Ring-opening polymerization of e-caprolactone initiated by rare earth alkoxides and borohydrides: a comparative study

The polymerization of epsilon-caprolactone initiated by trivalent rare earth (Ln) derivatives, namely the isopropoxide 'Ln(OiPr)(3)' opposed to the borohydride Ln(BH4)(3)(THF)(3) (Ln = La, Nd, Sm) and (CP*)(2)Sm(BH4)(THF) (Cp* = (eta-C5Me5)), has been evaluated. The comparison, based on the structure of the initiator and on the polymerization features (molar mass, molar mass distribution, side reactions) reveals some differences especially in the number of active polymer chains per metal and in the occurrence of some transfer reactions with Ln(BH4)(3)(THF)(3). Polymerizations performed with borohydride derivatives lead to a gel as a result of van der Waals interactions, while the reaction medium remains fluid with the alkoxide species. A typical coordination-insertion mechanism with oxygen-acyl bond cleavage of the monomer prevails with both types of initiators. However, while the alkoxide complexes lead to alpha,omega-hydroxyalkoxypoly(epsilon-caprolactone), the borohydride compounds allow the synthesis of alpha,omega-dihydroxypoly(epsilon-caprolactone). Both polymerization processes are well controlled under specific conditions