Structural and Kinetics Studies of N^O‐, O^N^O‐, and N^N‐ Schiff Base Mn Complexes as Highly Active Catalysts in the Ring‐Opening Polymerization of <i>rac‐</i>Lactides

A series of manganese complexes derived from (imino)phenol/pyridine proligands have been synthesized, structurally characterized and used as catalysts in the ring‐opening polymerization (ROP) of rac ‐lactides ( rac ‐LA). Reactions of MnCl 2 ·4H 2 O salt with 2‐[((2‐hydroxyethyl)imino)methyl]phenol ( L1H 2 ) afford a tetranuclear Mn(III) complex [Mn( L1 )Cl] 4 ( Mn1 ). Separately, treatment of MnCl 2 ·4H 2 O with 2‐[((2‐methoxyethyl)imino)methyl]phenol ( L2H ) gave the mononuclear Mn(II) complex [Mn( L2 ) 2 (CH 3 OH) 2 ] ( Mn2 ). Further, reactions of (imino)pyridine proligands ((E)‐2‐((pyridine‐2‐ylmethylene)amino)ethan‐1‐ol ( L3H ) and (E)‐N‐(2‐methoxyethyl)‐1‐(pyrid‐2‐yl)methanimine ( L4 )) with MnCl 2 ·4H 2 O afforded polynuclear Mn(II) complexes [Mn( L3H )Cl 2 ] 3 ( Mn3 ) and [Mn( L4 )Cl 2 ] 3 ( Mn4 ), respectively. The molecular structure of Mn1 established the tridentate binding mode of the dianionic alkoxy‐(imino)phenol proligand (( L1 2− )) through the phenoxo and pendant‐arm alkoxy‐oxygen and the imine‐nitrogen atoms. In contrast, the molecular structure of Mn2 showed that the ether‐(imino)phenol pro‐ligand ( L2H ) is monoanionic ( L2 − ) and bidentately coordinated to the Mn(II) metal center through the phenoxo‐oxygen and the imine‐nitrogen atoms. The dinuclear complex Mn3 contains a neutral N^N bidentately bound proligand ( L3H ). All the complexes ( Mn1 – Mn4 ) formed active catalysts in ROP of rac ‐LA with the propagation rate constant k p of up to (4.25 ± 0.15) × 10 −2 L mol −1 s −1 . The polymers obtained were atactic biased ( P r = 0.55–0.59), produced with moderate control over average‐number molecular weights and were moderately dispersed ( Đ up to 1.8) under melt conditions.