Reverse orders of reactivities in the polymerization of cyclic esters using N₂O₂ aluminium alkoxide complexes

Three aluminium alkoxide complexes containing N₂O₂ bis(phenoxy)-amine ligands were synthesized from reactions of the corresponding N₂O₂ ligands with Al(OⁱPr)₃ in toluene. Different amine side chains of the ligands included pyridine (1), CH₂NMe₂ (2), and CH₂NEt₂ (3). The related chloro aluminium analog (4) was prepared from a reaction between AlCl₃ and the potassium salt of the N₂O₂ ligand having CH₂NMe₂ side chain. X-Ray crystallography reveals that complexes 3 and 4 have a monomeric five-coordinate aluminium center. Complexes 1-3 catalyzed the polymerization of ε-caprolactone (ε-CL) at 70 °C in toluene with the relative reactivities of 1 < 2 < 3. In contrary, only complex 1 was active for the polymerization of lactide under the same polymerization conditions. ¹H NMR spectroscopy shows that treatment of 2 with 1 equivalent of lactide afforded the ring-opened product L²Al-OCH(Me)C(O)OCH(Me)C(O)OⁱPr. Electronic effects are believed to be responsible for the observed trend in the ε-CL polymerization rates. On the other hand, steric hindrance at the amine side chain is the main contributor to the observed rates of lactide polymerization.