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

Abstract

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.