Group Transfer Polymerization of (Meth)acrylic Monomers Catalyzed by N-Heterocyclic Carbenes and Synthesis of All Acrylic Block Copolymers: Evidence for an Associative Mechanism

摘要

N-Heterocyclic carbenes (NHCs), namely, 1,3-bis-(diisopropyl)imidazol-2-ylidene (1) and 1, 3-bis(di-tert-butyl)imidazol-2-ylidene (2) were employed as neutral organocatalysts to bring about the group transfer polymerization (GTP) of both methacrylic and acrylic monomers, including methyl methacrylate (MMA), tert-butylacrylate (tBA), and n-butylacrylate (nBA). This could be achieved at room temperature using 1-methoxy-2-methyl-1-trimethylsiloxypropene (MTS) as initiator in polar or apolar medium. In this way, polymethacrylates and polyacrylates with molar masses in the range 10 000-300 000 g.mol(-1), corresponding to the initial [monomer]/[MTS] ratio and with polydispersities lower than 1.2, were obtained in quantitative yields. The kinetics of GTP of MMA catalyzed by 1 or 2 was further investigated. Though the first-order kinetic plot ln[M](0)/[M] versus time deviated from linearity at high monomer conversion, no inhibition period was noted at low monomer conversion. Moreover, the polymerization rate dramatically increased as the concentration of initiator increased, with first-order dependence in initiator. When mixed in 1/1 molar ratio, MTS and NHC I did not reveal the formation of enolate-type species by Si-29 or 13 C NMR spectroscopy. Based on these observations, we propose that NHCs activate the silyl ketene acetal initiator and further propagate GTP via an associative mechanism. The fact that In[M](0)/[M] does not evolve linearly with time in the terminal phase of the polymerization can be understood by a reduced diffusion of the catalyst to the trimethylsilyl end groups. The proposed associative mechanism can also account for the successful control of NHC-catalyzed GTP of acrylates during which termination reactions such as backbiting or internal isomerization could be drastically minimized. Next, was described the synthesis of all acrylic block copolymers based on polyacrylates and polymethacrylates (e.g., PMMA-b-PnBA-b-PMMA), utilizing the same NHC as catalyst in sequential GTP. It is again argued that such block copolymer formation is favored by an associative mechanism forming highly unstable activated silicon intermediates.