Metallocene systems

Two interesting developments should also be noted syndiotactic polypropylene produced by a novel metallocene system and polypropylene grafted with styrene and/or maleic anhydride marketed by Montell as Hivalloy. 

These are some key advantages that the metallocene catalysts have over conventional Ziegler-Natta catalysts and hence it is highly probable that inter-and intra-chain heterogeneity expected in ethylene-a-olefins copolymers can be controlled through the use of the metallocene system. 

For the two metallocene systems, V(Cp)2 (d3), and Ni(Cp)2 (ds), the only acceptable assignments locate the 4> level between the two II levels in each case, and thus, using the data of Prins and Van Voorst (4 7), yield the parameters Ds = 3543 cm"1, Dt = 2074 cm"1 for the vanadium complex, and Ds = 3257 cm 1,Dt = 1806 cm-1 for the nickel derivative. Moreover, these authors, treating the available data for Fe(Cp)2, derived parameters corresponding to Ds = 5100 cm-1 and Dt = 2740 cm-1, from which it may be noted that, although the individual values of Ds and Dt vary appreciably from one complex to another, the ratio Dt/Ds remains approximately constant and shows the values 0.585,0.554,and 0.537 for the V, Ni, and Fe compounds respectively. More recently Sohn, Hendrickson, and Gray (48, 49) have successfully analysed the d-d spectra of several d6 systems in terms of parameters which lead to the ratios 0.525, 0.540, and 0.585 for Dt/Ds in the complexes Fe(Cp)2, Co(Cp)2+, and Ru(Cp)2 respectively, and it therefore seems reasonable to adopt an average value of 0.55 for this ratio for all metallocene systems. 

E2x E2 A l +A2 +E j, and/Sj x E2 - Ex +E2, the results of Table 12 are easily found. Clearly therefore a polarised spectrum measurement, coupled with data relating to the appropriate band intensities (and their proportionality to the coth (hvj2 kT) function) at various temperatures, would provide considerable insight into the vibronic coupling mechanism for metallocene systems. 

In contrast to the useful conceptual framework provided by the approximate approach just described, the results of more detailed molecular orbital calculations have on the whole been rather disappointing. Thus, although some semi-empirical SCF treatments were attempted, most of the earlier MO calculations for metallocene systems (18, 161, 162, 163, 164,165) suffered from such deficiencies as the neglect of the a-framework, or the use of various one-electron Hamiltonians, for example the various Wolfsberg-Helmholz techniques. Of late, Drago and his coworkers have carried out further Extended Htickel type computations for a wide range of both metallocene and bis-arene species (153, 154), and similar... 

Using the B value obtained above this gives A2 as approximately 6.5 kK., which is not unreasonable for a metallocene system. 

Transition metal catalysis plays a key role in the polyolefin industry. The discovery by Ziegler and Natta of the coordination polymerization of ethylene, propylene, and other non-polar a-olefins using titanium-based catalysts, revolutionized the industry. These catalysts, along with titanium- and zirconium-based metallocene systems and aluminum cocatalysts, are still the workhorse in the manufacture of commodity polyolefin materials such as polyethylene and polypropylene [3-6],... 

Constrained-geometry catalysts for C2H4 polymerization 88 that are counterparts of well-known ansa-metallocene systems have been prepared and shown to be active, in combination with MAO, toward polymerization of ethylene the product is almost entirely polyethylene, with ca. 1% of 1-octene obtained. The titanium complex was found to be four times as active as the zirconium species.1... 

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. 

Most of the spectroscopic investigations discussed above were carried out on well-defined metallocene systems, either isolated species or those generated from a well-defined metallocene alkyl precursor activated with one equivalent of a borane or borate activator. Most practical polymerisation catalysts, on the other hand, include a scavenger, usually an aluminum alkyl, and may contain ill-defined activators such as methylaluminoxane (MAO), usually at high MAO/Zr ratios. Such systems are less amenable to quantitative studies nevertheless, the identifications of species such as those depicted in Schemes 8.5-8.8 has enabled similar compounds to be identified in more complex mixtures. An idea of the possible mode of action... 

A number of observations have been made which qualitatively suggest that carbonium ions adjacent to metallocene systems possess unusual stability. Ferro-cenecarboxaldehyde, for example, is soluble in dilute hydrochloric acid (5), ferrocenyl carbinols such as ferrocenyl phenyl carbinol form ethers with great ease (124), and ferrocenylmethylcarbinol can be dehydrated to vinylferrocene under exceedingly mild conditions (114). The concept of stabilizations of this type has also been used to explain certain anomalous ring substitution reactions. 

Huang, J. and Rempel, G.L. (1995) Ziegler—Natta catalysts for olefin polymerization — mechanistic insights from metallocene systems. Prog. Polym. Sci., 20, 459. 

