Stopped-Flow NMR and Quantitative GPC Reveal Unexpected Complexities for the Mechanism of NHC-Catalyzed Lactide Polymerization

Stopped-flow NMR spectroscopy provides the first direct, in situ observation of lactide epimerization during polymerization with the N-heterocyclic carbene organocatalyst 1,3-dimesitylimidazol-2-ylidene (IMes). Hexad analysis of the polymer microstructure using 13C NMR spectroscopy supports a chain-end-controlled mechanism for stereocontrol of the lactide polymerization. Data for both monomer consumption and molecular weight distribution (MWD) as a function of time have been examined using more than one dozen kinetic models. The most successful models feature reversible, unimolecular termination, first-order propagation in monomer, no backbiting term, and include a first-order catalyst death term. The developed modeling method allows insight into a challenging mechanistic problem by successfully modeling MWD evolution and monomer concentration with time.