N,N′-Bis(imidazolyl)guanidinylphosphines: powerful initiators for conjugate-addition polymerization of Michael-type monomers

Abstract

Electron-rich phosphines are vital species in metal-free polymerization and ligands in coordination chemistry. Herein, we report a simple and efficient synthetic strategy to access a novel class of N,N′-bis(imidazolyl)guanidine-substituted phosphines (BIG-PR₂, R = –ⁱPr, –^tBu, –cycloHex, and –Ph). Theoretical calculations show that introducing a BIG substitute enhances the electron-releasing ability of the resulting phosphines, leading to an electronically and sterically tunable, extremely electron-rich phosphorus center. Superbasic BIG-PR₂ can capture carbon disulfide molecules, thus effectively forming zwitterionic Lewis base-CS₂ adducts. More importantly, BIG-PR₂ exhibits unprecedented activity and excellent selectivity for the conjugate-addition polymerization of various Michael-type monomers (such as acrylates, bio-sourced α-methylene-γ-butrolactone, 4-vinylpyridine and dimethylacrylamide) under mild polymerization conditions. Both computational studies and matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy reveal the zwitterionic conjugate-addition polymerization initiated by superbasic BIG-PR₂ alone.