Ethylene/propylene copolymerization catalyzed by half-titanocenes containing monodentate anionic nitrogen ligands: effect of ligands on catalytic behaviour and structure of copolymers

Abstract

Various half-titanocenes containing monodentate anionic nitrogen ligands, CpTiCl₂[3-C₆H₅(CH₂N)(CH₂O)CN] (T1), CpTiCl₂[(C₆H₅O)(NⁱPr₂)CN] (T2), CpTiCl₂[1,3-(4-MeC₆H₄)(CH₂N)₂CN] (T3), CpTiCl₂[1,3-(2,4,6-Me₃C₆H₂)(CH₂N)₂CN] (T4), CpTiCl₂[1,3-(2,6-ⁱPr₂C₆H₃)(CH₂N)₂CN] (T5) and CpTiCl[1,3-(2,6-ⁱPr₂C₆H₃)(CH₂N)₂CN][CH₃(CH₂)₃CH(C₂H₅)COO] (T6), were synthesized and characterized. The molecular structures of T1, T2 and T5 were determined by X-ray crystallography. All the above half-titanocenes were employed as precatalysts for ethylene/propylene copolymerization after activation by methylaluminoxane (MAO) or modified MAO (MMAO). The catalytic activity and propylene incorporation into copolymers increased in the order of T1 < T2 < T3 < T4 < T5, which is attributed to the electron donating nature of the ligands. The half-titanocene containing the 2-ethylhexanoate ligand (T6) has good solubility in hexanes. An ethylene–propylene copolymer with high molecular weight (M_w = 530 kg mol⁻¹) could be obtained by the T6/MMAO catalytic system. The ethylene/propylene copolymerization catalyzed by the T6/TIBA/MMAO catalytic system was more controllable than that by the T5/MAO catalytic system. Importantly, both the T5/MAO and T6/TIBA/MMAO catalytic systems displayed a high catalytic activity of 2.4 × 10⁶ and 2.2 × 10⁶ g polymer per mol of Ti per h, respectively, even at a low Al/Ti molar ratio (Al/Ti = 300). All the values of r_E × r_P (r_E: reactivity ratio for ethylene; r_P: reactivity ratio for propylene) were around 0.9 in the copolymerization catalyzed by T2, T4, T5 and T6, indicating that nearly random ethylene–propylene copolymers could be prepared. Compared to the T5/MAO catalytic system, ethylene–propylene copolymers with higher molecular weight (M_w = 381 kg mol⁻¹) and higher propylene incorporation (43.6 mol%) could be produced by using the T6/TIBA/MMAO (TIBA/Ti = 50) system with a similar catalytic activity.