N-Heterocyclic Carbene-Catalyzed Ring Opening Polymerization of ε-Caprolactone with and without Alcohol Initiators: Insights from Theory and Experiment

Computational investigations with density functional theory (DFT) have been performed on the N-heterocyclic carbene (NHC) catalyzed ring-opening polymerization of e-caprolactone in the presence and in the absence of a methanol initiator. Much like the zwitterionic ring opening (ZROP) of δ-valerolactone which was previously reported, calculations predict that the mechanism of the ZROP of caprolactone that occurs without an alcohol present involves a high-barrier step involving ring opening of the zwitterionic tetrahedral intermediate formed after the initial nucleophilic attack of NHC on caprolactone. However, the operative mechanism by which caprolactone is polymerized in the presence of an alcohol initiator does not involve the analogous mechanism involving initial nucleophilic attack by the organocatalytic NHC. Instead, the NHC activates the alcohol through hydrogen bonding and promotes nucleophilic attack and the subsequent ring-opening steps that occur during polymerization. The largest free energy ba...