Methylaluminoxane’s Molecular Cousin: A Well-defined and “Complete” Al-Activator for Molecular Olefin Polymerization Catalysts

Francesco Zaccaria,Cristiano Zuccaccia,Alceo Macchioni,Antonio Vittoria,Vincenzo Busico,Peter H M Budzelaar,Christian Ehm,Roberta Cipullo

doi:10.1021/acscatal.0c05696

Francesco Zaccaria, Cristiano Zuccaccia + Show 6 more

Open Access

https://doi.org/10.1021/acscatal.0c05696

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Abstract

Catalytic activity in olefin polymerization depends not only on the catalyst but also, crucially, on activator/alkylator/scavenger “packages.” Along with binary mixtures containing Lewis or Brønsted acids and Al-alkyl systems, methylaluminoxane (MAO), a still ill-defined oligomeric compound, is the only single-component cocatalyst known to fulfill all three roles effectively and simultaneously. Herein, we report a simple molecular alternative, Al-H-Al+[B(C6F5)4]−, an unusual borate salt containing a homodinuclear Al-cation (Al-H-Al+ = [iBu2(DMA)Al]2(μ-H)+). Unlike the simpler [AliBu2]+[B(C6F5)4]−, this species is easily synthesizable and stable at room temperature. Importantly, Al-H-Al+[B(C6F5)4]− can be used as a stand-alone cocatalyst for molecular olefin polymerization catalysis, representing an unprecedented molecular activator able to completely activate dichloride metallocene and prototypical post-metallocene precatalysts. Furthermore, spectroscopic and polymerization studies suggest that Al-H-Al+ is the true activating species formed in situ in the binary cocatalyst [PhMe2NH]+[B(C6F5)4]−/AliBu3. As little as 50 equiv of Al-H-Al+[B(C6F5)4]− are required for efficient catalyst activation and impurity scavenging, orders of magnitude below the amounts usually required with MAO or AliBu3. The high, yet “tamed,” Lewis acidity of cationic Al-H-Al+ is likely responsible for the increased scavenging ability. Unlike MAO, the well-defined structure of Al-H-Al+[B(C6F5)4]− offers easy avenues for further tuning, making it the prototype of a promising cocatalyst family.

Highlights

Since its discovery in the 1950s,1,2 catalytic olefin polymerization has been intimately tied to the dual performance of a precatalyst and a cocatalyst
The identification of efficient activators leading to high activities/productivities has been essential for the success story of molecular olefin polymerization catalysts
No reproducibility or control issues were noted under these conditions. This is a far cry from the 5 × 103 equiv of Al usually used for TTB/TiBA activation[27] or the up to 104−105 equiv required by certain metallocene catalysts with MAO.[13,27]

Summary

■ INTRODUCTION

Since its discovery in the 1950s,1,2 catalytic olefin polymerization has been intimately tied to the dual performance of a precatalyst and a cocatalyst. These results indicate that Al-HAl+[B(C6F5)4]−, generated in situ, is the species responsible for the cocatalytic properties of AB/TiBA. Preliminary tests were carried out to explore the general applicability of Al-H-Al+[B(C6F5)4]− This cocatalyst was used to activate L2ZrCl2 in ethene/1-hexene copolymerization, that is, in the type of process for which molecular catalysts are more commercially relevant.[5] The results, reported in the Supporting Information, show that effective catalyst activation and polymerization can be achieved in copolymerization and at moderately high temperatures (100 °C). This is a far cry from the 5 × 103 equiv of Al usually used for TTB/TiBA activation[27] or the up to 104−105 equiv required by certain metallocene catalysts with MAO (entry 15).[13,27]

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES