BHT-Modified MAO: Cage Size Estimation, Chemical Counting of Strongly Acidic Al Sites, and Activation of a Ti-Phosphinimide Precatalyst

Abstract

The MAO/BHT (MAO = methylaluminoxane; BHT = 2,6-di-tert-butyl-4-methylphenol) cocatalyst for olefin polymerization has been investigated by NMR spectroscopy. It has been found that it consists of oligomeric [AlOMe 0.9 (bht) 0.1 ] n cages and monomeric MeAl(bht) 2 (bht = deprotonated BHT). Diffusion NMR indicates an average n for Al clusters of 62−96, i.e., 2−3 times higher than that estimated for unmodified MAO under analogous conditions (n ≈ 26− 41). The reactivity of MAO/BHT has been explored by monitoring the activation of the Cp*−phosphinimide titanium dichloride precatalyst Cp*(tBu 3 PN)TiCl 2 . Comparison with independently synthesized model species and DFT modeling allowed characterization of the reaction mixtures obtained at varying aluminum to titanium ratios. Homodinuclear adducts [Cp*(tBu 3 PN)TiX] 2 (μ-Y) + (X, Y = Me or Cl) forming outer sphere ion pairs (OSIPs) with MAO/ BHT-derived anions are dominant at low Al/Ti ratios, whereas mononuclear inner sphere ion pairs [Cp*(tBu 3 P N)TiX] + [MAO/BHT] − are formed at high Al/Ti ratios; both types of species are found to be viable precursors for the cationic active species. Activation of dibenzyl analogue Cp*(tBu 3 PN)TiBn 2 results in the clean formation of [Cp*(tBu 3 PN)Ti-Bn] + [MAO/BHT] − OSIP, giving sharp 1 H and 31 P NMR signals; this reaction was exploited to quantify the amount of strongly acidic sites on Al clusters, shedding further light on the structure and properties of MAO/BHT.

On the Nature of the Lewis Acidic Sites in “TMA‐Free” Phenol‐Modified Methylaluminoxane

“…Finally, the broadness of the resonances at 7.64 and 8.10 ppm might be traced to residual bipy molecules interacting with different Al sites on different MAO/BHT cages , . The decay of these signals over time, paralleled by the increase of the aforementioned sharp ones, indicates that bipy continues to react with the Al‐cages to slowly generate ionic fragments of the type described above.…”

“…As mentioned in the Introduction, the experimentally estimated Al‐Me/Al‐bht molar ratio of ca. 9:1 for MAO/BHT cages led to the following tentative formula: (AlOMe 0.9 bht 0.1 ) n or, equivalently, [(AlOMe) 0.9 (AlObht) 0.1 ] n . However, this formula does not account for the presence of “structural” AlMe 2 bht, that appears to be an integral part of the Al‐clusters.…”

“…MAO/BHT was prepared according to established procedures by adding BHT to a diluted, commercial solution of MAO in toluene . The Al‐cages are the only fraction of interest to explore the activation properties of MAO/BHT, as the bis‐phenolate complex is known not to react with typical group IV metal precatalysts , , , . To remove AlMe(bht) 2 and toluene, the MAO/BHT mixture was vacuum dried, washed with pentane and dried again .…”

“…The 1 H NMR spectrum of washed solid‐(MAO/BHT) [ws(MAO/BHT)] in [D 5 ]chlorobenzene is reported in Figure a. The broad resonances can be ascribed to Al‐Me and Al‐bht groups on MAO cages …”

“…The Me/bht molar ratio of 9:1 found for the cages led to the tentative formula [AlOMe 0.9 (bht) 0.1 ] n . Polymerization, , and spectroscopic, studies clearly indicate that the resulting modified MAO (MAO/BHT) effectively activates group IV precatalysts.…”