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

Abstract

Solutions of Al isopropoxide in toluene have been investigated by {sup 13}C and {sup 27}Al NMR spectroscopy. Since Al(O{sup i}Pr){sub 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(O{sup i}Pr{sub 3}) 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(O{sup i}Pr){sub 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(O{sup i}Pr){sub 3} {center_dot} 3BL] mixed tetramer coexisting with [Al(O{sup i}Pr){sub 3}]{sub 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(O{sup i}Pr){sub 3} {center_dot} 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.