Isotactic-hemiisotactic polypropylene

A stereochemical analysis has predicted the distribution of isotactic stereoblocks present in isotactic-hemiisotactic polypropylene. For an ideal case (without site epimerization and with perfect enantiofacial selectivity at the more stereoselective site), the derived equations depend only on... 

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 2.21 A NMR pentad analysis of nine different isotactic-hemiisotactic polypropylenes reveals an increase in catalyst isoselectivity with increasing 3-substituent (R) size, but a consistently low level of stereoerror pentads, mmrm, rrmr, and mrmr. 

FIGURE 2.22 The [m] dyad fraction and the polymer melting temperature of isotactic-hemiisotactic polypropylenes can be controlled by manipulation of the catalyst steric framework. 

Miller, S. A. Bercaw, 1. E. Isotactic-hemiisotactic polypropylene from Ci-symmetric ansa-metaUocene catalysts A new strategy for the synthesis of elastomeric polypropylene. Organometallics 2002, 21, 934-945. 

An analogous method was used to obtain a new class of macromolecular stereoisomers The hemitactic polymers (99-101). This term refers to a head-to-tail vinyl polymer in which the tertiary carbon atoms constitute two distinct series one, which includes monomer units 1,3,5, 7,..., possesses strict steric regularity, whereas the other, with monomer units 2, 4, 6, 8,. .., is completely at random. In such polymers only one in every two tertiary atoms is influenced by an ordering rule 58 and 59 show the schematic structure of the hemiisotactic and hemisyndiotactic polymers where the white circles indicate the positions of disordered s ubstituents. The hemiisotactic polypropylene was obtained by Farina, Di Silvestro, Sozzani and Savar6 (99, 101) by nonste-reoselective reduction,of. isotactic frans-l,4-poly-2-methylpentadiene. 

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]. 

Substituents in the distal position of the cyclopentadienyl ring in these cyclopentadienyl-fluorenyl catalysts has a profound effect on the polymer product produced. A small substituent like methyl produces a novel material, hemiisotactic polypropylene, in which methine carbons of specific conformation alternate with methine carbons of random conformation. A larger substituent hke r-butyl makes isotactic polypropylene (Figure 6). 

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. 

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.

Small structural changes in the catalysts can produce even more remarkable results. Substitution of a methyl group in the cyclopentadienyl ring of iPr(CpHu)ZrCl2 led to the produetion of hemiisotactic polypropylene. In this structure, every second stereogeruc carbon has an isotactic configuration, and the centers in between have atactic placements. Thus, there is order and disorder along the chain in a well-defined manner. 

Farina et al. first prepared hemiisotactic polypropylene in 1982 through the hydrogenation of isotactic poly(2-methylpenta-l,3-diene). The first direct, stereoselective synthesis of hemiisotactic polypropylene was reported by Ewen, et al. in 1991" with the methyl-substituted zirconocene (CH3)2C(3-CH3-C5H3)(Ci3H8)ZrCl2 (h-1) activated with Scheme 2.4 depicts... 

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. 

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. 

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

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... 

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