Fluorinated Nickel(II) Phenoxyiminato Catalysts: Exploring
the Role of Fluorine Atoms in Controlling Polyethylene Productivities
and Microstructures
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Abstract
A series
of neutrally charged Ni(II) phenoxyiminato catalysts with fluorine
atoms at different positions on the <i>N</i>-terphenyl motif
are synthesized, and their abilities to polymerize ethylene are compared.
At 25 °C, the <i>ortho</i>-fluorinated <b>Ni-5F</b>, <b>Ni-3F</b>′, and <b>Ni-2F</b> achieve significantly
higher polymerization activities than <b>Ni-3F</b> and <b>Ni-0F</b>. In addition, branch density and molecular weight of
the obtained polyethylenes vary gradually in the order of <b>Ni-5F</b>, <b>Ni-3F</b>, <b>Ni-3F</b>′, <b>Ni-2F</b>, and <b>Ni-0F</b>. Based on the X-ray crystal structure and <sup>19</sup>F NMR spectra, the <i>ortho</i> fluorine atoms
are found to make terphenyl groups more rigid and bulky. Theoretical
calculations suggest that the increased steric bulk of terphenyl motif
leads to an increase in the ground state energy of the resting state
species relative to the migratory insertion transition state, and
consequently, lowered migratory insertion barriers are expected in <b>Ni-5F</b>, <b>Ni-3F</b>′, and <b>Ni-2F</b>.
On the other hand, the weak hydrogen bonding between the <i>ortho</i> fluorine atoms and coordinated ethylene in insertion transition
state is also proposed in favor of insertion. Similar to previous
reports, polyethylene microstructure was mainly related to electronic
effects of fluorine atoms