Reactivity of Amine/E(C(6)F(5))(3) (E = B, Al) Lewis Pairs toward Linear and Cyclic Acrylic Monomers: Hydrogenation vs. Polymerization

This work reveals the contrasting reactivity of amine/E(C(6)F(5))(3) (E = B, Al) Lewis pairs toward linear and cyclic acrylic monomers, methyl methacrylate (MMA) and biorenewable γ-methyl-α-methylene-γ-butyrolactone ((γ)MMBL). While mixing of 2,2,6,6-tetramethylpiperidine (TMP) and B(C(6)F(5))(3) leads to a frustrated Lewis pair (FLP), Et(3)N reacts with B(C(6)F(5))(3) to form disproportionation products, ammonium hydridoborate ionic pair and iminium zwitterion. On the other hand, the stoichiometric reaction of either TMP or Et(3)N with Al(C(6)F(5))(3) leads to clean formation of a classic Lewis adduct (CLA). Neither TMP nor Et(3)N, when paired with E(C(6)F(5))(3), polymerizes MMA, but the Et(3)N/2B(C(6)F(5))(3) pair promotes transfer hydrogenation of MMA to form methyl isobutyrate. In contrast, the amine/E(C(6)F(5))(3) pairs promote rapid polymerization of (γ)MMBL carrying the more reactive exocyclic methylene moiety, achieving full conversion in less than 3 min even at a low catalyst loading of 0.0625 mol %. TMP is more effective than Et(3)N for the polymerization when paired with either the borane or the alane, while the alane exhibits higher polymerization activity than the borane when paired with Et(3)N. Overall, the TMP/Al(C(6)F(5))(3) system exhibits the highest polymerization activity, achieving a maximum turn-over frequency of 96,000 h(−1) at 0.125 mol % of catalyst loading, producing high molecular weight P(γ)MMBL with M(n) = 1.29 × 10(5) g∙mol(−1).