The commercially important polyester poly(ethylene terephthalate) (PET) has been prepared by an organocatalytic N-heterocyclic carbene (NHC)-catalyzed catalyzed process. PET is generally prepared in a two step process: the condensation of dimethyl terephthalate (DMT) with excess ethylene glycol (EG) in THF at room temperature to generate bis(2-hydroxylethyl terephthalate) (BHET), followed by the self-condensation of BHET at high temperatures (270-290 ºC) in the presence of organometallic catalysts. Using an alternative metal-free approach, the NHC was employed to catalyze condensation of DMT with excess EG (Scheme 1). Complete conversion of DMT to BHET was realized in 1 h. An ionic liquid media based on an imidazolium salt, in the presence of a base, also proved to efficient for this NHC-catalyzed process to form BHET which could easily be isolated from solvent extraction or precipitation. The melt condensation of BHET was formed in the presence of the carbene using a slow heating ramp to 280 ºC under vacuum to generated PET. Importantly, the spectral and thermal (m.p. of 280 ºC) of the produced polymer were identical to the commercial PET.

NHCs also effectively catalyze the depolymerization of polyesters. For example, the transesterification reaction of PET with methanol in the presence of a NHC catalyst yields DMT and ethylene glycol. This depolymerization method is performed under relatively mild conditions (typically at 80 ºC or less) and provides an approach to chemical recycling of commercial polymers such as PET.