Nickel catalysts bearing bidentate alpha-aminoaldimines for ethylene polymerization--independent and cooperative structure/reactivity relationship resulting from unsymmetric square planar coordination.
Ethylene polymerization catalyzed by Ni(ii) complexes that bear new bidentate ligands with a functional hybrid of amine and imine has been studied. A class of new alpha-aminoaldimines and their nickel complexes [R(1)R(2)NCMe(2)CH[double bond, length as m-dash]N(2,6-R(3)(2)C(6)H(3))]NiBr(2) (R(1) = R(2) = Me, R(3) = Me (Ni-); R(3) = (i)Pr (Ni-); R(1) = R(2) = Et, R(3) = H (Ni-); Me (Ni-); (i)Pr (Ni-); R(1) = R(2) = (n)Pr, R(3) = (i)Pr (Ni-); (R(1)R(2)) = c-C(3)H(6) R(3) = (i)Pr (Ni-); (R(1)R(2)) = c-C(4)H(8), R(3) = (i)Pr (Ni-)) were synthesized. The molecular structures of six nickel complexes were determined by X-ray crystallography, showing distorted tetrahedral configurations. The SQUID data of Ni- confirms its ground state of triplet spin. Using methylaluminoxanes (MAO) as the activator, the nickel complexes are found to catalyze ethylene polymerization under moderate pressure and ambient temperature. The activity reaches to 10(6) g PE mol Ni(-1) h(-1), and increases with the ethylene pressure in the range of 14-28 bar. The highly branching PE products have M(n) approximately 10(5) with PDI < 2. The amine and imine functionalities demonstrate independent control to the polymerization reactions, wherein the activity appears to be facilitated by using the catalysts installed with bulky imino substituents as well as with less sterically hindered amino substituents. This is ascribed to the C(2) unsymmetric coordination in the square planar resting state in which the bulky polymer chain prefers cis to the imine and the small ethylene monomer is cis to the amine.
Yang
Feng-Zhao
FZ
Department of Chemistry, National Taiwan University, No 1, Section 4, Roosevelt Road, Taipei, Taiwan106.
Chen
Yi-Chun
YC
Lin
Ya-Fan
YF
Yu
Kuo-Hsuan
KH
Liu
Yi-Hung
YH
Wang
Yu
Y
Liu
Shiuh-Tzung
ST
Chen
Jwu-Ting
JT