Dinickel Bisphenoxyiminato Complexes for the Polymerization of Ethylene and α-Olefins

Dinuclear nickel phenoxyiminato olefin polymerization
catalysts
based on rigid p-terphenyl frameworks are reported.
Permethylation of the central arene of the terphenyl unit and oxygen
substitution of the peripheral rings ortho to the aryl–aryl
linkages blocks rotation around these linkages, allowing atropisomers
of the ligand to be isolated. The corresponding syn and anti dinickel
complexes (25-s and 25-a) were synthesized
and characterized by single-crystal X-ray diffraction. These frameworks
limit the relative movement of the metal centers, restricting the
metal–metal distance. Kinetics studies of isomerization of
a ligand precursor (7-a) allowed the calculation of the
activation parameters for the isomerization process (ΔH⧧ = 28.0 ± 0.4 kcal ×
mol–1 and ΔS⧧ = −12.3 ± 0.4 cal mol–1 K–1). The reported nickel complexes are active
for ethylene polymerization (TOF up to 3700 (mol C2H4) (mol Ni)−1 h–1) and
ethylene/α-olefin copolymerization. Only methyl branches are
observed in the polymerization of ethylene, while α-olefins
are incorporated without apparent chain walking. These catalysts are
active in the presence of polar additives and in neat tetrahydrofuran.
The syn and anti isomers differ in polymerization activity, polymer
branching, and polymer molecular weight. For comparison, a series
of mononuclear nickel complexes (26, 27-s, 27-a, 28, 30) was prepared
and studied. The effects of structure and catalyst nuclearity on reactivity
are discussed.

本研究报道了基于刚性对三联苯骨架的双核苯氧亚胺基镍烯烃聚合催化剂。三联苯单元中心芳烃的全甲基化，以及芳基-芳基键邻位外围环的氧取代，可阻断这些键的旋转，从而得以分离得到配体的阻转异构体。合成了相应的顺式（syn）与反式（anti）双核镍配合物（25-s与25-a），并通过单晶X射线衍射对其进行了表征。此类骨架可限制金属中心的相对运动，从而约束金属-金属间距。通过对配体前驱体（7-a）的异构化动力学研究，可计算得到该异构化过程的活化参数（ΔH⧧ = 28.0 ± 0.4 kcal·mol⁻¹，ΔS⧧ = −12.3 ± 0.4 cal·mol⁻¹·K⁻¹）。所报道的镍配合物可高效催化乙烯聚合（转化频率（TOF）最高可达3700 (mol C₂H₄)·(mol Ni)⁻¹·h⁻¹）以及乙烯/α-烯烃共聚反应。乙烯均聚产物仅存在甲基支链，而α-烯烃的共聚则未发生明显的链行走过程。此类催化剂在极性添加剂存在下以及纯四氢呋喃溶剂中仍可保持催化活性。顺式与反式异构体在聚合活性、聚合物支化度以及聚合物分子量方面均存在差异。为进行对照研究，本工作还合成并研究了一系列单核镍配合物（26、27-s、27-a、28、30），并探讨了催化剂结构与核数对反应活性的影响。