Since the initial research into the use of bis(aryliminoethyl) pyridyliron(II) dichlorides as pre-catalysts for ethylene polymerization during the late 1990s, there have been an enormous numbers of such pre-catalysts reported, as illustrated by recent review articles. With regard to industrial considerations, any promising iron pre-catalysts would need to be able to act as single site active species in producing polyethylenes with a narrow molecular weight distribution. Furthermore, suitable reaction parameters are critically important to maintain the stability of the active species, in particular the operational parameters of ethylene pressure and reaction temperature.

Researchers from Institute of Chemistry, University of East Anglia and Lanzhou Institute of Chemical Physics have synthesized a series of iron(II) complexes bearing bulky unsymmetrical 2,6- bis(imino)pyridines. All pre-catalysts C1-C5 performed with very high activities for ethylene polymerization at 60 ℃ in the presence of MAO, and at 70 ℃ in the presence of MMAO under 10 atm ethylene. The polyethylenes obtained could be adaptable for multi-modal or monomodal features, and polyethylenes with quite narrow molecular weight distributions were obtained. Moreover, the molecular weights of the polyethylenes could be tuned from thousands to hundreds of thousands. The reaction temperatures (60 ℃ and 70 ℃) are heading toward those required for industrial operation, and the resultant polyethylenes are promising in terms of further functionalization and application.

The work has received support from the National High-tech R&D Program of China (863 Program). The findings have been published in Polymer (Polymer 53 (2012) 130e137).

Synthesis of ligands (L1-L5) and iron complexes (C1-C5)