Abstract
Polymerization of e-caprolactone (CL) using an aluminum alkoxide catalyst (1) designed to prevent unproductive trans binding was monitored at 110 degrees C in toluene-d(8) by H-1 NMR and the concentration versus time data fit to a first-order tate expression. A comparison of t(1/2) 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.