Detailed investigations of the polymerization of ethylene by (R-diimine)nickel(II) catalysts are reported. Effects of structural variations of the diimine ligand on catalyst activities, polymer molecular weights, and polymer microstructure are described. The precatalysts employed were 6). Active polymerization catalysts were formed in situ by combination of 4-6 with modified methylaluminoxane. In general, as the bulk and number of ortho substituents increase, polymer molecular weights, turnover frequencies and extent of branching in the homopolyethylenes all increase. Effects of varying ethylene pressure and temperature on polymerizations are also reported. The degree of branching in the polymers rapidly decreases with increasing ethylene pressure but molecular weights are not markedly affected. Temperature increases result in more extensive branching and moderate reductions in molecular weights. Catalyst productivity decreases above 60 °C due to catalyst deactivation.

Nickel-catalyzed ethylene oligomerization and propylene dimerization reactions are described. A series of aryl-substituted α-diimine ligands 1 (ArNC(R)C(R)NAr, R,R ≡ 1,8-naphth-diyl, Ar ≡ XC6H4, X = p-CF3 (a), p-H (b), p-Me (c), p-OMe (d), o-Me (e)) and their corresponding Ni(II) dibromide complexes 2 were prepared. Treatment of the Ni(II) dibromide complexes 2 with aluminum alkyl activators such as MAO (methylalumoxane), modified MAO (MMAO), or Et2AlCl in toluene generates active cationic catalysts in situ that oligomerize ethylene to a Schulz−Flory distribution of linear α-olefins. Reaction conditions can be adjusted that lead to selectivities as high as 96% for linear α-olefins. These catalysts are highly active, with ethylene turnover frequencies as high as 1.4 × 105 mol of C2H4/((mol of Ni)h) observed. Schulz−Flory α values range from 0.59 to 0.81 and are dependent on the reaction temperature, ethylene pressure, and nature of the aluminum cocatalyst. The active catalysts dimerize propylene, generating product mixtures that have roughly equal compositions of n-hexenes and 2-methylpentenes. 2,3-Dimethylbutenes are minor products, usually comprising less than 10% of the total product distribution. Propylene dimers and their associated isomerization products are the only reaction products observed under the standard reaction conditions used. Propylene turnover frequencies as high as 2 × 104 mol of C3H6/((mol of Ni)h) were observed. Control experiments indicate that terminal olefins are slowly isomerized to internal olefins under propylene dimerization reaction conditions but dimer production is fast relative to product isomerization. Higher olefins such as 1-butene and 4-methyl-1-pentene are dimerized very slowly by these catalysts.

No abstract

The synthesis of a series of (alpha-diimine)NiR(2) (R = Et, (n)Pr) complexes via Grignard alkylation of the corresponding (alpha-diimine)NiBr(2) precursors is presented. Protonation of these species by the oxonium acid [H(OEt(2))(2)](+)[BAr'(4)](-) at low temperatures yields cationic Ni(II) beta-agostic alkyl complexes which model relevant intermediates present in nickel-catalyzed olefin polymerization reactions. The highly dynamic nature of these agostic alkyl cations is quantitatively addressed using NMR line broadening techniques. Trapping of these complexes with ethylene provides cationic Ni alkyl ethylene species, which are used to determine rates of ethylene insertion into primary and secondary carbon centers. The Ni agostic alkyl cations are also trapped by CH(3)CN and Me(2)S to yield Ni(R)(L)(+) (L = CH(3)CN, Me(2)S) complexes, and the dynamic behavior of these species in the presence of varied [L] is discussed. The kinetic data obtained from these experiments are used to present an overall picture of the ethylene polymerization mechanism for (alpha-diimine)Ni catalysts, including effects of reaction temperature and ethylene pressure on catalyst activity, polyethylene branching, and polymer architecture. Detailed comparisons of these systems to the previously presented analogous palladium catalysts are made.

No abstract