The ring-opening polymerization of ε-caprolactone has been initiated by [tris(hexamethyldisilyl)amide]yttrium in the presence of an excess of 2-propanol. This initiating system is very active even at high alcohol-to-Y molar ratio. Polymers with controlled molecular parameters (M n, end groups) and low dispersity are formed as result of fast alkoxide/alcohol exchange. At molar excess of 2-propanol higher than 100, this exchange is no longer quantitative. The in situ formed active species involved in this new initiating system are different from those ones present in the commercial yttrium oxo-isopropoxide Y5(μ-O)(OiPr)13 cluster. Evidence is provided by 1H NMR spectroscopy.

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[Tris(hexamethyldisilyl)amide] yttrium is a catalyst for the controlled ring-opening polymerization of ε-caprolactone when reacted with an excess of alcohol. Up to an alcohol-to-yttrium molar ratio of 50, the chain transfer to the alcohol is quantitative, and the molecular weight of the chains can be predicted from the monomer-to-alcohol molar ratio. At alcohol-to-yttrium ratios higher than 100, the alcohol is quantitatively consumed provided that 1,1,1,3,3,3-hexamethyldisilazane, which is formed as a byproduct, is removed from the reation medium. Depending on the structure of the alcohol, end-functional polyesters can be prepared (e.g., with N-pyrrolyl and 3-thienyl end groups). Well-defined block and random copolymers of ε-CL with lactides and γ-bromo-ε-caprolactone, respectively, have also been synthesized. Compared to aluminum alkoxides, the in situ formed yttrium alkoxides have the advantage of a much faster polymerization kinetics while preserving control.

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