Understanding the Mechanism of Polymerization of ε‑Caprolactone Catalyzed by Aluminum Salen Complexes

Studies of the kinetics of polymerization of ε-caprolactone (CL) by salen-aluminum catalysts comprising ligands with similar steric profiles but different electron donating characteristics (R = OMe, Br, or NO₂) were performed using high initial monomer concentrations (2 M < [CL]₀ < 2.6 M) in toluene-<i>d</i>₈ at temperatures ranging from 20 to 90 °C. Saturation behavior was observed, enabling determination of monomer equilibrium constants (<i>K</i>_eq) and catalytic rate constants (<i>k</i>₂) as a function of R and temperature. While <i>K</i>_eq varied only slightly with the electron donating properties of R (Hammett ρ = +0.16(8)), <i>k</i>₂ showed a more significant dependence reflected by ρ = +1.4(1). Thermodynamic parameters Δ<i>G</i>° (associated with <i>K</i>_eq) and Δ<i>G</i>^⧧ (associated with <i>k</i>₂) were determined, with the former being ∼0 kcal/mol for all catalysts and the latter exhibiting the trend R = OMe > Br > NO₂. Density functional theory (DFT) calculations were performed to characterize mechanistic pathways at a microscopic level of detail. Lowest energy transition-state structures feature incipient bonding of the nucleophile to the lactone carbonyl that is approaching the metal ion, but a distinct CL adduct is <i>not</i> an energy minimum on the reaction pathway, arguing against <i>K</i>_eq being associated with coordination of monomer according to the typical coordination–insertion mechanism. An alternative hypothesis is presented associating <i>K</i>_eq with “nonproductive” coordination of substrate in a manner that <i>inhibits</i> the polymerization reaction at high substrate concentrations

Understanding the Mechanism of Polymerization of ε‑Caprolactone Catalyzed by Aluminum Salen Complexes

Studies of the kinetics of polymerization of ε-caprolactone (CL) by salen-aluminum catalysts comprising ligands with similar steric profiles but different electron donating characteristics (R = OMe, Br, or NO₂) were performed using high initial monomer concentrations (2 M < [CL]₀ < 2.6 M) in toluene-<i>d</i>₈ at temperatures ranging from 20 to 90 °C. Saturation behavior was observed, enabling determination of monomer equilibrium constants (<i>K</i>_eq) and catalytic rate constants (<i>k</i>₂) as a function of R and temperature. While <i>K</i>_eq varied only slightly with the electron donating properties of R (Hammett ρ = +0.16(8)), <i>k</i>₂ showed a more significant dependence reflected by ρ = +1.4(1). Thermodynamic parameters Δ<i>G</i>° (associated with <i>K</i>_eq) and Δ<i>G</i>^⧧ (associated with <i>k</i>₂) were determined, with the former being ∼0 kcal/mol for all catalysts and the latter exhibiting the trend R = OMe > Br > NO₂. Density functional theory (DFT) calculations were performed to characterize mechanistic pathways at a microscopic level of detail. Lowest energy transition-state structures feature incipient bonding of the nucleophile to the lactone carbonyl that is approaching the metal ion, but a distinct CL adduct is <i>not</i> an energy minimum on the reaction pathway, arguing against <i>K</i>_eq being associated with coordination of monomer according to the typical coordination–insertion mechanism. An alternative hypothesis is presented associating <i>K</i>_eq with “nonproductive” coordination of substrate in a manner that <i>inhibits</i> the polymerization reaction at high substrate concentrations

Mechanistic Studies of ε‑Caprolactone Polymerization by (salen)AlOR Complexes and a Predictive Model for Cyclic Ester Polymerizations

Aluminum alkoxide complexes (<b>2</b>) of salen ligands with a three-carbon linker and para substituents having variable electron-withdrawing capabilities (X = NO₂, Br, OMe) were prepared, and the kinetics of their ring-opening polymerization (ROP) of ε-caprolactone (CL) were investigated as a function of temperature, with the aim of drawing comparisons to similar systems with two-carbon linkers investigated previously (<b>1</b>). While <b>1</b> and <b>2</b> exhibit saturation kinetics and similar dependences of their ROP rates on substituents X (invariant <i>K</i>_eq, similar Hammett ρ = +1.4(1) and 1.2(1) for <i>k</i>₂, respectively), ROP by <b>2</b> was significantly faster than for <b>1</b>. Theoretical calculations confirm that, while the reactant structures differ, the transition state geometries are quite similar, and by analyzing the energetics of the involved distortions accompanying the structural changes, a significant contribution to the basis for the rate differences was identified. Using this knowledge, a simplified computational method for evaluating ligand structural influences on cyclic ester ROP rates is proposed that may have utility for future catalyst design