Copolymerization of ε‑Caprolactone and l‑Lactide Catalyzed by Multinuclear Aluminum Complexes: An Immortal Approach

A series of aluminum complexes LaAl2Me4 (1), Lb2Al4Me4 (2), and LcAl2Me4 (3) have been prepared
from the reaction of
AlMe3 with Salan- and Salen-type ligands (LaH2 = [2-OH-3,5-tBu2-C6H2CH2N­(CH3)]2-(m-phenylene); LbH4 = [2-OH-3,5-tBu2-C6H2CH2NH]2-(m-phenylene); LcH2 = [2-OH-3,5-tBu2-C6H2CHN]2-(m-phenylene)), respectively. All these complexes were characterized
by NMR spectroscopy, X-ray diffraction, and elemental analyses, with
complexes 1 and 3 adopting binuclear structures,
while complex 2 being tetranuclear. In the presence of
alcohol, the binuclear complexes 1 and 3 catalyzed controlled ring-opening homopolymerizations of both ε-CL
and l-LA. In the copolymerization experiments, complexes 1 and 2 produced tapered copolymers of ε-CL
and l-LA, while complex 3 was able to provide
ε-CL-co-l-LA with tendentially random
structure indicated by the average lengths of the caproyl and lactidyl
sequences (LCL = 1.4; LLA = 2.6). Particularly, addition of excess alcohol into
the catalytic system of complex 3 established the first
“immortal” copolymerization of ε-CL/l-LA, which accelerated the polymerization rates of both monomers
and, thus, afforded random copolymers with predictable molecular weights
and narrow molecular weight distributions.