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

Abstract

AbstractThis 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‐2‐ylidene NHCs on the LPP of three classes of acrylic monomers, including linear methyl methacrylate (MMA), cyclic biorenewable γ‐methyl‐α‐methylene‐γ‐butyrolactone (γMMBL), and difunctional allyl methacrylate (AMA). For MMA polymerization, IiPr is not only the most active (∼3× and ∼120× 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 γMMBL, 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 PγMMBL can be tuned by adjusting the [γMMBL]/[NHC] ratio, and thus high MW polymers with relatively narrow MW distributions can be readily synthesized (e.g., from Mn=1.41×105 g mol−1, Đ=1.08 to Mn=4.89×105 g mol−1, Đ=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.