Copolymerization of ethylene with l-hexene using sterically hindered tris(pyrazolyl)borate titanium (IV) compounds

The copolymerization of ethylene/1-hexene was studied using {Tp(Ms)}TiCl3 ( 1) (Tp(Ms) = hydridotris(3-mesitylpyrazol- 1-yl) and (Tp(Ms*)) TiCl3 (2) (T p(Ms*) = hydrido-bis(3-mesitylpyrazol-1-yl)(5-mesitylpyrazol-1-yl)) compoun ds in the presence of methylaluminoxane at 65 degreesC. For both catalysts it was observed that the productivities decreased with increasing the 1-hex ene concentration in the milieu exhibiting a negative comonomer effect. The highest productivities and 1-hexene incorporations were achieved in the ca se of {Tp(Ms*)}TiCl3 catalyst system. The resulting copolymers were not sol uble in TCB even ata 165 degreesC. 1-hexene incorporations were very low: c a. 1.2 and 3.3 mol-% respectively for (1) and (2). The r(1) and r(2) parame ters for both (1) And (2)/MAO catalyst systems clearly indicated a strong t endency of these catalytic precursors to preferentially promote ethylene ho mopolymerization. According to X-ray photoelectron spectroscopy, from the a nalysis of the Ti2p(3/2) peak, Tp(Ms*) catalyst seems to present a stronger cationic character after MAO treatment, which can explain in part its high er catalyst activity.