Abstract
Ligand steric bulk is of paramount importance in late transition-metal mediated olefin polymerization. In this contribution, we designed and prepared two structurally similar bis(imino)pyridyl iron(II) complexes bearing 2,6-bis(dibenzhydryl)phenyl N-aryl moiety (Ph-Fe) and pentiptycenyl N-aryl moiety (Ipty-Fe) to reveal the huge effect of steric bulk on ethylene polymerization. Two iron(II) complexes were confirmed by NMR spectroscopy, mass spectroscopy, elemental analysis, and vital X-ray diffraction analysis, which indicates that Ph-Fe has a larger buried volume around the metal center than Ipty-Fe (%Vbur: 55.5 vs. 52.7). As a result, in the presence of Al reagent, Ph-Fe showed extremely low activity for ethylene polymerization, but Ipty-Fe with a less steric blockage exhibited much higher (two orders of magnitude) activity (up to 25,500 kg mol−1 h−1). Reaction temperature, time, pressure, type and amount of Al reagents were comprehensively studied to evaluate and compare these two iron(II) complexes. More interestingly, chain-end-functionalization of these obtained polyethylenes bearing vinylic end-groups could be achieved in situ by the Friedel-Crafts addition to aromatic toluene as the reaction solvent at elevated temperature or high ratio of [Al]/[Fe].
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