Macromolecular engineering of polylactones and polylactides. 14. A 13C and 27Al NMR study of the effect of γ-butyrolactone on the structure of aluminum isopropoxide in toluene

Solutions of Al isopropoxide in toluene have been investigated by C-13 and Al-27 NMR spectroscopy. Since Al(OiPr)3 is currently used as an initiator for the living polymerization of epsilon-caprolactone, special attention has been paid to the effect of the monomer on the coordinative structure of Al(OiPr3) in toluene. Gamma-butyrolactone (BL) has been chosen as a nonpolymerizable model, representative of epsilon-caprolactone. The NMR spectra have been recorded at various concentrations of gamma-butyrolactone and temperatures. As a rule, Al(OiPr)3 is coordinatively aggregated in toluene in such a way that tetrameric and trimeric species are in equilibrium. Upon the addition of BL, this equilibrium is shifted toward the formation of an octahedric [Al(OiPr)3.3BL] mixed tetramer coexisting with [Al(OiPr)3]4 tetramers, the external tetrahedric Al atoms of which are coordinated with extra BL molecules. In the presence of a large excess of BL, only the mixed tetramer persists, which suggests that the single [Al(OiPr)3.3M] species would be the actual initiator for the ring-opening polymerization of cyclic monomers M, such as epsilon-caprolactone, lactides, glycolide, and cyclic anhydrides.