Dimeric Aluminum Chloride Complexes of N-Alkoxyalkyl-β-ketoimines:  Activation with Propylene Oxide To Form Efficient Lactide Polymerization Catalysts

The ter- and tetradentate N-alkoxyalkyl-β-ketoimines CH₃C(O)CH₂C(NCH₂CHROH)CH₃ {L_1-3} react with diethylaluminum chloride to afford the dimeric chloride bridged complexes [{L_1-3}AlCl]₂ (1a−c), which are activated by addition of propylene oxide or cyclohexene oxide to afford efficient initiators for the ring-opening polymerization of (d,l)-lactide. The active species is believed to be a chloroalkoxide formed by nucleophilic ring opening of a coordinated PO by migration of the chloride coordinated to the adjacent aluminum center. The resulting polymers have a high molecular weight, close to that calculated for the monomer:initiator ratio of 100, and a narrow molecular weight distribution. While the corresponding aluminum methyl dimer [{L₁}AlMe]₂ (2a), formed by reaction of L₁ (R = H) with AlMe₃, is a poor inititaor for the polymeriation of (d,l)-lactide, addition of 2-chloroethanol affords a catalyst with an activity comparable to that of 1a/PO. The molecular weight and molecular weight distribution of the resulting polylactide is similar to that obtained with 1a/PO and consistent with formation of a similar chloroalkoxide initiator.