O'HARE GROUP

An insoluble form of methylaluminoxane, also known as solid polymethylaluminoxane (sMAO), has been synthesised by the controlled hydrolysis of trimethylaluminum (TMA) with benzoic acid, followed by thermolysis. Characterization of sMAO by multinuclear NMR spectroscopy in solution and the solid state reveals an aluminoxane structure that features “free” and bound TMA and incorporation of a benzoate residue. Total X-ray scattering (or pair distribution function, PDF) measurements on sMAO allow comparisons to be made with simulated data for density functional theory (DFT) modeled structures of methylaluminoxane (MAO). Several TMA-bound (AlOMe)n cage and nanotubular structures with n > 10 are consistent with the experimental data. The measured Brunauer–Emmett–Teller (BET) surface area of sMAO ranges between 312 and 606 m2 g–1 and shows an N2 adsorption/desorption isotherm consistent with a nonporous material. sMAO can be utilized to support metallocene precatalysts in slurry-phase ethylene polymerization reactions. Metallocene precatalyst rac-ethylenebis(1-indenyl)-dichlorozirconium, rac-(EBI)ZrCl2, was immobilised on sMAO samples, to afford solids which showed very high polymerization activities in hexane, comparable to those of the respective homogeneous catalysts formed by treatment of the precatalysts with MAO. rac-(EBI)ZrCl2 immobilised on an sMAO containing an Al:O ratio of 1.2 gave the highest ethylene polymerisation activity.

 

Postsynthesis modification of solid polymethylaluminoxane (sMAO) with tris(pentafluorophenyl)borane or pentafluorophenol produces highly active metallocene supports “sMMAOs” for use in slurry-phase ethylene polymerisation. Characterization of the sMMAOs using elemental analysis, BET isotherm, SEM-EDX, diffuse FT-IR, and solid-state NMR spectroscopy reveals that the surface methyl groups are exchanged for C6F5 and C6F5O moieties respectively, giving a material with reduced aluminum content and a lower specific surface area than sMAO. Rac-(EBI)ZrCl2 immobilized on B(C6F5)3- and C6F5OH-modified sMAO displayed activity increases of 66% and 71% respectively for ethylene polymerisation compared to the same zirconocene catalyst precursor on unmodified sMAO. 

Physicochemical surface-structure studies of highly active slurry-phase ethylene polymerisation catalysts has been performed. Zirconocene complexes immobilised on solid polymethylaluminoxane (sMAO) (sMAO–Cp2ZrX2), have been investigated using SEM-EDX, diffuse reflectance FT-IR (DRIFT) and high field (21.1 T) solid state NMR (ssNMR) spectroscopy. The data suggest a common surface-bound cationic methylzirconocene is the catalytically active species. 91Zr solid sate NMR spectra of sMAO–Cp2ZrCl2 and sMAO–Cp2ZrMe2 are consistent with a common surface-bound Zr environment. However, variation of the σ-donor (X) groups on the metallocene precatalyst leads to significant differences in polymerisation activity. We report evidence for X group transfer from the precatalyst complex onto the surface of the aluminoxane support, which in the case of X = C6F5, results in a 38% increase in activity.

 

Recent publication:

Physicochemical surface-structure studies of highly active zirconocene supported on solid polymethylaluminoxane polymerisation catalysts, A.F.R. Kilpatrick, N.H. Rees, Z.R. Turner, J.-C. Buffet and D. O’Hare, Materials Chem. Frontiers,  (2020), 4, 3226-3233. 

Metallocene polyethylene wax synthesis, J.V. Lamb, J.-C. Buffet, Z. R. Turner, T. Khamnaen, and D. O′Hare, Macromolecules, (2020),  53, 5847–5856. 

Synthesis and characterization of solid polymethylaluminoxane “sMAO”; a bifunctional activator and support for slurry-phase ethylene polymerization, A.F.R. Kilpatrick, J.-C. Buffet, S. Sripothongnak, P. Nørby, N.P. Funnell, and D. O’Hare, Chem Mater., (2016), 28 (20), 7444-7450.

Slurry-phase Ethylene Polymerization using Pentafluorophenyl- and Pentafluorophenoxy-Modified Solid Polymethylaluminoxanes, A.F.R. Kilpatrick, N.H. Rees, S.Sripothongnak, J.-C. Buffet and D. O’Hare, Organometallics, (2018), 37(1), 156–164.