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

• Salen-like and salan-like yttrium silylamido complexes have been explored.
• All complexes were highly active in the ring-opening polymerization of ɛ-caprolactone.
• Good control of macromolecular parameters in the ROP of lactides was shown.
• Highly heterotactic PLAs (Pr up to 0.91) were obtained.
• Tetrad analysis revealed a chain-end mechanism of stereocontrol.

The ring-opening polymerization (ROP) of cyclic esters such as ɛ-caprolactone (ɛ-CL), l- and rac-lactide, promoted by yttrium silylamido complexes bearing binaphthyl-bridged salen (1 and 2) and diamine bisphenolate salan ligands (3 and 4) is described.The yttrium silylamido complexes 1–4 are effective initiators for the ROP of ɛ-caprolactone, showing extremely high turnover frequencies (TOF up to 18000 h−1) under mild reaction conditions. All complexes promote the ROP of rac-lactide in toluene solution at room temperature, providing atactic polymers with controlled molecular weights and relatively narrow polydispersities (Mw/Mn = 1.73–1.99). Interestingly, in THF solution the same catalysts produce heterotactic polylactides with Pr between 0.58 and 0.91 via a chain-end stereocontrol mechanism. The salan complexes 3 and 4 are more active than the binaphthyl salen 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 selectivity of the corresponding complexes, while the effect of the steric hindrance of the ortho substituents at the phenoxy rings appears less significant.

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