A Mechanistic Investigation of the Polymerization of Ethylene Catalyzed by Neutral Ni(II) Complexes Derived from Bulky Anilinotropone Ligands

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Abstract
An extensive mechanistic investigation has been carried out on ethylene polymerizations catalyzed by neutral Ni(II) catalysts derived from bulky anilinotropone ligands. Complexes and precatalysts prepared include aryl derivatives [(2,6-i-Pr2C6H3)NC7H4O(7-Aryl)Ni(Ph)(PPh3)] (9, Aryl = phenyl(a), 1-naphthyl(b), p-methoxyphenyl(c), p-trifluoromethylphenyl(d)), alkyl derivatives [[(2,6-iPr2C6H3)NC7H5O]Ni(R)(2,4-lutidine)] (16, R = Et (a), n-Pr (b)) and [[(2,6-iPr2C6H3)NC7H5O]Ni(R)(PPh3)] (17, R = Et (a), n-Pr (b), n-hexyl (c), i-Pr (d)), and the nickel hydride complex [[(2,6-iPr2C6H3)NC7H5O]Ni(H)(PPh3)], 20. Branched polyethylenes are produced at 40−80 °C in toluene with Mn values in the 100−200K range and molecular weight distributions of ca. 1.4−2.2. Branching ranges from 15 to 64 branches/1000 carbons depending on temperature and ethylene pressure. The electron-withdrawing −CF3 substituent on the 7-aryl group increases activity but has little effect on branching and molecular weight. NMR experiments establish that in the case of the PPh3-substituted systems, the catalyst rests as an equilibrating mixture of the alkyl phosphine and the alkyl ethylene complexes. At high ethylene pressures, the turnover frequency saturates, indicating that the equilibrium has shifted nearly completely to the alkyl olefin complex. Under these conditions, the barriers to migratory insertion were determined to be ca. 16−17 kcal/mol for 9a, 9c, 9d, and 16a. Extraction of 2,6-lutidine from complexes 16a,b yields highly dynamic β-agostic alkyl complexes [[(2,6-i-Pr2C6H3)NC7H5O]Ni(Et)] 21 and [[(2,6-i-Pr2C6H3)NC7H5O]Ni(i-Pr)] 22. Free energy barriers to nickel−carbon bond rotation and β-hydride elimination of 11.1 and ca. 17 kcal/mol, respectively, were determined for 22. Themolysis of 17c at 50 °C generates hydride 20 and hexene and occurs by two pathways, one independent of [PPh3] and one retarded by PPh3. At much slower rates, hydride 20 reductively eliminates free ligand, which ultimately generates a bis-ligand complex, 25. Catalyst decay under polymerization conditions was shown to occur by a similar process to generate free ligand and a bis-ligand complex formed by reaction of free ligand with an active catalyst species. The major chain transfer route is a simple β-elimination process, not chain transfer to monomer.