Phosphonium ylide/organoaluminum-based Lewis pairs for the highly efficient living/controlled polymerization of alkyl (meth)acrylates

Abstract

The development of living/controlled polymerization technologies is of great importance in both fundamental studies and in industrial applications. Herein, we report for the first time the use of easily synthesized phosphonium ylides Ph₃P = CR¹R² (R¹,R² = H,H (P-ylide-1), H,Me (P-ylide-2), H,Et (P-ylide-3), H,Ph (P-ylide-4), Me,Me (P-ylide-5)) as the Lewis base (LB) in the Lewis pair polymerization (LPP) of (meth)acrylates. The P-ylide-2–P-ylide-5/Al(C₆F₅)₃, P-ylide-1–P-ylide-5/AlR(BHT)₂ (R = Me, Et, iBu; BHT = 2,6-tBu₂-4-MeC₆H₂O), and P-ylide-1–P-ylide-5/AliBu₂(BHT) systems are all highly effective initiators of methyl methacrylate (MMA) polymerization, of which the P-ylide-2/AlMe(BHT)₂ system is the best for living/controlled polymerizations, resulting in high initiation efficiencies (I* > 90%). The relative Lewis acidities of the Lewis acids (LAs), Al(C₆F₅)₃, AlR(BHT)₂, and AliBu₂(BHT), are evaluated using the Gutmann–Beckett method whilst the basicity of the P-ylides is evaluated through natural bond orbital computations. Model reactions using the P-ylides, LAs, and MMA were accomplished, and revealed that the active species in the MMA polymerization, the enolate Ph₃PCH(Me)CH₂C(Me)C[OAlMe(BHT)₂](OMe) (>99%), is efficiently generated. Furthermore, homopolymerizations and block copolymerizations of MMA, ethyl methacrylate, benzyl methacrylate, n-butyl methacrylate, n-butyl acrylate, and ethylhexyl acrylate were successfully realized using the P-ylide-2/AlMe(BHT)₂ system.