Modular design of a zwitterionic organocatalyst for bulk ring-opening polymerization of cyclic esters

Zwitterionic organocatalysis has been reported for the ring-opening polymerization (ROP) of cyclic esters due to the potency of the bifunctional activation of the monomer and chain-end by the corresponding cationic and anionic groups of the zwitterion. A series of phosphonium carboxybetaines (PCBs) were designed and synthesized by the Michael addition of tertiary phosphines to acrylic acid in a modular way. PCBs containing a quaternary phosphonium cation and carboxylate anion were exploited in the catalytic ROP of l-lactide (LLA), trimethylene carbonate (TMC), δ-valerolactone (δ-VL), and ϵ-caprolactone (ϵ-CL), and their homopolymers PLLA, PTMC, PVL, and PCL as well as the diblock copolymer PVL-b-PLLA were successfully prepared in bulk. The ROP of l-LA exhibited a controlled/living nature as indicated by the predicted molecular weight (M_n,NMR = 3.7-23.5 kg mol⁻¹) and narrow dispersity (Đ = 1.12-1.23). NMR titrations of the PCB catalyst against monomer l-LA and a hydroxyl-containing initiator showed appreciable interactions of the phosphonium cation with the monomer as well as interactions of the carboxylate anion with the initiator, which suggested a Lewis acidic and Lewis basic bifunctional activation mode of the zwitterionic PCB organocatalyst. In view of the universally accessible cations and anions in the modular design of zwitterionic molecules with catalytic functions, the present work enriched the toolbox of organocatalysis in polymerizations.