Aluminum Complexes of Tridentate Amine Biphenolate Ligands Containing Distinct N ‐alkyls: Synthesis and Catalytic Ring‐opening Polymerization

A series of aluminum complexes of amine biphenolate ligands that contain distinct N ‐alkyl substituents have been prepared and employed for ring‐opening polymerization (ROP) of ϵ‐caprolactone (ϵ‐CL). Alkane elimination of trimethylaluminum with H 2 [R‐ONO] ([R‐ONO] 2− = [RN(CH 2 ‐2‐O‐3,5‐C 6 H 2 ( t Bu) 2 ) 2 ] 2− ; R = t Bu ( 1a ), i Pr ( 1b ), n Pr ( 1c )) in toluene solutions at 25 °C produces colorless crystalline [R‐ONO]AlMe ( 2a‐c ). X‐ray diffraction studies of these organoaluminum complexes showed them to be four‐coordinate species having a distorted tetrahedral coordination core. In the presence of benzyl alcohol, complexes 2a‐c are all active catalyst precursors for ROP of ϵ‐CL. The ROP rates of ϵ‐CL are clearly a function of the steric sizes of the N ‐alkyl substituents, following an increasing order 2a < 2b < 2c . Interestingly, both 2b and 2c exhibit controlled ROP catalysis whereas 2a gives low molecular weight oligomers under identical conditions. In ROP catalyzed with 2b or 2c , the ϵ‐CL disappearance rates follow pseudofirst order kinetics, with the latter being about 2.86(2) times faster than the former. Complex 2b outperforms 2c in view of maintaining controlled ROP catalysis in reactions with extremely high monomerto‐catalyst ratios. The complex constitution‐reactivity relationship is discussed.