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

Abstract

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.

Synthesis and Catalytic Activity of Gallium Schiff‐base Complexes in the Ring‐Opening Homo‐ and Copolymerization of Cyclic Esters

“…The 1 H NMR spectrum of a control reaction of complex 2 and BnOH ([M] 0 [cat.] 0 =1 : 4) in CD 2 Cl 2 at 23 °C showed the formation of a Ga‐OCH 2 Ph species (Figure S23), probably due to protonation of the N,O‐chelating moiety of the ligand as was previously observed by Nomura et al for multinuclear aluminum methanetris(aryloxido) complexes [32] . These finding prove that the polymerization reactions are initiated by this alkoxide species, and the ROP occurs via coordination‐insertion‐mechanism (CIM).…”

Chirality-Induced Catalyst Aggregation: Insights into Catalyst Speciation and Activity Using Chiral Aluminum Catalysts in Cyclic Ester Ring-Opening Polymerization

“…, [ε-CL] 0 = 1.0 and 1.5 M, see details in Supporting Information), the reaction was found to have a partial positive order in [ε-CL] using both R , R - 1 and rac - 1 . While a first order dependency in ε-CL and lactide is generally reported in ROP, zero order or partial positive orders (usually between 0 and 1) are not unprecedented and have been proposed to originate from reversible catalyst-monomer binding equilibria, which can lead to “saturation kinetics” (e.g., Michaelis–Menten type kinetics). − This does not imply that a mechanism different to the well-known “coordination-insertion” process is operative. DFT calculations were carried out and support the favorable coordination of one molecule of ε-CL to both the mononuclear and dinuclear Al complexes (Figure S15).…”

Catalytic Activity of Various β‐Diketiminate Zinc Complexes toward the Ring‐Opening Polymerization of Caprolactone and Derivatives

“…[ 11–14 ] Hence, replacing tin‐based catalysts in the polymerization of cyclic esters by compounds that are less of an issue in terms of toxicity is of interest to the synthetic polymer community, particularly in view of the biomedical usage of such polymers. A wide range of elements is capable of binding / coordinating esters and promoting polymerization reactions, [ 15,16 ] and common metals that are used as the replacement of tin in the ROP are aluminum, [ 17–21 ] magnesium, [ 21–24 ] calcium, [ 24–26 ] and zinc. [ 20,21,24,26–38 ] Furthermore and in addition to the metal itself, an appropriate ligand system is crucial for an efficient ROP, and ligand design helps to optimize the catalytically active species in terms of activity and selectivity.…”