N-Heterocyclic Carbene Initiated Anionic Polymerization of (E,E)-Methyl Sorbate and Subsequent Ring-Closing to Cyclic Poly(alkyl sorbate).

A diene-based cyclic polymer has been synthesized by the anionic polymerization of methyl sorbate (MS) by an N-heterocyclic carbene (NHC) in the presence of a bulky aluminum Lewis acid. We first polymerized methyl sorbate (MS) initiated by NHC in N,N-dimethylformamide (DMF) at 25 °C, poly(MS) with a number-average molecular weight (M) of 3.5 × 10 (M/M = 2.1) was obtained with a conversion of 93%. The structure was confirmed by H and C NMR and IR spectra, which revealed that the propagation proceeded via 1,2-addition as well as 1,4-addition. Although the polymerization did not occur in toluene in the absence of any additive, quantitative monomer consumption was observed in the presence of methylaluminum bis(2,6-di-tert-butyl-4-methylphenoxide) (MAD) to afford the poly(MS) with a 1,4-trans structure, 86% of threo diastereoselectivity, and a M of 23.0 × 10 with narrow molecular weight distribution (M/M = 1.1). From the matrix assisted laser desorption/ionization (MALDI-TOF) mass spectra of poly(MS) and the hydrogenated analogue, ring-closing occurred by nucleophilic attack of the anionic propagating center into the adjacent carbon of the α-terminal imidazolimium group to afford cyclic poly(MS). The cyclic formation in the present synthesis system was confirmed by DSC and viscosity measurements.