In Situ Activation: Chances and Limitations to Form Ultrahigh Molecular Weight Syndiotactic Polypropylene with Metallocene Dichlorides

Abstract

Fluorenyl cyclopentadienyl C s -symmetric ansa-metallocene (M = Zr, Hf) complexes I−IV featuring different bridging motifs (C and Si) were synthesized and subsequently examined in the syndiospecific coordinative polymerization of propylene. All complexes were activated in situ with triisobutylaluminum (TIBA) and [Ph 3 C][B(C 6 F 5 ) 4 ] (TrBCF) in order to highlight the benefits of this in situ activation, resulting in a significantly higher molecular weight and a significantly increased productivity in comparison to methylaluminoxane (MAO). The isopropylidene-bridged zirconocene Ia (ZrCl 2 [Me 2 C(η 5 -Flu)(η 5 -Cp)]) exhibited a high productivity (80000 kg PP (mol cat h) −1 ) and stereoregularity ([rrrr] up to 93%) with a moderate molecular weight of polypropylene (PP), whereas the polymerization with the corresponding hafnocene Ib (HfCl 2 [Me 2 C(η 5 -Flu)(η 5 -Cp)]) resulted in a lower productivity and stereoregularity but yielded ultrahigh molecular weight polypropylene (M w = 1100 kg mol −1 ). The backbone in II (HfCl 2 [Ph 2 C(η 5 -Flu)(η 5 -Cp)]) was associated with a higher productivity and molecular weight, while the syndiotacticity was not affected. In contrast, SC-XRD revealed a reduced dihedral angle and D value for Si-bridged hafnocenes III (HfCl 2 [Me 2 Si(η 5 -Flu)(η 5 -Cp)]) and IV (HfCl 2 [Ph 2 Si(η 5 -Flu)(η 5 -Cp)]), resulting in a more constrained geometry of the catalyst. This led to an increased molecular weight, while the productivity as well as the syndiotacticity decreased due to these structural parameters. Activation of Ib and II−IV with n-octyl-modified methylaluminoxane (MMAO) resulted in a lower molecular weight of the polymer, because the transfer of the growing polymer chain onto the Al center was enhanced. Nevertheless, the stereoregularity of MMAO-activated catalysts was slightly increased, probably due to a coordination of the ill-defined MMAO anion during the polymerization. DSC analysis exposed for syndiotactic polypropylene (sPP) produced by the highly active zirconocene Ia a defined melting transition (T m up to 145.2 °C), whereas TIBA/TrBCF-activated hafnocenes Ib and II−IV gave polymers with no observable T m value.

Tailored Syndiotactic Polypropylene Feedstock Material for Laser-Based Powder Bed Fusion of Polymers: Material Development and Processability

“…39 However, these properties were only obtained for a medium-high molecular weight of 140 kg•mol −1 �nevertheless, the molecular weight may be beneficial for the anticipated enhanced mechanical properties by entanglement formation. 39 Zirconocene I was isolated in a two-step synthesis starting with fluorene as the basic moiety of the catalyst system (see Scheme 1). However, prior to polymerization, zirconocene I required activation to form the catalytic active cationic species.…”

“…Zirconocene I was previously proven to feature several advantages, if suitable polymerization conditions were applied: high productivity, a wide processing window with a peak melting temperature T m of 145.2 °C, and a suitable particle size with no need for further processing . However, these properties were only obtained for a medium-high molecular weight of 140 kg·mol –1 nevertheless, the molecular weight may be beneficial for the anticipated enhanced mechanical properties by entanglement formation . Zirconocene I was isolated in a two-step synthesis starting with fluorene as the basic moiety of the catalyst system (see Scheme ).…”

Perfectly Isotactic Polypropylene upon In Situ Activation of Ultrarigid meso Hafnocenes

“…With the isomerization using 1.00 equiv TIBA at 100 °C being more effective, we established a simple protocol for polymerizations using meso ‐ I to yield iPP with enhanced productivity. Initially, meso ‐ I was isomerized at 100 °C overnight—the isomerization was confirmed via NMR spectroscopy—and subsequently, the established in situ activation involving the pre‐activation with 200 equiv TIBA was executed [10b] . This procedure yielded perfectly isotactic PP with molecular weights, molecular weight distributions and productivities comparable to pure rac ‐ I for a meso/rac mixture while the high activity and thus temperature increase for pure meso ‐ I led to a broadened molecular weight distribution (see Table 1, entries 4 and 5).…”

Perfekt Isotaktisches Polypropylen nach In Situ Aktivierung von Ultrarigiden meso Hafnocenen

“…Zunächst wurde meso ‐ I über Nacht bei 100 °C isomerisiert – die Isomerisierung wurde über NMR Spektroskopie bestätigt – und anschließend wurde die etablierte in situ Aktivierung mit 200 Äquiv. TIBA durchgeführt [10b] . Dieses Verfahren lieferte für ein meso/rac Gemisch perfekt isotaktisches PP mit Molekuargewichten, Polydispersitäten und Produktivitäten vergleichbar zu reinem rac ‐ I , während eine hohe Aktivität und der damit einhergehende Temperatursprung für reines meso ‐ I zu einer verbreiterten Polydispersität führte (siehe Tabelle 1, Einträge 4 und 5).…”