Composition, Molecular Weight, and Structure of Modified Methylaluminoxane (MMAO): Implications for Selective Ethylene Tetramerization with Cr/PNP Catalysts

Modified methylaluminoxanes (MMAOs) are widely used activators in olefin polymerization and selective oligomerization, yet quantitative structure-performance correlations are often obscured by compositional heterogeneity and residual trialkylaluminum (R3Al) that biases bulk measurements. We prepare MAO/IBAO/MMAO as dry powders and quantify residual R3Al using a THF-probe 1H NMR method that separates sharp R3Al signals from the broad aluminoxane background. By removing the R3Al contribution, we derive corrected number-average molecular weights (Mn,corr) and Gas/Al ratios that precisely reflect the intrinsic aluminoxane framework. Combined with the measured framework Me/iBu ratios, these constraints establish how TMA/TIBA, H2O/Al, and the synthetic sequence control framework composition and condensation. Low-temperature 1H NMR shows extensive alkyl exchange in TMA/TIBA premixes, generating mixed-alkyl aluminum species prior to hydrolysis. In Cr/PNP-catalyzed ethylene tetramerization, the optimized MMAO with Me/iBu of 2.45, a (Gas/Al)corr of 1.41, and Mn,corr of 806 g/mol achieves high activity with 80.8 wt % 1-octene selectivity and minimal polyethylene formation (0.95 wt %). DFT calculations support an Al10O8Me10(iBu)4 cage as a thermodynamically preferred MMAO motif consistent with experiment. These results provide a corrected and quantitative basis for defining aluminoxane identity and rationalizing MMAO effects in selective oligomerization.