The ring-opening polymerization of -caprolactone initiated by Nd(BH4)3(THF)3, 1a, has been investigated. The polymerization occurs rapidly (<5 min) and quantitatively at room temperature to give, in high yields, polymers which exhibit controlled molecular weights and relatively low polydispersities. Initially, 3 mol of -caprolactone may coordinate the neodymium center to form the Nd(BH 4)3( -caprolactone)3 complex, 1b. Then, -caprolactone inserts into the metal-borohydride hydrogen bond of 1a or 1b to give the intermediate [(BH4)2Nd[O(CH2)5C(O)(HBH3)]], [i], in which each (HBH3) unit immediately interacts with the R-carbonyl group to form the corresponding neodymium(trisalkoxide) derivative [Nd[O(CH2)5CH2(OBH2)]3], 2a. Polymerization of -caprolactone by 2a then proceeds via a pseudoanionic mechanism through the alkoxide attack at the monomer resulting in the oxygen-acyl bond cleavage. Further treatment of the resulting product with benzyl alcohol gives R,ω-telechelic poly(caprolactone) diols, HO(CH2)5C(O){O(CH2)5C(O)}nO(CH2)6OH. The proposed mechanism is supported by 1 H and 13 C NMR, SEC, and MALDI-TOF analyses of the polymers. † Previously publishing under Sophie M. Cendrowski-Guillaume.

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