Chelation Effect in Polymerization of Cyclic Esters by Metal Alkoxides: Structure Characterization of the Intermediate Formed by Primary Insertion of Lactide into the Al−OR Bond of an Organometallic Initiator

Abstract

Well-defined dialkylaluminum alkoxides derived from 2-methoxyethanol [R2Al(μ-OCH2CH2OMe)]2 [where R = Me (1a) and tBu (1b)] and rac-ethyl lactate [Me2Al(μ-OCH(Me)CO2Et)]2 (2) were used as model complexes mimicking intermediate species in the initiation and propagation steps in the ring-opening polymerization of cyclic esters with metal alkoxides. Particularly, the effect of Lewis base termini of both the supporting ligand and the first polymer chain repeating unit on the aluminum alkoxides activity in the polymerization of ε-caprolactone (ε-CL) and rac-lactide (LA) were explored. It revealed that the initiation process is affected by the extent of chelation to a similar degree for ε-CL and LA as the character of the donor-functionalized alkoxide ligand determines the access of an incoming cyclic ester molecule to the effective coordinate site. On the other hand, a significant chelation effect on the propagation step was observed only in the polymerization of LA. The difference in the reactivity is rationalized on the basis of the first structurally authenticated intermediate [Me2Al(μ-OCH(Me)C(O))2O(CH2)2OMe]2 (1-LA) resulting from primary insertion of a lactide molecule into the aluminum−alkoxide bonds of the [Me2Al(μ-OCH2CH2OMe)]2 initiator. The direct relationship between the extent of intramolecular coordination and initiation and propagation steps of polymerization is also presented.