A systematic study of Nickel (II) α-diimine complex performance on ethylene polymerization: influence of cocatalyst nature

Ethylene polymerization was carried out with 1,4-bis(2,6-diisopropylphenyl) acenaphthene diimine-dibromo nickel (II) complex (1) combined with methyl aluminoxane (MAO) and ethylaluminium sesquichloride (EASC). The effect of three factors on three levels [temperature (10, 30, 50 °C), ethylene pressure (3, 5, 7 bar) and cocatalyst to catalyst ratio (CC) (1000, 2000, 3000)] on the polymerization process was investigated individually for both cocatalysts using regression models of responses [catalyst activity, weight average molecular weight (M _w) and crystallinity of polymer] and visualized via the response surface method (RSM). For both cocatalysts, activity responses show a second-order variation with temperature, while their pressure dependence is different. The maximum activity of catalyst 1 in the presence of EASC {3907.78 kg [(mol Ni)⁻¹ bar⁻¹ h⁻¹} is higher than maximum activity in the presence of MAO {1091.89 kg [(mol Ni)⁻¹ bar⁻¹ h⁻¹}. Polymerization conditions for reaching the maximum M _w are the same for both cocatalysts (10 °C, 7 bar, 3000); it is about 1,330,000 g mol⁻¹ for MAO and 1,512,297 g mol⁻¹ for EASC. Crystallinity responses reveal that EASC concentration has more significant influence on branching content of final polymer especially at lower polymerization temperature. For example at 10 °C and 3 bar, the increase of EASC concentration from 1000 to 3000 leads to the increase of crystallinity from 32 to 45 %. The obtained models provided a promising tool for designing the polymer properties by replacement of MAO with other cocatalysts.