Real-time analysis of methylalumoxane formation

Methylalumoxane (MAO), a perennially useful activator for olefin polymerization precatalysts, is famously intractable to structural elucidation, consisting as it does of a complex mixture of oligomers generated from hydrolysis of pyrophoric trimethylaluminum (TMA). Electrospray ionization mass spectrometry (ESI-MS) is capable of studying those oligomers that become charged during the activation process. We have exploited that ability to probe the synthesis of MAO in real time, starting less than a minute after the mixing of H2O and TMA and tracking the first half hour of reactivity. We find that the process does not involve an incremental build-up of oligomers; instead, oligomerization to species containing 12–15 aluminum atoms happens within a minute, with slower aggregation to higher molecular weight ions. The principal activated product of the benchtop synthesis is the same as that observed in industrial samples, namely [(MeAlO)16(Me3Al)6Me]−, and we have computationally located a new sheet structure for this ion 94 kJ mol−1 lower in Gibbs free energy than any previously calculated.


General Considerations
All experiments were performed under inert atmosphere using standard Schlenk and glovebox techniques. Fluorobenzene and 1,2-difluorobenzene was obtained from Oakwood Chemicals Ltd. and were refluxed over CaH 2 , distilled under N 2 , and stored over activated molecular sieves 4Å inside a glovebox for a minimum of 3 days prior to use. Me 3 Al (2M in toluene) and OMTS (98 %) was purchased from Sigma-Aldrich and was used as received. Cp 2 ZrMe 2 was purchased from Strem Chemicals.
All mass spectra were collected on a Micromass Q-ToF micro mass spectrometer in the positive or negative ion mode using electrospray ionization. Capillary voltage was set at 3000 V with source and desolvation gas temperature at 85 °C and 185 °C, respectively. The desolvation gas flow was set to 400 L h -1 . All MS/MS data was obtained as product ion spectra using 5.0 grade argon as the collision gas and a voltage range of 2-100 V. The ESI-MS spectra were recorded by injecting the solution from the glove box to the spectrometer via PTFE tubing (1/16″ o.d., 0.005″ i.d.) at a rate of 40 μL/min.
All experimental supplies, such as glass vial, syringes, gas-tight syringes, and PTFE tubings, were placed in a vacuum oven for 3 hours. Prior to the MS analysis, the materials were brought and stored inside a glovebox. Conditioning of the instrument was also done prior. A fresh capillary was used for each experiment to avoid unnecessary clogging issues. The baffle and sample cone were rinsed with dilute HCl/MeOH mixture between runs to ensure that they were free of Al 2 O 3 film formation (Shown below). Prior to analyzing these moisture and air sensitive samples, the PTFE tubing and attached instrument were conditioned by running a dilute solution (ca. 0.01 M) of Me 3 Al. This precaution ensures the PTFE tubing and the MS are free of trace amounts of moisture and oxygen. The probe tip was also adjusted for these experiments so that it was close (approximately 1 mm) to the sample cone. [14] Difluorobenzene was distilled over CaH 2 and then collected. To the distilled solvent water was added. Using a separatory funnel, the organic layer was collected and then degassed using freeze pump thaw technique. The wet difluorobenzene was then taken to the glovebox and the water content in the solvent was then estimated using 1 H proton NMR. Cp 2 ZrMe 2 (18.4 mg) was collected in a 10 mL glass vial with 0.5 mL of difluorobenzene and 0.2 mL of dry benzene-d 6 . A 1 H proton NMR was recorded and the amount of water in the solvent was calculated. The water concentration in the solvent was 0.055 M.

Monitoring experiments
A stainless-steel union tee was fitted with three lengths of PTFE tubing using PEEK ZDV low pressure nuts and ferrules. The two short, and equal lengths were fitted with Idex PEEK/PTFE syringe inlets using a PEEK union, and ZDV nut/ferrule. The entire apparatus was then dried in a vacuum oven before being transferred to a LC Technology Solutions LCB-120 glovebox. After flushing the entire apparatus with dry PhF via syringe, the measured, long length of PTFE tubing was connected to the QTOF Micro spectrometer. The dead time of this system was about 40 sec at a combined flow rate of 50 L/min. Syringe 1: 0.16 mL of 2 M Me 3 Al was added to 5 mL of degassed difluorobenzene in a vial and quickly loaded into a 1 mL gas-tight syringe. Syringe 2: 16.8 mg of OMTS was dissolved in 5 mL of dry difluorobenzene. 0.4 mL of this solution was diluted with 10 mL difluorobenzene, degassed and quickly loaded into a 1 mL gas-tight syringe. The two syringes were inserted into a dual syringe pump with PTFE tubing connected to the syringes. The tubing was connected to a mixing tee to combine the two solutions, and PTFE outlet tubing was attached to the mass spectrometer for analysis. The flow rates were set at 25 µL /min and ESI-MS data was collected continuously.  Figure S3.      The reaction is faster as compared to room temperature, but the spectrum is not as clean as the spectrum at room temperature.
Also 16,6, is not observed as a single dominant anion under these conditions.

MS/MS Spectra
The MS/MS product ion spectra show fragmentation exclusively through loss of Me 3 Al (-72 Da). The number of Me 3 Al losses exceed the y values for a given ion because at high energies the ion can rearrange to generate free Me3Al via the process 3(MeAlO)  Al 2 O 3 + Me 3 Al.