Ring-opening polymerization of ε-caprolactone and lactides promoted by salan- and salen-type yttrium amido complexes

The ring-opening polymerization (ROP) of cyclic esters such as ε-caprolactone (ε-CL), l- and rac-lactide,promoted by yttrium silylamido complexes bearing binaphthyl-bridged salen (1 and 2) and diaminebisphenolate salan ligands (3 and 4) is described.The yttrium silylamido complexes 1–4 are effective initiators for the ROP of ε-caprolactone, showingextremely high turnover frequencies (TOF up to 18000 h−1) under mild reaction conditions. All complexespromote the ROP of rac-lactide in toluene solution at room temperature, providing atactic polymers withcontrolled molecular weights and relatively narrow polydispersities (Mw/Mn= 1.73–1.99). Interestingly,in THF solution the same catalysts produce heterotactic polylactides with Prbetween 0.58 and 0.91 viaa chain-end stereocontrol mechanism. The salan complexes 3 and 4 are more active than the binaphthylsalen complexes 1 and 2, reasonably due to the presence of the more electron donating amino groups.On the other hand, the former resulted in a lower stereoselectivity than the latter. The rigidity of the“bridge” between the two nitrogen atoms seems to have a predominant role in governing the selectivityof the corresponding complexes, while the effect of the steric hindrance of the ortho substituents at thephenoxy rings appears less significant.