High-Throughput Discovery and Optimization of Hafnium Heteroaryl-amido Catalysts for the Isospecific Polymerization of Propylene

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Abstract

New nonmetallocene catalysts for the isospecific polymerization of propylene were discovered using high-throughput screening. These hafnium heteroaryl-amido catalysts are structural analogues of the highly successful hafnium pyridyl-amido catalysts. Initial primary and secondary screening of a library of five thiazole-amine ligands with Zr and Hf with propylene resulted in the discovery of highly active catalysts with high activity, high molecular weight capability, and stereoinduction that ranged from low to high, depending on the ligand and activation conditions. Subsequent screening revealed that bulky aryl substituents at the N-aryl (R1), thiazole ortho position (R2) and the bridge position (R3) were preferred for high activity, although stereoinduction was lower than that observed for the pyridyl-amidos. The thiazole-amine catalysts containing a 2-methoxyphenyl substituent at R2 and activation conditions that included B(C6F5)3 formed highly isotactic polypropylene with low catalyst activity. Subsequent complexation and reactivity studies revealed that in the presence of B(C6F5)3 and trialkylaluminum reagent, demethylation of the bound ligand ether occurred, resulting in binding of the ligand in a tridentate [O–, N, N–] fashion. The resulting thiazole-amido phenoxide complex was prepared independently and is a highly stereospecific catalyst for the polymerization of propylene, although it exhibited low activity relative to the other thiazole-amido catalysts. This structural clue suggested that expansion of the ring formed upon orthometalation at the R2 position might be advantageous for stereocontrol. A library of imidazole-amine and thiazole-amines containing 2-methylbenzothiophene (2-MeBZT) and 2-ethylbenzofuran substituents (2-EtBZF) at R2 was prepared, complexed to Hf, and screened with propylene at 110 °C. Many of the catalysts showed high activity, high molecular weight capability, and very good stereoinduction.

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