Metallocenes hemiisotactic

During the last decade, a variety of new catalysts have been presented for the stereospecific polymerisation of a-olefins, based on non-bridged metallocene or stereorigid ansa-metallocene as the procatalyst and a methylaluminoxane activator [29,30,37,105-107,112-114,116-135], Apart from isotactic [118,119,124, 131,132] and syndiotactic [23,118,124,133] polypropylenes and other poly(a-olefin)s [121], hemiisotactic [112,121,124], isoblock [131,132,134], syndioiso-block (stereocopolymer) [127], stereoblock isotactic [135] and stereoblock isotactic atactic [116,128,129] polypropylenes have been obtained using these new catalysts. 

Modifications of Cs-symmetric metallocenes may lead to Ci-symmetric metallocenes (Fig. 8). If a methyl group is introduced at position 3 of the cyclopentadienyl ring, stereospecificity is disturbed at one of the reaction sites so that every second insertion is random a hemiisotactic polymer is produced (276, 277). If steric hindrance is greater (e.g., if a /-butyl group replaces the methyl group), stereoselectivity is inverted, and the metallocene catalyzes the production of isotactic polymers (178-180). 

Ci symmetric metallocenes are able to produce new microstructures if consecutive insertions take place on the same active site in addition to chain migratory insertion. Polypropenes containing blocks of atactic and isotactic sequences are produced, the block lengths depending on the rate of chain stationary insertion or site isomerization vs chain migratory insertion [126-133], Thus hemiisotactic polypropene represents a special case, having a chain stationary/chain migratory ratio of 1 1. 

Metallocenes with diastereotopic sites for monomer coordination show quite an interesting polymerization behavior introduction of a methyl group in position 3 of the cyclopentadienyl ring in (21) disturbs the stereospecificity at this site, giving rise to hemiisotactic polypropylene [53], while a f-butyl group at the same position inverts the preferred mode of coordination (22) thus an isotactic polymer is generated [54]. Metallocene (8) has one nonspecific and one stereo-specific site, too at low temperature, hemiisotactic polypropylene is produced while at high temperatures site isomerization without insertion facilitates the formation of isoblock polypropylene. 

C. CrSymmetric Metallocenes from Hemiisotactic to Isotactic Polypropene... 

Isospecific Ci-Symmetric Metallocenes. Perfect hemiisotacticity requires that mmmm = 18.75%. Deviating from the structure of Ci-I-6 in general makes the complexes more isospecific. Fink studied the series Ci-I-6—Ci-I-9 where the R substituent on Cp increases from methyl to terf-butyl. While the ethyl derivative produces a polypropene very similar to that (prevailingly hemiisotactic) made with the methyl derivative Ci-I-6, the 3-i-PrCp derivative C -... 

Figure 22 deals with the MAO-activated propylene polymerization of a series of metallocene catalysts (see Figs. 18 and 19) having a substituent of increasing steric demand at the cyclopentadienyl ring. To each catalyst is appointed the syndio-tactic (rrrr), hemiisotactic, or isotactic (mmmm) NMR pentads (%). These pentad distributions are measured from samples polymerized at 10°C and 70°C. 

More importantly, while it is very difficult to control the nature of the site t)rpes on conventional heterogeneous Ziegler-Natta catalysts, metallocene catalysts can be designed to synthesize PP with different chain microstructures. PP chains with atactic, isotactic, isotactic-stereoblock, atactic-stereoblock and hemiisotactic configurations can be produced with metallocene catalysts (Figure 2). It is also possible to s)mthesize PP chains that have optical activity by using only one of the enantiomeric forms of the catalyst. Additionally, copolymers of propylene, ethylene and higher a-olefins made with metallocene catalysts have random (or near random) comonomer incorporation and narrow chemical composition distributions (CCD). 

