Ethylene Polymerizations with Unsymmetrical (α-Diimine)nickel(II) Catalysts

Abstract

Four unsymmetrical (-diimine)nickel(II) catalysts were synthesized by sequential stepwise condensation of acenaphthenequinone with the corresponding aniline compounds and subjected to ethylene polymerizations with MAO. Interestingly, when the catalysts have the same composition of the ortho substituents, the catalyst with two ortho isopropyl substitients on one side and two ortho methyl substituents on the other side produced polyethylenes with higher branching densities and lower molecular weights than the catalyst with one ortho isopropyl and one ortho methyl substituent on each side. However, branch distribution was not changed by the position of the ortho substituents. The catalysts have unique characteristic, production of branched polyethylene only from ethylene monomers. A chain walking (-hydride elimination and readdition to the other carbon) followed by ethylene coordination and insertion produces a methyl branch. Repeated chain walkings followed by ethylene coordination and insertion make a longer branch.Control of ortho substituents on aromatic rings of (-diimine)nickel(II) catalysts is important to achieve a proper catalytic activity. 13 The ortho substituents have been reported to locate at axial site of the catalysts, and they retard chain transfer reactions, promote the chain walking, and accelerate the rate of migratory insertion. Therefore, as steric bulkiness of the ortho substituents increases, molecular weights of polyethylenes, branching densities of polyethylenes, and polymerization activities increased.14 Some researchers further modified the ortho substituents of the (-diimine)nickel(II) catalysts to get novel polymerization results. Brookhart et al. reported that electron withdrawing ortho substituents of the (-diimine)-nickel(II) catalyst increased turnover frequency of ethylene polymerization presumably because electron withdrawing ortho substituents made nickel more electrophilic.14 Rieger et al. reported chiral C 2 symmetric (-diimine)nickel(II) catalysts bearing ortho phenyl substituents produced high molecular weight polyethylenes with high activities.15 Guan et al. reported cyclophane-based (-diimine)nickel(II) catalyst which was highly active for ethylene polymerizations even at high temperature ($90 C). 16 Zhu et al. reported that molecular weights of the polyethylenes produced by rac-(-diimine)-nickel(II) catalyst were significantly higher than those produced by meso-(-diimine)nickel(II) catalyst. 17Another way to modify the ortho substituents of the (-diimine)nickel(II) catalysts is making the (-diimine)nickel(II) catalysts as unsymmetric. But there have been few reports on ethylene polymerizations with unsymmetrical (-diimine)-nickel(II) catalysts due to the lack of proper synthetic method for unsymmetrically substituted -diimine ligands. [18][19][20][21][22] In this study, new unsymmetrical (-diimine)nickel(II) catalysts were synthesized by stepwise acid catalyzed condensation reaction followed by condensation with TiCl 4 /DABCO. The catalysts were subjected to ethylene poly...

Synthesis and X-ray crystallographic characterization of substituted aryl imines

“…When a-diimines are used as donor-ligands forming d 8 square planar coordination complexes, reactions including ethylene dimerization [5], ethylene oligomerization [6], ethylene polymerization [7][8][9][10][11], propylene polymerization [12], 2-butene polymerization [13], butadiene polymerization [14], CO and vinyl arene copolymerization generating atactic polyketones [15], and the CAH activation of methane [16] and benzene [17] are observed. Specifically, transition metal catalysts containing coordinated a-diimine 3 have been used for the synthesis of epoxides [18], N-arylpyrroles [19], hetero-Diels-Alder adducts [19], aniline from the reduction of nitrobenzene [20], and allylic amines [19,21].…”

Crystal structure of unsymmetrical α-diimine palladium(II) complex cis-[{ArN=C(Me)–(Et)C=NAr}PdCl2] [Ar = 2,6-(iPr)2C6H3]

“…-Diimine ligands may be conveniently prepared by condensation reactions between alkyl or aryl amine with 1,2-diketones. Most of the reported -diimine ligands possess molecular C2 symmetry, while very few unsymmetrical -diimine ligands, obtained by varying the substituents on the nitrogen atom, have been reported (Jeon & Kim, 2008). We report herein the synthesis and spectroscopic characterization of the unsymmetrical -diimine ligand [ArN C(Et)-(Me)C NAr], (I), [Ar = 2,6-i(Pr) 2 C 6 H 3 ] and the corresponding palladium complex cis-[PdCl 2 {I}] (II), where the -diimine ligand backbone contains methyl and ethyl substituents.…”

Non‐Symmetrical Tetraaryl‐α‐Diimines via Transimination: A Promising Route for Non‐Symmetrical 1,3,4,5‐Tetraarylimidazolium Chlorides

“…[10][11][12] One of the most common applications of these ligands are in α-olefin polymerization reactions, wherein the fine-tunable nature of the ligand plays a key role in controlling the microstructure of the polymer and its molecular weight. [13][14][15][16] These ligands were also found to be useful as precursors of chiral α-diamines [17] and 1,3-imidazolium salts, which are known to be stable versions of ready-to-use NHCs via cyclization followed by deprotonation. [18] Synthetic procedures for symmetrical N,N'-tetraaryl-α-diimines (ArÀ N=C(Ar)À C(Ar)=NÀ Ar) often involve the condensation of an α-diketone with aryl amines.…”