Ring-Opening Polymerization of ε-Caprolactone Initiated by Multinuclear Aluminum Methanetris(aryloxido) Complexes

Dinuclear aluminum complexes (2a-/2b-THF) supported by a methanetris(aryloxide) were synthesized by dealuminumation of the corresponding trinuclear aluminum complexes (la and 1b) in THF solvent. Treatment of methylaluminum derivatives la and 2a-THF with one equivalent of benzyl alcohol led to the formation of monobenzyloxides 3a and 4a with no nuclearity change, respectively. Ring-opening polymerization (ROP) of e-caprolactone by these multinuclear aluminum complexes were studied, and good polymerization activity and molecular weight control were observed when 4a was employed as the initiator. During the ROP, the three aryloxide moieties of the supporting ligand were observed to be equivalent by H-1 NMR spectroscopy, reflecting the rapid equilibrium of coordination between Al-2 centers and a Lewis base. Ester-tethered alkoxides 6-9 were synthesized and structurally characterized. On the basis of the ester adduct structures, both syn- and anti-CL adducts of methoxide 10 were probed using DFT calculation. Kinetic study for the 4a -initiated ROP was performed and revealed that the rate-determining step is switched by polymerization conditions. These results suggest that the two aluminum centers in the active Al-2 species cooperatively work as a coordination site toward Lewis bases such as CL and as a stabilization site for the insertion of an alkoxide terminus.