Journal of Physical Chemistry B, Vol.118, No.24, 6553-6560, 2014
Experimental and Computational Studies on the Mechanism of Zwitterionic Ring-Opening Polymerization of delta-Valerolactone with N-Heterocyclic Carbenes
Experimental and computational investigations of the zwitterionic ring-opening polymerization (ZROP) of delta-valerolactone (VL) catalyzed by the N-heterocyclic carbenes (NHC) 1,3-diisopropyl-4,5-dimethyl-imidazol-2-ylidene (1) and 1,3,4,5-tetramethyl-imidazol-2-ylidene (2) were carried out. The ZROP of delta-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 delta-valerolactone and its subsequent oligomerization. These studies have established that the initial steps of the mechanism involve nucleophilic attack of the NHC on delta-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 delta-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.