Random Copolymerization of ε‑Caprolactone and Lactides Promoted by Pyrrolylpyridylamido Aluminum Complexes

The monomethylaluminum complexes 1 and 2, bearing pyrrolylpyridylamido as dianionic [⁻NNN^–] tridentate ligands with general formula [NNN]­AlMe, were synthesized and tested as initiators in the ring-opening polymerization (ROP) of ε-caprolactone, l-lactide, and d,l-lactide. In the presence of 1 equiv of alcohol, compounds 1 and 2 were highly active initiators in the ROP of ε-CL (TOF up to 4000 mol_CL mol_Al^–1 h^–1), and they showed moderate activity in the ROP of lactides (TOF up to 1.7 mol_LA mol_Al^–1 h^–1). The polymerization processes proceeded with a living mechanism; moreover, the obtained PLAs resulted isotactic-enriched with P_m values up to 76%. More interestingly, this class of catalysts promoted the random copolymerization of ε-caprolactone and lactides. In particular, compound 1 allowed excellently controlled random copolymerization of ε-caprolactone and d,l-lactide as indicated by both the values of the reactivity ratios of the two monomers (r_LA = 1.17; r_CL = 1.36) and the average lengths of the caproyl and lactidyl sequences (L_CL = 2.0; L_LA = 2.5).