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

Abstract

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 is used in combination with an initiator, such as an alcohol, which generates an alpha-end group bearing the ester from the initiating alcohol upon ring-opening and a hydroxyl functional omega-chain end that 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 h. 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.

Computational Studies on the Selective Polymerization of Lactide Catalyzed by Bifunctional Yttrium NHC Catalyst

“…As for organocatalysts, the application of N-heterocyclic carbene (NHC) and its derivatives to ROP has attracted considerable attention. It can efficiently promote the polymerization of lactide and related monomers under mild conditions [23][24][25][26]. These organocatalysts often exhibit higher functional group tolerance [27] than metal-based complexes, and inherently work under milder conditions.…”

Asymmetric catalysis with N-heterocyclic carbenes as non-covalent chiral templates

“…Despite the well-established concept of asymmetric hydrogen bond catalysis [31][32][33][34] , NHCs have yet to identify themselves as good asymmetric Brønsted base catalysts for carbon-carbon bondforming reactions. Several transesterification reactions using NHCs have been reported, in which the carbene species were believed to exhibit Brønsted base characteristics [35][36][37][38][39] . Although transesterification of unactivated esters has been reported through the use of achiral NHC catalysts [35][36][37][38] , only highly reactive vinyl esters were successful in the asymmetric versions 39 .…”

“…Several transesterification reactions using NHCs have been reported, in which the carbene species were believed to exhibit Brønsted base characteristics [35][36][37][38][39] . Although transesterification of unactivated esters has been reported through the use of achiral NHC catalysts [35][36][37][38] , only highly reactive vinyl esters were successful in the asymmetric versions 39 . Alternative nucleophile catalysis by NHC may occur for these reactions, as supported by recent work by Chi and co-workers 25,40,41 .…”

1,3-Bis(2,6-diisopropylphenyl)-2-trichloromethylimidazolidine

“…Although carbenes are typically used as ligands for transition metals [11][12][13][14][15], or as nucleophilic organic catalysts [16][17][18][19][20][21][22][23][24][25], these carbene derivatives that contain an (H)CCl3 bound at the carbene C2 position have been found through reductive thermolysis, to give free carbene for reaction with metals [26].…”