Metallocene catalysts unbridged

Table 8. Isotactic poly(propylene) polymerized by bridged- and unbridged-metallocene catalysts... 

The isotacticities and activities achieved with nonbridged metallocene catalyst precursors were low. Partially isotactic polypropylene has been obtained by using a catalyst system of unbridged (non-ansa type) metallocenes at low temperatures [65]. A chiral zirconocene complex such as rac-ZrCl2(C5H4 CHMePh)2 (125) is the catalyst component for the isospecific polymerization of propylene (mmmm 0.60, 35% of type 1 and 65% of type 2 in Scheme Y) [161]. More bulky metallocene such as bis(l-methylfluorenyl)zirconium dichloride (126) together with MAO polymerized propylene to isotactic polypropylene in a temperature range between 40 and 70°C [162]. 

The issue begins with an article by Ittel, Johnson, and Brookhart on late metal catalysts for ethylene homo- and copolymerization. They detail the newest generation of catalysts to be commercially licensed. Alt and Koppl then introduce ethylene and propylene polymerization by metallocene catalysts. Structure-performance relationships for unbridged and bridged... 

Early Soluble Unbridged Metallocene Catalysts in Connection with Alkyl Aluminum Chlorides... 

Comparative studies of unbridged metallocene catalysts have indicated that, in general, alkylation of the Cp ring(s) causes an increase in polymerization activity, although substitution of all five carbons can lead to very low activities, as steric congestion prevents monomer approach. Bis(indenyl) and bis(fluorenyl)zirconium complexes have received far less attention, and in the latter case, this is almost certainly due to the greater lability of the fluorenyl-metal interaction. 

In addition to the bridged metallocene-based catalysts, a number of unbridged metallocene catalyst systems, exhibiting time-dependent C2-symmetry due to restricted ligand rotation around the ligand-metal axis, have been reported to produce polypropylenes with a range of tacticities and microstructures. In the context of the present discussion, the unbridged metallocene complexes are considered as pieM(io-C2-symmetric. 

In 1984, Ewen reported the first chain-end controlled polymerization of propylene to isotactic polypropylene with metallocene catalysts, using the unbridged C2v-symmetric bis(cyclopentadienyl)titanium diphenyl complex 26 at low temperature (Figure 1.16). With this catalyst system, partially isotactic PP with mmmm = 52% is obtained at -45 °C. 

Wihnes, G. M. Poise, J. L. Waymouth, R. M. Influence of cocatalyst on the stereoselectivty of unbridged 2-phenylindenyl metallocene catalysts. Macromolecules 2002, 35, 6766-6772. 

Chiral, Ci-symmetric (asymmetric) bridged metallocenes, 16 108-109 Chiral, C2-symmetric bridged metallocenes, 16 104-108 Chiral, C2-symmetric catalysts, racemic mixture of, 16 106 Chiral, C2-symmetric unbridged metallocenes, 16 108 Chiral catalysts, 16 395 Chiral centers, in biochemical compounds, 17 402... 

Unaccomplished moisture change, 9 97 Unagitated columns, in commercial extractors, 10 769-774 Unbound moisture, 9 97-98 Unbridged catalysts, oscillating, 16 109 Unbridged metallocenes... 

It is generally adopted that the catalytically active species in the metallocene-catalysed polymerization is a 14-electron cation. As an example, the mechanism of activation of an unbridged zirconocene catalyst is presented in Fig. 9.5-4, top. In the first two steps the activation by MAO, resulting in the 14-electron cation, is shown. The same cation can be generated by N,N -dimethylanilinium-tetrakis(pentafluorophenyl)borate and methylated metallocenes. As side-products methane and an amine are formed. TiBA can also be involved in the activation, which is not shown in Fig. 9.5-4, bottom. On the other hand, TiBA acts as a scavenger in the polymerization. The above-mentioned reactions take place in the absence of the monomer and are performed before the catalyst is used in the polymerization process. 

Unbridged, bridged, substituted, and half-sandwich complexes have been used as metallocenes for ethylene polymerization (Figs. 1 and 2). To compare the activities and molecular masses, the polymerizations are carried out under the same conditions (30°C, 2 bar ethylene pressure, with toluene as a solvent) (105). Table IV shows the polymerization behavior of various met-allocene/alumoxane catalysts. Generally, zirconium-containing catalysts are... 

Polymers of a similar microstructure are obtained if unbridged substituted metallocenes having a significant rotational isomerisation barrier are used as catalyst (Fig. 16). Early attempts concentrated on substituted cyclopentadienyl and indenyl compounds [134—138],... 

The chemistry of //// //-metallocene compounds has been the subject of several reviews. Structural aspects affecting the catalytic activity and the application of these complexes as catalysts for the homo- and co-polymerization of olefins have been considered.323 The evolution of the //// //-bridge complexes in terms of the various synthetic approaches used to construct the bridged ligand framework, the variety of bridges introduced, and the effect of the bridge on the structure and reactivity of ////.y//-titanocene and other transition metal complexes as compared with their unbridged counterparts has been reviewed.1634... 

Phillips catalysts tend to be more efficient than Ziegler catalysts, less efficient than tightly bridged metallocenes, and approximately equivalent in efficiency to unbridged metallocenes. CO-reduced catalysts incorporate 1-hexene about two to three times more efficiently than their... 

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