Unprecedented polymerization of epsilon-Caprolactone initiated by a single-site lanthanide borohydride complex, [Sm(eta-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 ring-opening polymerization of epsilon-caprolactone (epsilon-CL). The organometallic samarium(III) initiator 1 a produces, in quantitative yields, alpha,omega-dihydroxytelechelic poly(epsilon-caprolactone) displaying relatively narrow polydispersity indices (<1.3) within a short period of time (30 min). The polymers have been characterized by 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)(epsilon-CL)] (1b). Indeed, one molecule of epsilon-CL initially displaces the coordinated THF in 1 a to give 1 b. 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)6O(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)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.
Laboratoire de Chimie des Polymères Organiques, Unité Mixte de Recherche CNRS (UMR 5629), ENSCPB, Université de Bordeaux 1, 16 Avenue Pey-Berland, 33607 Pessac, France.
Isabelle
I
Palard
Alain
A
Soum
Sophie M
SM
Guillaume