Termination of Carbomethoxyethyl Radicals Generated by Br‐Atom Abstraction From MeCH(Br)COOMe With Metalloradicals: Implications for Polyacrylate Radical Termination

Acrylate radical termination is still controversial, with propositions spanning from 100% combination (Comb) to 100% disproportionation (Disp). This contribution analyzes the termination of CH 3 CH • (COOCH 3 ) (R H • ), generated by bromine abstraction from methyl 2‐bromopropionate (MBP) by metalloradicals ([(CO) 5 Mn • ], Me 2 E • (E = Si, Ge, Sn), n Bu 3 Sn • , Ph 3 Pb • , Ph 2 Sb • ). The Disp/Comb ratio strongly depends on the metalloradical nature (from 100:0 for Si 2 Me 6 to 37.5:62.5 ± 3.2 for Pb 2 Ph 6 ). Control experiments and comparative terminations of (CH 3 ) 2 C • (COOCH 3 ) (R Me • ), supported by DFT calculations, lead to a coherent mechanistic interpretation. Bimolecular termination of diffused R H • predominantly leads to Comb, while competing interception by a second metalloradical leads to 100% Disp via hydrogen‐atom transfer (HAT) in a {R H • ,metalloradical} caged pair, yielding a hydride intermediate ([(CO) 5 Mn‐H], Me 2 E‐H (E = Si, Ge, Sn), n Bu 3 Sn‐H, Ph 3 Pb‐H, or Ph 2 Sb‐H). An additional pathway, involving HAT within the caged {Me 3 E • , • EMe 3 } to yield Me 3 E‐H and Me 2 E=CH 2 intermediates, also contributes when E = Si and Ge. Selective generation of the {R H • ,metalloradical} caged pair by photolytic activation of PhTe‐R H or by the single‐electron transfer (SET) reaction of MBP with NaPbPh 3 leads to 100% Disp via the HAT process.