Synthesis of New Phenoxide-Modified Half-Titanocene Catalysts for Ethylene Polymerization

A series of half-titanocenes containing different trialkylsilyl para-phenoxy substituents, Cp*TiCl2(O-2,6-iPr2-4-R-C6H2) [Cp* = C5Me5; R = Si(n-Bu)3 (5), SiMe2(n-C8H17) (6), SiMe2(t-Bu) (7)], were prepared and identified. Catalytic activity in ethylene polymerization by Cp*TiCl2(O-2,6-iPr2-4-R-C6H2) [R = H (1), SiMe3 (2), SiEt3 (3), Si(i-Pr)3 (4), 5–7]–MAO (methylaluminoxane) catalysts increased in the following order (in toluene at 25 °C, ethylene 4 atm): R = H (1) < SiMe3 (2), SiEt3 (3), Si(i-Pr)3 (4) < SiMe2(t-Bu) (7) < SiMe2(n-C8H17) (6) < Si(n-Bu)3 (5, activity = 6.56 × 104 kg-PE/mol-Ti·h). The results thus suggest that the introduction of an alkyl group into a silyl substituent led to an increase in activity. The activities of 5 were affected by the Al/Ti molar ratio (amount of MAO charged), and the highest activity (7.00 × 105 kg-PE/mol-Ti·h) was observed under optimized conditions at 50 °C, whereas the activity decreased at 80 °C. In ethylene copolymerization with 1-dodecene, the Si(n-Bu)3 analog (5) exhibited remarkable catalytic activity (4.32 × 106 kg-polymer/mol-Ti·h at 25 °C), which was higher than those of the reported catalysts (1–3), affording poly(ethylene-co-1-dodecene)s with efficient comonomer incorporation as observed in 3 [rE = 3.77 (5) vs. 3.58 (3)].