Bimetallic Aluminum 5,6-Dihydro-7,7-dimethyl quinolin-8-olates as Pro-Initiators for the ROP of ε-CL; Probing the Nuclearity of the Active Initiator

Six examples of aluminum 5,6-dihydro-7,7-dimethylquinolin-8-olates, [{2-R¹-7,7-Me₂-8-R²C₉H₆N-8-O}AlR³₂]₂ (R¹ = R² = H, R³ = Me ; R¹ = R² = H, R³ = Et ; R¹ = R² = H, R³ = -Bu ; R¹ = Cl, R² = H, R³ = Me ; R¹ = H, R² = R³ = Me ; R¹ = Cl, R² = R³ = Me ), have been prepared by treating the corresponding pro-ligand ( ⁻ ) with either AlMe₃, AlEt₃ or Al( -Bu)₃. All complexes have been characterized by ¹H and C NMR spectroscopy and in the case of and by single crystal X-ray diffraction; dimeric species are a feature of their molecular structures. In the presence of PhCH₂OH (BnOH), ⁻ displayed good control and efficiency for the ROP of ε-CL with almost 100% conversion achievable in 10 min at 90 °C; the chloro-substituted and notably exhibited the lowest activity of the series. However, in the absence of BnOH, showed only low activity with 15% conversion achieved in 30 min forming a linear polymer capped with either a methyl or a group. By contrast, when one or more equivalents of BnOH was employed in combination with , the resulting catalyst was not only more active but gave linear polymers capped with BnO end-groups. By using ¹H and Al NMR spectroscopy to monitor solutions of , /BnOH and BnOH/10 ε-CL over a range of temperatures, some support for a monomeric species being the active initiator at the operational temperature is presented.