Structural Investigation of Methylalumoxane Using Transmission Electron Microscopy

Decades after its initial discovery and application as a potent activator for single-site catalysts, methylalumoxane (MAO) still remains a mysterious and poorly understood compound. Using an adapted cryo-TEM setup, for the first time images of dried MAO solutions have been obtained. Particle analysis, size comparison, and decomposition studies yield insights into the structural nature of the observed large MAO particles and can be correlated back to different MAO clusters and their behavior. The studied MAO samples were found to consist of highly aggregated fractals with the radius of the primary particles ranging from similar to 40 to 60 nm upon increasing sample age. These findings support a common particle sintering growing mechanism in which primary particles cluster together to form larger secondary particles. A repetition of this process could explain the dramatic change in structure, average molecular weight, and activity of MAO over time.