Neutral, Three-Coordinate, Chelating Diamide Aluminum Complexes:  Catalysts/Initiators for Synthesis of Telechelic Oligomers and High Polymers

Reaction of the bulky chelating diamine ligand ArNH(CH 2 ) 3 NHAr (Ar = 2,6- i Pr 2 C 6 H 3 ) with AlMe 3 cleanly affords the monomeric, three-coordinate, chelating diamide aluminum complex [N ∩ N]AlMe ( 1 ). The isobutyl derivative [N ∩ N]Al i Bu ( 2 ) is prepared in the same manner using triisobutylaluminum. The Al/B alkyl/aryl ligand exchange between 1 and B(C 6 F 5 ) 3 in refluxing toluene cleanly produces the pentafluorophenyl-substituted derivative [N ∩ N]AlC 6 F 5 ( 3 ). Studies of model reactions between 1 and ε-caprolactone as well as analyses of low-molecular-weight oligomers demonstrate the ability of 1 to produce telechelic oligomers. At high monomer/initiator ratios, all three complexes are active for ring-opening polymerization of ε-caprolactone and cyclohexene oxide, producing high-molecular-weight polymers. The isobutyl derivative 2 exhibits the highest activity for both polymerizations in this catalyst series and produces poly(ε-caprolactone) having a number-average molecular weight over 1,000,000 Da.