Ethylene Polymerization by Novel, Easily Accessible Catalysts Based on Nickel(II) Diazene Complexes

The vast economic importance of polyolefins is reflected by an annual production of more than 70 million tons of polyethylene and polypropylene. The major portion of these materials is produced by catalytic polymerization, employing Ziegler or Phillips catalysts based on early transition metals (Ti, Zr, Cr, V). Over the past decade, olefin polymerization by complexes of late transition metals has received increasing attention. 1 By comparison to early transition metal complexes, they are much less oxophilic and more tolerant toward polar reagents. In addition, polyolefins with novel microstruc-tures are accessible. Neutral nickel(II) complexes with bidentate κ 2-P,O ligands [(P ∧ O)Ni II R(L)] are the basis for the Shell higher olefin process (SHOP) for ethylene oligomerization to linear 1-olefins, developed in the 1960s. 2 By suitable variation of the ligands, linear polyethylene can also be obtained instead of low-molecular-weight oligomers. 3 In 1995, Brookhart reported on ethylene and 1-olefin polymerization by cationic Ni II or Pd II complexes with diimine ligands. 4 Ethylene homopolymers with various microstructures 5 can be obtained with high activities, and the cationic Pd II complexes can copolymerize ethylene and 1-olefins with simple polar-functionalized olefins such as acry-lates. 4 Recently, neutral Ni II complexes have received renewed interest, as they are expected to be more functional group tolerant than their cationic Ni II counterparts. 6,7 For example, such neutral complexes can polymerize ethylene in aqueous emulsion, affording stable polymer latices. 8 The quest for novel polymerization catalysts involves the search for new ligand structures with other donor functions. To date, this search is empirical to a large extent; albeit recent developments have provided somewhat more comprehensive guidelines for catalyst design. The known well-defined late transition metal systems for ethylene polymerization affording polymers of reasonably high molecular weight with good activities are based on a relatively limited number of donor functions, most prominently phosphine Ar 2 RP, imine R 1 R 2 CdNR 3 , pyridine, and alkoxide or phenolate RO-(R) alkyl, enyl, aryl) moieties. We now report on ethylene polym-erization by neutral nickel(II) complexes of bidentate ligands comprising a diazene function-NdN-as a donor and the properties of the polymers obtained. The ligands 1a-c were prepared from anilines and 2-naphthol by azo coupling. 9 Isoamylnitrite was employed as a reagent for generation of the diazene moiety. The intermediate diazonium salts were briefly isolated and reacted with 2-naphthol to afford 1a-c in good yields. (For experimental details and …