For metallocenes

Figure 24.10 Spin equilibrium in [Mn(ij -C5H4-Me)2] the orbitals shown here are the mainly metal-based orbitals in the centre of the MO diagram for metallocenes (see Fig. B, p. 939).

Table 1 Global Capacity for Metallocene-Based Polyolefins...

Development of a reduced-order model for metallocene-catalyzed ethylene-norbornene copolymerization reaction... 

Table 1 Comparison of degree of crystallinity for metallocene isotactic poly(propylenes) from wide-angle X-ray scattering analyzed by different methods...

Figure 19 (a) Percentage of y phase vs. crystallization temperature,Tc, for metallocene... 

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... 

The e (antibonding), a (nonbonding) and e 2 (bonding) are assumed to be the frontier orbitals for metallocenes. The non-bonding nature of the a orbitals, the HOMO, is in agreement with the observation that for ferrocene (18 valence electrons terminal electronic configuration e2a f) the removal of one electron to form the ferrocenium ion (17 valence electrons) does not substantially destabilize the molecular frame. 

Figure 1 Molecular orbital diagram for metallocenes (in the eclipsed configuration)...

Swogger, K. W., An Outlook for Metallocene and Single Site Catalyst Technology into the 2U Century, SPE ANTEC Tech. Papers, 44, 1790 (1998)... 

Zirconocene and Half-Sandwich Zirconium Derivatives The development of a single-site heterogeneous catalyst for metallocene-based polymerization catalysis has also been explored extensively with zirconocene and half-sandwich zirconium derivatives [32, 75, 91, 92]. 

Boron-based Lewis acids are useful coinitiators for metallocenes [Chen and Marks, 2000 Chen et al., 1998 Pedeutour et ah, 2001 Zhou et ah, 2001]. Organoboranes such as... 

K. Suzuki, H. Maeta, and T. Matsumoto, An improved procedure for metallocene-promoted glycosylation. Enhanced reactivity by employing 1 2 ratio of Cp2HfCl2-AgC104, Tetrahedron Lett. 30 4853 (1989), and references therein. 

According to TDDFT results by Boulet et al [84], for the CpNiNO complex, unlike for metallocenes, the BP functional improves the description of both the geometrical parameters and the excited states. 

In connection with the above characteristic features of metallocene methy-laluminoxane catalysts, it must be emphasised that alternative, potentially cheaper alkylaluminoxanes, such as ethyl or z-butyl derivatives, which are more soluble in aliphatic hydrocarbons than methylaluminoxane, or other alkylaluminium compounds used as activators for metallocene procatalysts, show inferior activity. 

Pathways (a)-(b)-(c)-(d) and (a )—(b )—(c )—(d ) correspond to the original mechanism proposed by Cossee [268,276,277] and are still valid, apart from some minor modifications [1], for heterogeneous catalysts. For metallocene-based catalysts of classes II and partially V, this mechanism gives rise to successive additions at the same site (from a configurational point of view) and is known as the chain stationary insertion mechanism ( chain skipped insertion or site isomerisation without insertion mechanism) [143, 146, 345], The (a)-(b)-(c)-(a )—(b )—(c ) pathway corresponds to the chain migratory insertion mechanism found in the case of metallocene catalysts of classes I, III, IV and partially V [143, 146]. 

At the end of considerations of the role of alkylaluminoxanes as activators for metallocenes as Ziegler-Natta olefin polymerisation catalysts, it should be noted that the analogy between methylaluminoxane and simple acidic moieties does not appear to hold. For example, highly Lewis acidic perfluorinated boranes, such as Bf CgFs, show enhanced activity as activators when compared with [Al(Me)0]x, but the stability of the resulting zirconocene-based catalyst is drastically lower than that of the catalyst formed with [Al(Me)0]x [98]. 

Comparing the three names developed so far for spiro compounds [(1) vs. (3) vs. (57)], and [(2) vs. (4) vs. (58)], one finds that the choice of which name is "better" is a strictly heuristic one. For example, choose the spherical name whenever the rings involved are congruent. At the other extreme, to use redundant paths for metallocenes would be, at best, pedantic, requiring the path to pass through the iron atom in ferrocene ten times. Additionally, as shall be demonstrated below, the redundant path technique is often the... 

Bonding of ligands to alkali, alkaline earth and lanthanoid metal ions is mainly electrostatic. Consequently, these bonds can be described as a combination of electrostatic and van der Waals terms (see Chapter 15). Similar approaches have also been used for metallocene compounds (see Chapter 14). 

For metallocene catalysts of general formula [H2SiCp2M-CH3]+ (M = Ti, Zr, Hf) the relative order of alkene insertion leading to polymerization is Zr > Hf > Ti. What sort of investigation may be required to rationalize this ... 

Figure 21 Changes in component ratio for metallocene (filled symbols) and Ziegler films (open symbols) (A) Mobile, (B) intermediate and (C) rigid amorphous components. The shaded area indicates the plateau stress region.

Fig. 18. Possible structures for metallocenes containing a functional group in the ligand either at the bridge or at the indenyl (cyclopentadienyl) group, making it possible to attach the metallocene to a carrier or even perform a polymerization using the metallocene as a comonomer...

Methylaluminoxane (MAO), sometimes referred to as polymethylaluminoxane (PMAO), currently gamers the most industrial interest of aU the alkylaluminoxanes owing to its activity as a cocatalyst for metallocene-catalyzed alkene polymerization (see Oligomerization Polymerization by... 

Amine elimination reaction developed for metallocenes is another method (see Section 2). Immobilized CG complexes can be prepared by this way to form heterogeneous phase systems (see Heterogenized Catalyst) (equation 15). ... 

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