Polypropylene syndiospecific

As stated above, we postulated that fast, reversible chain transfer between two different catalysts would be an excellent way to make block copolymers catalytically. While CCTP is well established, the use of main-group metals to exchange polymer chains between two different catalysts has much less precedent. Chien and coworkers reported propylene polymerizations with a dual catalyst system comprising either of two isospecific metallocenes 5 and 6 with an aspecific metallocene 7 [20], They reported that the combinations gave polypropylene (PP) alloys composed of isotactic polypropylene (iPP), atactic polypropylene (aPP), and a small fraction (7-10%) claimed by 13C NMR to have a stereoblock structure. Chien later reported a product made from mixtures of isospecific and syndiospecific polypropylene precatalysts 5 and 8 [21] (detailed analysis using WAXS, NMR, SEC/FT-IR, and AFM were said to be done and details to be published in Makromolecular Chemistry... 

Figure 1.10 Preinsertion intermediates for secondary propene insertion into primary polypropylene chain for (a) isospecific model complex based on (R, R)-coordinatedisopropyl-bis(l-indenyl) ligand and (b) syndiospecific model complex based on isopropyl(cyclopentadienyl-9-fluorenyl) ligand for R chirality at metal atom. Stereoselectivity of isospecific model site is in favor of opposite monomer prochiral faces for primary and secondary insertions (cf. Figures 1.4 and 1.10a). Stereoselectivity of syndiospecific model site is in favor of same monomer prochiral face for primary and secondary insertions (cf. Figures 1.6a and 1.1 Ob).

Such a catalytic centre is chiral, but interconversion between enantiomeric complexes is assumed to occur, after each insertion step, when the V atom is pentacoordinated. The analysis of the non-bonded interactions at the catalytic site suggests an si insertion of the last monomer unit (which generates what is called an si chain) to favour the formation of a A complex. This complex should in turn favour the re coordination and insertion of the successive monomer unit, thus ensuring syndiospecific polypropylene chain propagation. In other words, the chirality of the growing chain (expressed by the configuration of the last inserted monomeric unit) imposes the chirality of the coordinating monomer... 

The two positions of the growing polypropylene chain and of the coordinated monomer molecule are enantiotopic therefore, if enantioselectivity exists, the model is syndiospecific [23],... 

Figure 3.41 Models for the primary insertion of propylene into a polypropylene growing chain in a syndiospecific polymerisation with the Me2C(Cp)(Flu)ZrX2-based catalyst. The growing alkyl chain occupies an open sector of the ligand framework propylene enters the reaction complex with its methyl substituent away from the C/j atom of the last monomeric unit in the chain (the monomer methyl group is directed towards the mouth of the two O, rings). For the sake of clarity, only the C C bonds are sketched for the n ligands. O - Zr O - C or CH3 o H. Reproduced by permission from Ref. 30. Copyright 1995 Wiley-VCH Weinheim...

Contrary to i-PP, syndiotactic polypropylene (s-PP) has received little attention. This is due to the poor syndiospecificity of the Ziegler-Natta catalyst system. However, in the last few years, new metallocene catalysts have been developed, which allow very high syndiospecificity [15, 16]. 

Czv-Symmetric Catalysts. Syndiotactic polymers have been formed using metallocene catalysts where the polymer chain end controls the syndiospecificity of olefin insertion. Resconi has shown that Cp 2MCl2 (M = Zr. Hf) derived catalysts produce predominantly syndiotactic poly(l-butene) with an approximate 2 kcal/mol preference for syndiotactic versus isotactic dyad formation." At —20 °C. Cp 2HfCl2/MAO produces poly(l-butene) with 77% rr triads. Pellecchia had reported that the diimine-ligated nickel complex 30 forms moderately syndiotactic polypropylene at —78 °C when activated with MAO ([rr] = 0.80)." " Olefin insertion was shown to proceed by a 1.2-addition mechanism." in contrast to the related iron-based systems which insert propylene with 2.1-regiochemistry. ... 

The pooled library of salicylaldimine-titanium products was combined with MAO and 150 ml toluene in a pressure vessel, and screened at 20 °G under 2.7 atm of propylene for 6h. Although 90% of the polymers produced were determined to be atactic through extraction experiments with diethylether, the remaining 10% of the polymers produced were found to contain insoluble syndiotactic polypropylene. Small subsets of the complex libraries were then resynthesized and screened in order to ascertain the particular metal-ligand combination responsible for the stereocontrol. The authors concluded the study by identifying and isolating a new syndiospecific propylene polymerization catalyst 2. 

In 1962, Natta and Zambelli reported a heterogeneous, vanadium-based catalyst mixture which produced partially syndiotactic polypropylene at low polymerization temperatures.45 jjie regiochemistry of the insertion was determined to be a secondary insertion of propylene, and a chain-end control mechanism determined the syndiospecificity of monomer insertion. This system suffered from both low activity and low stereospecificity. 

In another significant development, Ewen reported a metallocene catalyst precursor (IV) that is highly active for the syndiospecific polymerization of propylene and higher aliphatic a-olefins (Fig.6). The degree of syndiospecificity for this catalyst is extraordinarily high and has provided for the first time a catalyst which is capable of producing highly syndiotactic crystalline polypropylene. 

The NMR spectrum for polypropylene produced at 50°C with this catalyst reveals a high degree of stereocontrol (81% rrrr pentads). Moreover, analysis of the stereochemical defects (predominantly rmmr pentads) were indicative of a site control mechanism. For a site control mechanism to operate in syndiospecific polymerization, the olefin must alternately bind to coordination sites with opposite enantioface selectivity. The model for this polymerization is shown in Scheme HI. 

Fig. 8 Representation of mechanism of syndiospecific polymerization and formation of syndiotactic polypropylene (top). Fischer projection of a perfect syndiotactic chain sequence (bottom)...

To obtain highly isotactic polypropylene chiral metallocenes have to be used. In this work it is shown that chirality is a necessary but not sufficient condition for high isospecifity. Variation of metallocene structure retaining chirality can lead to strongly reduced isospecifity. Breakdown of stereospecifity by slight structural variation can also be demonstrated with syndiospecific metallocene catalysts. 

In 2004, Marks and coworkers reported the synthesis of a novel polynuclear perfluoroarylaluminum cocatalyst 70, which was subsequently reacted with an ansa-zirconocene precursor [148]. NMR data of the resulting mixture are consistent with the formation of two products i.e., monomeric complex 71 and the p-Me Zr dimer 72 (Scheme 30), indicating that the A1 species 70 acts as an alkyl abstracting agent. The in situ generated 71/72 mixture efficiently and stereoselectively polymerizes propene for the production of highly syndiospecific polypropylene [activity 7.9 x 10 g polymer/((mol of catalyst) atm. h) in 3 min at 25 C, 89.3% rrrr] [148]. 

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