Syndiotactic-hemiisotactic polypropylene

FIGURE 2.25 Metallocenes used for the preparation of syndiotactic-hemiisotactic polypropylene. The [m] dyad fractions are for MAO-cocatalyzed polymerizations performed m liquid propylene at 0 °C. 

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. 

FIGURE 2.27 The [m] dyad fraction and the polymer melting temperature of syndiotactic-hemiisotactic polypropylenes can be controlled by manipulation of the catalyst steric framework. Flu = fluorenyl Oct = octamethyloctahydrodibenzofluorenyL... 

Figure 2.27 plots the observed polymer melting temperature as a function of the [m] dyad composition. The syndiotactic-hemiisotactic polypropylenes with [m] > 30% exhibit no strong endotherm through DSC and are essentially amorphous. Below [m] 25%, normal DSC traces are obtained and melting temperature is proportional to the syndiotacticity as quantified by 1 — [m]. For comparison, a data point for highly syndiotactic polypropylene from 5-90 (R = has been included. 

Figure 8 Segments of isotactic (a), syndiotactic (b), atactic (c), and hemiisotactic polypropylene (d) chains. Segments of erythro-6 soXaci c (e), f/ reo-diisotactic (f), and disyndiotactic (g) poly-diolefin chains. The modified Fischer projection is shown. For parts, (a)-(c) a zigzag representation is also reported.

The catalyst structures presented in the previous section have been used to prepare many types of polyolefins. The following several sections of this chapter provide an overview of the synthesis of polypropylenes containing various microstructures. These sections describe a selection of the catalysts that have been used to generate isotactic, syndiotactic, hemiisotactic, and stereoblock copolymers. Subsequent sections of this chapter describe catalysts that give rise to various copolymers of ethylene and propylene. Again, representative examples are provided, and the reader should consult other sources for comprehensive coverage of the synthesis and properties of each type of copolymer. ... 

In addition to polypropylenes in which the entire polymer chain consists of isotactic, syndiotactic, hemiisotactic, or atactic chains, polypropylenes in which the chain consists of alternating blocks of two microstructures have been prepared. Perhaps the most interesting and useful of these stereoblock polymers consists of the combination of a crystalline block, such as a unit of isotactic or syndiotactic polypropylene, and an amorphous block, such as atactic polypropylene. Polymers containing this combination of microstructures often behave as a thermoplastic elastomer and have properties... 

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... 

Using high-field NMR it is possible to detect the presence of these defects in isotactic and syndiotactic polypropylene prepared with special catalyst systems [20]. The 150-MHz C-NMR spectra (Fig. 7.14) are shown of the methyl region of four model polypropylene samples A-D dissolved in 1,1,2,2-tetrachloroethane-r/2 at 70°C. Polymer A was obtained in the presence of the catalyst system bis(cyclopentadienyl)-TiCh/methylalumoxane at low temperature (m 0.80) (see also Table 7.10). Polymer B was prepared with the catalyst system rac-ethylenebis(4,5,6,7-tetrahydro-l-in-denyl)ZrCl2 (m 0.70). Polymer C was prepared in the presence of (methyl)2C(cy-clopentadienyl)(9-fluorenyl)ZrCl2 (r 0.80). Polymer D is a hemiisotactic polypropylene obtained from the catalyst system (methyl)2C(cyclopentadienyl)(9-fluorenyl)-ZrCh/methylalumoxane. 

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. 

Propylene polymerisation with class I and class II catalysts gives rise, in principle, to atactic polypropylene, with class III catalysts to isotactic polypropylene [22] and with class IV catalysts to syndiotactic polypropylene [23], while for the less symmetric class V catalysts no general prediction is possible. In specific cases, however, isotactic, hemiisotactic, stereoblock isotactic attactic as well as syndiotactic polypropylenes can be obtained with class V catalysts, depending on their kind [107,112,116,124,127,137]. 

In which of the three groups (isotactic, syndiotactic or hemiisotactic) one may include syndioisostereoblock polypropylene ... 

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