From tetramerization to oligomerization/polymerization of ethylene by dinuclear pyridyl-imine Co- and Ni-based catalysts

A series of dinuclear pyridyl-imine Co- and Ni-based complexes (Co: C-1 and C-2 and Ni: C-3 and C-4) were prepared in reasonable yields through one-pot synthesis method. Toward ethylene oligomerization/polymerization, C-1 and C-2, activated by MMAO, were capable to produce oligomers with moderate activity (up to 5.1 x 10(5) g mol(-1) Co h(-1) for C-2) which alpha-C-8 was the major product. In contrast, C-3 and C-4 polymerized ethylene that the activity of C-4 was twofold greater than C-2. The high impact of o-substituent in C-2 and C-4 was along with dinuclearity effect leading to high productivity and selectivity for ethylene oligomerization and polymerization, respectively. Moreover, polymerization parameters had strong influence on catalytic behavior of C-4 and polyethylene samples made. For instance, high sensitivity of the structures led to formation of an oily branched oligoethylene to highly branched, high M-w polyethylene by changing the polymerization conditions. Polymerization of higher alpha-olefins such as 1-hexene and 1-octene, moreover, emphasized the effect of effective distance between the centers where an oily low Mw oligo-1-hexene and a solid poly(1-octene) having higher M-w were yielded. On the other side, quantum chemistry calculations were performed to investigate the structural properties and reactivity of the Ni- and Co-based species. The obtained results indicated that the Ni atoms have strong molecular orbital interactions with the C=C bond which may increase the reactivity of the catalyst in comparison with the Co metals.