Synthesis of Sodium Complexes Supported with NNO-Tridentate Schiff Base Ligands and Their Applications in the Ring-Opening Polymerization of l‑Lactide

A series of sodium complexes bearing
NNO-tridentate Schiff base ligands with an N-pendant arm were synthesized
and used as catalysts for the ring-opening polymerization of l-lactide (l-LA). Electronic effects of ancillary ligands
coordinated by sodium complexes substantially influence the catalysis,
and ligands with electron-donating groups increase the catalytic activity
of the sodium complexes for catalyzing l-LA polymerization.
In particular, a sodium complex bearing a 4-methoxy group has the
highest activity with conversion up to 95% within 30 s at 0 °C
and a low polydispersity index of 1.13, whereas the 4-bromo group
showed the poorest performance with regard to the catalytic rate of l-LA polymerization in the presence of benzyl alcohol (BnOH). 1H NMR pulsed-gradient spin–echo diffusion experiments
and single-crystal X-ray analyses showed that sodium complexes [LHNa­(THF)]2 and [L4‑ClNa­(THF)]2 were dinuclear species in both solution and the solid state.
The kinetic results indicated a first-order dependence on each of
[[L4‑ClNa]2], [l-LA], and [BnOH].