First Example of n-Heterocyclic Carbenes as Catalysts for Living Polymerization: Organocatalytic Ring-Opening Polymerization of Cyclic Esters

A novel metal-free, organocatalytic approach to living polymerization is presented. N-heterocyclic carbenes were employed as nucleophilic catalysts for the ring-opening polymerization (ROP) of cyclic ester monomers. The catalysts must be used in combination with an initiator, such as an alcohol, which generates an alpha-end group bearing the ester upon ring-opening and a omega-chain end with an alcohol which propagates the chain. This class of catalyst proved to be more reactive than tertiary amine and phosphine nucleophiles, producing narrowly dispersed polymers of predictable molecular weights at room temperature in 1 - 2 hours. Catalysis with respect to both initiating alcohol
and monomer was observed. Control of the alpha- and omega end-groups was demonstrated with a pyrene labeled initiator, allowing the preparation of well-defined macromolecular architectures. Analogous to the ROP of cyclic esters using biocatalysts, the polymeriztion pathway using the
N-heterocyclic carbenes is believed to ensue through a monomer-activated mechanism.