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

Abstract

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 (1) and 1,3,4,5-tetramethyl-imidazol-2-ylidene (2) 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.

Ion pairing effects in the zwitterionic ring opening polymerization of δ-valerolactone

“…18,22,27 The data in Fig. 2a and 3 implicate that initiation is more efficient in the presence of LiCl.…”

“…18,22 Computational studies imply that the ringopening of the initial NHC-monomer complex has a high energy barrier, which may account for the slow initiation. 27 The SEC traces of polymer samples obtained by the ZROP of valerolactone with NHC 1 (Fig. 1) revealed the presence of a high molecular weight shoulder which is evident throughout the polymerization.…”

From Zn(C6F5)2 to ZnEt2‐based Lewis Pairs: Significantly Improved Catalytic Activity and Monomer Adaptability for the Ring‐opening Polymerization of Lactones

“…The simple structure of ZnEt 2 /DMAP Lewis pair gives us an opportunity to gain insights into the reaction mechanism of this unique catalytic system by DFT calculations. All calculations were performed at the B3LYP level of theory, which has been widely used in the investigation of the ROP mechanism . Two possible reaction mechanisms involving single molecular initiation and biomolecular initiation were proposed in scheme .…”

A Simple and Facile Approach to Aliphatic N-Substituted Functional Eight-Membered Cyclic Carbonates and Their Organocatalytic Polymerization

“…Out-of-plane dihedral angles (oop) formed by the C-N bonds involving the nitrogen atom in the carbonate ring as an indicator of hydrogen bond acceptor strength (for the definition of the oop angle see Figure S12). If the rate-limiting step of the ROP was the opening of the ring (TS2, Figure 3) as observed in other systems, 43 Information) 46 was not attempted, instead structures were simply reoptimized. 46 These calculations reveal that the calculated enthalpy of ring-opening (∆H ro ) for the three cyclic carbonates differ significantly ( Table 1) 12…”