Real-time analysis of methylalumoxane formation

Area of Science:

• Polymer Chemistry
• Organometallic Chemistry
• Materials Science

Background:

• Methylaluminoxane (MAO) is a vital activator for olefin polymerization catalysts.
• The complex, oligomeric structure of MAO has historically hindered detailed structural analysis.
• Trimethylaluminum (TMA) hydrolysis is the standard method for MAO synthesis.

Purpose of the Study:

• To investigate the real-time synthesis and oligomerization process of MAO.
• To elucidate the early-stage structural evolution of MAO during activation.
• To identify and characterize the primary charged oligomers formed during MAO synthesis.

Main Methods:

• Electrospray ionization mass spectrometry (ESI-MS) was employed to analyze charged MAO species.
• The hydrolysis reaction between water (H2O) and trimethylaluminum (TMA) was monitored in situ.
• Computational methods were used to determine the Gibbs free energy of proposed MAO structures.

Main Results:

• MAO oligomerization occurs rapidly within the first minute, forming species with 12-15 aluminum atoms.
• Slower aggregation leads to higher molecular weight ions after initial rapid oligomerization.
• The dominant activated product, [(MeAlO)16(Me3Al)6Me]-, was identified, consistent with industrial MAO samples.

Conclusions:

• MAO formation is characterized by rapid initial oligomerization rather than a slow, incremental process.
• ESI-MS provides valuable real-time insights into the dynamic formation of MAO structures.
• A more stable, lower-energy sheet structure for the principal MAO ion was computationally identified.