o-Aryl Halogen Effect in Promoting The Synthesis of Linear Polyethylene with Varying Molecular Weights Catalyzed by Bulky Iron Complexes
The steric hindrance and electronic effects of ligands play a crucial role in the olefin poly-merization catalyzed by late transition metals.In this study,a series of o-aryl halogenated bis(imino)pyridyl ligands with bulky dibenzhydryl substituents and their corresponding iron catalysts(Fe1～Fe4)were prepared by acid-catalyzed condensation of 2-R-6-dibenzhydryl-4-methylaniline(A1～A4)with 2,6-diacetylpyridine.The obtained iron catalysts were comprehensively characterized by FT-IR,mass spectrometry,and elemental analysis.These iron catalysts exhibited high polymerization activity(＞106 g·mol-1·h-1),capable of producing highly linear polyethylene with high molecular weights(3.3～515.7 kg·mol-1)and broad molecular weight distributions(polymer dispersity index PDI=1.83～17.26).It's worth mentioning that the Fe4 catalyst can produce polyethylene with the highest molecu-lar weight(515.7 kg·mol-1)at 30 ℃ and exhibits the highest polymerization activity(9.96 ×106 g·mol-1·h-1)at 70 ℃.Notably,the polymerization temperature significantly affects both the mo-lecular weight and molecular weight distribution of the resulting polyethylene.Higher temperatures lead to the formation of polyethylene with lower molecular weights and narrower molecular weight distribu-tions.Interestingly,the o-aryl halogens have a significant impact on the molecular weight of polyethy-lene,with the atomic radius of the halogen substituents primarily determining the molecular weight of the polyethylene produced.
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