Roles of Monomer Binding and Alkoxide Nucleophilicity in Aluminum-Catalyzed Polymerization of ε-Caprolactone

Abstract

The kinetics of polymerization of ε-caprolactone (CL) initiated by aluminum-alkoxide complexes supported by the dianionic forms of N,N-bis[methyl-(2-hydroxy-3-tert-butyl-5-R-phenyl)]-N,N-dimethylethylenediamines, (LR)Al(Oi-Pr) (R = OMe, Br, NO2) were studied. The ligands are sterically similar but have variable electron donating characteristics due to the differing remote (para) ligand substituents R. Saturation kinetics were observed using [CL]0 = 2–2.6 M and [complex]0 = 7 mM, enabling independent determination of the substrate coordination (K eq) and insertion (k 2) events in the ring-opening polymerization process. Analysis of the effects of the substituent R as a function of temperature on both K eq and k 2 yielded thermodynamic parameters for these steps. The rate constant k 2, related to alkoxide nucleophilicity, was strongly enhanced by electron-donating R substituents, but the binding parameter K eq is invariant as a function of ligand electronic properties. Density functional calculations provide atomic-level detail for the structures of key reaction intermediates and their associated thermochemistries.

Aluminium salophen and salen initiators in the ring-opening polymerisation of rac-lactide and rac-β-butyrolactone: Electronic effects on stereoselectivity and polymerisation rates

“…Electron-donating groups do increase polymerisation rates slightly, likely due to a decreased electron density at the metal centre increasing lability of metal-alkoxide bonds and promoting acyl bond cleavage. This contrasts with previous reports which suggested that increased electron density at the metal centre results in stronger monomer binding and lower polymerisation rates [8]. Rates do change significantly, and reproducibly (3 repetitions minimum for each catalyst), at the latter stages of βBL polymerisation, potentially due to differences in transesterification reactivity.…”

Chiral (1,2)‐Diphenylethylene‐Salen Complexes of Triel Metals: Coordination Patterns and Mechanistic Considerations in the Isoselective ROP of Lactide

“…Yet, this is the opposite of the trend detected for Al‐salan complexes 23. 36 If it comes as no surprise that the introduction of bulky substituents in ortho position of the phenolates reduces substantially the reactions rates, the fact that it overall leads to increased isoselectivity could not have been surely anticipated: for instance, Gibson and co‐workers have instead observed that their aluminium–salan systems switched from isoselective to non‐stereoselective or even heteroselective upon replacement of o ‐H groups by o ‐Me, o ‐ t Bu and o ‐Cl 6a. On the other hand, we recently reported that with achiral 4‐coordinate aluminium complexes supported by bidentate phenoxy‐imine ligands, the presence of a bulky o ‐SiPh 3 proved mandatory (but not necessarily sufficient) to achieve significant isoselectivities 7d.…”

“…These considerations highlight the current difficulty in designing at will highly active and stereospecific ROP catalysts, even if some key features of ROP reactions are now better understood 20d. 29, 31a, 35, 36 Future efforts must focus on increasing the isoselectivity and catalytic activity of these ROP precatalysts through the tuning of steric and electronic properties of the chiral ancillary ligands, to prepare high molecular PLAs that will lend themselves well to the examination of their physical properties.…”

Dizinc Lactide Polymerization Catalysts: Hyperactivity by Control of Ligand Conformation and Metallic Cooperativity

“…Such rate independence in monomer is very unusual in ROP, however Tolman and co‐workers provide a compelling rationale for this in their study of Al catalysts. A Michaelis–Menten mechanistic model is invoked whereby the zeroth order is related to the pre‐equilibrium constant during reversible lactone binding to the metal ion . Monomer saturation kinetics are obtained if K m is much smaller than the monomer concentration (where K m is the Michaelis constant related to this fast pre‐equilibrium).…”