Ring-opening polymerization of β-butyrolactone catalyzed by efficient magnesium and zinc complexes derived from tridentate anilido-aldimine ligand

Magnesium (Mg) and zinc (Zn) complexes incorporating tridentate anilido-aldimine ligand, (E)-2, 6-diisopropyl-N-(2-((2-(piperidin-1-yl) ethylimino)methyl)phenyl)aniline (AA^Pip-H, 1), were synthesized and structurally characterized. The reaction of AA^Pip-H (1) with Mg ⁿBu₂ or ZnEt₂ in equivalent proportions afforded the monomeric complex [(AA^Pip)MgⁿBu] (2) or [(AA^Pip)ZnEt] (3), respectively. The coordination modes of these complexes differ in the solid state: Mg complex 2 shows a four-coordinated and distorted tetrahedral geometry, whereas Zn complex 3 adopts a trigonal planar geometry with a three-coordinated Zn center. Complexes 2 and 3 are efficient catalysts for the ring-opening polymerization of β-butyrolactone (β-BL) in the presence of 9-anthracenemethanol (9-AnOH). The polymerization of β-BL with the Zn catalyst system is demonstrated in a living fashion with a narrow polydispersity index, PDI = 1.01-1.10. The number-averaged molecular weight (M_n) of the produced poly(3-hydroxybutyrate) (PHB) is quite close to the expected M_n over diverse molar ratios of monomer to 9-AnOH. A greater ratio of monomer to alcohol catalyzed by Zn complex 3 served to form PHB with a large molecular weight (M_n > 60000). An effective method to prepare PHB-b-PCL and PEG-b-PHB by the ring-opening copolymerization of β-BL catalyzed by zinc complex 3 is reported. The ring-opening polymerization of β-butyrolactone catalyzed by efficient catalysts 2 and 3 in the presence of 9-anthracenemethanol or polyethylene glycol methyl ether was fully investigated. The catalytic system of complex 3 has demonstrated good activity and excellent control of molecular weight to produce PHB homopolymer, PHB-b-PCL, and PEG-b-PHB copolymers.