Lewis Pair Polymerization of Acrylic Monomers by N-Heterocyclic Carbenes and B(C6F5)3

This work has uncovered the first highly active and efficient Lewis pair polymerization (LPP) system based on N-heterocyclic carbene (NHC)/B(C6F5)(3) pairs for converting acrylic monomers into medium-to high-molecular weight polymers. The study has systematically examined steric and electronic effects of three 1,3-dialkyl(Me, iPr, tBu) imidazol-2ylidene NHCs on the LPP of three classes of acrylic monomers, including linear methyl methacrylate (MMA), cyclic biorenewable gamma-methyl-gamma-methylene-g-butyrolactone (gamma MMBL), and difunctional allyl methacrylate (AMA). For MMA polymerization, IiPr is not only the most active (similar to 3 x and similar to 120 x more active than IMe and ItBu, respectively), but also the most effective NHC, especially under low NHC loading conditions. Kinetic results are consistent with a bimolecular, activated monomer propagation mechanism. In the case of the more reactive gamma MMBL, the polymerization by NHC/B(C6F5)(3) in CH2Cl2 is extremely rapid, with all three NHCs achieving quantitative monomer conversion in 1 min and thus reaching a high turnover frequency of >= 48,000 h(-1). The molecular weight (MW) of P gamma MMBL can be tuned by adjusting the [gamma MMBL]/[NHC] ratio, and thus high MW polymers with relatively narrow MW distributions can be readily synthesized (e.g., from M-n=1.41 x 10(5) g mol(-1), D = 1.08 to M-n = 4.89 x 105 g mol(-1), D = 1.20). The LPP by NHC/B(C6F5)(3) is completely chemoselective, as demonstrated by the polymerization of AMA, which selectively polymerizes the conjugated vinyl group while leaving the non-conjugated vinyl group in the allyl moiety intact, thanks to its activated monomer propagation mechanism. The resulting PAMA is syndiotactic (rr=83%), uncross-linked, and soluble in common solvents, thus suitable for further functionalization. This quantitatively chemoselective polymerization by NHC/ B(C6F5)(3) should provide a facile, yet powerful, approach to functional acrylic polymers.