Theoretical investigation of the molecular structure of aluminium triisopropoxide and its complexes in ring-opening polymerization

We present a theoretical study on aluminium triisopropoxide (Al(O1Pr)3) using both empirical (Molecular Mechanics, MM, with Dreiding II force field) and quantum-chemical (Austin Model 1, AMI, semiempirical Hartree-Fock) techniques. We determine the most stable geometries for both the tetramer and tiimer of aluminium triisopropoxide as well as the thermodynamic characteristics of the equilibrium existing between these two aggregated structures. The theoretical results are compared to experimental data from X-ray diffraction and 27Al NMR measurements. For the tetramer, it appears that the optimal equilibrium geometries are in good agreement with the experimental X-ray diffraction geometry; another geometry is also obtained with both theoretical approaches, which is slightly less stable but of higher symmetry. On the basis of the most stable configurations for the tetramer and trimer aggregates, the variation of free enthalpy (AG) between the two aggregated structures has been estimated. The evolution of the theoretical AG values indicates a displacement of equilibrium towards the trimer species with temperature, in good agreement with experimental 1H and 27Al NMR data. Moreover, the AMI heats of formation show a gain of 33.9 kcal/mol due to the aggregation of four Al(O1Pr)3 instead of three, and thus a better stability of the tetramer. The molecular geometries being well described by the theoretical methods used in this study, we also present a model for the ring-opening polymerization complexes of ε-caprolactone and lactides.