Unprecedented Polymerization of ε‐Caprolactone Initiated by a Single‐Site Lanthanide Borohydride Complex, [Sm(η‐C5Me5)2(BH4)(thf)]: Mechanistic Insights

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Abstract
The monoborohydride lanthanide complex [Sm(Cp*)2(BH4)(thf)] (1 a) (Cp* = η-C5Me5), has been successfully used for the controlled ring-opening polymerization of ε-caprolactone (ε-CL). The organometallic samarium(III) initiator 1 a produces, in quantitative yields, α,ω-dihydroxytelechelic poly(ε-caprolactone) displaying relatively narrow polydispersity indices (1H and 13C NMR, SEC, and MALDI-TOF MS analyses. Use of the single-site initiator 1 a allows a better understanding of the polymerization mechanism, in particular with the identification of the intermediate compound [Sm(Cp*)2(BH4)(ε-CL)] (1 b). Indeed, one molecule of ε-CL initially displaces the coordinated THF in 1 a to give 1 b. Then, ε-CL opening (through cleavage of the cyclic ester oxygen–acyl bond) and insertion into the SmHBH3 bond followed by reduction of the carbonyl function by the BH3 end-group ligand, leads to the samarium alkoxyborane derivative [Sm(Cp*)2{O(CH2)6O(BH2)}] (2). This compound subsequently initiates the polymerization of ε-CL through a coordination–insertion mechanism. Finally, upon hydrolysis, α,ω-dihydroxypoly(ε-caprolactone), HO(CH2)5C(O){O(CH2)5C(O)}nO(CH2)6OH (4) is recovered. The stereoelectronic contribution of the two Cp* ligands appears to slow down the polymerization and to limit transesterification reactions.

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