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

Abstract

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°C to give threo (98%)‐disyndiotactic (91%) stereoregular poly(ES) in an excellent yield with narrow molecular weight distributions (M_n = 5.9 × 10³–8.4 × 10³, M_w/M_n = 1.14).