Synthesis of 5-halo-3-methoxy-salicylaldiminato nickel catalysts and their use for ethylene polymerization in organic and aqueous media

In the present work, we have synthesized nickel salicylaldiminato catalysts based on a 5-halo-3-methoxy ligand framework. The structures of the 5-chloro analog and the 5-bromo analog were successfully confirmed using NMR spectroscopy and X-ray crystallography. These syntheses have the advantage that starting materials are commercially available and only two steps are required to obtain the targeted catalysts. Both catalysts have been applied for polymerization of ethylene in organic media (toluene) and aqueous media. For polymerization in organic medium, the effectiveness of the catalysts was evaluated under different reaction conditions such as different temperatures and catalyst concentrations. The appearance and structure of the polymers changed from amorphous to rubbery with increase in temperature. The polymers obtained were found to be branched, as determined by NMR spectroscopy. Distinct methyl doublets were observed in the H NMR spectra. As the polymerization temperature increased, the branching also increased with an accompanying decrease in melting point. Remarkably, as the temperature of the polymerization was increased between 30 and 50oC, the molecular weight of polymer obtained was found to increase. This was accounted for the catalyst activity increasing as well in this temperature range. As the temperature is increased from 50 to 70oC, the molecular weight decreases as expected for coordinative polymerizations. The optimum temperature to obtain high activity, high molecular weight and relatively low branching was determined to be 50oC. When tried in aqueous medium, the percentage of polyethylene formed seemed to be very low (1-2 %). This is due to the low solubility of ethylene at the relatively low pressures we could apply in the glass reactor. We also applied a known nickel enolate catalyst bearing a CF3-ligand for the aqueous emulsion polymerizations to determine how the polymerization parameters influence the particle size. We found that in particular an increase in the stirrer speed helped to reduce the particle size