Tridentate [ONN] Iminophenolate Aluminum Complexes for Controlled Ring-Opening (Co)Polymerization of rac -Lactide, ε-Caprolactone, and δ-Valerolactone: Impact of Pendant Diamine Donor Arms and Ligand Substituents on Catalytic Performance

High Resolution Image Download MS PowerPoint Slide A series of tridentate [ONN] iminophenolate aluminum complexes ( 1 – 15 ) bearing hemilabile pendant diamine donor arms with tunable flexibility and systematically varied phenoxy substituents was successfully synthesized and structurally characterized in solution. All complexes promoted the controlled/living ring-opening polymerization of rac -lactide ( rac -LA), ε-caprolactone (ε-CL), and δ-valerolactone (δ-VL) in the presence of benzyl alcohol, affording polyesters with good control over molar mass and dispersity. Lewis acidity studies of representative complexes 1, 6, and 11, featuring ethylenedimethylamine, propylenedimethylamine, and 2,2-dimethylpropanedimethylamine donor arms, respectively, provided insights into the relationship between the accessibility of the metal center and catalytic performance. The accessibility of the aluminum center increased in the order 1 < 6 < 11, paralleling the enhancement in catalytic activity and establishing the flexibility of the pendant donor arm as a key structural parameter for improving the performance of aluminum complexes. Taking advantage of the controlled nature of these polymerizations, well-defined diblock copolymers, including poly(CL- b - rac- LA), poly(VL- b - rac -LA), poly(CL- b -VL), and poly(VL- b -CL), were successfully synthesized via sequential-feed copolymerization. In addition, single-feed copolymerization of ε-CL and δ-VL afforded the perfect random poly(CL- co -VL), with average caproyl and valeroyl sequence lengths ( L CL and L VL ) close to 2. These results establish hemilabile flexible pendant donor arms and phenoxy substituents as effective design elements for tuning tridentate aluminum catalysts in the controlled polymerization of cyclic esters.