Organometallic calcium and strontium borohydrides as initiators for the polymerization of ε-caprolactone and l-lactide: combined experimental and computational investigations

Abstract

[Ca(BH₄)₂(THF)₂] (1a), a known compound, was easily prepared following a convenient new procedure from [Ca(OMe)₂] and BH₃·THF in THF. Reaction of 1a with KCp* (Cp* = (η⁵-C₅Me₅)) and K{(Me₃SiNPPh₂)₂CH} in a 1 : 1 ratio in THF resulted in the corresponding dimeric heteroleptic mono-borohydride derivatives [Cp*Ca(BH₄)(THF)_n]₂ (2a) and [{(Me₃SiNPPh₂)₂CH}Ca(BH₄)(THF)₂] (3a), respectively. Both compounds were fully characterized and the solid-state structure of 3a was established by single crystal X-ray diffraction. Compounds 1a, 2a, and 3a, together with the earlier reported compounds [Sr(BH₄)₂(THF)₂] (1b), [Cp*Sr(BH₄)(THF)₂]₂ (2b), and [{(Me₃SiNPPh₂)₂CH}Sr(BH₄)(THF)₂] (3b), were used as initiators for the ROP of polar monomers. The general performances of the complexes in the ROP of ε-caprolactone and L-lactide demonstrate a relatively good control of the polymerization under the operating conditions established. α,ω-Dihydroxytelechelic poly(ε-caprolactone)s (PCLs) and poly(lactide)s (PLAs) were thus synthesized. DFT calculations on the initiation step of the ROP of ε-CL were carried out. Gibbs free energy profiles were determined for the three calcium complexes highlighting slightly more active calcium complexes as compared to strontium analogues, in agreement with experimental findings.