Expanding the scope of group transfer polymerization using N-heterocyclic carbenes as catalysts: Application to miscellaneous (Meth)acrylic monomers and kinetic investigations

1,3-Bis(di-isopropyl)imidazol-2-ylidene (1) and 1,3-bis(di-tert-butyl) imidazol-2-ylidene (2), which are N-heterocyclic carbenes (NHCs) are shown to catalyze the solution group transfer polymerization (GTP) of miscellaneous monomers in a controlled fashion at room temperature, in the presence of 1-methoxy-2-methyl-1-trimethylsiloxypropene (MTS) as initiator. The ability of seven distinct monomers, methyl methacrylate (MMA), tert-butyl acrylate (tBA), n-butyl acrylate (nBA), N,N-dimethylaminoethyl acrylate (DMAEA), N,N-dimethyl acrylamide (DMA), N,N-dimethylaminoethyl methacrylate (DMAEMA), and methacrylonitrile (MAN) to polymerize via GTP by NHC catalysis has been evaluated. The first-order kinetic plots, that is the evolution of ln[M] ₀/[M] versus time, systematically deviate from linearity, with the noticeable exception of GTP of DMA. A direct dependence of the rate of GTP on the concentration in MTS initiator is observed in the case of tBA carried out in THF, that is, the rate of polymerization increases with [MTS], with a first-order dependence on [MTS]. These results suggest the formation of hypervalent siliconate intermediates in NHC-induced GTP of acrylic monomers which proceeds via an associative mechanism. The nonlinear variation of ln[M]₀/[M] with time in the terminal phase of the polymerization of both acrylates and methacrylates may be explained by the development of strong interactions between the NHC and pendant ester groups of poly(meth)acrylates, limiting the availability of the catalyst for chain end activation. In contrast, interactions between the NHC and amide-type units of poly(DMA) are unlikely, NHCs being not known as effective catalysts for transamidation reactions. A first-order kinetic plot with a linear variation of ln[M]₀/[M] with time is thus observed for the NHC-catalyzed GTP of DMA.