Polypropylenes hemiisotactic

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. 

A deeper understanding of the NMR spectrum of polypropylene was obtained through the use of hemiisotactic polypropylene, 58, already mentioned (101). The strictly limited number of sequences (see Tables 2 and 5) greatly simplifies the spectrum, eliminating the major part of peak overlapping, especially in the methyl region furthermore, its high solubility permits the use of... 

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. 

As noted in Section 22.1.1, hemiisotactic polypropylene is a polymer in wliich every other methyl group off the main chain has the same stereochemical configuration, and the intervening methyl groups have a random stereochemical configuration. The NMR spectrum of hemiisotactic PP matches that of the material formed independently by hydrogenation of poly(trflns-2-methylpentadiene). ... 

Fig. 12. Hemiisotactic polypropylene. (Wavy lines indicate methine units of indeterminate chirality.)...

FIGURE 2.13 The pentad distribution of polymer formed by (CH3)2C(3-fert-butyl-4-methyl-C5H2)(Ci3Hg)ZrCl2/MAO (ih-S) at Tp = 20 °C is poorly described by a one-site enantiomorphic site control model. A superior fit is obtained with a two-site model that approximates hemiisotactic polypropylene. (Reprinted with permission from Miller, S. A. Bercaw, J. E. OrganometalUcs 2006, 25, 3576-3592. Copyright 2006 American Chemical Society.)... 

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... 

Naturally, this catalyst is not perfect and stereoerrors are invariably present in the formed polymer. The principal stereoerrors arise from enantiofacial misinsertion at the more stereoselective site and site epimerization. Such mistakes give rise to mr and rm triads and m and r dyads in an otherwise consistent procession of mm and rr triads (Figure 2.17). Thus, perfect hemiisotactic polypropylene... 

FIGURE 2.17 Enantiofacial misinsertion and site epimerization stereoerrors give rise to pentads containing isolated m and r dyads (in bold) such pentads are forbidden in perfectly hemiisotactic polypropylene. 

FIGURE 2.18 Ideal (bold) and actual pentad distributions for hemiisotactic polypropylene obtained with (CH3)2C(3-CH3-C5H3)(Ci3Hg)ZrCl2/MAO (/t-l/MAO) at varying polymerization temperatures. 

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. 

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