Experimental and Computational Studies on the Mechanism of Zwitterionic Ring-Opening Polymerization of δ‑Valerolactone with N‑Heterocyclic Carbenes

Experimental and computational investigations of the zwitterionic ring-opening polymerization (ZROP) of δ-valerolactone (VL) catalyzed by the N-heterocyclic carbenes (NHC) 1,3-diisopropyl-4,5-dimethyl-imidazol-2-ylidene (<b>1</b>) and 1,3,4,5-tetramethyl-imidazol-2-ylidene (<b>2</b>) were carried out. The ZROP of δ-valerolactone generates cyclic poly­(valerolactone)­s whose molecular weights are higher than predicted from [VL]₀/[NHC]_0. Kinetic studies reveal the rate of polymerization is first order in [VL] and first order in [NHC]. Density functional theory (DFT) calculations were carried out to elucidate the key steps involved in the ring-opening of δ-valerolactone and its subsequent oligomerization. These studies have established that the initial steps of the mechanism involve nucleophilic attack of the NHC on δ-valerolactone to form a zwitterionic tetrahedral intermediate. DFT calculations indicate that the highest activation barrier of the entire mechanism is associated with the ring-opening of the tetrahedral intermediate formed from the NHC and δ-valerolactone, a result consistent with inefficient initiation to generate reactive zwitterions. The large barrier in this step is due to the fact that ring-opening requires a partial positive charge to develop next to the directly attached NHC moiety which already bears a delocalized positive charge