1 H PGSE NMR measurements were performed by using a double stimulated echo pulse sequence (dstegp3s1d) on a Bruker AVANCE III HD 400 spectrometer equipped with a SmartProbe™ with a Z-gradient coil, at 298 K without spinning. For polydisperse species, the dependence of the resonance intensity (I) on a constant waiting time and on a varied gradient strength (G) is described by the equation 𝑖 where I = intensity of the observed spin echo, (I0)i = intensity of the spin echo in the absence of a gradient corresponding to the i-th species, Dt = self-diffusion coefficient of the i-th species, Δ = delay between the midpoints of the gradients, δ = length of the gradient pulse, and γ = magnetogyric ratio. The shape of the gradients was rectangular, their lengths were 4-5 msec, and their strengths G were varied during the experiments. In the present case, we tested both monoexponential (i = 1) and biexponential (i = 2) treatments by fitting the decays of I versus G with a non-linear regression algorithm and the best model for each sample was chosen by comparing the fitting errors and R values (see Supporting Information for this data). All spectra were recorded using 64 K points and a spectral window of 6400 Hz and they were processed with a line broadening of 1.0. Different values of Δ, G, and numbers of transients were used for different samples. The self-diffusion coefficient Dt, which is directly proportional to the slope (m) of the exponential fitting obtained by plotting I versus G was estimated by evaluating the proportionality constant using a sample of HDO (5%) in D2O (known diffusion coefficients in the range 274-318 K) under identical conditions as for the sample of interest. The solvent was taken as an internal standard in the case of toluene-d8 measurements. In the case of chlorobenzene-d5, overlap between aromatic signals hampered the precise estimation of Dt and hydrodynamic dimensions were obtained using dissolved toluene as internal standard. Dt data were treated as described in the literature in order to derive the hydrodynamic dimensions.24 NMR samples for PGSE NMR measurements were prepared inside a glovebox by successive dilution. After having inserted the sample into the NMR probe, the sample was equilibrated for at least 30 minutes before starting the experiment to achieve an optimal thermal stability of the solution. The actual concentration was estimated by integration relative to an external standard. The nominal concentration refers to Al-Me groups and considers both MAO and AlMe3.