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

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.