Fluorinated Nickel(II) Phenoxyiminato Catalysts: Exploring the Role of Fluorine Atoms in Controlling Polyethylene Productivities and Microstructures

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 ¹⁹F 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

Homo- and Heteroligated Salicylaldiminato Titanium Complexes with Different Substituents <i>Ortho</i> to the Phenoxy Oxygens for Ethylene and Ethylene/1-Hexene (Co)polymerization

A series of homoligated (<b>1c</b>–<b>1e</b>) and heteroligated (<b>2a</b>–<b>2e</b>) salicylaldiminato titanium dichloride complexes with different substituents <i>ortho</i> to the phenoxy oxygens were efficiently prepared. X-ray diffraction studies on these new dichloride complexes <b>2b</b>, <b>2d</b>, and <b>2e</b> reveal a distorted octahedral coordination of the central metal. In the presence of dried methylaluminoxane, all the complexes exhibit high ethylene polymerization productivities. Surprisingly, complex <b>1d</b> incorporating an <i>o</i>-(trimethylsilyl)­ethynyl group displays the highest activity [5.26 × 10³ kg of polyethylenes (mol Ti)⁻¹ h^–1]. In ethylene/1-hexene copolymerization, the heteroligated complexes <b>2a</b>–<b>2e</b> display improved activities and intermediate incorporation ability compared with their homoligated counterparts <b>1a</b>–<b>1f</b>. The activity and incorporation ability for 1-hexene are highly dependent on the nature of the <i>ortho</i>-substituents. Among them, (trimethylsilyl)­ethynyl-substituted precatalyst (<b>1d</b>) achieves the highest incorporation ratio (27.3 mol %), while ethynyl-substituted precatalyst (<b>2c</b>) achieves the highest copolymerization activity [2.89 × 10³ kg of copolymers (mol Ti)⁻¹ h^–1]

Ethylene (Co)polymerization by Binuclear Nickel Phenoxyiminato Catalysts with Cofacial Orientation

A series of neutral binuclear nickel phenoxyiminato catalysts connected by rigid skeletons of different lengths have been efficiently synthesized. The rigid skeleton and bulky <i>tert</i>-butyl groups together force two nickel coordination planes to get close and stack in an <i>anti</i> cofacial fashion. With reduced nickel–nickel distances, these binuclear nickel complexes displayed higher catalytic activity, produced polymers with higher molecular weight, and showed less inhibition by the presence of additional polar monomers in ethylene polymerization and copolymerization. We attributed these effects to a favorable consequence of the enhanced bimetallic effect and steric hindrance due to the cofacial orientation

Macrocyclic Trinuclear Nickel Phenoxyimine Catalysts for High-Temperature Polymerization of Ethylene and Isospecific Polymerization of Propylene

A series of macrocyclic multinucleating phenoxyimine ligands and the corresponding neutral binuclear (<b>2-Ni</b>_<b>2</b>) and trinuclear (<b>3-Ni</b>_<b>3</b>) nickel catalysts have been efficiently synthesized. The trinuclear nickel complex <b>3-Ni</b>_<b>3</b> showed high activity, high thermal stability, and slow chain transfer in ethylene polymerization, thus producing polyethylene with high molecular weight and low branch density. Highly regiospecific and isospecific polymerization of propylene was also achieved with <b>3-Ni</b>_<b>3</b>, generating regioregular and highly isotactic propylene with high <i>T</i>_m and crystallinity. This is the first example of regio- and stereocontrolled propylene polymerization promoted by nickel phenoxyimine catalysts. Statistical analysis suggested selective 1,2-insertion and enantiomorphic site control mechanism in the chain propagation step, likely caused by the unique steric effect of macrocyclic ligands and the potential cooperative effect