title: Role of Radical Species in Salicylaldiminato Ni(II) Mediated Polymer Chain Growth: A Case Study for the Migratory Insertion Polymerization of Ethylene in the Presence of Methyl Methacrylate.
abstract: To date, an inconclusive and partially contradictive picture exists on the behavior of neutral Ni(II) insertion polymerization catalysts toward methyl methacrylate (MMA). We shed light on this issue by a combination of comprehensive mechanistic NMR and EPR studies, isolation of a key Ni(I) intermediate, and pressure reactor studies with ethylene and MMA, followed by detailed polymer analysis. An interlocking mechanistic picture of an insertion and a free radical polymerization is revealed. Both polymerizations run simultaneously (25 bar ethylene, neat MMA, 70 °C); however, the chain growth cycles are independent of each other, and therefore exclusively a physical mixture of homo-PE and homo-PMMA is obtained. A Ni-C bond cleavage was excluded as a free radical source. Rather a homolytic P-C bond cleavage in the labile aryl phosphine ligand and the reaction of low-valent Ni(0/I) species with specific iodo substituted N^O (Ar-I) ligands were shown to initiate radical MMA polymerizations. Several reductive elimination decomposition pathways of catalyst precursor or active intermediates were shown to form low-valent Ni species. One of those pathways is a bimolecular reductive coupling via intermediate (N^O)Ni(I) formation. These intermediate Ni(I) species can be prevented from ultimate decomposition by capturing with organic radical sources, forming insertion polymerization active [(N^O)Ni(II)-R] species and prolonging the ethylene polymerization activity.
authors.affiliation: Chair of Chemical Materials Science, Department of Chemistry, University of Konstanz , D-78464 Konstanz, Germany.
authors.forename: Franz
authors.initials: F
authors.lastname: Ölscher
authors.affiliation: Chair of Chemical Materials Science, Department of Chemistry, University of Konstanz , D-78464 Konstanz, Germany.
authors.forename: Inigo
authors.initials: I
authors.lastname: Göttker-Schnetmann
authors.affiliation: Université de Lyon , Univ. Lyon 1, CPE Lyon, CNRS UMR 5265 Laboratoire de Chimie Catalyse Polymères et Procédés (C2P2), LCPP team Bat 308F, 43 Bd du 11 novembre 1918, F-69616 Villeurbanne, France.
authors.forename: Vincent
authors.initials: V
authors.lastname: Monteil
authors.affiliation: Chair of Chemical Materials Science, Department of Chemistry, University of Konstanz , D-78464 Konstanz, Germany.
authors.forename: Stefan
authors.initials: S
authors.lastname: Mecking