N-heterocyclic carbenes for the organocatalytic preparation of polyesters

N-Heterocyclic carbenes (NHCs) have emerged as a versatile class of ligands in transition metal catalysis and as organocatalysts for as variety of transformations. NHCs effectively promote the ring-opening polymerization (ROP) of epsilon-caprolactone and other strained cyclic esters to yield polyesters, an important class of biocompatible and biodegradable polymers. In the presence of benzyl alcohol initiator, poly(epsilon-caprolactone) has been synthesized with controlled high molecular weights and relatively narrow polydispersities (PDIs) (<1.3) using NHCs 1,3,4,5-tetramethylimidazol-2-ylidene and 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene as organocatalysts. These NHC-catalyzed polymerizations exhibit features of a living polymerization including a linear molecular weight versus conversion relationship and successful polymerization by chain extension. High alcohol initiation efficiency is demonstrated with polymerizations employing functionalized initiators. Multi-branched alcohol initiators allowed for the preparation of star-branched poly(epsilon-caprolactone)s. The proposed mechanism is a monomer-activated mechanism in which the nucleophilic carbene activates the cyclic ester to form an acyl-imidazolium intermediate for transesterification to the growing polymer chain. However, a chain-end activated mechanism whereby the NHC activates the alcohol toward nucleophilic attack cannot be ruled out. In the absence of an alcohol initiator, zwitterionic polymerizations of epsilon-caprolactone performed neat or in solution using 1,3,4,5-tetramethylimidazol-2-ylidene and 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene generated poly(epsilon-caprolactone)s with molecular weights (Mn) reaching over 100,000.; In addition, NHCs are efficient transesterification catalysts that mediate the step-growth polymerization of esters and the depolymerization of polyesters. Our study focused on commodity thermoplastic poly(ethylene terephthalate) (PET) which was readily synthesized from the polycondenstation of bis(2-hydoxyethyl terephthalate) (BHET) from using 1,3,4,5-tetramethylimidazol-2-ylidene as an organocatalyst. NHCs also provide mild, metal-free route for the chemical depolymerization of PET through glycolysis or methanolysis to produce the monomers BHET or dimethyl terephthalate (DMT). As an alternative to typical metal-based transesterification catalysts which generally require high temperatures and pressures, the depolymerization of PET proceeded at room temperature. NHCs prepared from the in situ deprotonation of commercially-available imidazolium ionic liquids such 1,3-dimethyl imidazolium methylsulfate and 1-butyl-3-methyl-imidazolium diethyleneglycol-monomethylethersulfate were highly efficient catalysts for the glycolysis reaction of PET generating BHET in high conversions (> 90%) under mild conditions after reaction times on the order of 1 h.