Homogeneous ethylene polymerization

Lewis-acid complexes 6 are formed upon addition of the aluminium alkyl to oxovanadium(v) species and are mildly active catalysts for homogeneous ethylene polymerization (16). System 6 appeared to be a promising candidate for an... 

Figure 8.24 The structure of the dication [YMe(THF)6] + [78]. (Reproduced with permission from S. Arndt, etal., Homogeneous Ethylene-Polymerization Catalysts Based on Alkyl Cations of the Rare-Earth Metals Are Dicationic Mono(aUcyl)Complexe s the Active Species , Angewanrffe Chemie International Edition, 2003, 442, 5075-5079 (Eigure 2). Wiley-VCH Verlag GmbH Co. KGaA.)...

The complexes [C9H6(CH2)2NEt]Ti(NEt2)2 and [Ci3H8(CH2)2NEt]Ti(NEt2)2 have been synthesized and characterized by spectroscopy techniques. These compounds activated by MAO are used for homogeneous ethylene polymerizations.745... 

The mixed-ring titanium compound Cp(Ind)TiCl2 is prepared by the reaction of Lilnd with GpTiCl3 and shows high catalytic activity as homogeneous ethylene polymerization catalyst in combination with MAO as co-catalyst.1036... 

Lohrenz, J. C. W. Woo, T. K. Ziegler, T. A density functional study of the origin of the propagation barrier in the homogeneous ethylene polymerization with Kaminsky-type catalysts. J. Am. Chem. Soc. ms, 117, 12793-12780. 

David Breslow of the Hercules Research Center recognized in the mid-1950s that efforts to identify a homogeneous ethylene polymerization catalyst system would be an important step in understanding the nature of... 

Two additional chromium-based cationic complexes were reported by Gibson et al that also exhibit high activity as homogeneous ethylene polymerization catalysts [36]. These cationic complexes were prepared from the Bis(imido) Chromium (VI) complex (R-N=),Cr(CH,CHJ, where R is t-butyl. The structure of this precursor complex is illustrated in Figure 3.29. 

In 1988 Howard W. Turner reported the preparation of a solid catalyst component based on the reaction of (BuCpl ZrCl and methylalumoxane at room temperature. The solid was isolated by cooling the reaction vessel to -30°C for one hour and then decanting the liquid layer and washing the solid material with pentane to yield a glassy solid. The solid with an Al/ Zr ratio of 20 was examined as a homogeneous ethylene polymerization catalyst using toluene as a solvent. Catalyst activity was 164 g PE/g catalyst at 80°C, 35 psi ethylene in 10 minutes [45]. 

The polyhomologation reaction has been used to prepare polymethylene with aMn as high as 354 KDa corresponding to an intermediate tris(polymethylene)borane with Mn> 1.0 x 10 (Busch et aL, 2002). A of 1.06 million for the star tris(polymethylene)borane represents approximately 7.6 x 10 turnovers per boron atom. The reaction is complete in <10 min, from which one estimates a lower limit turnover frequency of 6.4 x lO g of polymethylene (mol boron) h at 120 °C. The turnover frequency for the boron catalytic center is comparable to some of the most efficient homogeneous ethylene polymerization catalysts, such as neutral anilinotropone-Ni(II) (8.8 x 10 g of polyethylene (mole catalyst) h ) (80 °C, 200 psi ethylene) (Hicks and Brookhart, 2001). 

The second type of solution polymerization concept uses mixtures of supercritical ethylene and molten PE as the medium for ethylene polymerization. Some reactors previously used for free-radical ethylene polymerization in supercritical ethylene at high pressure (see Olefin POLYMERS,LOW DENSITY polyethylene) were converted for the catalytic synthesis of LLDPE. Both stirred and tubular autoclaves operating at 30—200 MPa (4,500—30,000 psig) and 170—350°C can also be used for this purpose. Residence times in these reactors are short, from 1 to 5 minutes. Three types of catalysts are used in these processes. The first type includes pseudo-homogeneous Ziegler catalysts. In this case, all catalyst components are introduced into a reactor as hquids or solutions but form soHd catalysts when combined in the reactor. Examples of such catalysts include titanium tetrachloride as well as its mixtures with vanadium oxytrichloride and a trialkyl aluminum compound (53,54). The second type of catalysts are soHd Ziegler catalysts (55). Both of these catalysts produce compositionaHy nonuniform LLDPE resins. Exxon Chemical Company uses a third type of catalysts, metallocene catalysts, in a similar solution process to produce uniformly branched ethylene copolymers with 1-butene and 1-hexene called Exact resins (56). 

