Methyaluminoxane (MAO) Polymerization Mechanism and Kinetic Model from Ab Initio Molecular Dynamics and Electronic Structure Calculations

Abstract

MAO is the co-catalyst in metallocene catalytic systems, which are widely used in single-site olefin polymerization due to their high stereoselectivity. To date, the structures of the catalytically active compound or compounds in MAO have eluded researchers. Although many structural models have been proposed, none are generally accepted. In this study, aspects of the formation mechanism of MAO are addressed. Molecular dynamics simulations at the MP2 level of theory were carried out for presumed elementary steps in MAO formation via hydrolysis of trimethylaluminum (TMA). Methane production was observed, in agreement with experiment, as well as intermediate species that are consistent with the known structural features of MAO and similar to isolated and structurally characterized aluminoxanes. A (CH3)3Al-OH2 species, which we denote as TMA-OH2, containing a stable Al-O single bond emerged as the building block molecule. From this species, a hexameric cage was formed and activation barriers for the various reactions were calculated. Three distinct channels were identified for growth beyond the hexameric cage. It was concluded that MAO formation is a step polymerization through a bifunctional monomer, with [(CH3)Al-O] as the structural unit and a kinetic model was proposed. The structures that emerged were in agreement with the crystallographic evidence for aluminoxanes and support the experimental data regarding the MAO chemical composition.

Screening the Thermodynamics of Trimethylaluminum‐Hydrolysis Products and Their Co‐Catalytic Performance in Olefin‐Polymerization Catalysis

“…[6] Stoichiometric Al/O/Me (1:1:1) ratios in the proposed cage structures do not fit well with the experimentally measured molecular formula of [Me 1.4-1.5 AlO 0.80-0.75 ] n , as reported in 1998 by Imhoff et al [24] The measured stoichiometries are understood by considering the preparation of MAO from the hydrolysis of trimethylaluminum (TMA), as has been demonstrated by computations of the TMA+H 2 O reaction pathways at the MP2 level. [25][26][27] These reactions eventually lead to the association of TMA into the (AlOMe) n core, to generate a general molecular formula of (AlOMe) n -A C H T U N G T R E N N U N G (AlMe 3 ) m , and to Al/O/Me stoichiometries that are in accordance with experimental results. There is also evidence for the critical role of associated TMA in the catalyst-activation process.…”

“…[34] A stoichiometric reaction between TMA and H 2 O initiates oligomerization reactions that ultimately lead to the formation of MAOs; the initial steps in this reaction series have been documented in detail in previous reports. [25][26][27] The first reaction yields a water/TMA adduct: Al 2 Me 6 +2 H 2 O!2 AlMe 3 OH 2 . At the MP2/TZVP level of theory, the reaction is barrier-less and spontaneous (DG = À30.0 kJ mol À1 ).…”

Ethylene homo- and copolymerization chain-transfers: A perspective from supported ( n BuCp) 2 ZrCl 2 catalyst active centre distribution

“…See Equations A24 and A31, respectively (Appendix A). 11,17,19,[31][32][33][34][35][36][37][38][39][40][41][42][43][44] These equations show how they correlate to the different apparent propagation rate constants and the momoner concentrations. Second, we note that each active site type copolymerization rate is higher than the corresponding ethylene homopolymerization rate.…”

“…Next, it abstracts the chloride ligand to generate the active metallocenium cation (Zr + ). 17,19,[30][31][32][33][34][35][36][37] See Equation A1. Reversible complex formation 11,[38][39][40][41] with the transition metal active site, as per the trigger mechanism of Ystenes, 42 is a pre-requisite to propagation.…”

Metallocene-catalyzed ethylene−α-olefin isomeric copolymerization: A perspective from hydrodynamic boundary layer mass transfer and design of MAO anion

“…The observed common composition distribution broadness demonstrates the presence of strong micromixing/segregation effect [58][59][60]. This phenomenon can be ascribed to the three-dimensional MAO cage structures that feature the following [61][62][63][64][65][66][67][68]:…”