Direct Observation of a Cyclic Vinyl Polymer Prepared by Anionic Polymerization using N-Heterocyclic Carbene and Subsequent Ring-Closure without Highly Diluted Conditions

A 1:1 adduct of methyl sorbate (MS) and 1,3-di-tert-butylimidazol-2-ylidene (NHCtBu) initiates anionic polymerization of a nonconjugated polar alkene, allyl methacrylate (AMA) in toluene at −20 °C. After the monomer is consumed quantitatively using a bulky aluminum Lewis acid, methylaluminum bis(2,6-di-tert-butyl-4-methylphenoxide) (MAD), as an additive, successive ring-closure occurs without highly dilute conditions to give a cyclic poly(AMA) containing α-terminal MS unit, and an M_n of 8.8 × 10³−58.5 × 10³ with a narrow molecular dispersity index (M_w/M_n = 1.1₄–1.3₇). The lack of a need for dilution is due to the fact that an α-terminal NHCtBu group is acting as the counter cation for the propagating center in the polymerization. From ¹H NMR and matrix assisted laser desorption/ionization (MALDI-TOF) mass spectra, combined with transmittance electron microscope (TEM) observation of a synthesized poly(AMA) with longer alkyl side chains prepared via a thiol-ene click reaction, it is concluded that once the monomer is consumed, nucleophilic attack at the neighboring methine of the α-terminal NHCtBu residue by the propagating anionic center causes ring-closing to cyclic poly(AMA).