The first example of homogeneous transition metal catalysis in an ionic liquid was the platinum-catalyzed hydroformylation of ethene in tetraethylammonium trichlorostannate (mp. 78 °C), described by Parshall in 1972 (Scheme 5.2-1, a)) [1]. In 1987, Knifton reported the ruthenium- and cobalt-catalyzed hydroformylation of internal and terminal alkenes in molten [Bu4P]Br, a salt that falls under the now accepted definition for an ionic liquid (see Scheme 5.2-1, b)) [2]. The first applications of room-temperature ionic liquids in homogeneous transition metal catalysis were described in 1990 by Chauvin et al. and by Wilkes et ak. Wilkes et al. used weekly acidic chloroaluminate melts and studied ethylene polymerization in them with Ziegler-Natta catalysts (Scheme 5.2-1, c)) [3]. Chauvin s group dissolved nickel catalysts in weakly acidic chloroaluminate melts and investigated the resulting ionic catalyst solutions for the dimerization of propene (Scheme 5.2-1, d)) [4]. 

On the other hand, the polymer prepared by the embedded catalyst shows T around 130 °C, which is a typical melting temperature of high density polyethylene. There was little activity difference between the polyethylene produced by embedded particles and those by homogeneous catalysts. The results of ethylene polymerization using embedded catalyst and homogeneous catalyst are summarized in Table 1 and Fig. 2,... 

Fig. 2. Activity profiles of ethylene polymerization (a) with homogenous catalyst ...

Table 1. Results of ethylene polymerization with embedded catalyst and homogeneous...

Non-metallocene complexes, such as aryloxide 31 and amide 138, have also been utilized as catalyst systems for the polymerization of a-olefins. Moreover, the homogeneous olefin polymerization catalysts have been extended to metals other than those in Group 4, as described in Sect. 7. Complexes such as mono(cyclopentadienyl)mono(diene) are in isoelectronic relationship with Group 4 metallocenes and they have been found to initiate the living polymerization of ethylene. These studies will being further progress to the chemistry of homogeneous polymerization catalysts. 

Sivaram S, Srinivasa RS (1995) Homogeneous metallocene-methylaluminoxane catalyst systems for ethylene polymerization. Prog Polym Sci 20 309-367... 

Twenty years later Reichert [9] and Breslow [10] discovered that the addition of small amounts of water to the alkylaluminum chloride co-catalysts resulted in a one to two orders of magnitude increase in ethylene polymerization activity. In the 1980s the first reports appeared concerning the homogeneous stereospecilic polymerisation, but they received relatively little attention because in the same period the first highly active, supported,... 

Figure 14.6 A heterobimetallic Ti-Mg silsesquioxane, a homogeneous catalyst for ethylene polymerization after AlEtj activation, a possible model for the heterogeneous catalyst TiCl4/MgCl2/Si02 (one of the proposed surface structures) and a patented route to precatalyst, active after activation with MAO.

The silica-supported chromate can be activated directly to a very efficient ethylene polymerization catalyst by ethylene itself or by reduction under CO, to yield active Cr(ll) bisiloxy species, ](=SiO)2Cr] [8]. While the silsesquioxane Cr derivative on its own does not lead to an active polymerization catalyst under ethylene (albeit only low ethylene pressure were tested), the silsesquioxane chromate ester can yield an active polymerization catalyst by addition of methyl-aluminoxane as co-catalyst. Comparison between the two catalytic systems is therefore possible but suffers from the lack of molecular definition of the active homogeneous species obtained after activation with the alkylating agent (Scheme 14.11). 

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