A Computational Analysis of the Ring-Opening Polymerization of rac-Lactide Initiated by Single-Site β-Diketiminate Metal Complexes:  Defining the Mechanistic Pathway and the Origin of Stereocontrol

The ring-opening polymerization of rac-lactide at a β-diketiminate magnesium center, [HC{CMeN-2,6-ⁱPr₂C₆H₃}₂]Mg(OMe)(THF), has been investigated using a B3-LYP density functional procedure employing three different layers of basis set:  6-311G(3d) at the Mg center, 6-31G(d) for both the ligand skeleton and the monomer, and a STO-3G basis set at the bulky 2,6-diisopropylphenyl substituents. By studying the consecutive ring-opening of two lactide molecules, clear conclusions are drawn regarding both the mechanism of ring-opening and the origin of heterotactic stereocontrol observed with such initiators. Polymerization proceeds via two major transition states, an observation applicable to other coordinative initiator systems, with the highest energy transition state dictating the stereochemistry of monomer insertion. In the β-diketiminate magnesium system, a detailed examination of the rate-limiting second transition state geometries reveals that heterotactic poly(lactic acid) arises via the minimization of several steric interactions, possibly reinforced by an attractive CH···π interaction.