文摘
Polymerization of ε-caprolactone (CL) using an aluminum alkoxide catalyst (<b>1b>) designed to prevent unproductive trans binding was monitored at 110 °C in toluene-db>8b> by 1H NMR and the concentration versus time data fit to a first-order rate expression. A comparison of tb>1/2b> for <b>1b> to values for many other aluminum alkyl and alkoxide complexes shows much lower activity of <b>1b> 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 <b>1b> 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 <b>1b> 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.