Copolymerization of Ethylene and 1-olefins in Supercritical Co2 by an Electron Poor Ni(ii) Complex

SUPERCRITICAL CO2 BY AN ELECTRON POOR NI(II) COMPLEX Damien Guironnet and Stefan Mecking* Chair of Chemical Materials Science, Department of Chemistry, University of Konstanz, Germany. Introduction Late transition metal catalysts for polymerization of olefins have been studied intensely. Due to their functional group tolerance, polymerizations can be carried out in oxygenated reaction media, amongst others dense carbon dioxide. Dense carbon dioxide, that is liquid or supercritical CO2 (scCO2), possesses unique properties, such as the possibility of variation of its density and solvent properties over a wide range. In polymerization processes and polymer processing, dense carbon dioxide can be useful as a solvent or suspension medium. It can be removed conveniently by reducing the medium density, resulting in a dry polymer powder. Ethylene polymerization has been studied in scCO2 with cationic Pd(II)diimine catalysts. Highly branched amorphous polyethylene is obtained invariably, which is due to a high rate of ‘chain walking compared to the rate of chain growth. An activity of 2×10 TO h was reported under the conditions studied. We have reported ethylene homopolymerization with CO2-soluble neutral Ni(II) salicylaldiminato complexes. Activities of up to 5×10 TO h were observed under the conditions studied. Linear semicrystalline polyethylenes with molecular weights up to Mn = 2.4×10 g mol were obtained. Polymer crystallinities could be varied in a limited range via the choice of catalyst, by introduction of methyl branches as a result of ‘chain walking’. However, with increased branching molecular weights decrease, as ß-hydride elimination is a key step both for chain walking and chain transfer. By copolymerization polymer crystallinites could be controlled over a wide range, and without an intrinsic correlation with molecular weight. This requires a suitable catalyst.