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

Stasiw, D. E.; Mandal, M.; Neisen, B. D.; Mitchell, L. M.; Cramer, C. J.; Tolman, W. B.
Inorg. Chem. 2017, 56, 725 (doi:10.1021/acs.inorgchem.6b02849)

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.