Base-assisted anionic polymerization of alkyl sorbates and subsequent ring-closure without extreme dilution for synthesis of cyclic stereoregular poly(alkyl sorbates)

A typical base, lithium diisopropylamide (LDA) or lithium bis(trimethylsilyl)amide (LiHMDS), was found to initiate the anionic chain-growth polymerization of ethyl sorbate (ES) in the presence of an aluminum Lewis acid, that is, methylaluminum bis(2,6-di-tert-butyl-4-methylphenoxide) (MAD) and, after ES consumption, ring-closure occurred without highly dilute conditions to give the cyclic poly(ES). The finding of ring closure was supported by matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectroscopy and direct transmittance electron microscope (TEM) observations, in which a series of spectral patterns separated by the mass of ES and cyclic objects with 7.4-25.9 nm rings were observed, respectively. The results indicated that instead of the use of an expensive N-heterocyclic carbene (NHC), employing these commodity bases to initiate chain polymerization enabled us to fabricate the intended cyclic vinyl polymers without highly dilute conditions. Also, this polymerization proceeded even at -80 degrees C to give threo (98%)-disyndiotactic (91%) stereoregular poly(ES) in an excellent yield with narrow molecular weight distributions (Mn = 5.9 x 103-8.4 x 103, Mw/Mn = 1.14). image