Polymerization of ε-Caprolactone and Copolymerization with rac-Lactide Catalyzed by Mono(amidinate) Aluminum Complexes

Mono(amidinate) aluminum complexes showed high catalytic activity for the ring-opening polymerization of e-caprolactone in toluene at ambient temperature. Monomer conversion up to 91% could be reached within 1 h when using [{PhC(N-2,6-(Pr2C6H3)-Pr-i)(2)}AlMe2] (C1) as initiator at 25 degrees C. 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-substituents on one of the N-phenyl rings were more active than the analogues bearing one ortho-substituent. Although high molecular weight poly(epsilon-caprolactone)s were obtained by using these aluminum catalysts, the polymerization of epsilon-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 e-caprolactone and rac-lactide using mono(amidinate) aluminum complexes afforded poly(epsilon-caprolactone)-poly(rac-lactide) diblock copolymer, which was characterized well by NMR spectroscopy and gel permeation chromatography analysis.