Dimethylaluminum complexes bearing bidentate amidate, oxypyridine, and salicylaldimine N,O-ligands and tridentate N,N',N"-pyridyliminoamide ligands were synthesized and spectroscopically characterized. The complexes were investigated in both neutral and borane-activated cationic forms, along with bidentate N,N'-ligated aluminum amidinates, as catalysts for the polymerization of methyl methacrylate, E-caprolactone, and propylene oxide. The neutral complexes generally did not carry out polymerization, but the polymerization/oligomerization of all three monomers was achieved when the various catalysts were activated with B(C6F5)(3) or FPh3C](+)[B(C6F5)(4)](-). The N,O-ligated cations were much less active for polymerization than the analogous, more stable N,N'-ligated amidinate cations; both types of cationic complexes catalyzed the ring-opening cationic polymerization of tetrahydrofuran. B(C6F5)(3) and [Ph3C](+)[B(C6F5)](-) also independently carried out the oligomerization of propylene oxide.