Half-titanocenes catalysts based on titamium-phosphinimide for producing atactic elastomeric polypropylenes with high molecular weight

Two half-titanocenes, containing a phosphinimide ligand [(t-Bu)₃P=N]CpTiCl2 (PT1) and [(t-Bu)₃P=N]CpTiMe2 (PT2) as the main catalysts for olefin polymerization, were examined in propylene polymerization in the presence of methylaluminoxane (MAO) or [Ph₃C][B(C₆F₅)4] as the co-catalyst. The catalytic performance was also compared with typical {rac-[En(IndH4)2]ZrC12} (1), CpTiCl₃ (2) and Cp₂TiCl₂ (3) catalysts. The effects of experimental conditions, including feeding molar ratio of the co-catalyst to the main catalyst [Al]/[Ti], temperature, propylene pressure and reaction time, on the catalytic activity of the polymerization, the molecular weight and molecular weight distribution of the resulting polypropylenes (PPs) were investigated in detail. A maximal activity of 3.25 × 10⁶ g PP/molTi·h was attained using PT2/[Ph₃C][B(C₆F₅)4] in the presence of MAO at [Al]/[Ti] = 800. Meanwhile, a highest molecular weight of M_w = 4.42 × 10⁵ with a molecular weight distribution of 1.73 was reached with PT1/MAO at [Al]/[Ti]=1200. The molecular weights were increased with decreased polymerization temperature, increased reaction pressure and polymerization time (t_p). Interestingly, it was noted that an asymptotic plot of M_w versus t_p was almost linear up to 15 min, indicating a “single-site” catalytic polymerization. ¹³C-NMR and IR analyses revealed that the resulting polymers were atactic PPs, which were more or less syndiotactically enriched atactic ones with higher diads r (62.28%) than m (37.72%). Furthermore, a calculated value of 1.04 for [mr/(2mm + mr)] + [mr/(2rr + mr)] indicated that the polymerization was conducted in agreement with the statistical model of Bernoullian. DSC measurements further confirmed the atactic microstructure of the PPs with a glass transition temperature (T_g) ranged from ?3.7 ºC to ?2.6 ºC without any melting endotherm. Accordingly, the PPs produced in this study by the phosphinimide catalysts were mainly made up of 1,2-insertion units together with a small number of 2,1-insertion units.