Metal and Ligand-Substituent Effects in the Immortal Polymerization of rac-Lactide with Li, Na, and K Phenoxo-imine Complexes

A series of lithium, sodium, and potassium complexes with phenoxo-imine ligands M[(O-2-(RN=CH)-C6H4] [R = C6H5; 2-(BuC6H5)-Bu-t; 2,6-(Pr2C6H3)-Pr-i] and [O-2-(RN=CH)-4,6-(Bu2C6H4)-Bu-t] [R = C6H5; 2-(BuC6H5)-Bu-t; 2,6-(Pr2C6H3)-Pr-i] 1-3(a-f) have been synthesized. The molecular structures in the solid state of some of these complexes have been determined by X-ray diffraction. These compounds show different nuclearities and geometries around the metal center depending on the nature and the pocket of the ligand substituents. Of particular interest is the structure of compound 3e, being the first example of a potassium cubane complex obtained with this kind of ligand. The structural behavior in solution has also been studied by diffusion-ordered NMR spectroscopy (DOSY), showing a direct correlation between aggregation behavior and polymerization activity. Compounds 1-3(a-f) are extremely active catalysts in the ring-opening polymerization (ROP) of rac-lactide, achieving conversions of 100% in less than 1 min and heterorich-PLA that is modified by the metal atom and the ligand substituents. BnOH was used as co-initiator, and the presence of large amounts of the alcohol produces the immortal polymerization of rac-lactide in a more controlled process. Stoichiometric reactions involving the catalysts, BnOH, and lactide demonstrated an activated monomer mechanism for the polymerization of rac-lactide.