Ring-opening polymerization of epsilon-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 epsilon-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 2methyl group of L6MeZnEt (40 min, 94% conversion at 25 degrees C, [CL] = 2.0 M) and the 2,4-di-tert-butyl groups of LBuZnEt (55 min, 93% conversion at 25 degrees 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 L4MeZnEt (55 min, 90% conversion at 50 degrees C, [LA] = 0.5 M) and the 2,4-di-tert-butyl group of LBuZnEt (60 min, 90% conversion at 50 degrees 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.