Macromolecular Engineering of Polylactones and Polylactides. 8. Ring-Opening Polymerization of ϵ-Caprolactone Initiated by Primary Amines and Trialkylaluminum

Whenever added with triethylaluminum, primary amines have been found to be effective initiators for ϵ-caprolactone polymerization in both toluene and THF at 40 °C. The IR and NMR analysis of the polyester has supported a polymerization mechanism proceeding through a “coordination-insertion” pathway and the selective rupture of the acyl-oxygen bond of the monomer. The alkylaluminum activates the carbonyl group of the monomer and accordingly favors the nucleophilic addition of the amine, which is the actual initiation step. Propagation is typically a living process, and the molecular weight distribution is controlled by both AlEt₃/amine molar ratio and solvent. As an extension of that mechanism, diethylaluminum ω-amino alkoxide has been prepared on purpose and successfully used as an initiator for the synthesis of α,ω-dihydroxypoly(ϵ-caprolactone).