Homolytic Pd^II–C Bond Cleavage in the MILRad Process: Reversibility and Termination Mechanism

Click to copy article linkArticle link copied!

This work probed the thermal “switchability” from ethylene coordination/insertion to controlled radical polymerization of methyl acrylate (MA) for Brookhart-type α-diimine Pd^II catalysts. The investigation focused on the extremely bulky 2,6-bis(3,5-dimethylphenyl)-4-methylphenyl (Xyl₄Ph) α-diimine N-substituents to probe reversible Pd^II–C bond activation in the MA-quenched Pd-capped PE intermediate and reversible trapping during radical MA polymerization. The substituent steric effect on the relative stability of various [PE–MA–Pd^II(ArN═CMeCMe═NAr)]⁺ chain-end structures and on the bond dissociation-free energy (BDFE) for the homolytic Pd^II–C bond cleavage has been assessed by DFT calculations at the full quantum mechanics (QM) and QM/molecular mechanics (QM/MM) methods. The structures comprise ester-chelated forms with the Pd atom bonded to the α, β, and γ C atoms as a result of 2,1 MA insertion into the PE–Pd bond and of subsequent chain walking, as well as related monodentate (ring-opened) forms resulting from the addition of MA or acetonitrile. The opened C_α-bonded form is electronically favored for smaller N-substituents, including 2,6-diisopropylphenyl (Dipp), particularly when MeCN is added, but the open C_γ-bonded form is preferred for the extremely bulky system with Ar = Xyl₄Ph. The Pd_α–C bond is the weakest one to cleave, with the BDFE decreasing as the Ar steric bulk is increased (31.8, 25.8, and 12.6 kcal mol^–1 for Ph, Dipp, and Xyl₄Ph, respectively). However, experimental investigations on the [PE–MA–Pd^II(ArN═CMeCMe═NAr)]⁺ (Ar = Xyl₄Ph) macroinitiator do not show any evidence of radical formation under thermal activation conditions, while photolytic activation produces both TEMPO-trapped (TEMPO = 2,2,6,6-tetramethylpiperidinyloxy) and unsaturated MA-containing PE chains. The DFT investigation has highlighted a low-energy pathway for termination of the PE–MA^• radicals by disproportionation, promoted by β-H elimination/dissociation and H-atom abstraction from the Pd^II–H intermediate by a second radical. This phenomenon appears to be the main reason for the failure of this Pd^II system to control the radical polymerization of MA by the OMRP (OMRP = organometallic-mediated radical polymerization) mechanism.