Computational Mechanism for Initiation and Growth of Poly(3-hexylthiophene) Using Palladium <i>N</i>‑Heterocyclic Carbene Precatalysts

The living, chain-growth polymerization of (hetero)­aryl monomers by carbene-ligated palladium precatalysts gives conjugated polymers with narrow dispersities as well as specified molecular weights and sequences. Despite this success, the mechanism for Pd precatalyst initiation and subsequent polymer growth is unknown. A quantum chemical study is presented herein to provide insight into thiophene homopolymerization initiated by (IPr)­PdCl₂(3-chloropyridine) (IPr = 1,3-bis­(2,6-diisopropyl­phenyl)­imidazol-2-ylidene) using novel reaction discovery tools for generating and evaluating mechanistic hypotheses. This study reveals the role for the “throw-away” 3-chloropyridine ligand during initiation via a stepwise transmetalation pathway that involves pyridine dissociation, followed by pyridine reassociation to liberate the Grignard byproduct (MgCl₂) and stabilize the resulting Pd intermediate. In contrast, 3-chloropyridine association to Pd during propagation hinders catalyst turnover by producing a low-energy, off-cycle intermediate. Throughout these studies, explicit solvent molecules (THF) coordinated to the Grignard reagent are shown to be needed for accurate modeling. Finally, the energetic span model (ESM) reveals that the turnover-limiting step for initiation is transmetalation with pyridine dissociation/reassociation, while the turnover-limiting step for propagation is also transmetalation that is inhibited by pyridine coordination