NacNac-zinc-pyridonate mediated ε-caprolactone ROP

Area of Science:

• Organometallic Chemistry
• Polymer Chemistry
• Catalysis

Background:

• Zinc complexes with bulky β-diiminate (NacNac) ligands are known catalysts for polymerization.
• Amide ligands, such as pyridonates, are less explored as initiating groups in zinc-catalyzed ring-opening polymerization (ROP).

Purpose of the Study:

• To synthesize and characterize new zinc-amidate complexes.
• To investigate their activity in the ring-opening polymerization (ROP) of ε-caprolactone (PCL).
• To elucidate the polymerization mechanism and the role of the amidate ligand.

Main Methods:

• Synthesis and isolation of novel zinc-amidate complexes.
• Solution state characterization using 1H DOSY NMR spectroscopy.
• Computational studies using DFT calculations.
• Polymerization experiments under exogenous initiator-free conditions.
• Product characterization using MALDI-ToF mass spectrometry.

Main Results:

• Two new zinc complexes featuring a 2,6-diisopropylphenyl (Dipp) NacNac ligand and either a 2-pyridonate or 6-methyl-2-pyridonate amidate ligand were synthesized.
• Both complexes exhibit monomeric behavior in solution and initiate ROP via a single-site monometallic mechanism.
• High molecular weight PCL was obtained using both catalysts under exogenous initiator-free conditions.
• Increased steric bulk on the pyridonate ligand (6-methyl vs. 6-H) enhanced catalytic activity, achieving full monomer conversion in 1 hour.
• Pyridonate end-groups were observed in the resulting PCL, and cyclic PCL species were detected, suggesting a role for the pyridonate as a leaving group.

Conclusions:

• These novel zinc-amidate complexes are effective catalysts for exogenous initiator-free ROP of ε-caprolactone.
• The pyridonate ligand plays a crucial role in the initiation and termination steps of the polymerization.
• These findings represent the first examples of pyridonate-initiated ROP using zinc complexes.