Ring-opening polymerization of ε-caprolactone and L-lactide using ethyl salicylate-bearing zinc complexes as catalysts

A series of zinc complexes bearing ethyl salicylates were synthesized, and their catalytic activity in the polymerization of ε-caprolactone (CL) and L-lactide (LA) was investigated. In CL polymerization, the catalytic activity of Zn complexes was promoted by ligands with the substituents at the ortho position of phenolates, such as the 2-methyl group of L^6MeZnEt (40 min, 94% conversion at 25 °C, [CL] = 2.0 M) and the 2,4-di‑tert‑butyl groups of L^BuZnEt (55 min, 93% conversion at 25 °C, [CL] = 2.0 M). In LA polymerization, the catalytic activity was promoted by ligands with electron-donating substituents, such as the 4-methyl group of L^4MeZnEt (55 min, 90% conversion at 50 °C, [LA] = 0.5 M) and the 2,4-di‑tert‑butyl group of L^BuZnEt (60 min, 90% conversion at 50 °C, [LA] = 0.5 M). Density functional theory calculations revealed that the polymerization requires the detachment of a bridging alkoxide group from one Zn center to form an intermediate II-CL; bulky ligands may facilitate this unfavored dissociation process (from I-CL to II-CL). In LA polymerization, ligands with electron-withdrawing groups increase the Lewis acidity of Zn centers; the detachment of coordination atoms from the Zn atom is difficult; hence, the carbonyl group of ring-opening LA back binds to the Zn center to form a low-energy intermediate trip and then hinders another LA coordination.