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

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.

研究人员通过高通量筛选（high-throughput screening）发现了用于丙烯（propylene）等规聚合（isospecific polymerization）的新型非茂金属催化剂（nonmetallocene catalysts）。该类铪基杂芳基酰胺基催化剂（hafnium heteroaryl-amido catalysts）是当前应用广泛的铪基吡啶基酰胺基催化剂（hafnium pyridyl-amido catalysts）的结构类似物。研究人员对包含5种噻唑胺配体（thiazole-amine ligands）的库进行初步初筛与复筛，将其与锆（Zr）、铪（Hf）络合后用于丙烯聚合，最终得到活性优异的催化剂体系：这类催化剂不仅具备高聚合活性与高分子量调控能力，其立体定向诱导（stereoinduction）性能还可随配体结构与活化条件变化，覆盖低至高的调控范围。后续筛选结果表明，尽管该类催化剂的立体定向诱导性能略逊于吡啶基酰胺基催化剂，但在N-芳基位点（R1）、噻唑邻位（R2）与桥联位点（R3）引入大位阻芳基取代基（bulky aryl substituents）时，可获得更高的聚合活性。当R2位点带有2-甲氧基苯基取代基，且活化条件使用三(五氟苯基)硼烷（B(C6F5)3）时，所得噻唑胺基催化剂可制备得到高全同立构聚丙烯（isotactic polypropylene），但催化剂活性较低。后续络合与反应性研究表明，在B(C6F5)3与三烷基铝试剂存在下，配体醚键会发生脱甲基化（demethylation）反应，最终使配体以三齿[O–, N, N–]配位模式结合。研究人员独立合成了对应的噻唑酰胺基苯氧络合物，该络合物是一种高立体专一性的丙烯聚合催化剂，但其聚合活性相较于其他噻唑酰胺基催化剂更低。该结构线索提示，在R2位点发生邻位金属化（orthometalation）时形成的环系拓展，或许有利于提升立体定向调控效果。研究人员构建了一类咪唑胺配体（imidazole-amine）库与噻唑胺配体库，在R2位点引入2-甲基苯并噻吩（2-MeBZT）与2-乙基苯并呋喃（2-EtBZF）取代基，将其与铪络合后，在110 ℃下用于丙烯聚合筛选。结果显示，多数催化剂展现出高聚合活性、高分子量调控能力与优异的立体定向诱导性能。