Seven magnesium complexes (1-7) were synthesized by reaction of new (L-3-H-L-5-H) and previously reported ketoimine pro-ligands with dibutyl magnesium and were isolated in 59-70% yields. Complexes 1-7 were characterized fully and consisted of bis-ligated homoleptic ketoiminates coordinated in distorted octahedral geometry around the magnesium centers. The complexes were investigated for their ability to initiate the ring opening polymerization (ROP) of l-lactide (L-LA) to poly-lactic acid (PLA) and e-caprolactone (eCL) to poly-caprolactone in the presence of 4-fluorophenol co-catalyst. For L-LA polymerization, complexes containing ligand electron-donating groups (1-5) achieved >90% conversion in 2 h at 100 degrees C, while the presence of CF3 groups in 6 and 7 slowed or resulted in no PLA detected. With eCL, ROP initiated with 1-7 resulted in lower percentage conversion with similar electronic effects. Moderate molecular weight PLA polymeric material (14.3-21.3 kDa) with low polydispersity index values (1.23-1.56) was obtained, and ROP appeared to be living in nature. Copolymerization of L-LA and eCL yielded block copolymers only from the sequential polymerization of eCL followed by L-LA and not the reverse sequence of monomers or the simultaneous presence of both monomers. Polymers and copolymers were characterized with NMR, gel permeation chromatography, and differential scanning calorimetry. (c) 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 48-59