Comparative Study of Aluminum Complexes Bearing N,N′-Diaryldioxalamidate and N,N′-Diaryldithiooxalamidate Ligands in Ring-Opening Polymerization of ε-Caprolactone

A series of Al catalysts with N,N′-diaryloxalamidate and N,N′-diaryldithiooxalamidate ligands were synthesized, and their ε-caprolactone polymerization rates were determined in this study. Of the Al complexes bearing N,N′-diaryldithiooxalamidate, [PhHNCS]2Al2Me4 exhibited the highest catalytic activity (conv. of poly-ε-caprolactone = 94%; [ε-caprolactone]:[[PhHNCS]2Al2Me4]:[benzyl alcohol] = 100:0.5:2 with [ε-caprolactone] = 2 M at 25 °C after 20 min), and of the Al complexes bearing N,N′-diaryldioxalamidate, [PhHNCO]2Al2Me4 exhibited the highest catalytic activity (conv. of poly-ε-caprolactone = 94%; [ε-caprolactone]:[OHAl2Me4]:[benzyl alcohol] = 100:0.5:2 with [ε-caprolactone] = 2 M at 25 °C after 30 min). The Al complexes bearing N,N′-diaryldithiooxalamidate demonstrated a greater catalytic activity (1.4–7.3-fold) than Al complexes bearing N,N′-diaryldioxalamidate. Density functional theory calculations revealed that [PhiPrNCS]2Al2(OMe)4 with a weaker Al–OMe bond and less repulsion of the highest transition state TS1 revealed the lower potential energy of TS1 compared to [PhiPrNCO]2Al2(OMe)4 and further exhibited a higher polymerization rate than [PhiPrNCO]2Al2(OMe)4.