Why so slow? Mechanistic insights from studies of a poor catalyst for polymerization of ϵ-Caprolactone

Daniel Stasiw, Mukunda Mandal, Benjamin D. Neisen, Lauren Mitchell, Chris Cramer, William B Tolman

Research output: Contribution to journalArticle

11 Scopus citations

Abstract

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 highframework 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.

Original languageEnglish (US)
Pages (from-to)725-728
Number of pages4
JournalInorganic Chemistry
Volume56
Issue number2
DOIs
StatePublished - Jan 17 2017

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