Metallocene activation

Scheme 8.3. Glycoside formation from glycosyl fluorides with a metallocene activator.

Since a positive correlation between the electrophilicity of the active sites and ethylene-polymerization activity can be expected and has actually been observed in a metallocene activated with homologous perfluoroarylboranes, counteranions that are sterically and electronically less coordinative have been pursued in order to render the active species more electrophilic. To this end, many sterically encumbered activators incorporating extended conjugation... 

Interestingly, the metallocene-activated C-F activation is not limited to tertiary C F bonds (in contrast to the Na/benzophenone system). Perfluorocyclohexane has been defluorinated in the presence of bis(r/5-cyclopentadienyl)zirconium(IV) chloride or bis(t/5-cyclopentadienyl)-titanium(IV) chloride. This reaction is accompanied by hydrogen transfer from the solvent onto the substrate yielding 1,2,4,5-tetrafluorobenzene as product.210... 

Fig. 1.6 Metallocene activation by MAO, structure of the active site, and ethylene polymerization.

Metallocenes such as Cp2TiCl2 and Cp2ZrCl2 alone are capable of polymerising styrene to an atactic polymer (involving a free radical propagation mechanism) [97]. The same metallocenes activated with methylaluminoxane form active catalysts for the polymerisation of styrene their productivity and syn-diospecificity, however, are not very high. In contrast, when activated with aluminium alkyls, these metallocenes do not afford catalysts that might be active in the polymerisation of styrene [98,99]. 

Run Metallocene, Activator Temp. Time Polymer Prod.b A/w MJ... 

The results demonstrate the degree to which ethene polymerisations tend to be mass-transport limited. They also highlight the effect that can be achieved by altering the mode of catalyst activation a catalyst produced by activating rac-Me2Si(Ind)2ZrMe2 with AlBu j /14 proved 36 times more active than the same metallocene activated with MAO under closely comparable conditions. 

The electronic features and steric environment inherent in perfluoroaryl borane abstractors employed for alkyl abstraction reactions are key to the metallocene activation process for polymerization catalysis and the structure of the... 

Figure 3 Selected examples of metallocene activation processes.

Propagation rates of first order in monomer concentration have been reported for ethylene and for propylene in the case of aspecific metallocenes266 as well as for propylene polymerization with isospecific metallocenes activated with MAO, B(C6F5)3, and [PhsCHBlCftFsL].290 297 Moreover, first-order kinetics were also observed for 1-hexene polymerization with the [ra(r-G2H4( 1 -Ind)2ZrMc [ McB(Gf,l 3)3. 156... 

Metallocene activation involves the reaction of a base (the metallocene) with an add (the support). Because the support contains a variety of acid sites, varying in add strength, not all of them are capable of ionizing metallocenes. 

M.P. McDaniel, M.D. Jensen, K. Jayaratne, K.S. Collins, E.A. Benham, N.D. McDaniel, P.K. Das, J.L. Martin, Q. Yang, M.G. Thom, A.P. Masino, Metallocene activation by solid acids, in J. Severn, J.C. Chadwick (Eds.), Tailor-Made Polymers Via Immobilization of Alpha-Olefin Polymerization Catalysts, Wiley-VCH, Weinheim, 2008, pp. 171-210 XVI, Chapter 7. 

Figure J.19 Possible mechanism for metallocene activation on clay surface proposed by McDaniel (reproduced from Ref [44]).

Metallocene Activity Molecular weight M (10 g/mol) PP melting temperature ( O... 

Other Methods of Activation. Despite some early claims (31), the evidence for metallocene activation by simple trialkylaluminum compoimds has not accumulated. Activation by alternate aluminoxanes such as isobutylaluminoxane (IBAO) has also been attested (32). The fact that these much cheaper methods have not consigned MAO to the ash-heap of history speaks, perhaps, of some limitations in these alternatives. 

Preparing the metallocene/activator complex in solution, then treating the support with the complex... 

Supporting the Metallocene, then Treating with the Activator. Because of their method of preparation, silicas have a high concentration of surface hydroxyl groups and complexed water. Normally both hydroxyl groups and water will promote decomposition of a metallocene or metallocene/activator complex. Therefore it is necessary to remove the water either by thermal or chemical dehydration. Full dehydration of silica can occur at 150°C. At this point the silica gel will have a surface that is fully hydroxylated. Even these hydroxyl groups can decompose some types of metallocenes. In most cases the silica gel can be further dehydrated at 400°C (see Fig. 19). 

The energetics associated with stereo-, regio-, and monomer selectivity can be reproduced by modem theoretical methodologies. The counteranion, the polymer conformation, and the metallocene active site shape and van der Waals attraction are each computed to play a role in dictating selectivity. 

All chain transfer reactions are influenced by the nature of the metallocene catalyst (steric/electronic factors) and polymerization conditions (temperature/pressure). Table 10.1 summarizes the effect of the ligand structure in the metallocene active site on chain transfer mechanisms and the structure of the growing polymer chain in propylene polymerization. 

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