Copolymerization of Ethylene with 1-Hexene Using Sterically Hindered Tris(pyrazolyl)borate Titanium (IV) Compounds

Abstract

The copolymerization of ethylene/1-hexene was studied using {TpMs}TiCl3 (1) (TpMs = hydridotris(3-mesitylpyrazol-1-yl) and {TpMs*}TiCl3 (2) (TpMs* = hydridobis(3-mesitylpyrazol-1-yl)(5-mesitylpyrazol-1-yl)) com- pounds in the presence of methylaluminoxane at 65°C. For both catalysts it was observed that the productivities decreased with increasing the 1-hexene concentration in the milieu exhibiting a negative comonomer effect. The highest productivities and 1-hexene incorporations were achieved in the case of {TpMs*}TiCl3 catalyst system. The resulting copolymers were not soluble in TCB even at 165°C. 1-hexene incorporations were very low: ca. 1.2 and 3.3 mol-%, respectively for (1) and (2). The r1 and r2 parameters for both (1) and (2)/MAO catalyst systems clearly indicated a strong tendency of these catalytic precursors to preferentially promote ethylene homopolymerization. According to X-ray photoelectron spectroscopy, from the analysis of the Ti2p3/2 peak, TpMs* catalyst seems to present a stronger cationic character after MAO treatment, which can explain in part its higher catalyst activity.