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

Aluminum
alkoxide complexes (2) 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 (1). While 1 and 2 exhibit saturation kinetics and similar dependences of their
ROP rates on substituents X (invariant Keq, similar Hammett ρ = +1.4(1) and 1.2(1) for k2, respectively), ROP by 2 was significantly
faster than for 1. 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.