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 (1a and 1b) in THF solvent. Treatment of methylaluminum
derivatives 1a 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 ε-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 1H NMR spectroscopy, reflecting the rapid equilibrium of coordination
between Al2 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 Al2 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.