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Rare earth tris(borohydride) complexes, Ln(BH4)3(THF)3 (Ln = Sc, Y, La and Dy), are highly efficient single-component catalysts for ring opening polymerizations of α-amino acid N-carboxyanhydrides (NCAs) as presented by H. Peng, J. Ling, Y. Zhu, L. You, and Z. Shen on page 3016. Homopolypeptides and block copolypeptides with relatively narrow molecular weight distributions (as low as 1.14) can be prepared. Polymerization mechanism studies show that at room temperature, two reaction pathways occur simultaneously producing α-hydroxyl-ω-aminotelechelic and α-carboxylic-ω-aminotelechelic polypeptides. However, when the temperature is decreased to 0°C, one of the reaction pathways can be efficiently depressed affording α-hydroxyl-ω-aminotelechelic polypeptide exclusively.