Abstract: Mono(amidinate) aluminum complexes showed high catalytic activity for the ring-opening polymerization of ε-caprolactone in toluene at ambient temperature. Monomer conversion up to 91% could be reached within 1 h when using [{PhC(N-2,6-ⁱPr₂C₆H₃)₂}AlMe₂] (C1) as initiator at 25 ºC. The electron-donating groups on the ortho-positions of the N-substituted phenyl ring were superior to the electron-withdrawing groups for the enhancement of the catalytic activity, but both brought an increase in activity when compared with the un-substituted complex C7. The complexes bearing two ortho-substitutents on one of the N-phenyl rings were more active than the analogues bearing one ortho-substitutent. Although high molecular weight poly(ε-caprolactone)s were obtained by using these aluminum catalysts, the polymerization of ε-caprolactone was not well controlled as indicated by the broad molecular weight distributions (PDI = 1.43~1.85) and the deviation of the number average molecular weight from the theoretical values. Sequential copolymerization of ε-caprolactone and rac-lactide using mono(amidinate) aluminum complexes afforded poly(ε-caprolactone)-poly(rac-lactide) diblock copolymer, which was characterized well by NMR spectroscopy and gel permeation chromatography analysis.

Key words: aluminum amidinate complex, ε-caprolactone, rac-lactide, ring-opening polymerization, copolymerization