Enhanced Reactivity of Aluminum Complexes Containing P-Bridged Biphenolate Ligands in Ring-Opening Polymerization Catalysis

Aluminum complexes containing [RP(O)(2-O-3,5- Bu C H ) ] [R = Bu ( ), Ph ( )] have been synthesized, structurally characterized, and their reactivity studied in comparison with those of their [RP(2-O-3,5- Bu C H ) ] [R = Bu ( ), Ph ( )] analogs. Treating AlMe with one equiv of H [ ] in THF at 0°C affords quantitatively [ ]AlMe, subsequent reactions of which with benzyl alcohol in THF at 25°C generate {[ ]Al(μ -OCH Ph)} . The methyl [ ]AlMe and the benzyloxide {[ ]Al(μ -OCH Ph)} are all active for catalytic ring-opening polymerization (ROP) of ε-caprolactone and -lactide ( -LA). Controlled experiments reveal that {[ ]Al(μ -OCH Ph)} is competent in living polymerization. Kinetic studies indicate that [ ]AlMe, in the presence of benzyl alcohol, catalyzes ROP of -LA at a rate faster than [ ]AlMe and [ ]AlMe(THF) by a factor of 1.8 and 23.6, respectively, highlighting the profound reactivity enhancement in ROP catalysis by varying the P-substituents of these biphenolate complexes of aluminum.