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1.
J Am Chem Soc ; 137(46): 14819-28, 2015 Nov 25.
Artículo en Inglés | MEDLINE | ID: mdl-26571229

RESUMEN

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.

2.
Chemistry ; 19(34): 11409-17, 2013 Aug 19.
Artículo en Inglés | MEDLINE | ID: mdl-23843135

RESUMEN

Formation of Ni-polymeryl propagating species upon the interaction of three salicylaldiminato nickel(II) complexes of the type [(N,O)Ni(CH3 )(Py)] (where (N,O)=salicylaldimine ligands, Py=pyridine) with ethylene (C2 H4 /Ni=10:30) has been studied by (1) H and (13) C NMR spectroscopy. Typically, the ethylene/catalyst mixtures in [D8 ]toluene were stored for short periods of time at +60 °C to generate the [(N,O)Ni(polymeryl)] species, then quickly cooled, and the NMR measurements were conducted at -20 °C. At that temperature, the [(N,O)Ni(polymeryl)] species are stable for days; diffusion (1) H NMR measurements provide an estimate of the average length of polymeryl chain (polymeryl=(C2 H4 )n H, n=6-18). At high ethylene consumptions, the [(N,O)Ni(polymeryl)] intermediates decline, releasing free polymer chains and yielding [(N,O)Ni(Et)(Py)] species, which also further decompose to form the ultimate catalyst degradation product, a paramagnetic [(N,O)2 Ni(Py)] complex. In [(N,O)2 Ni(Py)], the pyridine ligand is labile (with activation energy for its dissociation of (12.3±0.5) kcal mol(-1) , ΔH(≠) 298 =(11.7±0.5) kcal mol(-1) , ΔS(≠) 298 =(-7±1) cal K(-1) mol(-1) ). Upon the addition of nonpolar solvent (pentane), the pyridine ligand is lost completely to yield the crystals of diamagnetic [(N,O)2 Ni] complex. NMR spectroscopic analysis of the polyethylenes formed suggests that the evolution of chain-propagating species ends up with formation of polyethylene with predominately internal and terminal vinylene groups rather than vinyl groups.

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