Catalysts syndioselective

In an attempt to combine the syndioselectivity of half-sandwich titanium catalysts with the living characteristics of anionic polymerization initiators, the use of half-sandwich calcium-based catalysts has been described.363 364 In neat styrene complex (152) affords 76% rr triad PS. However, polydispersities are still quite high (Mw/Mn > 2.2)... 

Some chiral initiators have structures such that alternate monomer placements occur with opposite faces of the monomer to yield the syndiotactic polymer. This is syndioselective polymerization proceeding with catalyst site control and is usually observed only with some homogeneous initiators, both traditional Ziegler-Natta and metallocene. 

Not all syndioselective polymerizations proceed with polymer chain end control. Some metallocene initiators yield syndioselective polymerization through catalyst site control (Sec. 8-5). 

Syndioselective polymerization by a Cs metallocene such as Me2C(Cp)(Flu)ZrCl2 proceeds by catalyst site control. A statistical model for syndioselective catalyst site control has been described in terms of the parameter p [Resconi et al., 2000]. Parameter p is the probability of a monomer with a given enantioface inserting at one site of the initiator p is also the probability of the monomer with the opposite enantioface inserting at the other site of the initiator. The pentad fractions are given by... 

Having established that a particular polymerization follows Bemoullian or first-order Markov or catalyst site control behavior tells us about the mechanism by which polymer stereochemistry is determined. The Bemoullian model describes those polymerizations in which the chain end determines stereochemistry, due to interactions between either the last two units in the chain or the last unit in the chain and the entering monomer. This corresponds to the generally accepted mechanism for polymerizations proceeding in a nonco-ordinated manner to give mostly atactic polymer—ionic polymerizations in polar solvents and free-radical polymerizations. Highly isoselective and syndioselective polymerizations follow the catalyst site control model as expected. Some syndioselective polymerizations follow Markov behavior, which is indicative of a more complex form of chain end control. 

Site-controlled syndioselectivity of C/-symmetric metallocenes is unprecedented and represents a major achievement of the rational molecular design of single-site catalysts. Recent highly syndioselective metallocenes are listed in Figure The flat fluorenyl ligand has been further spatially extended by substituents at the 2,6- and/or 3,7-... 

All aspects of syndioselective propagation with ( -symmetric catalysts, the influence of ligand,794,797-807 metal,806,808,809 and counteranion100,104 variations, as well as the influence of the polymerization conditions,805,810 have been studied in detail and reviewed.162,181,209,288,811... 

A different, possibly more productive, research direction has been the development of less syndioselective catalysts, based on modifications of the silyl-bridged cyclopentadienyl-amido titanium complex. 

Figure 31 shows the correlation between melting points and stereoregularity in the syndiotactic domain. As is the case of isoselective single-center catalysts, the degree of stereocontrol decreases by increasing the polymerization temperature for the syndioselective ones. This behavior is shown in Figure 32. 

Of all the hafnium compounds listed in Figure 2.7, none excels their zirconium counterparts in syndioselectivity. This observation is sometimes attributed to hafnium s decreased rate of bimolecular propagation relative to unimolecular site epimerization. Lower overall activities with hafnium catalysts corroborate this argument. The most syndioselective hafnium catalyst appears to be Cl-symmetric i-92-Hf, which produces syndiotactic polypropylene with [r] = 92.2% and... 

The zirconium-based catalyst 5 -136/MAO is highly syndioselective ([r] > 99%, Tp = 0 C) and apparently provides the highest-melting syndiotactic polypropylene known The unannealed Tm for the polypropylene is 164 °C, and the annealed Tm is 174 °C. Stereoerrors have not been detected in this polymer by NMR, but pentads presumably attributable to site epimerization (single m mistakes) can be found if polymerizations are conducted in dilute propylene at elevated temperatures. 

The fluorenyl ligand has proven to be an essential component in many homogeneous, stereoselective olefin polymerization catalysts. Since the first fluorenyl-containing metallocene was employed in 1988 for syndioselective propylene polymerization, other tacticities have been accessed isotactic, hemiisotactic, isotactic-hemiisotactic, and syndiotactic-hemiisotactic. Indeed, fluorenyl-containing metallocenes are remarkably versatile, valuable for mechanistic inquiry, and functional in the synthesis of polymers with novel structures and properties. 

Peculiar propylene polymerization behaviors of FI catalysts including syndioselective living polymerization were first disclosed in the patent application by Mitsui Chemicals, Inc. filed in January 2000 and have been published in the subsequent scientific papers by the same group. Coates and coworkers at Cornell University independently conducted intensive work on Ti-FI catalysts, and reported their results on syndioselective propylene polymerization in June 2000 and on syndioselective living propylene polymerization in February 2001 Since then, a great deal of work aiming... 

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