A DFT study on the mechanism for polymerization of δ-valerolactone initiated by N-heterocyclic carbene (NHC) catalysts

Abstract Polymerization reactions using renewable raw material as substrate (namely, δ-valerolactone) and N-heterocyclic carbenes (NHC) as organocatalysts to form polyesters were investigated by computational approaches. Two routes were investigated for the reaction, either with the NHC acting as a Bronsted base and activating an alcohol used as co-initiator or by direct nucleophilic attack of the NHC on the carbonyl carbon of the lactone, forming a zwitterionic intermediate. In agreement with previous studies, the lowest energy pathway leading to polymerization is that where the NHC activates the alcohol co-initiator, yielding a partially charged alkoxide that then performs a nucleophilic attack on the lactone. The proton affinity of the NHCs shows a high correlation with the activation enthalpy for the first reaction step. Thus, NHCs with high proton affinity stabilize the first intermediate and make the lactone ring-opening the rate-determining step for the reaction.