Diffusion of methylaluminoxane (MAO) in toluene probed by H-1 NMR spin-lattice relaxation time

A critical evaluation of model-fitted and observed H-1 NMR spin-lattice rel axation time data (T-1) of methylaluminoxane (MAO) dissolved in toluene sug gests that the relaxation time of MAO is predominantly controlled by rotati onal motions. Within the Al concentration (C-Al) range (10(-3) M< C-Al < 1 M) and the temperature range (90 degreesC > T > 20 degreesC) studied, the d iffusion of MAO contributes with less than 20% (at the maximum concentratio n investigated) to the overall relaxation rate. For concentrations smaller than 0.5 M, the diffusion has no significant impact on the spin-lattice rel axation time (<5%). Nevertheless, the diffusion coefficient of MAO can be e stimated from the rotational correlation time, as derived from a spin-latti ce relaxation time model discussed thoroughly in this work. This model give s information on the molecular dimension of MAO ((19.4 +/- 0.4) <Angstrom>) , the activation energy of the diffusion process-((11.2 +/- 0.7) kJ/mol), a nd the temperature/concentration dependence of the viscosity of the MAO sol ution.