Why So Slow? Mechanistic Insights from Studies of a Poor Catalyst for Polymerization of ε‑Caprolactone

Polymerization
of ε-caprolactone (CL) using an aluminum alkoxide catalyst (1) designed to prevent unproductive trans binding was monitored
at 110 °C in toluene-d8 by 1H NMR and the concentration versus time data fit to a first-order
rate expression. A comparison of t1/2 for 1 to values for many other aluminum alkyl and alkoxide complexes
shows much lower activity of 1 toward polymerization
of CL. Density functional theory calculations were used to understand
the basis for the slow kinetics. The optimized geometry of the ligand
framework of 1 was found indeed to make CL trans binding
difficult: no trans-bound intermediate could be identified as a local
minimum. Nor were local minima for cis-bound precomplexes found, suggesting
a concerted coordination–insertion for polymer initiation and
propagation. The sluggish performance of 1 is attributed
to a high-framework distortion energy required to deform the “resting”
ligand geometry to that providing optimal catalysis in the corresponding
transition-state structure geometry, thus suggesting a need to incorporate
ligand flexibility in the design of efficient polymerization catalysts.