Diffusion of Methylaluminoxane (MAO) in Toluene Probed by1H NMR Spin-Lattice Relaxation Time

Abstract

A critical evaluation of model-fitted and observed 1H NMR spin-lattice relaxation time data (T1) of methylaluminoxane (MAO) dissolved in toluene suggests that the relaxation time of MAO is predominantly controlled by rotational motions. Within the Al concentration (CAl) range (10–3 M < CAl < 1 M) and the temperature range (90°C > T > 20°C) studied, the diffusion of MAO contributes with less than 20% (at the maximum concentration investigated) to the overall relaxation rate. For concentrations smaller than 0.5 M, the diffusion has no significant impact on the spin-lattice relaxation time (< 5%). Nevertheless, the diffusion coefficient of MAO can be estimated from the rotational correlation time, as derived from a spin-lattice relaxation time model discussed thoroughly in this work. This model gives information on the molecular dimension of MAO ((19.4 ± 0.4) Å), the activation energy of the diffusion process ((11.2 ± 0.7) kJ/mol), and the temperature/concentration dependence of the viscosity of the MAO solution.