A computational insight into a metal mediated pathway for the ring-opening polymerization (ROP) of lactides by an ionic {(NHC)₂Ag}⁺X⁻ (X = halide) type N-heterocyclic carbene (NHC) complex

Abstract

A metal mediated coordination–insertion pathway for the ring-opening polymerization (ROP) of L-lactide by an ionic {(NHC)₂Ag}⁺X⁻ (X = halide) type silver complex of N-heterocyclic Carbene (NHC) has been investigated using the Density Functional Theory (DFT) method. A clear insight into the lactide insertion process could be obtained by modeling two consecutive monomer addition steps with the first one mimicking chain initiation with the second representing a propagation step. In particular, in each of the cycles, the reaction initiates with the formation of a lactide coordinated species, [1+LL] and [2+LL] that transforms into a metal bound cyclic lactide intermediate, I([1+LL] → 2) and I([2+LL] → 3), which subsequently ring opens to give the lactide inserted products, 2 and 3. The estimated overall activation barrier for the initiation step is 42.0 kcal mol⁻¹ while the same for the propagation step is 31.5 kcal mol⁻¹. Studies on higher monomer insertions showed a decrease in the relative product energies as anticipated for an addition polymerization pathway.