Unprecedented Polymerization of ε‐Caprolactone Initiated by a Single‐Site Lanthanide Borohydride Complex, [Sm( η ‐C 5 Me 5 ) 2 (BH 4 )(thf)]: Mechanistic Insights

The monoborohydride lanthanide complex [Sm(Cp*) 2 (BH 4 )(thf)] ( 1 a ) (Cp* = η ‐C 5 Me 5 ), 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 (<1.3) within a short period of time (30 min). The polymers have been characterized by 1 H and 13 C 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 (BH 4 )(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 SmHBH 3 bond followed by reduction of the carbonyl function by the BH 3 end‐group ligand, leads to the samarium alkoxyborane derivative [Sm(Cp*) 2 {O(CH 2 ) 6 O(BH 2 )}] ( 2 ). This compound subsequently initiates the polymerization of ε‐CL through a coordination–insertion mechanism. Finally, upon hydrolysis, α,ω‐dihydroxypoly(ε‐caprolactone), HO(CH 2 ) 5 C(O){O(CH 2 ) 5 C(O)} n O(CH 2 ) 6 OH ( 4 ) is recovered. The stereoelectronic contribution of the two Cp* ligands appears to slow down the polymerization and to limit transesterification reactions.