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

Thiswork probed the thermal switchability fromethylene coordination/insertion to controlled radical polymerizationof methyl acrylate (MA) for Brookhart-type & alpha;-diimine Pd-II catalysts. The investigation focused on the extremely bulky2,6-bis(3,5-dimethylphenyl)-4-methylphenyl (Xyl(4)Ph) & alpha;-diimine N-substituents to probe reversible Pd-II-Cbond activation in the MA-quenched Pd-capped PE intermediate and reversibletrapping during radical MA polymerization. The substituent stericeffect on the relative stability of various [PE-MA-Pd-II(ArN CMeCMe NAr)](+) chain-end structuresand on the bond dissociation-free energy (BDFE) for the homolyticPd(II)-C bond cleavage has been assessed by DFT calculationsat the full quantum mechanics (QM) and QM/molecular mechanics (QM/MM)methods. The structures comprise ester-chelated forms with the Pdatom bonded to the & alpha;, & beta;, and & gamma; C atoms as a resultof 2,1 MA insertion into the PE-Pd bond and of subsequent chainwalking, as well as related monodentate (ring-opened) forms resultingfrom the addition of MA or acetonitrile. The opened C-& alpha;-bonded form is electronically favored for smaller N-substituents, including 2,6-diisopropylphenyl (Dipp), particularlywhen MeCN is added, but the open C-& gamma;-bonded form ispreferred for the extremely bulky system with Ar = Xyl(4)Ph. The Pd-& alpha;-C bond is the weakest one tocleave, with the BDFE decreasing as the Ar steric bulk is increased(31.8, 25.8, and 12.6 kcal mol(-1) for Ph, Dipp, andXyl(4)Ph, respectively). However, experimental investigationson the [PE-MA-Pd-II(ArN CMeCMe NAr)](+) (Ar = Xyl(4)Ph) macroinitiator do not show any evidenceof radical formation under thermal activation conditions, while photolyticactivation produces both TEMPO-trapped (TEMPO = 2,2,6,6-tetramethylpiperidinyloxy)and unsaturated MA-containing PE chains. The DFT investigation hashighlighted a low-energy pathway for termination of the PE-MA(& BULL;) radicals by disproportionation, promoted by & beta;-Helimination/dissociation and H-atom abstraction from the Pd-II-H intermediate by a second radical. This phenomenon appearsto be the main reason for the failure of this Pd-II systemto control the radical polymerization of MA by the OMRP (OMRP = organometallic-mediatedradical polymerization) mechanism.

Automated summary

This work investigated the thermal switchability of ethylene coordination/insertion to controlled radical polymerization of methyl acrylate using Brookhart-type α-diimine Pd-II catalysts. The impact of bulky α-diimine N-substituents was explored, revealing the reversible activation of the Pd-II-C bond and trapping during radical MA polymerization. DFT calculations evaluated the sterics effect on the stability of PE-MA-Pd-II chain-end structures and the bond dissociation-free energy for Pd(II)-C bond cleavage. Experimental investigations showed the failure of the Pd-II system to control radical polymerization of MA through OMRP mechanism.