Correlating the TMA Content with Structural Evolution and Catalytic Behavior of dMAO in Ethylene Polymerization

Methylaluminoxane (MAO) and trimethylaluminum (TMA) are essential cocatalysts in olefin polymerization and oligomerization, yet their interplay and impact on catalytic performance remain incompletely understood. In this study, TMA-depleted MAO (dMAO) was obtained via vacuum drying and redissolved, followed by controlled reintroduction of TMA to investigate its structural and catalytic effects. Diffusion-ordered NMR spectroscopy (DOSY) revealed that untreated MAO exhibits a broad oligomeric size distribution (5.99–10.4 Å) in toluene. Vacuum drying removes free and loosely bound TMA, yielding more uniform MAO clusters (9.10 Å). Catalytic studies demonstrate that in Cp2ZrCl2-catalyzed ethylene polymerization, TMA acts as a chain-transfer agent, with optimal activity at a TMA/dMAO ratio of 1:3. In Fe(acac)3-catalyzed oligomerization, moderate TMA levels enhance α-olefin selectivity (up to 90%) and suppress wax formation. Similarly, in Cr/PNP-catalyzed tetramerization, TMA improves 1-octene selectivity to 77.1% while minimizing PE byproducts. These findings underscore the importance of TMA control in optimizing the MAO structure and catalytic efficiency for polyolefin production.