Unprecedented polymerization of ε-caprolactone initiated by a single-site lanthanide borohydride complex, [SM(η-C5Me5)2(BH4)(thf)]:: Mechanistic insights

The monoborohydride lanthanide complex [SM(CP*)(2)(BH4)(thf)] (1a) (Cp* = eta-C5Me5), has been successfully used for the controlled ringopening polymerization of epsilon-caprolactone (epsilon-CL). The organometallic samarium(III) initiator la produces, in quantitative yields, alpha,omega-dihydroxytelechelic poly(epsilon-caprolactone) displaying relatively narrow polydispersity indices (<1.3) within a short period of time (30min). The polymers have been characterized by H-1 and C-13 NMR, SEC, and MALDI-TOF MS analyses. Use of the single-site initiator 1a allows a better understanding of the polymerization mechanism, in particular with the identification of the intermediate compound [SM(CP*)(2)(BH4)(epsilon-CL)] (1b). Indeed, one molecule of F-CL initially displaces the coordinated THF in 1a to give 1b. Then, epsilon-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)(6)O(BH2)}] (2). This compound subsequently initiates the polymerization of epsilon-CL through a coordination-insertion mechanism. Finally, upon hydrolysis, alpha,omega-dihydroxypoly(epsilon-caprolactone), HO(CH2)(5)C(O){O(CH2)(5)C(O)}(n)O(CH2)(6)OH (4) is recovered. The stereoelectronic contribution of the two Cp* ligands appears to slow down the polymerization and to limit transesterification reactions.