4 research outputs found
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)Âethynyl group displays the highest
activity [5.26 × 10<sup>3</sup> kg of polyethylenes (mol Ti)<sup>−1</sup> h<sup>–1</sup>]. 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)Âethynyl-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 × 10<sup>3</sup> kg of
copolymers (mol Ti)<sup>−1</sup> h<sup>–1</sup>]
Fluorinated Nickel(II) Phenoxyiminato Catalysts: Exploring the Role of Fluorine Atoms in Controlling Polyethylene Productivities and Microstructures
A series
of neutrally charged NiÂ(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
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
Binuclear Heteroligated Titanium Catalyst Based on Phenoxyimine Ligands: Synthesis, Characterization, and Ethylene (Co)polymerization
A binuclear heteroligated titaniumÂ(IV) catalyst based
on phenoxyimine ligands (<b>FI</b><sup><b>2</b></sup><b>-Ti</b><sub><b>2</b></sub>) with its crystal structure elucidated
has been developed for the first time for olefin (co)Âpolymerization.
In ethylene polymerization, the activity of the binuclear catalyst <b>FI</b><sup><b>2</b></sup><b>-Ti</b><sub><b>2</b></sub> can reach 3.0 × 10<sup>6</sup> g mol<sup>–1</sup> h<sup>–1</sup>, and the polydispersity of the resulting polymers
is narrow. In copolymerization of ethylene and other olefins, similar
incorporation ratios for monoenes are obtained for <b>FI</b><sup><b>2</b></sup><b>-Ti</b><sub><b>2</b></sub> and <b>FI-Ti</b><sub><b>1</b></sub>. However, the incorporation
ratio of 1,5-hexadiene increases from 3.2% using <b>FI-Ti</b><sub><b>1</b></sub> to 8.8% with <b>FI</b><sup><b>2</b></sup><b>-Ti</b><sub><b>2</b></sub>. In comparison,
the binuclear monophenoxyimine catalyst, [<b>FI</b><sup><b>2</b></sup><b>-Ti</b><sub><b>2</b></sub><b>(THF)</b><sub><b>2</b></sub>], exhibits higher catalytic activity and
incorporates more α-olefins than its mononuclear analogue [<b>FI-Ti</b><sub><b>1</b></sub><b>(THF)</b>] in the copolymerization.
These results are interpreted as consequences of two effects. First,
the cooperativity between the two metal centers facilitates the coordination
of olefin. Second, the substitutions near the active sites exert a
steric effect by blocking and suppressing the binding of comonomers