For the synthesis of stereospecific metallocene catalysts for propylene polymerization, C2 symmetric precursors are necessary to obtain a catalyst for isospecific polymerization, and C5 symmetric precursors to produce a catalyst for syndiospecific polymerization. Asymmetric precursors can be used to synthesize metallocene catalysts that produce hemiisotactic and isotactic-stereoblock PP. Farina et al. [7] have proposed an useful classification of metallocene catalysts based on their symmetry (Figure 3). 

Figure 2.19 depicts several reported hemiisoselective fluorenyl-containing metallocenes. The parent catalyst, h-1, appears to be the most hemiisoselective with [m] = 49.6% at Tp = 20 C. Any deviation in catalyst stracture tends to effect less hemiisotactic polymers. For example, ethyl substitution (h-S) instead of methyl substitution (h-l) results in a measurable increase in syndiose-lectivity. The indenyl variant h-4 commits a considerable fraction of stereoerrors ([mmrm] + [rrmr] + [mrmr] = 7.5%at7 p = 10°C). " In addition, the hafnium complex h-l-Hf is more isoselective than the parent zirconium complex h-1 under identical conditions ([mmmm] = 24.2% versus [mmmm] = 15.1% at Tp = 60 °C). ... 

FIGURE 2.20 Metallocenes used for the preparation of isotactic-hemiisotactic polypropylene. The [m] dyad fractions are for MAO-cocatalyzed polymerizations performed in liquid propylene at 0 °C. Hafnocene [m] values are given in parentheses. 

FIGURE 2.26 A C NMR pentad analysis of five different syndiotactic-hemiisotactic polypropylenes reveals a strong dependence on the steric substitution of the metallocene precatalyst. A consistently low level of stereoerror pentads, mmrm, rrmr, andmrmr, is observed. Flu = fluorenyl Oct=octamethyloctahydrodibenzofluorenyl. 

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. 

Miller, S. A. Bercaw, J. E. Isotactic-hemiisotactic polypropylene from Cj-asymmelric ansa-metallocene catalysts A new strategy for the synthesis of elastomeric polypropylene. Organometallics 2002, 21, 934-945. 

Table 12.3 summarizes stereochemical control in propylene homopolymerization for different catalytic systems. Metallocenes are the only SSCs currently able to produce the entire variety of possible propylene sequence microstructures (isotactic, syndiotactic, and hemiisotactic). As mentioned above. 

Further support for this mechanism was provided by Ewen in the form of a catalyst which polymerizes propylene to hemiisotactic polypropylene. The metallocene shown in Scheme IV has two different coordination sites, one which is isospecific and one which is aspecific.51 When used for propylene polymerization, the alternation between iso- and aspecific sites results in a hemiisotactic polymer (Scheme IV). The polymer was readily characterized due to the pioneering work of Farina, who independently prepared this material previously. The rational synthesis of isotactic, syndiotactic, and hemiisotactic polyolefins represents a crowning achievement in the application of transition metal catalysts in stereocontrolled reactions. 

A model assuming that Cp substituents distal to the bridge experience steric non-bonded contacts with the monomer methyl group, perhaps mediated by the chain end, accounts for the specificity of the chiral metallocenes that produce isotactic, atactic, syndiotactic, hemiisotactic, and random or block cotactic polypropylenes. The tacticities as well as the microstructures of these polymers are accomodated by these simple concepts, the geometry of the metallocene ligands, and by generally accepted fundamental aspects of the polymerization and stereochemical control mechanisms. 

Various chiral metallocenes with methylaluminox-ane cocatalysts (MAO) are specific for the preparation of isotactic, syndiotactic, atactic and hemiisotactic polypropylenes and higher polyolefins [72]. In hemiisotactic polypropylene (hit-PP) every other methyl is placed isotactically, the remaining methyls randomly. This type of polypropylene has served as a keypoint in the elucidation of the polymerization mechanism with metallocene catalysts [73]. The chiral metallocene catalysts are not as stereorigid as the conventional heterogeneous systems. Consequently under some... 

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