Highly Syndiotactic PPs

An FI catalyst normally assumes a C2-symmetric trans-O, ds-N, and d.s-Cl configuration as the predominant isomer. In addition, DFT calculations suggest that a catalytically active species derived from an FI catalyst favors a C2-symmetric configuration with a trans-O, cis-N, and d.v-polymer chain/coordinated olefin arrangement. Thus, FI catalysts have been targeted as catalysts capable of producing iPP via a site-control mechanism. 

With MAO activation, Zr- and Hf-FI catalysts 1 and 3 exhibit fairly high reactivity toward propylene and produce propylene oligomers [64, 65], Conversely, the corresponding Ti-FI catalyst/MAO 2 forms semicrystalline PP (1 °C polymerization), which displays a peak melting temperature of 97 °C, indicative of the formation of a stereoregular polymer. To our surprise, microstructural analysis by 13C NMR indicates that the resultant polymer is syndiotactic (rr 19%), and that a chain-end control mechanism is responsible for the observed stereocontrol, regardless of the C2 symmetric catalyst ([28] for the first report on syndiospecific propylene 

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The production of highly isotactic PPs with Zr- and Hf-FI catalysts//-Bu3Al/ Ph3CB(C6F5)4 (phenoxy-amine complexes site-controlled polymerization with 1,2-insertion) is in sharp contrast to that of highly syndiotactic PPs with Ti-FI cata-lysts/MAO (phenoxy-imine complexes chain-end controlled polymerization with 2,1-insertion), which will be described later [64]. 

Additionally, research on FI catalysts has provided useful information on high catalytic activity, the formation of highly syndiotactic PPs with C2-symmetric catalysts, the origin of highly controlled living olefin polymerization, and the high incorporation capability for higher a-olefins. 

FIGURE 4.13 Expanded pyrograms for the Cjo region for the highly syndiotactic PPs prepared with metallocene (ML) and Zieglar-Natta (ZN) type catalyst. 

Highly isotactic PP can be produced with C2-sym-metric metallocene catalysts. In such isotactic propagation, monomer insertion is to take place on two active sites on the metal center alternately, and the stereochemistry on one site must be the same as that on the other site. Highly syndiotactic PP has been... 

It was later found that the incorporation of fluorinated iV-aryl moieties into the bis(phenox5dmine) ligand framework could provide catalyst precursors for the syndiotactic and living polymerization of propylene. At 0°C, 50/MAO (Figure 15) produced highly syndiotactic PP ([rrrr] = 0.96), which had a peak melting temperature of 148 The polymerization... 

Complex 6.21 represents a class of precatalysts that are active for both ethylene and propylene polymerization reactions. When Ar is phenyl, highly syndiotactic PP of low molecular weight is obtained. However, when Ar is pentafluro phenyl, highly syndiotactic living PP is the product. The same catalyst can also produce ethylene-propylene block copolymers. 

By referring to the symmetries of 6.12 and 6.13, the effects of the catalyst structures on the microstructures of PP could be explained. Complex 6.12 gives highly isotactic PP, whereas with 6.13 highly syndiotactic PP is obtained. It is important to note that during chain propagation, the polymer chain and the alkene keep exchanging places between two adjacent coordination sites (compare 6.26 with 6.28). 

By contrast, the synthesis of syndiotactic PP, s-PP, is generally catalyzed by Cs-symmetry ansa- metallocenes. For example, (16)/MAO affords PP with a pentad (rrrr) content of 86% at 25 °C.77 The stereoselectivity is highly sensitive to ligand variation. For example, substitution at the 3-position of the Cp ring with a methyl group affords heterotactic PP,78 whilst the Bu analog favors i-PP production.50,75,79... 

Single crystals with a Tm of 423 K have been produced from low-density polyethylene (ldpe). Isotactic PP crystals have a Tm of 444 K and syndiotactic PP has a Tm of 411 K, whereas atactic PP is amorphous and has a Ts of 255 K. Isotactic polyolefins with pendant groups, such as polyhexene, have high Tm values. Random copolymers of ethylene and propylene are amorphous, but block copolymers of these monomers are crystalline. 

Isotactic polypropylene does not exist in the two zig-zag and helix conformations, but syndiotactic PP is a good candidate for this search. Indeed, according to the preparation procedure of the polymer ( ), it can exist in a helix or zig-zag planar conformation. Syndio-PP films were prepared following both ways, their conformation checked by IR, and studied by XPS. Their valence band spectra again show distinct differences in the C2s band (Figure 16) for zig-zag PP that is probably highly amorphous, the C-C band width increased by about leV, whereas the bonding subband increased in intensity and became more structured ( ). 

With the replacement of Cp by a 9-fluorenyl moiety, the metallocene promotes syndiotactic polymerization of propylene under site control. Syndiotactic PP with (rrrr) as high as 0.77 was obtained with Me2Si(9-Flu) (N-t-Bu)ZrCl2/MAO catalyst. ° The stereoselectivity is due to the (pseudo)Cs-symmetry of the catalytic complex, and the stereocontrol mechanism is analog to that for the Cs-symmetric awM-metallocenes. 

The first metallocene -and the first catalyst in general- able to produce highly syndiotactic polypropene (s-PP), was the Q-symmetric Me2C (Cp) (9-Flu)-ZrCp (Cs-1 in Chart 18)." The behavior of this catalyst and the characterization of 5 pp397-407 have been extensively reviewed. A number of studies on the thermal behavior, crystal structures, and morphology of 5-PP have appeared in the litera-... 

Fig. 14 Three-site model proposed by Busico et al. [63] (reproduced frran [75]). Active site models relevant to the productirai of (a) syndiotactic (or atactic), (b) isotactoid (or isotactic), and (c) highly isotactic PP. A growing chain and propylene monomer occupy the chained squares. M = Ti, Mg, or Al L = Cl, donor, or alkylaluminum moiety [64]...

FIGURE 4.8 Typical high-resolution hydrogenated pyrograms of (A) isotactic PP (ZN-Ij), (B) atactic-PP (ML-A), and syndiotactic-PP (ML-S,). i, s, and h represent isotactic, syndiotactic, and heterotactic products, respectively [22]. 

When polymer scientists discuss the stereochemical features of PP, they usually discuss it in terms of tacticity or percent tacticity of polypropylene, and in the marketplace the term polypropylene is generally used to refer to a material that has high tacticity, meaning high isotactic content. The high tacticity PP materials have desirable physical, mechanical, and thermal properties in the solid state. Atactic material is a soft, sticky, gummy material that is mainly used in sealants, caulks, and other applications where its stickiness is desirable. Syndiotactic PP, not a large volume commercial material, is far less crystalline than isotactic PP. 

After 1980 continuous growth was recorded with the development of a number of high performance polymers that could compete with traditional materials such as polyetheretherketone, polyetherimide (1982), polyamide 4,6 (1987), syndiotactic PS (1989), metallocene polyolefins, polyphthalamide (1991), styrene-ethylene copolymer, syndiotactic PP in 1992 and nanocomposites [15]. 

Syndiotactic PP with high molecular weight, narrow MWD and high productivity can be synthesized with C5 symmetric metallocenes such as... 

Isotactic polypropylene is currently of great industrial interest (the degree of crystallisation is 40 to 60 %). Non-crystalline atactic PP is used as elastomer components in PP copolymers. The production of syndiotactic PP has only recently become possible through the progress made in catalyst research. It is characterised by a high flexibility, though it crystallises slower and to the same extent as iPP. 

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