In this work, four novel binuclear sodium and potassium complexes bearing pyridine-2,6-diyl-bis(methylene)- bridged bis(aminophenolate) ligands were synthesized via the reactions of corresponding bisphenol proligands L¹H₂~L³H₂ with excess NaH or KH in tetrahydrofuran at room temperature. All the newly synthesized proligands and complexes were characterized by ¹H NMR, ¹³C NMR and high-resolution mass spectrometry (HRMS) /or elemental analysis. The X-ray diffraction analysis of complex Na2 being obtained as a pair of racemic isomers shows that the molecule possesses two non-symmetrically coordinated sodium centers, with one sodium center five-coordinated by all of the heteroatoms of the multidentate ligand, the other one four-coordinated by two tetrahydrofuran molecules in addition to the two phenolate oxygen atoms, and the two sodium centers are bridged via two phenolate oxygen atoms, leading to a Na•••Na distance of 0.3041 nm. Without the addition of alcohol, the catalytic activity of Na1 towards the ring-opening polymerization (ROP) of rac-lactide (rac-LA) was very low (turnover frequency (TOF)=225 h^-1), and the molecular weight of the obtained polymer was much lower than the theoretical one. When benzyl alcohol was used as an initiator, all these sodium and potassium complexes showed high catalytic activities and low isoselectivities of P_m=0.51~0.57 towards the polymerization of rac-LA. The electron-donating effect of the substituents on the skeleton nitrogen atoms of the ligand is beneficial to the improvement of catalytic activity of the corresponding complex, while increasing the steric bulkiness of those substituents is unfavorable to the reaction. Among them, complex Na3 with tert-butyl groups substituted on the skeleton nitrogen atoms showed the highest catalytic activity in the presence of four equiv. of benzyl alcohol (TOF up to 46552 h^-1). In addition, with the decrease of the polymerization temperature, the selectivity of complexes for rac-LA polymerization gradually increased. For example, complex Na1 showed a low isoselectivity of P_m=0.57 at room temperature, which could be improved to P_m=0.65 when the polymerization was carried at －50 ℃. The ¹H NMR tracking experiments and matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) analysis indicated that, in the absence of benzyl alcohol, the reaction of typical complex Na1 with 20 equiv. of rac-LA afforded cyclic polymers, which is proposed to be formed via an anionic polymerization pathway initiated by the anion generated through the abstraction of lactide methine proton by complex Na1. The NMR scale reactions of complex Na1 with 1 to 4 equiv. of benzyl alcohol supported the presence of weak interactions between Na1 and BnOH instead of the direct alcoholysis reaction; upon further treated with 20 equiv. of rac-LA, active propagation PLA chain end-capped by benzyloxy group at one end could be identified mainly, which was companied by partial decomposition of Na1. On the basis of NMR tracking experiments as well as MALDI-TOF mass spectrometry analysis, it is proposed that, in the presence of benzyl alcohol, the polymerization of rac-LA catalyzed by Na1 processes via ligand assisted monomer activation pathway mainly and the anionic pathway is inhibited partially.

Key words: pyridine-2,6-diyl-bis(methylene)-bridged bis(aminophenol), alkaline metal complex, racemic lactide, ring-opening polymerization, isoselectivity