No matter the order of monomer addition for the synthesis of well-defined block copolymers by sequential group transfer polymerization using N-heterocyclic carbenes as catalysts
Résumé
Unsaturated N-heterocyclic carbenes (NHCs) such as 1,3-bis(di-isopropyl)imidazol-2-ylidene (1) and 1,3-bis(di-tert-butyl)imidazol-2-ylidene (2) are shown to catalyze the sequential group transfer polymerization (GTP) of (meth) acrylic monomers. A variety of block copolymers including not only alkyl methacrylate but also alkyl acrylate monomer units as well as blocks deriving from N,N-dimethylacrylamide and methacrylonitrile were thus obtained at room temperature, using 1-methoxy-2-methyl-1-trimethylsiloxypropene (MTS) as initiator in THF as solvent. Block copolymerizations could be achieved, starting indifferently from the GTP of the acrylic monomer to that of the methacrylic one or vice versa, that is, regardless of the order of addition of the two monomers, in contrast to most examples of block copolymer synthesis by "controlled/living" sequential polymerization. It is postulated that these NHC-catalyzed GTPs of (meth) acrylics proceed via a single step concerted-like associative mechanism, involving the formation of thermodynamically unstable intermediates or transition states, likely hypervalent siliconates, with no detectable anionic enolates formed.