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 C1; R¹ = R² = H, R³ = Et C2; R¹ = R² = H, R³ = i-Bu C3; R¹ = Cl, R² = H, R³ = Me C4; R¹ = H, R² = R³ = Me C5; R¹ = Cl, R² = R³ = Me C6), have been prepared by treating the corresponding pro-ligand (L1⁻L4) with either AlMe₃, AlEt₃ or Al(i-Bu)₃. All complexes have been characterized by ¹H and 13C NMR spectroscopy and in the case of C1 and C4 by single crystal X-ray diffraction; dimeric species are a feature of their molecular structures. In the presence of PhCH₂OH (BnOH), C1⁻C6 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 C4 and C6 notably exhibited the lowest activity of the series. However, in the absence of BnOH, C1 showed only low activity with 15% conversion achieved in 30 min forming a linear polymer capped with either a methyl or a L1 group. By contrast, when one or more equivalents of BnOH was employed in combination with C1, the resulting catalyst was not only more active but gave linear polymers capped with BnO end-groups. By using ¹H and 27Al NMR spectroscopy to monitor solutions of C1, C1/BnOH and C1/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.
Keywords: aluminum complexes; bidentate; reaction mechanism; ring opening polymerization; ε-caprolactone.