A comparative kinetic study of ethylene polymerization mediated by iron, cobalt and chromium catalysts bearing the same N,N,N-bis(imino)trihydroquinoline

The iron(II), cobalt(II) and chromium(III) chlorides, [2-{(2,4,6-Me₃C₆H₂)NCMe}-8-{N(2,4,6-Me₃C₆H₂)}C₉H₈N]MCl_n (n = 2, M = Fe LFeCl₂, Co LCoCl₂; n = 3, M = Cr LCrCl₃), each bearing the same N,N,N-bis(imino)trihydroquinoline chelating ligand, have been employed as precatalysts for ethylene polymerization with modified methylaluminoxane (MMAO) as the co-catalyst. The kinetic profiles for these homogeneous polymerizations are reported in addition to the properties of the resultant polymers under comparable reaction conditions. All the experimental data indicate that the active metal center plays a key role on the catalytic performances of the complexes, especially the polymerization activity, thermal stability and lifetime of the active species. Under optimized conditions the iron catalyst displays the highest rate of polymerization but displays this for only a short period, while the chromium catalyst shows a lower maximum polymerization rate but sustains its performance over a longer period and at a higher temperature. In terms of the polymer properties, all three metal catalysts afford highly linear polymers with the metal center influencing the molecular weight and type of end group. Specifically, the cobalt and chromium catalysts produce narrowly dispersed low molecular weight polymers incorporating vinyl end groups, while the iron catalyst affords polymers of higher molecular weight displaying broad molecular weight distributions, with both fully saturated and unsaturated chain ends.