)3]". One aspect of our research recently has been the attempt of building the Lewis acidic activator function into the metallocene precursor complex, in an effort to synthesise self-activating systems. The principle is illustrated... 

He is currently supervising a group of 15 students and scientists in the field of pyrolysis of plastic waste in a fluidized bed process and a group in the field of metal-locene/methylaluminoxane chemistry. His past experience includes the development of pilot plants for the feedstock recycling of plastic wastes and scrap tires, and discovering highly active metallocene catalysts for the polymerization of olefins. He has published more than 300 papers and books and holds 20 patents. He has organized several international symposia in the field of pyrolysis and olefin polymerization. 

Kaminsky, Sinn and coworkers discover high activity metallocene SSC using MAO as cocatalyst... 

Metallocene catalysis is an alternative to the traditional Ziegler-Natta vanadium-based catalysis for commercial polyolefin production, e.g. the use of metallocene-catalyzed ethylene alpha-olefin copolymers as viscosity index modifiers for lubricating oil compositions [23]. The catalyst is an activated metallocene transition metal, usually Ti, Zr or Hf, attached to one or two cyclopentadienyl rings and typically activated by methylaluminoxane. Metallocene catalysis achieves more stereo-regularity and also enables incorporation of higher alpha-olefins and/or other monomers into the polymer backbone. In addition, the low catalyst concentration does not require a cleanup step to remove ash. 

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