Synthesis and Characterization of Nickel(II) Complexes for Catalyzing the Ring-Opening Polymerization of l-Lactide

Industrial polylactide (PLA) production typically uses stannous octoate as a catalyst for lactide (LA) polymerization, which has drawback that PLA degradation caused by tin residual. This study explores the synthesis and application of tetradentate nitrogen-coordinated Ni(II) complexes (L1Ni–L4Ni) as alternative catalysts for the ring-opening polymerization of unpurified l-LA. The robust Ni(II) complexes were synthesized and characterized, and their catalytic performance was evaluated. Results indicated that in the presence of benzyl alcohol, the Ni(II) complexes demonstrated relatively good catalytic activity, achieving over 90% conversion of l-LA within 4 h. The polymerization reaction even can be operated in air without affecting the catalytic effect. The molecular weight of the resulting PLA showed a linear relationship with the [LA]/[Ni] ratio, indicating characteristics of living polymerization. The polymerization mechanism was proposed based on MALDI–TOF, NMR, FT-IR and DFT calculation.