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-R(1)-7,7-Me(2)-8-R(2)C(9)H(6)N-8-O}AlR(3)(2)](2) (R(1) = R(2) = H, R(3) = Me C1; R(1) = R(2) = H, R(3) = Et C2; R(1) = R(2) = H, R(3) = i-Bu C3; R(1) = Cl, R(2) = H, R(3) = Me C4; R(1) = H, R(2) = R(3) = Me C5; R(1) = Cl, R(2) = R(3) = Me C6), have been prepared by treating the corresponding pro-ligand (L1–L4) with either AlMe(3), AlEt(3) or Al(i-Bu)(3). All complexes have been characterized by (1)H and (13)C 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 PhCH(2)OH (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 (1)H and (27)Al 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.