Unveiling the mutual effects of hydroxyl and dibenzhydryl groups in α‐diimino‐nickel precatalysts for direct synthesis of polyethylene elastomers

Abstract

The properties and practical applications of polyethylene are closely associated with the polymer molecular weight, polydispersity, degree of branching, and the type of branches and its arrangement within microstructure. In this study, a set of structurally diverse unsymmetrical 1‐(2,6‐dibenzhydryl‐4‐hydroxylphenylimino)‐2‐(2,6‐(R)‐4‐(R1)phenylimino)acenaphthene‐nickel dibromide precatalysts (where Ni2Me [R = Me, R1 = H], Ni2Et [R = Et, R1 = H], Ni2iPr [R = iPr, R1 = H], Ni3Me [R, R1 = Me], Ni2Et,Me [R = Et, R1 = Me]) has been prepared and studied for ethylene polymerization. Molecular structure analysis of Ni2Me and Ni2Et complexes revealed distorted tetrahedral geometries at nickel. The polymerization activities are extremely high for all precatalysts upon activation with either EASC or MMAO aluminum reagents, a trend notably pronounced in the case of the EASC‐activated systems, entering activity in the range of 10 million g mol−1 h−1 at 30°C. The Ni2Me demonstrated exceptional performance at higher temperatures, achieving an activity of 2.4 × 106 g mol−1 h−1 at 100°C and generated high molecular weight polyethylene (Mw = 105 g mol−1) with narrow polymer dispersity across all reaction temperatures (Ð ≈ 1.5). High temperature 13C NMR spectra identified 97–142/1000C branches in resulting polyethylene, a feature significantly dependent on the reaction temperature. In terms of mechanical properties, stress–strain analysis indicated that polyethylene synthesized at lower temperatures displayed superior tensile strength compared with that produced at higher reaction temperatures, while a reverse trend was observed in strain recovery analysis. The high strain recovery (up to 72%) of these polyethylene highlights the characteristic properties of thermoplastic elastomers.

Thermostability promotion of α-diimine nickel precatalysts tailored with 2,6-bis(bis(4-fluorophenyl)methyl)-3,4,5-trimethoxyaniline for PE elastomer synthesis

“…31,32 The noteworthy results were found when X = OH was incorporated, displaying excellent activity of 2.4 × 10 6 with high polymer molecular weights and narrow dispersity at 100 °C. 34 On the other hand, complexes with fluorinated-benzhydryl steric substituents displayed superior catalytic performance, particularly in thermal stability ( VI , Fig. 1): X = F, activity 0.24 × 10 6 at 80 °C; X = NO 2 , activity 6.01 × 10 6 at 80 °C; X = OMe, activity 4.82 × 10 6 at 50 °C; X = Me, activity 7.08 × 10 6 at 80 °C; X = t Bu, activity 4.34 × 10 6 at 90 °C.…”

“…1 ( II–VII ). 20–41 In-depth investigations into ethylene polymerization have shown that the inclusion of benzhydryl-containing aniline significantly enhances the thermal stability of the catalyst by effectively shielding the axial sites of the metal center (Fig. 1, IV–VII ).…”

“…1, IV–VII ). 25–41 In addition, electronic modifications at the para -position of aniline have proven to be an effective method for fine-tuning both monomer insertion and chain propagation rates. For example, nickel complexes incorporating electron-withdrawing groups (X = para -NO 2 , F, or Cl) exhibited activity in the range of 1.7 to 3.8 × 10 6 g mol −1 h −1 with moderate thermal stability of up to 60 °C; 28–30 replacing them with electron-donating groups (X = para -Me or t Bu) further increased the activity in the range of 3.9 to 4.5 × 10 6 g mol −1 h −1 at 60 °C with no improvement in the thermal stability (Fig.…”