Multisite catalysts

Chromocene) deposited on silica reaction of the metallocene with hydroxyl gronps on the silica snrface generates the active catalyst (eqnation 15). These ill-defined heterogeneous systems give high-density polyethylene with broad molecular weight distributions, indicative of multisited catalysts. 

NMR is usually performed at pentad level (J). This analysis is not adequate only in the following cases /) polymers from multisite catalysts (at least 5 adjustable parameters for only 3 different active species) //) polymers from fluctuating catalysts (at least 4 adjustable parameters) iii) weakly tactic polymers (all models degenerate into the Bernoullian statistics). In most such cases, however, unambiguous conclusions can be reached from the analysis of high-field NMR data at heptad/nonad level. 

One of the key features of metallocene catalysts is their ability to copolymerize a wide variety of olefins as well as styrene without sacrificing the extraordinary xmiformity with respect to narrow molecular weight distribution and especially molecular weight independent comonomer incorporation. Most conventional catalysts are multisite catalysts and contain catalytically active centers with greatly different reactivity towards insertion of ethylene and other olefins. Frequently,... 

Figure 11 Comparison of conventional multisite catalyst and modem single site metallocene catalysts in copolymerization.

Polymerization of butadiene and isoprene under action of microheterogeneous titanium based catalyst with ultrasonic irradiation of the reaction mixture at the initial time is studied. It is shown that ultrasonic irradiation causes transformation multisite catalyst in a quasi-single site. At that activity of dominant site depends on the monomer nature. 

FIG. 31 Metallocene single-site versus Ziegler Natta multisite catalyst. (From Ref. 55.)... 

Vast arrays of metal-containing polymers have been produced that offer a wide variety of properties. Key milestones in the history of this diverse topic and a sense of its growth and importance were discussed in this chapter. While initial efforts focused on polysiloxanes, today s efforts are quite diverse and include the production of multisite catalysts, variable oxidation state materials, and smart materials where the precise structure can be changed through the introduction of different counterions. These polymers have been produced by all of the well-established polymerization methodologies. The metal atoms reside as part of the macromolecular backbone, in sidechains, coordinated to the backbone, and as integral parts of dendrites, stars, and rods. Truly, many of tomorrow s critically important materials will have metal atoms as an integral part of the polymer framework, which will allow the materials to function as demanded. 

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