Rare Earth Metal Tris(borohydride) Complexes as Initiators for ε-Caprolactone Polymerization:  General Features and IR Investigations of the Process

Trivalent rare earth metal borohydride complexes Ln(BH4)3(THF)3 (Ln = La, Nd, Sm) initiate the ring-opening polymerization of ε-caprolactone (ε-CL) to give, in quantitative yields and in a few minutes, α,ω-dihydroxytelechelic poly(ε-caprolactone) displaying a relatively narrow molar mass distribution (≤1.3). The polymer features depend on both the metal and the solvent. A good agreement between M̄n(theo) and M̄n(exp) is observed for low [monomer]0/[initiator]0 values whereas a deviation (M̄n(exp) < M̄n(theo)) is obtained at higher ratios; this behavior has been rationalized by the occurrence of some transfer reactions. The gel formed during the polymerization arises from the involvement of the −(OBH2) end groups of the active polymer chains in several inter- and intramolecular van der Waals interactions, as revealed by in-depth infrared investigations of the various intermediates implicated in the whole polymerization mechanism. Finally, the polymers synthesized display only traces of residual nontoxic rare earth metals, thus making them suitable for biomedical applications.