A DFT Study of the Mechanism of Polymerization of ε‐Caprolactone Initiated by Organolanthanide Borohydride Complexes

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Abstract
The mechanisms of polymerization of ε-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=η5-C5H5; 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)5C(O)H}], is the first-formed initiating species. In contrast, for the reaction of CL with the borohydride complexes [(Lx)Eu(BH4)] (Lx=Cp2 or N2NN′), an aliphatic alkoxide with a terminal CH2OBH2 group, [(Lx)Eu{O(CH2)6OBH2}] is formed and subsequently propagates the polymerization. The present DFT investigations are fully compatible with previously reported mechanistic studies of experimental systems.

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