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iPP–sPP Stereoblocks or Blends? Studies on the Synthesis of Isotactic–Syndiotactic Polypropylene Using Single C1-Symmetric {Ph2C-(Flu)(3-Me3Si-Cp)}ZrR2 Metallocene Precatalysts

Abstract : Single isoselective C1-symmetric {Ph2C-(Flu)(3-Me3Si-Cp)}ZrR2 (R = Cl (1b), Me (2b)) metallo­cene precatalysts, upon activation with MAO or [PhNMe2H]+[B(C6F5)4]−/iBu3Al, produce isotactic polypropylene (PP) (up to 80% mmmm), which is more enriched with syndiotactic pentads (0.9–2.2% rrrr) than the statistical model assumes. Similar or even more syndiotactically enriched PPs (3.0–25.4% rrrr) are obtained using combinations of 1b or 2b with Cs-symmetric syndioselective {Ph2C-(Flu)(Cp)}ZrR2 (R = Cl (1a), Me (2a)) metallo­cenes. While NMR spectroscopy, differential scanning calorimetry (DSC), and gel per­meation chromatography (GPC) analyses do not provide explicit evidence on the nature of these polymers, temperature rising elution fractionation (TREF) unambiguously identifies them as blends of iPP and sPP polymers and not discrete iPP−b−sPP stereoblock copolymers. The formation of sPP fractions from the a priori isoselective 1b and 2b systems is attributed to a desilylation side reaction, which takes place in the precatalyst synthesis step and possibly also during the polymerization, resulting in the generation of the Me3Si-free syndioselective catalytically active species. Activation of 2a with B(C6F5)3 and [PhNMe2H]+[B(C6F5)4]− selectively leads to the corresponding cationic complexes 3a and 4a, as confirmed by NMR spectroscopic methods and the solid-state structure of {Ph2C-(Flu)(Cp)}ZrMe(μ-Me)B(C6F5)3 (3a). On the other hand, the activation of 2b with the same molecular activators is less selective, and the formation of the corresponding cationic products 3b and 4b along with unidentified impurities is monitored by NMR spectroscopy. A possible chain-shuttling process between isoselective and syndioselective active species, mediated by Me3Al or iBu3Al, appears to be inoperative under given conditions.
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https://hal-univ-rennes1.archives-ouvertes.fr/hal-01114327
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Submitted on : Monday, February 9, 2015 - 11:29:00 AM
Last modification on : Friday, July 10, 2020 - 4:21:46 PM

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Gabriel Theurkauff, Thierry Roisnel, Jeroen Waassenaar, Jean-François Carpentier, Evgeny Kirillov. iPP–sPP Stereoblocks or Blends? Studies on the Synthesis of Isotactic–Syndiotactic Polypropylene Using Single C1-Symmetric {Ph2C-(Flu)(3-Me3Si-Cp)}ZrR2 Metallocene Precatalysts. Applied Macromolecular Chemistry and Physics, 2014, 215 (21), pp.2035-2047. ⟨10.1002/macp.201400338⟩. ⟨hal-01114327⟩

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