Polymerization of ε-Caprolactone Initiated by Nd(BH4)3(THF)3: Synthesis of Hydroxytelechelic Poly(ε-caprolactone)

The ring-opening polymerization of ε-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 ε-caprolactone may coordinate the neodymium center to form the Nd (BH4) 3 (ε-caprolactone) 3 complex, 1b. Then, ε-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 α-carbonyl group to form the corresponding neodymium (trisalkoxide) derivative Nd[O (CH2) 5CH2 (OBH2) 3], 2a. Polymerization of ε-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 α, ω-telechelic poly (ε-caprolactone) diols, HO (CH2) 5C (O) O (CH2) 5C (O) nO (CH2) 6OH. The proposed mechanism is supported by 1H and 13C NMR, SEC, and MALDI−TOF analyses of the polymers.