Ring opening polymerization and copolymerization of L‐lactide and ɛ‐caprolactone by bis‐ligated magnesium complexes

Abstract

ABSTRACTSeven 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 ɛ‐caprolactone (ɛCL) 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 °C, while the presence of CF3 groups in 6 and 7 slowed or resulted in no PLA detected. With ɛCL, 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 ɛCL yielded block copolymers only from the sequential polymerization of ɛCL 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. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 48–59