Alcohol Adducts of <i>N</i>-Heterocyclic Carbenes:  Latent Catalysts for the Thermally-Controlled Living Polymerization of Cyclic Esters

Abstract

Alcohol adducts of the N -heterocyclic carbene, 1,3,4-triphenyl-4,5-dihydro-1 H -1,2-triazol-5-ylidene (NHC), function as excellent single-component catalyst/initiators for the ring-opening polymerization of lactide and β-butyrolactone. Their reversible dissociation at elevated temperatures generates alcohol and triazolylidene carbene to provide a facile entry to polymerization of cyclic esters on demand. Under optimum conditions, adverse transesterification reactions are minimized, and importantly, upon complete consumption of monomer, a second monomer addition facilitates additional polymer growth, even after precipitation. Block copolymers were prepared by combining disparate polymerization techniques from the use of oligo-adducts and bifunctional initiators. Additionally, more complex polymer architectures were prepared from multifunctional or dendritic initiators, further demonstrating the versatility of the N -heterocyclic carbene platform.