Anionic Polymerization of Polar Conjugated Diene Monomers with Phosphine as the Initiator and Subsequent Ring Closure without Highly Dilute Conditions

The pairing of tri-n-butyl phosphine (PnBu3) and methylaluminum bis(2,6-di-tert-butyl-4-methylphenoxide) (MAD) acts as a frustrated Lewis pair for the controlled anionic polymerization of ethyl sorbate (ES). In this study, PnBu3 initiated the controlled anionic polymerization of ES, yielding poly(ES) with a very high initiation efficiency and a narrow molecular weight distribution. When we used dimethylphenylphosphine (PPhMe2) as the initiator, ES underwent anionic polymerization in toluene. After monomer depletion, ring closure partially occurred at 70 °C without the need for highly dilute conditions (1 M) because of the effect of the neighboring α-terminal phosphonium group during polymerization. The cyclic topology of poly(ES) was confirmed by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry, and it was also apparent in TEM images. PPhMe2 both initiated the anionic polymerization of ES in the presence of MAD and acted as the leaving group after monomer depletion in order to give the corresponding cyclic structure via a nucleophilic pathway. This procedure enabled us to synthesize cyclic vinyl polymers using phosphine instead of expensive N-heterocyclic carbenes (NHCs), which greatly widens the research range of cyclic polymer science in both academic and industrial fields.