Separation of propene/1-alkene and ethylene/1-alkene copolymers by high-temperature adsorption liquid chromatography

Abstract

A high performance liquid chromatography column (HPLC) Hypercarb® packed with porous graphite has proven to discriminate polyolefin molecules due to differences in their adsorption and desorption behaviour. While linear polyethylene (PE) and syndiotactic polypropylene (sPP) are adsorbed on the graphite packing, isotactic polypropylene (iPP) is not adsorbed. The column operates at 160°C with 1-decanol as sample solvent and mobile phase. We have now tested this HPLC system for separations of random propene/1-alkene and ethylene/1-hexene copolymers: While copolymers of propene with 1-butene, 1-hexene and 1-octene copolymers eluted in size exclusion mode without adsorption, propene/1-octadecene and ethylene/1-hexene copolymers are strongly retained and eluted only after application of a linear gradient starting from 1-decanol and ending with pure 1,2,4-trichlorobenzene. The retention of propene/1-alkene (>11 carbons in the side chain) copolymers increases with the concentration of comonomer, making this HPLC system suitable to separate these copolymers according to their chemical composition. In contrast, the retention of ethylene/1-hexene samples decreases with increasing 1-hexene content. Branching in this case shortens the length of continuous methylene sequences of the polymer backbone, which are expected to adsorb in a planar conformation to the graphite layers. This is the first report on the separation of short chain branched polyolefins by high-temperature adsorption liquid chromatography.