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-beta-ketoimines CF3C(O)CH2C(NCH2CHROH)CH3 {L1-3} react with diethylaluminum chloride to afford the dimeric chloride bridged complexes [{L1-3}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 [{L-1}AlMe](2) (2a), formed by reaction of L-1 (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.