Oxidation of Methylalumoxane Oligomers: A Theoretical Study Guided by Mass Spectrometry

Abstract

Methylalumoxane (MAO) plays a critical role in catalytic polymerization of olefins, both as activator of the precatalyst and scavenger of impurities. The latter involves oxidation of MAO, which is not well understood at a molecular level. On the basis of our previous computational explorations of the structure of MAO and electrospray ionization mass spectrometry (ESI-MS) studies of its oxidation, we report here a systematic theoretical study to shed light on the oxidation process. We identify the structural features of the MAO that are essential for oxidation by screening the thermodynamics of the oxidation reactions as a function of shape and size of the previously justified model MAOs. In correlation with previously reported ESI-MS studies of the corresponding anions (Chem.

Modifying methylalumoxane via alkyl exchange

“…Substitution by 10 Et 3 Al is expected to be much more like the self-exchange process involving Me 3 Al. Indeed, Et/Me exchange is extremely fast and depending on the amount of Et 3 Al that was added, 16,6 derivatives with over 24 Et groups could be observed (Figure 4a and S5 Figure S5). It is possible that the ion-pairs are more reactive towards exchange than the neutrals in the case of Et 3 Al at low levels of substitution.…”

“…To better understand the R 3 Al/MAO-Me exchange process we set out to study the reaction in real-time using pressurized sample infusion (continuous injection of solution into the mass spectrometer using a variant of cannula transfer). 26 Upon addition of 1% iBu 3 Al to MAO rapid exchange is observed resulting in the formation of the one, two, and three iBu/Me substituted 16,6 derivatives (see Supporting Information Figure S6). These species equilibrate within a minute and 12 their ion counts thenceforth remain stable.…”

“…Its average composition, (Me 1.4-1. 5 AlO 0.75-0.80 ) n , 4 molecular weight (MW, ~ 1200-2000) 5 have been established 2 and, in combination with computational studies 6 and structurally characterized aluminoxanes 7 it is generally thought that MAO is made up of cage-like structures that have the general formula (MeAlO) n (Me 3 Al) m .…”

Catalyst Deactivation Processes during 1-Hexene Polymerization

“…25,26 We have also studied ion-speciation in the case of additive such as octamethyltrisiloxane (OMTS) which forms well-defined ion-pairs [Me 2 Al(OMTS)][(MeAlO) 16 (Me 3 Al) 6 Me] through the process of [Me 2 Al] + abstraction from the MAO. 27 This additive has proven useful in monitoring both aging and oxidation of MAO, 28,29 features known to be important in affecting the efficacy of this elusive but important activator. [30][31][32] We have also studied the reaction of MAO-activated Cp 2 ZrMe 2 with ethylene in toluene solution at low ethylene pressures.…”

Preparation of Methylaluminoxane from CO2 and Me3Al

“…Methylaluminoxane (MAO) is known as a versatile activating reagent of transition‐metal catalyst for coordination polymerization and industrially very important reagent for the manufacture of polyolefins Generally, MAO plays two roles on the activation of the catalysts, which is alkylation of metal center to render an initiating group on it and abstraction of one of the anionic ligands from the neutral metal precursor to generate cationic active species. MAO consists of oligomers with various numbers of –(Me)Al–O– repeating units, and recent study applying mass spectrometry, NMR analysis and theoretical calculation reveal more about the precise structural dynamics . MAO was first prepared over several decades before by the partial hydrolysis of Me 3 Al, which is a highly exothermic reaction and hard to control.…”