A DFT study of the mechanism of polymerization of epsilon-caprolactone initiated by organolanthanide borohydride complexes

The mechanisms of polymerization of c-caprolactone (CL) initiated by either the rare-earth hydride [Cp2Eu(H)] or the borohydrides [Cp2Eu(BH4)] or [(N2NN')Eu(BH4)] were studied at the DFT level (Cp = eta(5)-C2H5; N2NN' = (2-C5H4N)CH2(CH2-CH2NMe)(2)). For all compounds the reaction proceeds in two steps: a hydride transfer from the rare earth initiator to the carbonyl carbon of the lactone, followed by ring-opening of the monomer. In the last step a difference is observed between the hydride and borohydride complexes, because for the latter the ring-opening is induced by an additional B-H bond cleavage leading to a terminal -CH2OBH2 group. This corresponds to the reduction by BH3 Of the carbonyl group of CL. Upon reaction of [Cp2Eu(H)] with CL, the alkoxy-aldehyde complex produced, [Cp2Eu{O(CH2)(5)C(O)H}], is the first-formed initiating species. In contrast, for the reaction of CL with the borohydride complexes [(L-x)Eu(BH4)] (L-x = Cp-2 or N2NN'), an aliphatic alkoxide with a terminal -CH2OBH2 group, [(L-x)Eu{O(CH2)(6)OBH2}] is formed and subsequently propagates the polymerization. The present DFT investigations are fully compatible with previously reported mechanistic studies of experimental systems.