Comparison of imidazolium and pyridinium ionic liquids as the media for biphasic ethylene polymerization in the presence of titanocene catalyst

Abstract

Tetrachloroaluminate ionic liquids having 1-n-octyl-3-methylpyridinium [C 8 -β-mpy] + [AlCl 4 ] ― or 1-n-octyl-4-methylpyridinium cation [C 8 -γ-mpy] + [AlCl 4 ] ― , respectively, were investigated in the biphasic ionic liquid/hexane mode ethylene polymerization as media of bis(cyclopentadienyl)titanium(IV)dichloride (Cp 2 TiCl 2 ) titanocene catalyst activated by AlEtCl 2 alkylaluminum compound. The results were compared to those obtained with using 1-n-octyl-3-methylimidazolium tetrachloroaluminate ionic liquid ([C 8 -mim] + [AlCl 4 ] ― ). It was found that the pyridinium ionic liquids enable to obtain better polyethylene (PE) yields in total than the imidazolium ionic liquids. The best results were obtained using [C 8 -γ-mpy] + [AlCl 4 ] ― , in which the alkyl substituents were in para (γ) positions at the pyridinium cation. The considerable amount of PE formed in the ionic liquid phase, largely contaminated, was observed at higher activator/catalyst (Al/Ti) molar ratio. It results from the mass transfer limitation through the phases. This obstacle disappears at lower activator/catalyst molar ratio and polyethylene produced is easily transported into the hexane phase. The effects of Al/Ti ratio and polymeryzation time on the total PE yield were also studied. PE obtained in the biphasic ionic liquid/hexane mode reveals molecular characteristics similar to polyethylenes produced in the presence of both homogeneous (low molecular weight) or heterogeneous (broad molecular weight distribution) metallocene catalyst systems. Additionally, it shows some unique features, such as high crystallinity degree and bulk density. .