Within the metallocene systems, there are basically three catalyst variations that have been developed. The first to be introduced was Cp2MMe2 (M = Ti, Zr Harrod4), closely followed by CpCp M(SiR3)X (M = Zr, Hf R = Me, Ph, SiMe3 X = C1, alkyl Tilley43) and the combination catalyst, Cp2MCl2/ BuLi (M = Ti, Zr, Hf Corey45). 

Even non-conjugated di-olefins may be used in these systems as precursors for the (conjugated diene)metallocene synthesis. They become isomerized under the reaction conditions at the bent metallocene system.81 Even vinylcyclopropanes yield the corresponding (l,3-diene)metallocene complexes when treated with the reagent 71. The three-membered ring is readily opened under the typical reaction conditions.82... 

Butadiene)zirconocene reacts with metal carbonyls in a similar way as with ketones. Carbon-carbon coupling between a butadiene terminus and the carbonyl carbon atom occurs readily to yield a metallacyclic product, only that generally a metallacyclic n-allyl metallocene system is obtained from the reactions with metal carbonyls.8 Sometimes, O-zirconated Fischer-type carbene complexes are readily formed in this way that cannot be easily obtained by other routes. A typical example is the... 

The specific geometry of the Z-configurated [(CgFs B-CFy-allyl ligand prohibits the formation of a stabilizing Zr-F(CAr) interaction instead these systems feature a pronounced internal [B]-CH2/Zr ion pair interaction (see Fig. 19). The fluorenyl-derived ansa-metallocene system 139 shows a similar structural behavior.134... 

This mechanism, although understandable in a conceptual sense, is not fully understood in a mechanistic sense. For instance, the exact nature of active species and the role of the activator and/or counterion is a subject of debate this concerns methylaluminoxane-activated group 4 metallocene systems in particular. Methylaluminoxane may act to generate the active species and remove impurities from the polymerisation system as well as playing a more fundamental role such as assisting in the insertion of each monomer unit or reactivating dormant sites [358]. 

Two major mechanisms have been proposed for alkene polymerization. These are the Cossee-Arlman mechanism and the Green-Rooney mechanism. A modified version of the latter has also been considered to explain the behavior of homogeneous, metallocene catalysts. The original Cossee-Arlman mechanism was proposed for the TiCl3 based heterogeneous catalyst. In the following sections we discuss these different mechanisms in some detail. In the following discussion in accordance with the results obtained from the metallocene systems, the oxidation states of the active surface sites are assumed to be 4+. 

It is quite remarkable that with all the effort on dinitrogen fixation with the titanium metallocene systems very little is known about the reactions of these complexes with the oxides of nitrogen. Salzmann (119) reported the formation of a polymeric complex [(tj-CsHs TiNO] from the reaction of 3 and NO in toluene solutions. Later, Bottomley and Brintzinger reported that (T7-C5H )2Ti(CO)2 (39) reacts with NO to yield [( -CsHs TiO] as well as an apparent isocyanate complex (112). With an excess of (tj-C5H5)2Ti(CO)2 to nitric oxide, CO, C02, N2, and a Ti-isocyanate species were observed. With N20, (T -C5H5)2Ti(CO)2 yields N2, CO, and various Ti-oxo species. In contrast to our studies with 37 and CO, (tj-C5H5)2Ti(CO)2 and NH3 did not generate an isocyanate species. 

It is important to point out that the zirconium benzamidinate complexes 1 and 5 have lower catalytic activity in the a-olefin polymerizations than the corresponding metallocene systems (lxlO7 and 6xl06 g polymer-mol Zr -h 1 for ethylene and propylene, respectively [70]). The difference in activity can be rationalized by the structural environment of the metal (Zr) center. In the metallocenes, the cone angle is normally the key feature for unsaturation, whereas in the octahedral complexes the groups at the N-moiety may induce an electronic unsaturation at the metal center. Thus, the lower polymerization activities of the complexes 1 and 5 may be a consequence of the partial saturation at the metal orbitals as compared to the metallocenes [71-73]. 

Breslow and Newburg [14] used bis-cyclopentadienyltitaniumdichloride (Cp2TiCl2) together with diethylaluminumchloride for the polymerization of ethene. Subsequent research on this and other metallocene systems with various alkyl groups has been performed by Natta et al. [15], Belov et al. [16], Dyachkovskii et al. [17], Patat and Sinn [18], Chien and Hsieh [19], Clauss and Blstian [20], Henrici-Olive and Olive [21], Reichert and Schoetter [22], and... 

It is estimated that by the year 2000 nearly every second new polyolefin plant will run with metallocene systems. A lot of polymer-producing companies try to use mainly supported metallocene catalysts in their running plants, the so-called drop in technology others build plants specially equipped for these new types of catalysts. 

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