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

Aluminum alkoxide complexes () of salen ligands with a three-carbon linker and para substituents having variable electron-withdrawing capabilities (X = NO, 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 (). While and exhibit saturation kinetics and similar dependences of their ROP rates on substituents X (invariant , similar Hammett ρ = +1.4(1) and 1.2(1) for , respectively), ROP by was significantly faster than for . 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.